Efficiency of Steam and Hot Water

Heat Distribution Systems

Gary Phetteplace September 1995

CRREL REPORT 95-18
Abstract

This report will provide some general guidance on the selection of distribution medium (steam or hot water) and temperature for heat distribution

systems. The report discusses the efficiency of both steam and hot water

heat distribution systems in more detail. The results of several field studies

using data from boiler plant logs and measured heat losses are given. For

steam, an efficiency analysis for the steam heat distribution system at

Hawthorne Army Ammunition Plant is summarized. This analysis is based

on the limited data available from the boiler logs maintained at the central

plant. From this information, along with energy and mass balances that

are constructed from the central plant data, gross measures of efficiency

are obtained. The results of the analysis show that only 43.5% of the

steam input to the distribution system is used to meet the required space

heating load. The results also indicate that on average only 46.2% of the

steam that leaves the plant returns as condensate. By converting this

steam distribution system to a low temperature hot water heat distribution

system, savings would exceed $292,000 for the 181-day study period,

which represents a typical heating season. For hot water based systems

this report describes two field projects underway at U.S. Army bases. At

Fort Jackson, South Carolina, a medium-temperature hot water heat distribution system has been monitored. Three different types of system construction have been instrumented: pipes enclosed in a shallow concrete

trench, steel conduit with supply and return pipes in common conduit, and

separate steel conduits for supply and return pipes. At Ft. Irwin, California,

a low-temperature hot water system has been monitored. Two sites have

been instrumented on this direct buried system that consists of steel carrier

pipes insulated with polyurethane foam protected by a fiberglass jacket.

The data provide a clear illustration of the much lower heat losses from the

low temperature water systems.

For conversion of SI units to non-SI units of measurement consult ASTM

Standard E380-93, Standard Practice for Use of the International System

of Units, published by the American Society for Testing and Materials,

1916 Race St., Philadelphia, Pa. 19103.

Cover: Simplicity of the low temperature heat distribution system (bottom);

complexity of high temperature heat distribution system (top).
CRREL Report 95-18

Efficiency of Steam and Hot Water

Heat Distribution Systems

Gary Phetteplace September 1995

Prepared for

OFFICE OF THE CHIEF OF ENGINEERS

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
ii

PREFACE

This report was prepared by Gary Phetteplace, Mechanical Engineer of the Applied Research Division, Research and Engineering Directorate, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire. Funding was

provided by DA Project 4A762784AT42, Research in Snow, Ice and Frozen Ground, Task

CO, Work Unit M06. The author thanks Barry Coutermarsh and Herbert Ueda of

CRREL for technical review of the manuscript.

This field work was primarily sponsored by the Army’s Facilities Engineering

Applications Program (FEAP), which seeks to demonstrate and facilitate the implementation of appropriate commercially available technologies for Army facilities.

The author acknowledges the support of Bernie Wasserman of the Army’s Center for

Public Works, Satish Sharma of the Office of the Assistant Chief of Staff for Installation Management, and Dale Otterness, Headquarters U.S. Army Corps of Engineers,

in obtaining funding for this study. Frank Hall, U.S. Army Corps of Engineers Area

Office at Ft. Jackson, provided a great deal of assistance in installing and maintaining

the equipment. Neil Lowell, Eric Swedberg, and Jim Blodgett all of the U.S. Army

Corps of Engineers Sacramento District provided assistance on both the Ft. Irwin and

Hawthorne AAP studies. The Hawthorne AAP studies were aided by Minnie Williams and Alfred Bandoni of the Directorate of Engineering and Housing office. At Ft.

Irwin, Orvie Dahl of the Directorate of Engineering and Housing, as well as several

people from the Area Office of the U.S. Army Corps of Engineers, Sacramento District, provided us with support. David Carbee of CRREL has been responsible for

most of the data acquisition from these sites. Much of the data reduction was conducted by Rosanne Stoops and John Bement of CRREL.

The contents of this report are not to be used for advertising or promotional

purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.
iii

CONTENTS

Page

Preface ................................................................................................................................. ii

Introduction ....................................................................................................................... 1

Design considerations for heat distribution systems .............................................. 1

Distributed media selection ...................................................................................... 2

Low temperature water design issues .................................................................... 3

Efficiency analysis of a stream heat distribution system........................................ 5

Heat generation and distribution at Hawthorne AAP ....................................... 5

Heating loads on the Hawthorne AAP system .................................................... 6

Results for the Hawthorne system .......................................................................... 7

Conclusions regarding the Hawthorne AAP steam system .............................. 11

Heat loss measurements ................................................................................................. 13

MTHW common conduit system ............................................................................. 13

MTHW individual conduit system ......................................................................... 13

MTHW shallow concrete trench system ................................................................ 13

LTHW preinsulated system, site 1........................................................................... 14

LTHW preinsulated system, site 2........................................................................... 14

Instrumentation, data logging and communication systems ........................... 15

Data analysis ................................................................................................................ 15

Insulation method ....................................................................................................... 15

Soil method, one pipe or conduit ............................................................................ 17

Method for two buried pipes in individual conduits ......................................... 17

Method for two pipes buried in a common conduit ........................................... 17

Soil method for two direct buried pipes ................................................................ 17

Results of heat loss measurements .......................................................................... 17

MTHW trench site ....................................................................................................... 17

MTHW common conduit site ................................................................................... 18

MTHW individual conduit site ................................................................................ 19

LTHW preinsulated system, site 1........................................................................... 20

LTHW preinsulated system, site 2........................................................................... 21

Conclusions drawn from heat loss measurements .............................................. 22

Conclusions ....................................................................................................................... 23

Selected bibliography ...................................................................................................... 23

Abstract .............................................................................................................................. 25

ILLUSTRATIONS

Figure

1. An example of temperature and flow rate modulation ................................. 4

2. Steam flow from the plant as a function of degree days ............................... 7

3. Adjusted average makeup flow rate as a function of degree days ............. 10

4. Adjusted makeup rate as a function of degree days ...................................... 11

5. Construction details for the MTHW common conduit site ........................... 13
iv

Figure Page

6. Construction details for the MTHW individual conduit site ..................... 13

7. Construction details for the MTHW trench site ............................................ 14

8. Construction details for the LTHW site 1 ....................................................... 14

9. Construction details for the LTHW site 2 ....................................................... 15

10. Heat losses for the MTHW trench site ............................................................. 18

11. Heat losses from the MTHW common conduit site ...................................... 19

12. Heat losses from the MTHW individual conduit system ............................ 20

13. Heat losses from the LTHW site 1 system....................................................... 21

14. Heat losses from the LTHW site 2 system....................................................... 22

TABLES

Table

1. Sample data and calculated results for the Hawthorne AAP steam

system .............................................................................................................. 8

2. Averages of some calculated and measured values for the study sites ... 16
INTRODUCTION

Most major Department of Defense (DoD) facilities are heated using central heat distribution systems. The heat from the central heating plants usually is distributed to the buildings as high-temperature hot water or steam through buried piping systems. DoD has approximately 10,000 km (6000 mi)

of heat distribution piping systems in service (Segan

and Chen 1984). Many of these systems are old and

in need of major repairs or replacement. To replace

these systems currently costs about $1000/lineal m

($300/lineal ft). Thus, the DoD is facing monumental costs for replacement. In addition, the technology currently being used by DoD is problematic.

Many systems that have been recently replaced have

failed prematurely. A previous study done by the

Corps of Engineers (Segan and Chen 1984) identified many problems caused by improper design,

installation, and maintenance. Most of these problems led to premature failure of the systems. For the

Army’s 1992 fiscal year the annual maintenance

costs were over $24 million (U.S. Army 1992). This

does not include any of the larger replacement

projects. In addition, as will be shown later, the cost

of heat loss is much greater.

In an effort to reduce the installation and operating cost of these systems, the Army has an active

research and development program in heat distribution technology. The objective of DoD heat distribution research is to identify improvements in methods and systems that will prove to be less costly and

problematic. This report gives sample results from

several of the Army’s research projects.

One of the major components of the Army’s heat

distribution research is the accurate assessment of

heat losses. As well as being an essential part of the

design of these systems, determination of heat losses

is a primary factor in making repair/replace decisions. If we assume an optimistic value for average

heat loss from current systems of 48 W/m (50 Btu/

hr-ft) (for older systems and those with large pipes,

greater than 4 in. or 100 mm, a value of several times

this is likely) and a cost of $9.48/GJ ($10/million

Btu’s) for heat energy, we find that heat losses cost

the Army around $85 million per year. Before giving details on several of the heat distribution efficiency studies that CRREL has done, we will outline

some of the basic design considerations for these

systems.

DESIGN CONSIDERATIONS FOR

HEAT DISTRIBUTION SYSTEMS

The design of heat distribution systems is much

more complicated than the design of other piped

utilities such as potable water and sewage systems.

Because of the need to conserve thermal energy as

well as the hot water or steam itself, the systems

become much more complicated and costly, from

both the installed cost and operations and maintenance standpoints. The major challenge in the design of heat distribution systems is keeping the thermal insulation dry so that it remains effective. This

becomes a formidable task because the system is

usually buried in the ground, which is frequently

saturated with water, and it is carrying water inside

the pipe. Fortunately, the heated pipes tend to drive

moisture out of the insulation. Unfortunately, the

elevated temperatures accelerate corrosion as well,

which tends to increase the frequency of leaks. Leaks

are very costly in heat distribution systems because

not only is the treated water itself costly, but the

thermal energy contained in the water is also lost

when a leak exists.

Thus, the design of heat distribution systems

requires careful examination of the alternatives once

Efficiency of Steam and

Hot Water Heat Distribution Systems

GARY PHETTEPLACE
the site conditions and project requirements are

known. Some of the basic issues that impact distribution system efficiency are addressed below. For a

more complete discussion of the issues and alternatives Phetteplace and Meyer (1990) and ASHRAE

(1992) should be consulted.

Distributed media selection

District heating began in the United States in the

late 19th century. These early systems used steam as

the heat-carrying medium. The steam-based systems thrived with the inexpensive energy prices of

the time and many systems were built, with a large

number of them still operational today. The very

high load density of the densely developed downtown areas these steam systems serve allows them

to tolerate the large distribution losses and even

today remain competitive with alternative means of

building heating.

After World War II many district heating systems were built in European cities. Both steam and

hot water-based systems were built. As the technology for the hot water systems evolved, it became

clear that they were a good alternative to the steam

systems. The hot water systems eliminated the problems of condensate handling and were easier to

control than the steam systems. Thus, most systems

built in Europe after the war were hot water based.

This technology was brought to the United States in

the early 1950s and a number of hot-water based

systems were built in the United States, particularly

on DoD facilities. These systems used pressurized

water at temperatures above 175°C (350°F) in most

cases. Hot water systems are normally broken into

three temperature classes. Systems with supply temperatures over approximately 175°C (350°F) are usually considered to be high temperature hot water

(HTHW) systems. Medium temperature hot water

(MTHW) systems have supply temperatures in the

range of 120 to 175°C (250 to 350°F). Low temperature hot water (LTHW) systems have supply temperatures of 120°C (250°F) or lower. In practice, the

return temperature from all these types of systems

varies widely, with the higher temperature systems

tending to have higher return temperatures. Achieving a large temperature difference between supply

and return is desirable because this reduces the

amount of water that needs to be circulated for a

given heat delivered.

