NAT'L INST. OF STAND & TECH

Economizer Algorithms for Energy

Management and Control Systems

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Building Equipment Division

Washington, DC 20234

February 1 984

Sponsored by:

Office of Building and Community Systems

U.S. Department of Energy

< “QC -'-s100

,s’ Navy Civil Engineering Laboratory

Department of Defense

. U5o

M-2352

NBSIR 84-2832

ECONOMIZER ALGORITHMS FOR ENERGY

MANAGEMENT AND CONTROL SYSTEMS

WATIGHAr, BUKE ' ; OF E7AVOA:'(D£ TKaC.

QC

1 OO

• U 5(c ^ 4

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^

Cheol Park

George E. Kelly

James Y. Kao

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Building Equipment Division

Washington, DC 20234

February 1984

Sponsored by:

Office of Building and Community Systems

U.S. Department of Energy

U.S. Navy Civil Engineering Laboratory

U.S. Department of Defense

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Economizer cycles have been recognized as important energy conservation

measures for building air handling systems and have been included in most

Energy Management and Control Systems (EMCS). This report describes the

psychrometric processes of the most commonly used economizer cycles and

presents algorithms for implementing these cycles on a typical Energy

Management and Control System.

Economizer cycles included in this study are dry-bulb and enthalpy types, as

applied to both dry coils and sprayed coils. In addition, an enhancement to

the normal enthalpy economizer cycle algorithm is presented for dual-duct or

multi-zone system which takes into account differences in the costs of

heating energy and cooling energy. Computer program listings of the

algorithms and sample input/output data are shown in the appendices. A brief

discussion of common types of air handling systems is also given to help the

reader better understand the application of the algorithms presented in this

report

.

Key words: control strategies; cooling energy; dry-bulb economizer cycle;

energy management and control system; enthalpy economizer cycle;

heating energy.