While the high temperature hot water systems

proved to be much less problematic than steam

systems, they still suffered from relatively high levels of heat loss, as well as other problems associated

with elevated temperatures and pressures. However, with energy costs still low, they offered a good

compromise because large temperature differentials

could be achieved and thus mass flow rates and

pipe sizes remained lower than was possible with

water at lower temperatures. With low temperature

hot water it would have been more difficult to

achieve temperature differentials of the same magnitude with the heat exchanger technology of the

time. In Europe, however, energy costs were higher

and lower temperatures were shown to offer lower

life cycle costs, particularly since the lower temperatures were much more favorable for cogeneration of heat and electricity. A number of other material developments, such as suitable nonmetallic jacket

materials and polyurethane foam insulation, applicable only to lower temperature systems, further

widened the advantage these systems held. These

materials either were not able to withstand the higher

temperatures or degraded very rapidly under such

conditions. Currently nearly all systems in Europe

use low temperature hot water. Such systems have

been so successful in Europe that the market penetration is very high, for example, approximately

50% in Denmark and Finland. The major advantage

that low temperature water holds with regard to

heat losses is best illustrated by actual field measurements made on low temperature hot water and

medium temperature hot water systems installed

on Army bases. A study that made such measurements is described later in this report.

Regardless of whether steam or hot water is chosen, the temperature and pressures used should be

only as high as required to satisfy the requirements

of the consumers. This cannot be overemphasized.

Higher temperatures result in higher heat losses, as

will be shown from the data presented later. In

addition, higher temperatures and pressures result

in numerous other problems such as accelerated

corrosion, higher rates of leakage, and lower safety

and comfort levels for operators and maintenance

personnel. Higher temperatures also may require

higher pressure ratings for piping and fittings and,

in addition, may preclude the use of desirable materials such as polyurethane foam insulation and nonmetallic conduits. For hot water systems it is im–

portant to design for a high temperature drop at the

consumer. This reduces the flow rate through the

system and thus the pumping power required. Higher temperature drops at the consumer also result in

lower return temperatures, which in turn result in

lower return line heat loss, as the data to be presented will show.

 Although hot water is widely regarded as the

best alternative where either medium will meet the

2
requirements, in many instances existing equipment

and processes will require the use of steam. Some

common examples of where steam may be required

on DoD installations are laundry facilities, mess halls,

motor pools, and hospitals. A careful study should

be made of the alternatives before selecting the distribution medium and temperature. A list of common attributes with a brief discussion of the relative

merits of each medium is given below.

Heat capacity

Steam has a distinct advantage as it relies primarily on the latent heat of water rather than the sensible heat. The net heat effect for saturated steam at

6.9 bars (100 psig) (170°C or 338 °F) condensed and

cooled to 82°C (180°F) is approximately 2.42 mJ/kg

(1040 Btu/lbm). For hot water cooled from 175° to

120°C (350° to 250°F) the net heat effect is 0.24 mJ/kg

(103 Btu/lbm), or only about 10% as much. Thus, a

hot water system must circulate about 10 times more

mass than a steam system of similar heat capacity.

Pipe sizes

Although much less mass of steam is required

for a given heat load, steam usually requires a larger

pipe size for the supply line due to its lower density.

This is compensated by a much smaller condensate

return pipe. Piping costs for steam and condensate,

as opposed to hot water supply and return, are

comparable. With regard to the temperature level in

hot water systems, given an equal temperature difference between supply and return, the temperature level of the supply will have no effect on the

mass flow rate. Thus, aside from any minor effects

caused by viscosity and density differences, a low

temperature water system with a temperature difference of 55°C (100°F) will have the same size pipes

as a high temperature system with a temperature

difference of 55°C (100°F). Because the heat can be

used only down to some practical lower limit of

temperature, low temperature hot water systems

may not have as high values of temperature difference between supply and return. Thus, if temperature difference were the only consideration, higher

temperature hot water systems would be favored.

However, when the increased heat losses, maintenance and system complexity that result from higher

temperatures is considered, lower hot water temperatures become more favorable. With proper design

they will not require significantly larger piping.

Return system

Condensate return systems have proven to be

very problematic. Corrosion of the piping is a major

problem. Nonmetallic piping has been used with

only limited success. For these reasons condensate

was seldom returned from older systems. However, at current energy costs it becomes imperative

to do so. Condensate drainage systems (steam traps,

condensate pumps, and receiver tanks) have also

proven to be problematic, corrosion again being

one of the major sources of problems. Although

much developmental effort has been expended on

steam traps, the best designs today still have a relatively short (less than five years) life expectancy and

will leak several pounds per hour of steam in the

closed condition even when new. In summary, problems in condensate collection and return are the

major disadvantages of steam systems.

Pressure requirements

Flowing steam and hot water both incur pressure losses. Hot water systems may use intermediate booster pumps to increase the pressure at points

between the plant and the consumer. Pressure variations due to elevation differences within hot water

systems are much greater than for steam systems

due to the higher density of water. This can adversely affect the economics of a hot water system

by requiring the use of a higher pressure class of

piping and/or booster pumps.

Low temperature

water design issues

Because low temperature hot water systems are

the least common system type in the United States,

designers are often not as familiar with these systems. For this reason we will address a few issues

with hopes of dispelling some common myths.

From a designer’s perspective low temperature

water systems are no more difficult, and in most

cases simpler, than steam and high temperature

water systems. Distribution system design is straightforward and is less complicated than steam or high

temperature hot water because of the lower temperatures and pressures involved. Because of the

high degree of standardization of these systems, a

number of specialized components are available that

simplify the design process.

Low temperature water is well suited to the gradual change-over of the distribution medium from

steam or high temperature hot water. When major

distribution replacement projects are under consideration, or a new area of a base or community is

being expanded into, low temperature water can be

used simply by putting a heat exchanger station

between the existing network and the new low temperature system. In this way gradual conversion

3
can take place as portions of the distribution system

are replaced, thus making the economics more attractive.

Despite their widespread acceptance in Europe

and several successful recent installations in the

United States, low temperature hot water systems

are still associated with a number of myths. Some of

the more commonly held misconceptions are described below.

Low temperatures are not suitable for large systems.

One only needs to look at some of the enormous

systems in Europe to be convinced that this is clearly not the case. For example, all of Copenhagen and

the surrounding towns are tied into one big low

temperature system. St. Paul, Minnesota, has a large

system and several other towns in the United States

have fairly large systems. Because of the high efficiency of urethane insulation and the lower temperatures, heat losses are low (approximately 5% of

system capacity) and significant temperature drops

during transport are simply not a problem. For example, consider the 150-mm (6-in.) LTHW system

at Ft. Irwin, discussed in more detail later. The average heat loss from the supply pipe is only 20 W/m

(21 Btu/hr-ft) (Phetteplace 1992). If a modest flow

velocity of 1.5 m/s (5 ft/s) is assumed the water in

this supply pipe would experience a temperature

drop of only about 0.3°C (0.6°F) over a transport

distance of one mile.

Low temperature systems require much larger piping.

As stated earlier, for a given flow rate, the amount

of heat conveyed depends only on the temperature

difference between supply and return, not the absolute value of the supply temperature. With proper

design, low temperature water systems can and do

achieve temperature differences of 55°C (100°F), just

like medium and high temperature water systems.

Low temperatures and pressures make it easier to

control the building space heating and domestic hot

water heating systems; thus, it is easier to maintain

significant temperature differences between supply

and return even in times of lower load. In larger

systems, the flow rate is usually modulated along

with the temperature in order to meet the varying

load conditions while keeping the cost of pumping

and heat losses as low as possible. The temperature

modulation is done in a manner similar to the way

the temperature of hydronic heating systems in

buildings are reset based on outdoor air temperature. An example of temperature and flow rate

modulation is given in Figure 1 (from Phetteplace

and Labbe 1978).

Even when temperature differences between supply and return are somewhat smaller, pipe sizes do

not need to be increased significantly. For example,

consider the Ft. Irwin LTHW system and the Ft.

Jackson MTHW system, which will be discussed in

more detail later. Assume a flow velocity of 1.5 m/s

(5 ft/s) in each system and a temperature difference

of 36°C (65°F) for the LTHW system and

55°C(100°F) for the MTHW system. The heat conveyed by the MTHW system with 125-mm (5-in.)

piping is about 3.81 MW (13.0 MBtu/hr). For the

LTHW system with 150-mm (6-in.) piping the heat

conveyed is only slightly lower at 3.72 MW (12.7

MBtu/hr). Despite the fact that the flow rate in the

LTHW system would need to be about 50% higher

than for the MTHW system, the pumping energy

costs would only be about 25% higher as a result of

the larger pipe size. Pumping energy is usually a

0 10 20 30 (kW)

0 20 40 60 80 100 103 (Btu/hr)

Used Load (kW)

200

180

160

140

0.20

0.15

0.10

0.05

0

0.1

0.2

0.3

0.4

Mass Flow Rate

(lbm/s) (kg/s)

100

90

80

70

60

Supply Temperature

(°C) (°F)

Design Conditions

Flow Rate

Modulation

Temperature

Modulation

0.179 0.396 95 203

7.49

26

Figure 1. An example of temperature and flow rate modulation.

4
small portion of the delivered energy (2–5%), so this

additional cost is insignificant when compared to

the greatly reduced heat losses.

Low temperature systems require larger heat exchangers. This is true; however, it does not become a

major disadvantage for several reasons. Because of

the lower temperatures, it is often possible to eliminate the need for heat exchangers for building heating altogether. Since higher temperatures must ultimately be reduced within the building systems under most circumstances, lower supply temperature

is seldom a problem in space heating applications.

If heat exchangers are needed or desired for isolation purposes, larger buildings can use plate-andframe heat exchangers which, because of their large

surface area to volume ratios, are economical for

LTHW applications. For domestic hot water production, careful design of the heat exchange system

can yield a very close approach temperature between the LTHW supply and the delivered domestic hot water, again eliminating any problems with

inadequate temperature at the end user. In smaller

buildings, “brazed” heat exchangers, which are very

similar to plate and frame units except they cannot

be disassembled, are becoming very popular in Europe because of their low cost and extreme compactness. For example, a stainless steel brazed heat

exchanger including preformed insulation, for a residential size application is about the size of a lunch

box, and would cost about $200.

The large surface area to volume ratios which

can be achieved in plate type heat exchangers make

their performance impressive. Consider, for example, a typical design for a brazed plate unit that

might be used on a small space heating or domestic

water heating load of about 20 kW (70,000 Btu/hr).

With a primary water supply temperature of 120°C

(250°F) and design secondary side temperatures of

90°C (195°F) supply and 70°C (160°F) return, the

approach temperature would be less than 0.5°C (1°F).

This means that the primary return water would be

at about 70.5°C (161°F), within 0.5°C (1°F) of the

theoretical minimum of the entering secondary water temperature. The flow rates through the heat

exchanger would be about 6 liters/min (1.6 gpm)

on the primary side and 14.8 liters/min (3.9 gpm)

on the secondary side. Pressures losses would be

about 34.5 kPa (0.5 psi) on the primary side and 145

kPa (2.1 psi) on the secondary side. The heat exchanger used for this sample design would be about

53.3 cm (21 in.) long, 11.4 cm (4.5 in.) wide, and 4.1

cm (1.6 in.) thick with 14 plates and an empty weight

of less than 5.4 kg (12 lb).