iii
TABLE OF CONTENTS

Pa*e

ABSTRACT iii

LIST OF FIGURES iv

1. INTRODUCTION 1

2. COMMON TYPES OF HVAC SYSTEMS 3

3. THE DRY-BULB ECONOMIZER CYCLE 11

4. DRY-BULB ECONOMIZER ALGORITHM 22

5. ENTHALPY ECONOMIZER CYCLE 28

6 . ENTHALPY ECONOMIZER ALGORITHM 33

7. DUAL-DUCT SYSTEMS 38

8. ALGORITHM ENHANCEMENT FOR DUAL-DUCT AND MULTI-ZONE SYSTEMS 41

9. SUMMARY 45

REFERENCES 47

APPENDIX A. COMPUTER PROGRAM LISTING OF THE DRY-BULB ECONOMIZER

ALGORITHM (DBE), AND SAMPLE INPUT AND OUTPUT A-l

APPENDIX B. COMPUTER PROGRAM LISTING OF THE ENTHALPY ECONOMIZER

ALGORITHM (ETC1), AND SAMPLE INPUT AND OUTPUT B-l

APPENDIX C. DERIVATION OF THE DIMENSIONLESS FACTOR, X C-l

APPENDIX D. COMPUTER PROGRAM LISTING OF THE ENTHALPY ECONOMIZER

ALGORITHM WITH ENHANCEMENT (ETC2) , AND SAMPLE INPUT AND

OUTPUT D-l

iv
LIST OF FIGURES

Page

Figure 1. Single-zone air handling unit 4

Figure 2. Single-zone air handling unit with reheat system 6

Figure 3. Single-zone air handling unit with sprayed coil 7

Figure 4. Variable air volume (VAV) system 8

Figure 5. Dual duct system . 10

Figure 6 . Psychrometric chart of dry-bulb economizer cycle for

systems not using sprayed coils or air washers 12

Figure 7 . Operation of a dry-bulb economizer cycle on a single-zone

air handling unit (refer to figures 1 and 6) 15

Figure 8. Operation of a dry-bulb economizer cycle on a single-zone

reheat system (refer to figures 2 and 6) 17

Figure 9. Psychrometric chart of dry-bulb economizer cycle for

systems employing sprayed coils or air washers 18

Figure 10. Operation of a dry-bulb economizer cycle on an air

handling system with sprayed coils or air washers

(refer to figures 3 and 9) 20

Figure 11. Logic flow diagram of dry-bulb economizer cycle 26

Figure 12. Psychrometric chart of enthalpy economizer cycle for

systems which do not employ sprayed cooling coils or

air washers 30

Figure 13. Psychrometric chart of enthalpy economizer cycle for

systems with sprayed cooling coils or air washers 31

Figure 14. Logic flow diagram of the enthalpy economizer cycle 36

v
Figure 15. Psychrometric charts of the dual-duct system without

Page

sprayed coil or air washer 40

Figure 16. Logic flow diagram of the enhancement routine for

dual-duct and multizone systems 43

vi
1 . INTRODUCTION

Rising energy costs, accompanied by decreasing prices for computers and

microprocessors, have resulted in a rapid increase in the use of Energy

Management and Control Systems (EMCS) in new and existing buildings. With the

proper application algorithms, these EMCS are capable of implementing building

control strategies which minimize the use of heating and air conditioning

equipment to save appreciable amounts of energy. One such control strategy

which is commonly found on today's EMCS is the use of outdoor air for cooling

a building's interior when outdoor temperature and humidity conditions permit.

Such a strategy is usually referred to as an economizer cycle.*

There are two basic types of economizer cycles. A dry-bulb economizer, which

is the simpler of the two, utilizes the outdoor and return air dry-bulb

temperatures to position the outdoor, return, and relief air dampers. An

enthalpy economizer performs the same functions using the enthalpies of the

outdoor and return air. The latter has been shown to save more energy

[1,2,3], but requires the installation of additional sensors to measure the

relative humidities or dew point temperatures of the outdoor and return air.

These sensors require frequent maintenance and calibration to prevent

measurement errors which can lead to a significant increase in building energy

consumption [4] through the improper use of outdoor air for cooling.

*The phrase "economizer cycle" is misleading since there is really no thermodynamic "cycle" involved. However, this terminology has been commonly

used in the literature in the past and it is adopted in this paper to avoid

additional confusion.
This report describes both the dry-bulb and enthalpy economizer cycles and

presents algorithms for implementing them on a typical Energy Management and

Control System. In addition, an enhancement to the enthalpy economizer is

presented for dual-duct systems which takes into account the relative costs of

heating and cooling the air supplied to the conditioned space.

Computer programs, written in Fortran 77, are presented in the appendices for

the two economizer algorithms, and for the enhancement covering dual-duct

systems, with calling routines that can be used to check the operation of the

economizer programs. Sample input data and resulting output are also

included. It should be pointed out, however, that these programs do not

contain software for reading sensors, actuating the outdoor, return, and

relief air damper motors, or positioning these dampers so as to obtain the

desired mixed air temperature, since these functions are typically system

dependent

In addition, since there are many different kinds of systems and many ways of

controlling each type, it is always important to thoroughly analyze an

application before implementing a new control strategy. The algorithms

presented in the report, while covering most typical HVAC installations, may

require some modifications before being used in the actual operation of a

specific building system.

2
2 . COMMON TYPES OF HVAC SYSTEMS

The numerous types of HVAC systems used in buildings and the various methods

employed for controlling their operation have been discussed extensively

elsewhere [5,6,7]. While it is beyond the scope of this report to try and

review this information, it is useful to briefly discuss some of the more

common systems found in commercial buildings in the United States. This will

serve as background material for better understanding the algorithm presented

in the following sections.