The above discussion clearly illustrates that low

temperature hot water is a feasible, and in most

cases, desirable heat distribution medium for DoD

facilities. When new systems or major replacement

projects are planned, serious consideration should

be given to this technology.

To illustrate the efficiency of several types of

heat distribution we will present results from several field studies conducted by CRREL over the last

few years. First we will discuss an efficiency analysis conducted on a steam system at Hawthorne AAP.

EFFICIENCY ANALYSIS OF A STEAM

HEAT DISTRIBUTION SYSTEM

The following efficiency analysis was conducted

on data obtained from Hawthorne Army Ammunition Plant (Hawthorne AAP). This facility is located

in Mineral County, Nevada, about 220 km (135 mi)

southeast of Reno. Hawthorne AAP is located in a

high desert region 1276 m (4,186 ft) above sea level.

The 99% dry bulb heating design temperature is

–13.9°C (7°F) and the 97.5% dry bulb heating design

temperature is –11.7°C (11°F) (U.S. Army 1978). This

climate accumulates 3060°C-days (18.3°C base)

(5508°F-days, 65°F base) during an average heating

season (U.S. Army 1978).

The compound of Hawthorne AAP covers about

5.87 × 108 m2 (145,000 acres) and has 2,873 buildings. The land and buildings can be divided into

two categories: industrial and ordnance. This study

addresses only the buildings in the industrial area

of Hawthorne AAP. These buildings consist of offices, shops, housing units, and recreational and

other facilities. Most of these buildings are heated

by steam from a central distribution system. The

steam is generated in a central plant using fuel-oilfired boilers. The conversion of this central heat

distribution system from steam to low temperature

hot water was the subject of a detailed study

(Phetteplace 1991a).

Heat generation and distribution

at Hawthorne AAP

The existing steam distribution system at Hawthorne AAP consists of a steam supply line and a

condensate return line. The distance along the steam

line from the central plant to the most distant consumer is less than 1 mile. The steam pressure is

normally around 6.9 bars gage (100 psig) at the

entrance to the distribution system. This pressure is

reduced in the buildings to a nominal 1 bar (15

psig). In the housing area, a central pressure reducing station reduces the pressure to approximately

5
of increased maintenance on the boiler plant equipment or distribution system was apparent to the

operators at the time of the study, the long-term

effect due to thermal cycling of the system is likely

to be detrimental.

All of the boilers are fired with no. 2 fuel oil,

which is delivered by truck and stored in several

large holding tanks. Measurements on the boilers

using combustion analysis have shown the efficiency

to be around 85%. No steam flow data from the

central plant are available due to lack of a functional

flow meter. The individual buildings are not

equipped with flow meters, consistent with standard practice on most DoD installations. Daily boiler logs are maintained for the heating plant. These

boiler logs contain various temperature and pressure readings taken on a hourly basis. Records of

daily fuel used, in storage, and delivered are maintained on the boiler logs as well. By using limited

data available from the boiler logs with heat and

mass balances for the system and some statistical

analysis, we can find some estimates of the losses in

which we are reasonably confident. The details on

the development of the model used for this purpose

are given in Phetteplace (1991a).

Heating loads on the

Hawthorne AAP system

Without meters on the buildings or at the plant it

becomes necessary to estimate the heating load on

the system from weather data. The concept of the

heating degree day has been used extensively for

this purpose. Details on its use can be found in

ASHRAE (1985). For the purposes of this analysis

we need only to assume that the heating energy

required over a specified period of time is proportional to the degree days accumulated over that

same period of time. Werner (1984) proposes a much

more detailed model of the heat load in district

heating systems. From his study of the degree-day

concept he concludes that it “... does not accurately

explain the composition of the space heating demand.” Although it is agreed that the degree-day

concept does not accurately account for such factors

as solar gain and infiltration, we feel that for the

purposes of an efficiency analysis such as this, its

use is acceptable as an alternative to more complicated models such as those proposed by Werner

(1984). Data from a fairly large (approximately 75-

MW or, 255 MBtu/hr) steam district heating system

on Ft. Wainwright, Alaska, (Phetteplace et al. 1981)

indicate that it does track the heat load reasonably

well if the effects of the distribution system losses

and domestic tap water heating are considered.

For residential and office buildings normally

0.34 bars gage (5 psig) before distribution to the

individual family housing units.

Most of the main steam lines for the distribution

system are contained in shallow concrete trenches

whose covers are at grade level and used as sidewalks. A small portion of the steam distribution

piping is directly buried in either steel conduit systems or loose fill insulation materials. For a more

detailed description of the construction details for

these types of systems, see Phetteplace and Meyer

(1990). In most all cases the condensate return piping is routed entirely independently of the steam

piping. Most of the return piping is buried directly

in the soil and is uninsulated brass or steel piping.

The condensate return is by gravity in most places.

An unusual arrangement called a “sand pit” is used

in the condensate return system. The sand pits resemble sewer manholes in that the incoming condensate flows into a concrete basin and then out into the

outflow piping at the level of the condensate in the

sand pit. Unlike a sewer manhole, the bottom of the

sand pit is not concrete but is formed by the native

sandy soil. To the extent that this soil is not clogged

with particulates, the condensate is allowed to percolate into it. This arrangement may account for a

significant portion of the mass loss from the distribution system, as will be discussed later.

Very few of the buildings at Hawthorne AAP are

equipped to use steam from the central system to

heat hot water for domestic uses. Electric hot water

heaters are used in most cases. The notable exceptions to this are two apartment buildings that have

large hot water storage tanks in their basements.

These are equipped with steam heat exchangers as

well as electric heating elements. Although steam

has heated these tanks in the past, the electric heaters were in use at the time of our study. Personnel

from the operating contractor indicated that the electric heaters have been used exclusively in recent

years. Earlier they had been operated only when the

heat distribution system was not in operation.

At the time of our analysis the operating policy

was to turn off the central steam distribution system

and the heating plant during times of low load. A

portion of the system that serves a residential section of the base was sometimes shut down prior to

the remainder of the system. The entire system was

shut down for the summer months and was occasionally shut down and restarted during the “swing”

periods of the fall and the spring seasons as well.

This practice was instituted several years earlier by

the operating contractor as an energy conservation

measure. Because the system is not used to generate

domestic hot water, this presents no problem from

a heat supply standpoint. Although no evidence

6
heated to about 21°C (70°F) the base temperature

for the degree day calculation is taken as 18.3°C

(65°F). For other types of buildings that are not

heated to as high a level, lower base temperatures

can be used. The portion of Hawthorne AAP under study had residential and office buildings that

would normally be heated to about 21°C (70°F) as

well as shop type buildings that would not be

heated to as high a level. In order to more accurately model the energy use of the shop buildings, a base temperature of 12.8°C (55°F) was used

for these buildings. The degree days were calculated on a daily basis for each of the two base

temperatures. These were then combined by forming a weighted average of the two degree-day

figures obtained. The weighting factors used were

determined by taking the relative fraction of total

floor area comprised by each building type. The

floor area of the residential and office buildings

comprised 63.2% of the total heated floor area

and the remaining 36.8% was from shop buildings. For the month of December 1987 the degree

days computed for each of the two base temperatures, as well as the weighted average, are presented in Table 1.

Results for the Hawthorne system

For each of the 181 days of the continuous

period of boiler operation in the 1987/88 heating

season, the quantities in Table 1 were either measured or calculated based on formulas presented

in Phetteplace (1991a). The measured data were

input manually into a commercially available

spreadsheet program, which was used to calcuFigure 2. Steam flow from the plant as a function of degree days.

late the remaining quantities. This information was

then transferred to a commercially available statistical/graphical software package where the remaining analysis was performed.

The first set of results we will present is intended

to enforce the premise that the heating degree day

data provides a reasonable representation of the

heat load from the buildings. Figure 2 shows the

average steam flow rate from the plant as a function

of the weighted average of the heating degree-day

data. Although there is considerable scatter in these

data, they clearly support the concept of the degree

day. The r2 value for the linear relationship that was

fitted to the data is 72%. Since this is the total steam

flow from the plant, it represents all the losses from

the system that would be manifested by steam consumption as well as all energy consumption at the

buildings. The components that make up the total

steam flow out of the plant are the following:

1. Leaks from the steam piping into the environment.

2. Heat losses from the steam lines that cause

the saturated steam to condense. The condensate is then subsequently drained from

the steam pipe and passes through a steam

trap into the condensate return system.

3. The heating load in the building that causes

steam to be condensed to meet this load.

Then the condensate passes through steam

traps into the condensate return system. During the time period when our data were

collected, no steam was used for domestic

tapwater heating.

0

0 8 16 24 32 40 (°F-days)

4 8 12 16 20 24 (°C-days)

Steam Flow Rate

Weighted Average of Degree Days

0.4

4.0

3.2

2.4

1.6

0.8

1.2

1.6

2.0

(lbm/s) (kg/s)

7
8

Table 1. Sample data and calculated results for the Hawthorne AAP steam system.

 Avg. Degree days Weighted Adjusted

 Julian air temp 18.3°C (65°F) 12.8°C (55°F) average DD Fuel use Make-up flow Heat rate Blow Blow down rate Steam flow Make-up make-up

Date date °C (°F) base base °C-day (°F-day) L/hr (gal./hr) kg/sec (lbm/sec) kW (MBtu/hr) downs kg/sec (lbm/sec) kg/sec (lbm/sec) rate rate