Figure 1 shows a typical single-zone air handling unit found in many small

buildings [6]. This system is controlled to provide either heating or

cooling, but not both at the same time. During cooling, there is no direct

control of the humidity level in the conditioned space, although some indirect

control can be achieved by properly selecting the cooling coil to provide the

desired ratio of sensible to total cooling capacity at full load.

Two techniques are commonly employed for implementing dry-bulb economizer

control on the above system [1]. The first utilizes an outside, return, and

relief air damper controller to regulate the mixed air temperature to a given

set point. The second sequences the dampers and cooling coil flow rate

control valve so that the cooling coil discharge controller regulates the

damper position before the cooling coil valve opens. Although the former is

easier to implement with pneumatic controls, the latter is slightly more

3
DAMPER RETURN AIR FAN

RELIEF

AIR

\\

Figure 1. Single-zone air handling unit

4
general in that it assures that there is no conflict between control of the

cooling coil valve and control of the dampers. In addition, it is capable of

providing proper control for both dry coil and sprayed coil systems. Because

of these features, the second control technique (i.e., the one utilizing the

cooling coil discharge temperature) was selected as the basis of the

economizer algorithms presented in this report.

In order to limit the maximum relative humidity in a conditioned space [6], a

HVAC system designer will often interchange the relative position of the

cooling and heating coils in the system discussed above. This results in the

single-zone reheat system shown in figure 2. If more than one zone is

involved, the single heating coil can be replaced by reheat coils or induction

reheat units in each zone.

If tighter humidity control in the conditioned space is desired, a designer

will often use a sprayed cooling coil or even an air washer in place of a

cooling coil [6]. The former system is shown in figure 3. Again, the single

reheat coil in these figures can be replaced by multiple reheat coils or

induction reheat units in each of the different zones.

Another system that is enjoying immense popularity these days because of high

energy costs is the Variable Air Volume (VAV) system shown in figure 4 [6,8].

The rate of air flow may be controlled by a fan inlet (vortex) damper, a fan

discharge damper, mechanical or electrical fan speed control, variable volume

boxes in each zone, or some combination of these.

5
DAMPER RETURN AIR FAN

Figure 2. Single-zone air handling unit with reheat system

6
DAMPER RETURN AIR FAN

RELIEF

AIR

\\\

SPRAYED

COOLING REHEAT

COIL COIL SUPPLY AIR FAN

OUTDOOR

AIR, OA

^^

h;

FILTER

X X ! L

^=50-

MAKEUP

WATER

Figure 3. Single-zone air handling unit with sprayed coil
Figure 4. Variable air volume (VAV) system

8
Applications involving multiple zones are also handled using dual-duct and

multi-zone systems to provide conditioned air to the occupied spaces in a

building [6]. Both of these systems mix chilled and heated air to obtain the

desired supply air conditions. The only difference is that the dual-duct

system, shown in figure 5, mixes the cold and hot air at each zone, while the

multi-zone system mixes it at the air handling unit and then ducts the mixed

air to the zone.

9
DAMPER RETURN AIR FAN

Figure 5. Dual-duct system

10
3 . THE DRY-BULB ECONOMIZER CYCLE

Dry bulb economizer cycles employing outdoor air for "free cooling" have

become a common feature on air handling systems. For systems not using

sprayed cooling coils or air washers, the basic dry bulb economizer cycle can

be described using the sketch of the single air handling unit in figure 1 and

the psychrometric chart shown in figure 6.

When the outdoor air temperature is above a changeover temperature, T

CQ , and

in region lb in figure 6, it is always desirable to minimize the amount of

outdoor air used because the enthalpy of the outside air will be greater than

the enthalpy of the return air. In region la, the enthalpy of the outside

air is less than the enthalpy of the return air, and, under certain

conditions, there could be some advantage in using the maximum amount of

outdoor air. However, to utilize outdoor conditions in this entire region

requires measurement of the thermodynamic states of the outdoor air and return

air. These measurements are, however, not available since if they were, an

enthalpy eonomizer cycle would be used. For this reason, the outdoor and

relief air dampers should be at their minimum open position and the return air

damper (see figure 1) at its maximum open position whenever the outdoor air is

in either Region la or lb. These damper positions should also be set to

satisfy the minimum fresh air requirements of the zone served by the air

handling unit.