01 Dec 87 335 7.2 (44) 11.7 (21) 6.1 (11) 9.6 (17.3) 376 (99.4) 0.758 (1.67) 3451 (11.78) 3 0.0329 (0.072) 1.313 (2.895) 0.577 0.55202 Dec 87 336 6.7 (45) 11.1 (20) 5.6 (10) 9.1 (16.3) 339 (89.5) 0.679 (1.50) 3108 (10.61) 3 0.0329 (0.072) 1.182 (2.606) 0.574 0.54703 Dec 87 337 8.3 (47) 10.0 (18) 4.4 (8) 7.9 (14.3) 308 (81.3) 0.675 (1.49) 2825 (9.64) 3 0.0329 (0.072) 1.068 (2.354) 0.632 0.60104 Dec 87 338 9.4 (49) 8.9 (16) 3.3 (6) 6.8 (12.3) 352 (92.9) 0.675 (1.49) 3227 (11.01) 3 0.0329 (0.072) 1.231 (2.713) 0.548 0.52105 Dec 87 339 5.3 (41.5) 13.1 (23.5) 7.5 (13.5) 11.0 (19.8) 373 (98.5) 0.670 (1.48) 3421 (11.67) 3 0.0329 (0.072) 1.309 (2.887) 0.512 0.48706 Dec 87 340 8.9 (48) 9.4 (17) 3.9 (7) 7.4 (13.3) 315 (83.2) 0.683 (1.51) 2890 (9.86) 3 0.0329 (0.072) 1.093 (2.410) 0.625 0.59507 Dec 87 341 5.3 (41.5) 13.1 (23.5) 7.5 (13.5) 11.0 (19.8) 345 (91.1) 0.688 (1.52) 3165 (10.80) 3 0.0329 (0.072) 1.204 (2.655) 0.571 0.54408 Dec 87 342 4.4 (40) 13.9 (25) 8.3 (15) 11.8 (21.3) 373 (98.6) 0.767 (1.69) 3424 (11.68) 3 0.0329 (0.072) 1.301 (2.868) 0.589 0.56409 Dec 87 343 7.2 (45) 11.1 (20) 5.6 (10) 9.1 (16.3) 345 (91.1) 0.609 (1.34) 3165 (10.80) 3 0.0329 (0.072) 1.212 (2.673) 0.502 0.47510 Dec 87 344 8.9 (48) 9.4 (17) 3.9 (7) 7.4 (13.3) 262 (69.1) 0.561 (1.24) 2401 (8.19) 3 0.0329 (0.072) 0.908 (2.002) 0.617 0.58111 Dec 87 345 5.0 (41) 13.3 (24) 7.8 (14) 11.3 (20.3) 246 (64.9) 0.569 (1.26) 2254 (7.69) 3 0.0329 (0.072) 0.848 (1.870) 0.672 0.63312 Dec 87 346 –1.7 (29) 20.0 (36) 14.4 (26) 17.9 (32.3) 394 (104.2) 0.644 (1.42) 3619 (12.35) 4 0.0438 (0.097) 1.391 (3.066) 0.463 0.43213 Dec 87 347 –2.8 (27) 21.1 (38) 15.6 (28) 19.1 (34.3) 541 (142.9) 0.705 (1.55) 4962 (16.93) 6 0.0657 (0.145) 1.924 (4.242) 0.367 0.33214 Dec 87 348 –6.1 (21) 24.4 (44) 18.9 (34) 22.4 (40.3) 460 (121.5) 0.788 (1.74) 4221 (14.40) 6 0.0657 (0.145) 1.616 (3.562) 0.488 0.44715 Dec 87 349 –4.4 (24) 22.8 (41) 17.2 (31) 20.7 (37.3) 495 (130.8) 0.885 (1.95) 4544 (15.50) 6 0.0657 (0.145) 1.736 (3.828) 0.510 0.47216 Dec 87 350 0.6 (33) 17.8 (32) 12.2 (22) 15.7 (28.3) 486 (128.3) 0.937 (2.07) 4457 (15.21) 6 0.0657 (0.145) 1.696 (3.739) 0.553 0.51417 Dec 87 351 0.6 (33) 17.8 (32) 12.2 (22) 15.7 (28.3) 464 (122.5) 0.850 (1.87) 4256 (14.52) 6 0.0657 (0.145) 1.623 (3.579) 0.523 0.48318 Dec 87 352 –0.6 (31) 18.9 (34) 13.3 (24) 16.8 (30.3) 452 (119.5) 0.780 (1.72) 4150 (14.16) 6 0.0657 (0.145) 1.588 (3.501) 0.491 0.45019 Dec 87 353 0.6 (33) 17.8 (32) 12.2 (22) 15.7 (28.3) 400 (105.6) 0.841 (1.85) 3668 (12.52) 6 0.0657 (0.145) 1.386 (3.057) 0.607 0.55920 Dec 87 354 –2.8 (27) 21.1 (38) 15.6 (28) 19.1 (34.3) 390 (102.9) 0.771 (1.70) 3574 (12.19) 6 0.0657 (0.145) 1.356 (2.989) 0.569 0.52021 Dec 87 355 0.0 (32) 18.3 (33) 12.8 (23) 16.3 (29.3) 412 (108.8) 0.819 (1.81) 3778 (12.89) 6 0.0657 (0.145) 1.433 (3.160) 0.572 0.52622 Dec 87 356 0.6 (33) 17.8 (32) 12.2 (22) 15.7 (28.3) 396 (104.7) 0.775 (1.71) 3635 (12.40) 6 0.0657 (0.145) 1.380 (3.042) 0.562 0.51423 Dec 87 357 –3.9 (25) 22.2 (40) 16.7 (30) 20.2 (36.3) 440 (116.1) 0.828 (1.83) 4033 (13.76) 6 0.0657 (0.145) 1.535 (3.385) 0.539 0.49624 Dec 87 358 –5.6 (22) 23.9 (43) 18.3 (33) 21.8 (39.3) 499 (131.8) 0.819 (1.81) 4578 (15.62) 6 0.0657 (0.145) 1.757 (3.874) 0.466 0.42925 Dec 87 359 –7.2 (19) 25.6 (46) 20.0 (36) 23.5 (42.3) 465 (122.9) 0.828 (1.83) 4267 (14.56) 6 0.0657 (0.145) 1.630 (3.595) 0.508 0.46826 Dec 87 360 –6.1 (21) 24.4 (44) 18.9 (34) 22.4 (40.3) 422 (111.5) 0.780 (1.72) 3874 (13.22) 6 0.0657 (0.145) 1.476 (3.254) 0.528 0.48427 Dec 87 361 –4.4 (24) 22.8 (41) 17.2 (31) 20.7 (37.3) 465 (123.0) 0.793 (1.75) 4270 (14.57) 6 0.0657 (0.145) 1.635 (3.605) 0.485 0.44528 Dec 87 362 –2.8 (27) 21.1 (38) 15.6 (28) 19.1 (34.3) 457 (120.8) 0.885 (1.95) 4196 (14.32) 6 0.0657 (0.145) 1.596 (3.518) 0.555 0.51329 Dec 87 363 –3.3 (26) 21.7 (39) 16.1 (29) 19.6 (35.3) 451 (119.2) 0.898 (1.98) 4140 (14.13) 6 0.0657 (0.145) 1.571 (3.465) 0.571 0.53030 Dec 87 364 0.0 (32) 18.3 (33) 12.8 (23) 16.3 (29.3) 450 (118.8) 0.810 (1.79) 4125 (14.08) 6 0.0657 (0.145) 1.575 (3.472) 0.515 0.47331 Dec 87 365 –6.1 (21) 24.4 (44) 18.9 (34) 22.4 (40.3) 456 (120.5) 0.845 (1.86) 4183 (14.27) 6 0.0657 (0.145) 1.594 (3.516) 0.530 0.489Totals 547 (985.0) 375.0 (675.0) 483.5 (870.3) 151Averages 0.7 (33.2) 17.7 (31.8) 12.1 (21.8) 15.6 (28.1) 407 (108) 0.762 (1.680) 3737 (13) 4.9 0.0533 (0.118) 1.425 (3.141) 0.543 0.506
The steam use due to leaks in the steam distribution piping is basically only a function of the pressure in the steam supply pipe. Since the supply

pressure at the inlet to the distribution system is

maintained at a reasonably constant level, the rate

of steam leakage is also fairly constant. When higher

loads are experienced at the buildings, the increased

steam flow rate will result in greater pressure losses

and thus the pressure will be lower under the higher

load conditions. This effect will of course increase as

the distance from the plant increases. The Hawthorne AAP system is a relatively small system with

the longest distance along the pipe from plant to

consumer being about 1035 m (3400 ft). For this

reason, the amount of steam used due to leaks will

be assumed to be essentially independent of the

space heating load.

Steam use resulting from heat losses in the distribution system is a function of the temperature of the

surroundings, and thus it is also correlated with the

degree days accumulated. This function is basically

proportional to the temperature difference between

the steam in the pipe and the adjacent surroundings. Since most of the main steam lines are contained in concrete trenches whose covers are at grade

level, there is little time lag between the time of an

air temperature change and the time at which the

steam system experiences heat losses proportional

to the new temperature. The dependence of the rate

of heat loss from the steam pipe on the ambient air

temperature is not nearly as dramatic as that of the

building heating load. Since the elevated temperature of the steam pipe results in a large temperature

difference between itself and the environment, the

changes in this temperature difference due to

changes of the environmental temperature are relatively small. For example, during the 1987/88 heating season the largest temperature difference between the mean ambient air temperature and the

steam pipe occurred on 25 December when the temperature difference was 177.20°C (170 to –7.2) (319°F).

By contrast this temperature difference was at its

lowest value during the 1987/88 heating season on

27 October when it was 154.4°C (170 to 15.6) (278°F).

Thus, this temperature difference is only 12.8% less

at its minimum than when at its maximum. The rate

of heat loss will vary by a somewhat lesser amount,

since the thermal mass and thermal resistance of the

concrete trench and surrounding soil will tend to

dampen the effective temperature difference between the steam pipe and the environment. Hence,

during this period when the space heating load

would range from essentially zero to its maximum

value, the heat losses from the steam distribution

system will undergo much less variation. For this

reason we will assume that the heat losses from the

steam pipe are constant over the heating season.

This allows us to lump the steam use due to these

heat losses with the steam use due to leaks.

The remaining steam load on the system, that

due to the space heating requirements of the building, will vary substantially with the outdoor air

temperature and thus the degree days accumulated.

In fact, if our degree-day model were an exact representation, this load would disappear when the average outdoor air temperature for the day reached

18.3°C (65°F), the point at which no degree days

would be accumulated for the day. In reality this is

not true for several reasons, the primary one being

inadequate control of the heating system in the buildings. Some of the building heating systems at

Hawthorne AAP do not shut off completely when a

specified indoor temperature level is achieved. This

is a common problem in steam-heated buildings,

resulting primarily from leaking valves that do not

completely close. Thus, some portion of the load

due to space heating at the buildings is residual

after degree days are no longer being accumulated.

This must be lumped with the “fixed” portion of the

load on the system, resulting from steam leaks and

heat losses from the steam distribution piping.

A linear function has been regressed onto the

data of Figure 2 as shown. The slope of the regressed line is 0.0445 kg/s–°C-day (0.0545 lbm/

s–°F-day) and the intercept is 0.663 kg/s (1.46

lbm/s). Based on the discussion presented above,

the intercept would represent the sum of the loads

due to steam leaks, heat losses from the steam

piping, and residual heating of the building due

to inadequate building control systems. The slope

of the regressed line would represent the rate of

increase of steam use as a function of increasing

degree days. One measure of the efficiency of the

distribution system is the ratio of the useful heat

delivered to the total heating energy that leaves

the plant. We can find the useful heat delivered

using the slope of the regressed line from above.

To do so we multiply the slope value from the

regressed function by the length of a day and

then in turn multiply the resulting value by the

total number of degree days for the heating season. For the period of the 1987/88 heating season

under study, the total accumulation of the

weighted average degree days was 2026.5°C-days

(3647.7°F-days). Thus, the total useful steam delivered was 7,791,000 kg (17,176,000 lbm). The

total steam delivered will include, in addition to

this useful steam, that which was consumed due

9
0

0 8 16 24 32 40 (°F-days)

4 8 12 16 20 24 (°C-days)

Average Adjusted Makeup Flow

Weighted Average of Degree Days

0.2

0.8

1.2

1.6

2.0

0.4

0.6

0.8

1.0

(lb/s) (kg/s)

to heat losses and leaks as well as that unnecessarily used for building heating due to inadequate

control. This remaining quantity is found by multiplying the intercept of the regressed line by the

length of the period of the heating season under

study (181 days), the result being 10,368,000 kg

(22,832,000 lbm). Thus the total steam supplied was

18,159,000 kg (40,008,000 lbm). The distribution efficiency, that is the useful steam delivered divided by

the total steam leaving the plant, is 42.9%.

The amount of water which must be added to

the system to account for that lost is called the

makeup. The amount of makeup water is another

important measure of the efficiency of a heat distribution system. Figure 3 shows the average makeup

water flow rate as a function of the weighted average degree days accumulated for each of the 181

days in the study period. This makeup water flow

rate has been adjusted to be more representative of

that due to losses in the distribution system and

terminal equipment by subtracting the flow rate of

water used in boiler blowdowns. Boiler blowdown

is the term used to describe the periodic discharge

of water from the bottom of the boiler. This water is

discharged because it becomes high in contaminants

as the pure water is boiled off.

Mass losses in the distribution system and terminal equipment occur in the following areas:

1. Steam leaks from the steam supply piping.

2. Steam and condensate leaks at the heat consumer’s terminal equipment.

3. Condensate discarded at the consumers due

to lack of a condensate return system.

4. Condensate leaks from the condensate return system.

The first two sources of leaks listed, those in the

steam supply line and the consumer’s equipment,

will be primarily functions of pressure and thus

fairly independent of heating load. To some extent

this is also true of the leaks from the condensate

return system. This system operates by gravity in

most places. The condensate return piping is in poor

condition in many places, however, and larger flow

rates can result in slightly higher pressures or more

completely filled pipes in cases where the pipes are

not always flowing full. Thus, higher rates of leakage could accompany the increased flow resulting

from higher heating loads. The remaining source of

condensate loss, discarding at the consumer, will be

highly dependent on heat load. With the exception

of the portion of the heat load that results from

excessive building heating due to inadequate control, all of the condensate loss from this source will

be proportional to the load. A linear function has

been regressed onto the data of Figure 3. The intercept of this linear function is 0.390 kg/s (0.860

lbm/s) and its slope is 0.020 kg/s-°C-day (0.0247

lbm/s-°F-day). In this case it is more difficult to

determine what sources are responsible for the

“fixed” losses represented by the intercept portion

of the regressed function and what sources are responsible for the “variable” losses represented by

Figure 3. Adjusted average makeup flow rate as a function of degree days.

(lbm/s)

10
the slope of the regressed function. It is fairly certain, however, that most of the variable portion must

be due to the condensate return system or lack

thereof. Since leaks in the steam supply piping and

terminal equipment were not prevalent during site

visits, most of the fixed portion of the leakage can

also be assumed to be in the form of condensate.

Since the condensate has a much lower enthalpy

than the steam, on a unit mass basis, the condensate

leaks are of much less consequence than steam leaks

would be. If we assume that all the mass loss is in

the form of condensate, the equivalent energy loss

would be the difference between the enthalpy of the

condensate and that of the makeup water multiplied

by the total mass loss from the system over the

study period. From the regression equation the total

mass loss is found to be 9,601,000 kg (21,234,000

lbm) and the enthalpy difference is 255.6 KJ/kg

(109.9 Btu/lbm), so that the total equivalent energy

loss from this mass leakage is 2,450 GJ (2,334 MBtu)

for the study period. If 10% of the mass loss were to

be in the form of steam the equivalent energy loss

would be nearly double at 4,820 GJ (4,588 MBtu).

The most common measure of distribution system leakage is the makeup rate, as discussed earlier.

We have adjusted the makeup rate by subtracting

the makeup flow required for boiler blowdown.

This yields a more representative picture of the mass

losses in the distribution system and terminal equipment. Figure 4 shows the adjusted makeup rate as a

function of the weighted average degree days accumulated for each day of the study period. A linear

function has also been regressed onto these data.

The r

2 for this regressed function is only 3.7%, however, so it cannot be considered statistically significant. The average of all the adjusted makeup rate

data for the entire study period is 54%.

Conclusions regarding the

Hawthorne AAP steam system

With limited data we have constructed some

estimates of the thermal and mass losses from the

Hawthorne AAP steam heat distribution system.

These estimates indicate that losses from the system

are very high. Our estimates indicate that only 43%

of the steam leaving the plant is ultimately used for

required space heating. The remainder (57%) is consumed in steam and condensate leaks, heat losses,

and unnecessary overheating of buildings due to

poor control. Most of these losses would be drastically reduced if a low temperature hot water heat

distribution system were retrofitted to Hawthorne

AAP. For example, Werner (1984) reports the heat

losses from six Swedish low temperature hot water

heating systems as ranging from 4.9% to 7.7% of the

total heat production, with the average being 6.6%.

It is also clear from Werner’s (1984) data that the

heating load drops to 10%, or less, of its maximum

value during times of low load. Since this includes

domestic tap water heating as well as distribution

system heat losses, it is apparent that control of the

building heating systems on a low temperature hot

water system must be much more effective than on

the Hawthorne AAP steam system. Based on the

Figure 4. Adjusted makeup rate as a function of degree days.

0

0 8 16 24 32 40 (°F-days)

4 8 12 16 20 24 (°C-days)

Adjusted Makeup Rate

Weighted Average of Degree Days

0.3

0.4

0.5

0.6

0.7

0.8

0.9

11
experiences of the European systems (Werner 1984)

it would seem reasonable to assume that the distribution efficiency of a low temperature water system

might be of the order of 90% or more. This compares very favorably with our estimate of 43% for

Hawthorne AAP.

Our estimates of mass losses from the Hawthorne

AAP steam system are also cause for concern. On

the average only 46% of the steam that leaves the

plant is returned as condensate. Relatively high mass

losses are commonplace in steam systems, primarily due to the problems associated with condensate

collection and return. Makeup rates of 10 to 20% are

normally considered good for steam systems. The

Hawthorne system, however, has a makeup rate of

54%. Some steam systems are even worse than the

Hawthorne AAP system. For example, the author

was recently told of a 10-year-old steam system

designed to return all of the condensate that was in

fact returning only 10% of the condensate, i.e., a

makeup rate of 90%. For a low temperature hot

water system, the makeup rate would most likely

be 5% or less. Here again a significant improvement

can be made by switching to low temperature hot

water.

Economics is usually the driving force behind a

decision to retrofit existing steam and high temperature hot water systems to low temperature hot

water operation. If we make the conservative assumption that there are no leaks in the Hawthorne

steam supply line, we can make some estimates of

the cost savings that could be obtained from converting to low temperature hot water operation.

The fuel use for a low temperature system would

drop to about 48% (43%/90%) of the existing value

just due to excess steam now used due to heat losses

for the distribution system, condensate leaks, and

overheating of the building due to inadequate control. If we assume a value of $10/GJ for energy

costs, the total fuel consumption for the 181-day

study period of 1,445.250 L (381,836 gal) would be

reduced to 693.720 L (183,281 gal), representing a

savings of about $292,000.

In addition, the need to heat makeup water would

be reduced by a significant amount, resulting in

additional savings. If we include the energy loss

due to mass leakage based on our previous calculations, we can estimate these additional savings. In

this case if we assume that 10% of the mass lost is in

the form of steam and the remainder is condensate

then the total savings would be increased to $322,880

for the study period.

The study period did not represent an entire

heating season, only that portion of the season when

boiler plant operation was continuous. Savings in

an entire year would be somewhat greater. Additional savings might also be possible by switching

the buildings from heating domestic hot water with

electricity to using the low temperature system for

that purpose. This would require that the system be

left in operation over the entire year, rather than

being shut down for the summer as is done now.

With the greatly reduced rate of heat loss from the

low temperature hot water system, the cost of heat

losses from the system during the summer months

might not make this economically prohibitive, as it

would be with the existing steam system.

To determine if it would be economically feasible to convert the steam system at Hawthorne

AAP to a low temperature hot water system, CRREL

contracted with the Sacramento District of the US

Army Corps of Engineers to prepare a detailed cost

estimate and preliminary design for the conversion.

The design work consisted of a concept design for

the entire system and a detailed design for conversion of three of the buildings. The estimated cost

of the conversion contract was approximately $5.8

million and with contingencies, supervision and inspection, etc., approximately $6.8 million was estimated for the entire project. Of this cost, the building conversion costs were about $2.1 million, with

almost half of that cost being associated with asbestos abatement due to pipe insulation containing asbestos. The distribution system cost was approximately $2.6 million. A temporary boiler required during the plant conversion was estimated

to cost about $0.6 million.

From the projected project costs and savings given above it was clear that this project was not economically viable on the basis of energy savings

alone. This result could be entirely different if, due

to other reasons such as health considerations or

required replacement of equipment, some portion

of the work was justifiable by other means. For

example, assume that the distribution system has

deteriorated to the point where it needs to be replaced regardless of whether the conversion to

LTHW is considered. Thus the $2.6 million cost for

the distribution system would be subtracted from

the $5.8 million project total and the cost of the

conversion would be lowered to about $3.2 million

plus contingencies, supervision and inspection, etc.

Under this condition the project would most likely

just meet the current Energy Conservation Investment Program (ECIP) criteria of 10-year payback.

Another possibility might be if renovations of the

buildings were being accomplished independently

of a decision to convert to LTHW. In this case the

12
Figure 5. Construction details for the MTHW common

conduit site.

Figure 6. Construction details for the MTHW individual

conduit site.

Thermocouple

Heat Flux Sensor

38 mm (1.5 in.)

Thick Insulation

125 mm (5 in.) Supply

and Return Pipe

500 mm (20 in.) Conduit

1.12 m

(44 in.)

~36 mm

(~1.4 in.)

1.78 m

(70 in.)

Heat Flux Sensor

100 mm (4 in.) Supply

and Return Pipe Thermocouple

64 mm (2.5 in.)

Insulation

400 mm

(16 in.)

Conduit

260 mm

(10.25 in.)

economics are not quite as favorable, but a detailed

cost analysis and refinement of the LTHW design

might still identify an economically viable project

under ECIP criteria.

HEAT LOSS MEASUREMENTS

Over the past few years we have instrumented

operating systems on Army facilities in order to

make measurements of heat losses under realistic

field conditions. Ft. Jackson, South Carolina, was

selected as one site because a large replacement

project was underway there on what would be classified as a medium-temperature hot water (MTHW)

system consistent with the definitions given in

ASHRAE (1992). Three types of heat distribution

piping systems were instrumented.

In addition to the three sites at Ft. Jackson, two

sites were instrumented on a low-temperature hot

water (LTHW) system at Ft. Irwin, California. The

LTHW system was a new system built to serve a

new barracks complex. The specifics of the construction of all of these systems are discussed below.

MTHW common conduit system

The common conduit system has both the supply and return piping in the same steel conduit (Fig.

5). This is a prefabricated system that conforms to

the Federal Agency Criteria for a class A system. This

type of system is designed and installed in accordance with Corps of Engineers Guide Specification

(CEGS) 02695 (U.S. Army Corps of Engineers 1989).

The class A conduit system used at Ft. Jackson

consists of schedule 40 steel supply and return pipes

of approximately 125 mm (5-in. nominal pipe size

[NPS]). These pipes are insulated with a mineral

wool insulation of 38 mm (1.5-in.) thickness. The

insulated supply and return pipes are encased in a

spiral wound steel conduit, which is approximately

3.2 mm (1/8 in.) thick. The supply and return pipes

are positioned within the conduit with the supply

pipe on top of the return pipe. The conduit has an

outer diameter of approximately 500 mm (20 in.),

thus allowing for an air space between the pipe

insulation and the inside of the conduit.

MTHW individual conduit system

The individual conduit system (Fig. 6) employs

the same construction features as the common conduit system described above. In this case the supply

and return pipes are approximately 100-mm (4-in.

NPS) Schedule 40 steel and each is encased in its

own individual conduit of approximately 400-mm

(16-in.) outer diameter. The insulation on the pipes

is mineral wool and is 64 mm (2.5 in.) thick in each

case.

MTHW shallow concrete trench system

The shallow concrete trench system (Fig. 7) consists of a cast-in-place concrete trench with cast-inplace concrete covers. The interior dimensions of

the shallow concrete trench at the Ft. Jackson test

site are 1 m (40 in.) in width and 550 mm (21.5 in.) in

height. The trench walls are 140 mm (5.5 in.) thick

and the trench covers are 150 mm (6 in.) thick, having a lip of about 25 mm (1 in.) at the outside edge so

that the portion resting on the trench wall is about

125 mm (5 in.) thick. The supply and return piping

is approximately 125-mm (5-in. NPS) Schedule 40

steel. Each pipe is insulated with 64 mm (2.5 in.) of

mineral wool pipe insulation.

13
LTHW preinsulated system, site 2

The pre-insulated piping system used at Ft. Irwin

Site 2 is similar in construction to that used at Ft.

Irwin Site 1, but is of a different size (Fig. 9). At Ft.

Irwin Site 2 the supply and return pipes are approximately 75 mm in diameter (3-in. NPS). These

pipes are insulated with a polyurethane foam insulation of approximately 32-mm (1.25-in.) thickness.

Each insulated pipe is encased in its own fiberglassreinforced plastic (FRP) jacket, which is approximately 2.5 mm (0.10 in.) thick. The jacket has an

outer diameter of approximately 160 mm (6.25 in.).

Again the insulation was “foamed-in-place” so that

no air space exists between the pipe insulation and

inside of the jacket.

~135 mm

(~5.4 in.)

335 mm

(13.25 in.)

150 mm

(6 in.)

Thermocouple

64 mm (2.5 in.)

Insulation

Heat Flux Sensor

125 mm (5 in.) Supply

and Return Pipe

185 mm

(7.25 in.)

140 mm

(5.5 in.)

1 m

(40 in.)

G. PHETTEPLACE

GP 23

Figure 7. Construction details for the

MTHW trench site.

Figure 8. Construction details for the LTHW site 1.

~200 mm (8 in.)

PVC Pipes

2.5 mm (0.1 in.)

FRP Jackets

CHILLED 

WATER

HOT

WATER

2.82 m (9.26 ft)

~150 mm (6 in.)

Steel Pipes

Supply Return Return Supply

100 mm (4 in.) Concrete Slab

Silty Gravelly Sand 

61 cm

(24 in.)

60.96 cm

(24 in.)

59.7 cm

(23.4 in.)

32 mm (1.25 in.)

Polyurethane Insulation

LTHW preinsulated system, site 1

The pre-insulated piping system used at Ft. Irwin

consists of schedule 40 steel pipes. At Ft. Irwin Site 1

the supply and return pipes are of approximately

150-mm diameter (6-in. NPS). These pipes are insulated with a polyurethane foam insulation of approximately 32 mm (1.25-in.) thickness. Each insulated pipe is encased in its own fiberglass-reinforced

plastic (FRP) jacket, which is approximately 2.5 mm

(0.10 in.) thick. The jacket has an outer diameter of

approximately 240 mm (9.4 in.). The insulation was

“foamed-in-place,” thus allowing for no air space

between the pipe insulation and inside of the jacket.

Figure 8 shows the construction details of both of

the LTHW sites.

14
more than one of the three system types while others are applicable for only one type of system. Each

of these methods will be briefly described here and

the systems for which each is applicable will be

given. More detail on the actual methods used is

included in Phetteplace et al. (1991). Worked examples of heat transfer calculations for these types of

systems may be found in Phetteplace and Meyer

(1990).

Insulation method

This method is applicable to all system types.

With the observed temperatures on the inside and

outside of the pipe insulation, the heat flow through

the insulation can be easily calculated using the

standard formulas for a concentric cylindrical cross

section and the thermal conductivity of the insulation. In using this method on the MTHW systems,

we assume that these temperatures are reasonably

uniform around the circumference of the insulation.

For the individual conduit system, this assumption

is supported by the data of Lunardini (1989). Air

temperature measurements within the concrete

trench and common conduit systems in this study

also support this assumption. For the LTHW system at Ft. Irwin, temperatures are measured at four

equally spaced points around the perimeter of the

piping system. These temperatures are then averaged and subsequently used to find the heat flow.

Because of the difficulty of placing thermocouples

between the jacket and insulation of the LTHW

system, temperatures are measured on the outside

of the jacket rather than at the outside of the insulation at these sites. An appropriate thermal resistance for the jacket has been added to the insulation

thermal resistance in these cases.

~75 mm (3 in.)

Steel Pipes

100 mm (4 in.) Concrete Slab

Silty Gravelly Sand 

HOT

WATER

CHILLED

WATER

2.41 m (7.9 ft)

Supply Return Return Supply

~100 mm (4 in.)

PVC Pipes

54.6 cm

(21.5 in.)

63.5 cm

(25 in.)

66.0 cm

( 26 in.)

2.5 mm (0.1 in.)

FRP Jackets

32 mm (1.25 in.)

Polyurethane Insulation

Figure 9. Construction details for the LTHW site 2.

Instrumentation, data logging

and communication systems

The instrumentation consists primarily of type T

copper–constantan thermocouples constructed

from 20 AWG thermocouple extension wire. Detailed information on the location of the 167 thermocouples used on the MTHW sites at Ft. Jackson can

be found in Phetteplace et al. (1991). A report currently in preparation will describe in detail the instrumentation of the Ft. Irwin LTHW sites. Heat flux

sensors were used at all sites, but the results from

these are inconclusive for reasons described

in Phetteplace et al. (1991). On each of the systems

instrumented, one site along the length of the system was selected. Each of the sites at Ft. Jackson was

located at least 7.5 m (25 ft) from the closest manhole

and was chosen as representative of the remainder

of the system segment between manholes. On the

LTHW system at Ft. Irwin, no manholes are used with

all piping junctions buried directly in the ground. The

sites at Ft. Irwin are at least 2.4 m (8 ft.) from the

closest junction or change in direction of the pipes.

Three microprocessor-controlled data loggers

were used in this study. Each of these units was

equipped with an RS-232 interface and modem so

that we could transfer the data from the field sites at

Ft. Jackson and Ft. Irwin directly to our personal

computer at CRREL for processing. Again, more

details on the extent of the data collected at Ft. Jackson can be found in Phetteplace et al. (1991), and for

Ft. Irwin will be described in a future report.

Data analysis

Several different procedures are used to calculate

the heat losses from the data collected at the five test

sites. Some of these procedures are applicable to

15
16

Table 2. Averages of some calculated and measured values for the study sites.

 Jacket or

Supply Return Soil temp. conduit temp Heat loss by insulation method Heat loss by soil method Heat loss by CEGS method

temp. temp. @ pipe depth supply/return Supply Return Total Supply Return Total Supply Return Total

Site °C (°F) °C (°F) °C (°F) °C (°F) W/m (Btu/hr-ft) W/m (Btu/hr-ft) W/m (Btu/hr-ft)

MTHWTrench 163.6 (326.4) 107.8 (226.0) — 46.7/40.9 (116.0/105.6) 52.1 (54.2) 28.4 (29.6) 80.5 (83.8) — — — — — —MTHWCommonConduit 164.8 (328.7) 120.8 (249.4) 17.4 (63.3) 54.1 (129.4) 63.3 (65.9) 40.7 (42.4) 104.0 (108.3) — — 104.9 (109.2) 74.6 (77.6) 42.8 (44.5) 117.4 (122.1)MTHWIndividualConduit 165.3 (329.6) 111.6 (232.8) 14.1 (57.3) 28.7/28.1 (83.6/82.5) 48.5 (50.5) 28.8 (30.0) 77.3 (80.5) — — — 46.0 (47.9) 25.9 (27.0) 71.9 (74.9)LTHWSite 1 85.5 (185.9) 81.2 (178.2) 25.5 (77.9) 43.6/42.4 (110.5/108.3) 18.7 (19.5) 17.3 (18.0) 36.0 (37.5) 20.8 (21.6) 18.9 (19.7) 39.7 (41.3) — — —LTHWSite 2 83.5 (182.3) 80.3 (176.6) 25.6 (78.0) 37.6/37.1 (99.7/98.7) 12.3 (12.8) 11.6 (12.1) 23.9 (24.9) 13.2 (13.7) 12.3 (12.8) 25.5 (26.5) — — —
(3 Btu/hr-ft2-°F) has been assumed in the calculational procedure outlined by U.S. Army Corps of

Engineers (1989). This heat transfer coefficient is based

on the outer surface area of the insulation. The validity of this assumption, based on some results from

this study, is discussed in Phetteplace (1991b).

Method for two pipes

buried in a common conduit

This method is applicable only to the case where

both the supply and return pipes are in a common

conduit. Here the same assumption as in the section

above is made regarding heat transfer within the air

space. The equations used are again those prescribed

by U.S. Army Corps of Engineers (1989) and are

referred to as the CEGS-02695 method here.

Soil method for

two direct buried pipes

For the LTHW sites, the soil method uses the

coupled two-pipe resistance formulation presented

by Phetteplace and Meyer (1990). The same assumption made above regarding soil temperatures has

again been made here. The thermal properties of the

soil were estimated from published data (Kersten

1949) using measured soil moisture and classification.

Results of heat loss measurements

Most of the results will be presented in graphical

form in order to present a large amount of information within a reasonable space. Table 2 provides a

summary of some of the more important measured

parameters and calculated results. In the graphical

data presented in Figures 10–14, the sharp fluctuations sometimes observed usually result from shortterm transients in the supply and return temperatures. Such transients can result from a number of

causes such as the startup or shutdown of pumps,

boilers, or building equipment.

MTHW trench site

The average temperature of the supply pipe was

163.6°C (326.4°F) and for the return pipe 107.8°C

(226.0°F). The average air temperature within the

trench was 37.2°C (99.0°F). This temperature is the

average of four air temperatures measured within

the trench.

During the study period the average heat loss

from the trench system was 80.5 W/m (83.8 Btu/hrft). This value was calculated using the “insulation

method” described earlier, the only method used for

the trench system. Figure 10 shows the heat loss

from the shallow trench for the entire study period.

Soil method, one pipe or conduit

This method is applicable only to the common

conduit type of system. For the soil thermal resistance we use the formula for an isothermal cylinder

buried in a semi-infinite medium having an isothermal surface (Holman 1972). The temperature of the

cylinder and its radius are taken as those of the

outside of the conduit. However, the isothermal soil

surface temperature is not used as prescribed by this

formula. Instead, the temperature of the soil at the

depth of the pipe is used for reasons described below.

Soil temperatures vary with depth due primarily

to changes in the air temperature. The thermal properties of the soil damp the amplitude of the temperature fluctuations at the surface and also cause a time

delay for a temperature disturbance at the surface to

reach the soil at some depth below. To accurately

model the variations in heat transfer rate from a

buried heat distribution system due to temperature

variations at the surface requires a transient solution

to the problem. Unfortunately, no closed-form transient solution is available for the case of a buried

pipe. Numerical methods can be used to find very

good approximate solutions to such problems, but

they require much more effort than the closed form

steady-state solutions. To account for the transient

nature of the problem an approximation can be made

by using the undisturbed soil temperature at burial

depth instead of the ground surface temperature in

the steady-state solution for a buried pipe (Janson

1963). We have made this substitution in the calculations for the results presented here, including those

made with the other methods described below where

soil surface temperature is also required.

Method for two buried pipes in

individual conduits

This method is a combination of the two methods

outlined above, but it also accounts for the thermal

resistance of the air space and the interaction of the

two conduits. For the MTHW results presented here,

the method given by U.S. Army Corps of Engineers

(1989), referred to as the CEGS-02695 method, has

been used. This method uses steady-state conductances for the pipe, insulation, and soil. The heat

transfer across the air space between outer surface of

the insulation and the conduit is approximated as

described below.

The actual heat transfer processes within the air

space are far too complicated to warrant a complete

treatment for the purpose of determining the heat

losses from such systems. As an approximation, an

effective heat transfer coefficient of 17 W/m2-°C

17
(Btu/hr-ft) (W/m)

OND J FMAM J J A SON F MAM J J JFMA M J

1986 1987 1988 1989

100

90

80

70

60

50

40

100

90

80

70

60

50

40

110

Heat Loss

Figure 10. Heat losses for the MTHW trench site.

The reduction in heat losses during 1988 and 1989

over the previous years is attributable to the reduced return temperature during that time period.

The cause of this temperature reduction is not

known. This is a fairly significant reduction and

provides a clear example of the benefits of keeping

the temperature differential between supply and

return as large as possible, thus resulting in lower

return temperature. Not only will heat losses be

reduced by lower return temperatures, but pumping costs are also reduced since less mass will need

to be circulated. Of course, this assumes that the

thermal load is constant and that some method of

reducing pumping power input, such as variable

speed drives or multiple pumps, is available.

MTHW common conduit site

Figure 11 shows the heat loss from the MTHW

common conduit site over the study period. The

temperature of the supply at this site during the

study period averaged 164.8°C (328.7°F) and the

return averaged 120.8°C (249.4°F) for the same period of time. The supply temperature is somewhat

higher (1.3°C, or 2.3°F) than the supply temperature

observed at the trench site. The return temperature

averaged about 130°C (23.4°F) higher at this site as

well. This would tend to make heat losses higher at

this site if all else were equal, as of course is not the

case. The temperature of the outer surface of the

conduit averaged 54.1°C (129.4°F), while the undisturbed ground temperature at approximately the

same depth as the centerline of the conduit averaged 17.4°C (63.3°F). This illustrates the rather dramatic effect of the buried conduit on surrounding

soil temperatures.

The heat losses for the common conduit system

were calculated by three of the methods described

earlier. The method referred to as the “soil method”

uses the single buried pipe equation and the conduit outer surface temperature to calculate the heat

flow. The thermal conductivity of the soil is taken as

1.08 W/m-°C (7.5 Btu-in./hr-ft2-°F) in this and the

CEGS-02695 method, which is believed to be a realistic average value based on the observed soil type

and moisture content (Phetteplace et al. 1991) and

published data (Kersten 1949). Because of the inhomogeneous nature of soil and the difficulty in

making thermal measurements on soils, this value

is considered to be accurate only to within 25%.

The average of the values computed by the three

methods is 108.8 W/m (113.2 Btu/hr-ft). The highest of the methods (CEGS-02695) was approximately

7.6% greater than the average value and the lowest

(insulation method) was 4.3% below the average.

Considering the difficulty involved in making thermal measurements of this nature, we believe this

agreement is very good. This is particularly true

when one considers that the CEGS-02695 method

18
(W/m)

CEGS - 02695

Q Soil 7.5

Insulation

J FMAMJ J A SON FMAMJ J J FMAM J

1987 1988 1989

140

120

100

80

60

40

140

120

100

80

60

40

(Btu/hr-ft) Heat Loss

Figure 11. Heat losses from the MTHW common conduit site.

may have conservative assumptions (in that they

would under-predict the actual thermal resistance)

regarding the heat transfer across the air space, as

discussed in Phetteplace (1991b).

The heat losses for the entire study period for the

common conduit site are shown in Figure 11. Note

that during the early spring (around March), for

each of the three years that we have data during this

time period, the results from the insulation method

increase to a value greater than those for the soil

method. Some time during the fall (about mid-September for 1987, the only year for which we have

data during this time period) the trend is reversed.

The soil moisture content offers one possible explanation for this. Soil samples taken for this study

(Phetteplace et al. 1991) indicate that the soil moisture content is higher during the spring and summer than in the winter. One plausible explanation

for this would be that it takes a considerable length

of time for the winter precipitation to diffuse into

the low-permeability soil found at Ft. Jackson. Subsequently the soil dries out slowly during the summer reaching its lowest moisture content in the fall

and winter.

If the moisture content of the soil around the

conduit, particularly that between the conduit and

the ground surface, increases during the spring and

summer months, then the thermal conductivity of

the soil will increase during that time period as

well. This will reduce the thermal resistance of the

soil in an absolute sense as well as relative to the

other thermal resistances in the system. Presumably the other thermal resistances remain fairly constant year-round, notably the insulation thermal

resistance which is much greater than the thermal

resistance of the soil or any other thermal resistance

in the system. Thus, the overall thermal resistance

will be reduced by a much smaller relative amount

than the soil thermal resistance. With the lower

thermal resistance the temperature drop across the

soil from the conduit casing to the ground surface

will decrease relative to the other temperature drops

in the system. However, we have assumed that the

thermal conductivity of the soil is constant yearround in our soil method, and thus with the lower

actual resistance and relative temperature drop measured we will underpredict the heat flow.

MTHW individual conduit site

At the individual conduit site, the temperature

of the supply during the study period averaged

165.3°C (329.6°F) and the return averaged 111.6°C

(232.8°F) for the same period of time. The temperature of the outer surface of the conduit averaged

28.7°C (83.6°F) for the supply and 28.1°C (82.5°F)

for the return. The undisturbed ground temperature at approximately the same depth as the

centerline of the conduit averaged 14.3°C (57.3°F).

19
Total Insulation

Total CEGS - 02695

Supply Insulation

Supply CEGS - 02695

Return Insulation

Return CEGS - 02695

90

80

70

60

50

40

30

20

100

90

80

70

60

50

40

30

20

10

Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May J

1988 1989

Heat Loss

(Btu/hr-ft) (W/m)

Figure 12. Heat losses from the MTHW individual conduit system.

Here the average temperature difference between

the outside of the conduit and the undisturbed soil

temperature at the burial depth is only –3.4°C

(25.8°F). This can be compared to a temperature

difference of about 36.7°C (66°F) at the common

conduit site. The primary reason for this much lower

temperature difference at the individual conduit

site is the increased insulation thickness at that site.

Because the thermal resistance of the insulation is

much larger at the individual conduit site, the thermal resistance of the soil becomes a much smaller

fraction of the total, and thus the corresponding

temperature drop across that resistance decreases.

The heat losses for the individual conduit system

were calculated by the insulation and CEGS-02695

methods described earlier. As in the calculations for

the common conduit site, the thermal conductivity

of the soil was taken as 1.08 W/m-°C (7.5 Btu-in./

hr-ft2-°F) for the CEGS-02695 method. The average

of the heat loss values computed by the two methods is 74.7 W/m (77.7 Btu/hr-ft). The highest of the

methods (Insulation Method) was approximately

7.5% greater than the lowest (CEGS-02695 Method).

Again, considering the difficulty involved in making thermal measurements of this nature, we believe this agreement is very good. The heat losses

for a portion of the study period for the individual

conduit site are shown in Figure 12.

LTHW preinsulated system, site 1

On the LTHW system at Site 1, the temperature

of the supply during the study period averaged

85.5°C (185.9°F) and the return averaged 81.2°C

(178.2°F) for the same period of time. The relatively

small temperature difference between supply and

return must be attributed to the mass flow rate

being in excess of what is required. It is not known if

this resulted from an error in design or from the

substitution of a larger pump in the construction

process. The temperature of the outer surface of the

FRP jacket averaged 43.6°C (110.5°F) for the supply

and 42.4°C (108.3°F) for the return. The undisturbed

ground temperature at approximately the same

depth as the centerline of the pipe averaged 25.5°C

(77.9°F). Here the average temperature difference

between the outside of the jacket and the undisturbed soil temperature at the burial depth is 17.5°C

(31.5°F). This can be compared to a temperature

difference of about 36.7°C (66°F) at the common

conduit site and 14.3°C (25.8°F) at the individual

conduit site. As with the individual conduit site, the

primary reason for this much lower temperature

20
06-01-89 12-01-89 12-01-90 12-01-91 06-01-90 06-01-91

Date

20

25

30

35

40

45

50

50

45

40

35

30

25 Soil Method


Insulation Method

Heat Loss

(Btu/hr-ft) (W/m)

difference compared to the common conduit site is

the increased insulation resistance at this site. Because the pipes at this site are buried very deep (3.0

m or 9.7 ft) this site has a relatively significant thermal resistance in the soil which results in significant

temperature drops in the soil. Thus, the temperature drop between the jacket and the undisturbed

soil at this site is slightly more than at the individual

conduit site even though the heat losses are much

lower.

The heat losses for the LTHW site 1 system were

calculated by the insulation and soil methods described earlier. The thermal conductivity of the soil

was taken as 1.46 W/m-°C (10 Btu-in./hr-ft2-°F) for

the soil method. This value is based on published

data (Kersten 1949) and measured soil moisture and

classification. The average of the heat loss values

computed by the two methods is 37.9 W/m (39.4

Btu/hr-ft). The highest of the methods (soil method)

was approximately 10.1% greater than the lowest

(insulation method). The heat losses for the LTHW

system site 1 for the study period are shown in

Figure 13.

LTHW preinsulated system, site 2

For the LTHW system at site 2, the temperature

of the supply during the study period averaged

83.5°C (182.3°F) and the return averaged 80.1°C

(176.2°F). The temperature of the outer surface of

the FRP jacket averaged 37.6°C (99.7°F) and 37.1°C

(98.7°F) for the supply and return, respectively. The

undisturbed ground temperature at approximately

the same depth as the centerline of the pipe averaged 25.5°C (78.0°F). Here the average temperature

difference between the outside of the jacket and the

undisturbed soil temperature at the burial depth is

11.7°C (21.2°F), the lowest of any of the direct buried systems in this study. Despite the fact that this

system is buried deep (2.5 m or 8.2 ft.), the relatively

high level of insulation results in less significant

temperature drops in the soil compared to the LTHW

system site 1, where the same thickness of insulation is used on a pipe nearly twice as large in outer

diameter. Thus, the temperature drop between the

jacket and the undisturbed soil at this site is considerably less than for site 1 on the LTHW system.

As was done for site 1 on the LTHW system, heat

losses from site 2 were calculated by the insulation

and soil methods described earlier. Again, the thermal conductivity of the soil was taken as 1.46 W/m-

°C (10 Btu-in./hr-ft2-°F) for the soil method. The

average of the heat loss values computed by the two

methods is 24.7 W/m (25.7 Btu/hr-ft). The highest

of the methods (soil method) was approximately

6.4% greater than the lowest (insulation method).

The heat losses from site 2 of the LTHW system

over the course of the study period are shown in

Figure 14.

Figure 13. Heat losses from the LTHW site 1 system.

21
the supply and return piping is approximately

150 mm (6 in.) in diameter with approximately 32

mm (1.25 in.) of polyurethane foam insulation.

The average value of the heat loss from this system using the two methods of measurement described earlier is about 37.5 W/m (39 Btu/hr-ft).

For the medium temperature hot water system at

Ft. Jackson, South Carolina, the common conduit

site has approximately 125-mm (5-in.)-diameter

supply and return piping with about 38 mm (1.5

in.) of mineral wool insulation. The heat losses of

this system average 108.6 W/m (113 Btu/hr-ft)

using the average of the three different methods

of measurement described earlier. With proper

design, the 150 mm (6-in.) low temperature piping is able to convey as much heat as the 125 mm

(5-in.) medium temperature water piping. An example of the heat carrying capacity is given in

Phetteplace (1992). Thus, the losses are reduced

by a factor of nearly three. To put this in perspective, consider what this would do to the heat

losses from the Army’s 5600 km (3500 mi) of heat

distribution piping (U.S. Army 1992). At 108.6

W/m (113 Btu/hr-ft) and $9.48 per GJ ($10 per

million Btu) the annual cost of heat lost would be

over $182 million. For 37.5 W/m (39 Btu/hr-ft)

and the same cost of heat the annual cost of heat

losses would be about $63 million. Thus, the Army

could save nearly $120 million per year from heat

that is now lost to the environment. This assumes,

of course, that all the existing systems have rates

of heat loss similar to those of the Ft. Jackson

system. Assuming that the size of the Ft. Jackson

06-01-89 12-01-89 12-01-90 12-01-91 06-01-90 06-01-91

Date

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20

25

30

35 36

32

28

24

20

16

Soil Method


Insulation Method

Heat Loss

(Btu/hr-ft) (W/m)

Figure 14. Heat losses from the LTHW site 2 system.

Conclusions drawn from

heat loss measurements

The objective of our heat loss studies has been

to achieve some insight into several aspects of

thermal analysis of heat distribution systems. Of

primary importance is the actual rate of heat losses

from operating heat distribution systems. We have

presented several years of data for five different

sites on four types of heat distribution systems. In

all but one case these heat losses have been measured under actual field conditions by more than

one method. By making heat loss calculations using several different methods using independent

measurements, we have established the validity

of our techniques for systems similar to those

studied. The methods used to make these measurements have proven to be very reliable and

repeatable. In general, agreement between methods has been very good in terms of the magnitude

of the heat losses as well as the trends over the

yearly cycle.

The relative levels of heat losses found in our

studies give rise to some significant conclusions regarding the cost of heat losses for the DoD and our

ability to impact these costs. We can use these results to make an estimate of heat loss costs for DoD

and also the savings which could be achieved by

converting our high temperature water and steam

systems over to low temperature hot water.

As an example, consider the reduction in heat

losses possible with low temperature water as

illustrated by some of the measurements taken on

the Ft. Irwin system. At one of the Ft. Irwin sites

22
system is representative, this is a conservative assumption, since the Ft. Jackson system is relatively

new and is undoubtly more thermally efficient than

the Army’s average system.

While these savings are significant, the cost of

replacing the piping systems would be enormous.

Current replacement cost for high temperature water and steam systems is approximately $1000 per

lineal meter ($300 per lineal ft). If we assume that

the cost of the LTHW systems is 50% of the cost of

the high temperature water and steam systems

they would replace, it would cost about $2.8 billion

to replace all the Army’s 5600 km (3500 mi) of systems. Thus, the simple payback would be unacceptable, at about 23 years. But since these systems

are continuously being replaced, many opportunities exist to convert to LTHW in those instances.

Even when building conversion costs are considered, conversion to LTHW may be less costly than

replacing in-kind from a capital cost standpoint

alone. When the reduced operating costs of LTHW

are considered, LTHW will have the lowest life cycle

cost in many cases.

CONCLUSIONS

The objective of this report was to present information on the efficiency of heat distribution

systems. Data were presented from several field

studies conducted by CRREL on operating systems of Army bases. After studying this report

the reader will conclude that low temperature hot

water is a superior means of heat distribution.

The primary advantages of low temperature water systems are listed below:

1. Capital costs are only about 50% of those for

high temperature water and steam systems.

This is a result of the much simpler system

with fewer provisions for expansion and

high pressures. In addition, the insulation

and jacketing materials that can be used with

LTHW not only perform better but they are

less costly.

2. Heat losses are drastically reduced. As illustrated by the data given above, CRREL’s

measurements have shown the heat losses

to be about 35% of those for a high temperature water system.

3. Leakage is usually much lower due to lower

temperature and pressure. This is particularly true when compared to steam systems

where problems with condensate return usually result in significant mass losses. For example, the CRREL analysis of the Hawthorne

AAP steam system described above has

shown that about 50% of the mass is lost in

distribution.

4. Maintenance is lower and the systems are

safer as a result of lower temperatures and

pressures. Frequently buildings can be connected directly to the central system without heat exchangers because of the lower

temperatures and pressures.

5. Cogeneration of electricity is more favorable because the lower condensing temperature results in higher efficiency of the power

generating cycle.

6. Lower density loads can be served economically. Because of the lower piping cost and

simpler systems, it becomes economical to

extend the distribution system into areas

where the loads are smaller and farther apart.

For this reason, even residential areas are

served by low temperature systems in Europe.

7. Low temperature systems are more easily

adaptable to many alternate sources of energy, such as solar, geothermal, and waste

heat.

We hope that this report has helped provide

evidence of the major advantages of low temperature hot water systems. Perhaps the strongest support for the concept comes from Europe

where the vast majority of the systems are LTHW.

There the much higher penetration of district heat

as a space heating means is primarily due to the

efficiency of the LTHW systems. If the DoD is to

remain committed to district heating technology

on its bases we must begin to convert existing

high temperature water and steam systems to

LTHW. If this is not done, it will become increasing difficult to justify the use of district heating.

This is true, even when all the inherent advantages

of district heating from energy efficiency and environmental impact standpoints are considered,

due to increased competition from alternate fuels, especially natural gas.

SELECTED BIBLIOGRAPHY

ASHRAE (1985) ASHRAE Handbook., 1985 Fundamentals Volume, Chapter 15. Atlanta, Georgia: American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

23
ASHRAE (1992) ASHRAE Handbook, 1992 HVAC

Systems and Equipment Volume, Chapter 11. Atlanta,

Georgia: American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

ASME (1977) ASME Steam Tables, Third Edition.

New York: American Society of Mechanical Engineers.

Holman, J.P. (1972) Heat Transfer, Third Edition.

New York: McGraw-Hill Book Co.

Janson, L. (1963) Water supply systems in frozen

ground. In Proceedings, 1st International Conference

on Permafrost, 11–15 November, Lafayette, Indiana.

Washington, D.C.: National Academy of Sciences,

National Research Council, Publication 1287.

Kersten, M.S. (1949) Laboratory research for the

determination of the thermal properties of soils.

Engineering Experiment Station, University of

Minnesota.

Lunardini, V.J. (1989) Measurement of heat losses

from a conduit type heat distribution system. USA

Cold Regions Research and Engineering Laboratory, preliminary report (unpublished).

Lunardini, V.J. (1990) Measurement of heat losses

from a conduit-type heat distribution system. 1990

Army Science Conference (ASC), 12–15 June 1990,

Durham, North Carolina, Proceedings ASC.

Phetteplace, G.E. (1990) Low temperature hot water heat distribution systems. USA Cold Regions

Research and Engineering Laboratory, Facilities

Engineering Applications Program (FEAP) Fact

Sheet.

Phetteplace, G.E. (1991a) Efficiency analysis of a

steam heat distribution system. In Proceedings of

the International Symposium on Fluids for District

Heating, Copenhagen, Denmark, 10–11 April, 1991,

p. 199–213.

Phetteplace, G.E. (1991b) Measurement of heat

losses from a buried heat distribution system. Presented at 1991 National Heat Transfer Conference,

Minneapolis, Minnesota, American Society of Mechanical Engineers (ASME), HTD, vol. 172, p. 47–54.

Phetteplace, G.E. (1992). Low temperature water

for heat distribution systems. In Proceedings of the

U.S. Army Corps of Engineers Electrical/Mechanical

Conference, Dallas, Texas, July 1992, p. 139–145.

Phetteplace, G.E. and N. Labbe (1978) Heat distribution with low temperature hot water. USA

Cold Regions Research and Engineering Laboratory, Technical Note (unpublished).

Phetteplace, G.E. and V. Meyer (1990) Piping for

thermal distribution systems. USA Cold Regions

Research and Engineering Laboratory, Internal

Report 1059 (unpublished).

Phetteplace, G.E., D. Carbee and M. Kryska

(1991) Field measurements of heat losses from

three types of heat distribution systems. USA Cold

Regions Research and Engineering Laboratory,

Special Report 91-19.

Phetteplace, G.E., W. Willey and M.A. Novick

(1981) Losses from the Fort Wainwright heat distribution system. USA Cold Regions Research and

Engineering Laboratory, Special Report 81-14.

Segan, E.G. and C. Chen (1984) Investigation of

tri-service heat distribution systems. U.S. Army

Construction Engineering Research Laboratory,

Champaign, Illinois, Technical Report M-347.

U.S. Army (1978) Engineering Weather Data. TM 5-

785. Washington, D.C.: Department of the Army.

U.S. Army (1992) Facilities Engineering and Housing Annual Summary of Operations, Volume III: Installations Performance, Fiscal Year 1992. Office of

the Assistant Chief of Engineers, U.S. Army Engineering and Housing Support Center.

U.S. Army Corps of Engineers (1989) Underground heat distribution systems (preapproved

systems). CEGS 02695, U.S. Army Corps of Engineers, Washington, D.C.

Werner, S.E. (1984) The heat load in district heating systems. Chalmers University of Technology,

Goteborg, Sweden.

24
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This report will provide some general guidance on the selection of distribution medium (steam or hot water) and

temperature for heat distribution systems. The report discusses the efficiency of both steam and hot water heat distribution systems in more detail. The results of several field studies using data from boiler plant logs and measured heat

losses are given. For steam, an efficiency analysis for the steam heat distribution system at Hawthorne Army Ammunition Plant is summarized. This analysis is based on the limited data available from the boiler logs maintained at the

central plant. From this information, along with energy and mass balances that are constructed from the central plant

data, gross measures of efficiency are obtained. The results of the analysis show that only 43.5% of the steam input to

the distribution system is used to meet the required space heating load. The results also indicate that on average only

46.2% of the steam that leaves the plant returns as condensate. By converting this steam distribution system to a low

temperature hot water heat distribution system, savings would exceed $292,000 for the 181-day study period, which

represents a typical heating season. For hot water based systems this report describes two field projects underway at

U.S. Army bases. At Fort Jackson, South Carolina, a medium-temperature hot water heat distribution system has

been monitored. Three different types of system construction have been instrumented: pipes enclosed in a shallow

concrete trench, steel conduit with supply and return pipes in common conduit, and separate steel conduits for

supply and return pipes. At Ft. Irwin, California, a low-temperature hot water system has been monitored. Two sites

have been instrumented on this direct buried system that consists of steel carrier pipes insulated with polyurethane

foam protected by a fiberglass jacket. The data provide a clear illustration of the much lower heat losses from the low

temperature water systems.

September 1995

Efficiency of Steam and Hot Water Heat Distribution Systems PE: 6.27.84A

PR: 4A762784AT42

TA: CO

WU: M06

Gary Phetteplace

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 95-18

Hanover, New Hampshire 03755-1290

Office of the Chief of Engineers

Washington, D.C. 20314-1000

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161

32

Efficiency analysis Hot water heat distribution systems

Heat distribution systems Steam heat distribution systems

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