11
DRY-BULB TEMPERATURE

Figure 6. Psychrometric chart of dry-bulb economizer cycle for

systems not using sprayed coils or air washers

12

HUMIDITY

RATIO
aen re <

• ..cor ai is in region Ila of figure 6, its dry-bulb temperature

is greater than (or equal to) the dry-bulb set point temperature of the

cooling coil discharge air, T^gp, but less than the changeover temperature,

T

co

. In this region, the maximum amount of outdoor air should be used, along

with mechanical cooling to reduce the temperature of the mixed air to T

c^g

p.

The changeover temperature, T^q, should be selected so as to maximize the sum

of the positive savings from region lie and the negative savings (loss) from

region lib over the entire year. The latter is due to the high humidity

conditions that exist when the outdoor air conditions are in this region

(i.e., lib). The value of T

co varies with the climate and the typical

conditions assumed for the thermodynamics state of the return air. Typical

values of T

co

are given in reference [8] for most of the major U.S. cities.

For outdoor air dry-bulb temperatures below T

CASP» the cooling coil valve

should be closed and the outdoor, return and relief air dampers modulated to

maintain a cooling coil discharge temperature equal to TCAgp. Under these

conditions, the outdoor and relief air dampers will be at their maximum open

position when the outdoor air dry-bulb temperature is near TCASp and gradually

close, until they reach their minimum open positions, as the outdoor

temperature continues to decrease. The return air damper operates in the

opposite direction - gradually opening as the outdoor temperature falls below

TCASP *

For systems with preheat coils, such as the one in figure 2, the outdoor and

relief air dampers go from whatever position they are in to a minimum open

13
position when the outdoor dry-bulb temperature drops to some minimum value,

TmIN* In region IV, with outdoor temperature below T Mjjj, only the minimum

amount of outside air, needed to satisfy the building's fresh air

requirements, is used and the preheat coils are operated to raise the mixed

air temperature to T^gp. The value of T MIN may be selected to correspond to

that outdoor temperature which results in a mixed air temperature of T^gp

when the minimum amount of outside air is mixed with return air. However,

this may not protect the preheat coils from freezing and, depending on the

system employed, it may be necessary to select a higher value for T Mpjj.

All of the systems shown in figures 1 through 5 are usually controlled in the

manner described above when a dry-bulb economizer cycle is employed.

However, there are some interesting differences in the actual operation of the

systems that are worth discussing. For the single-zone air handling unit in

figure 1, the outdoor and return air are mixed to obtain a temperature TCASP

leaving the cooling coil (with no mechanical refrigeration) when the outdoor

temperature is in region III of figure 6. Maximum and minimum outdoor air,

plus mechanical refrigeration, is used to maintain this same cooling coil

discharge temperature when the outdoor temperature is in region II and I,

respectively. Operation in these three regions is shown in figure 7. The

value of T^gp is selected to satisfy the space conditioning needs of the zone

served. For the reheat system in figure 2, TCA3p

is usually lower than the

supply air temperature needed to cool the conditioned space. This is done to

remove moisture from the air and thus limit the maximum relative humidity

14
DRY-BULB TEMPERATURE

Figure 7. Operation of a dry-bulb economizer cycle on a single-zone

air handling unit (refer to figures 1 and 6)

15

HUMIDITY

RATIO
allowed in the space and/or to satisfy a diversity of cooling loads among a

number of zones. As a result, the air leaving the cooling coil must be

reheated, either using a single reheat coil as illustrated in figure 8, or

using separate coils in each of the different zones.

Figure 9 shows the psychrometric chart for a system using a sprayed coil (as

shown in figure 3) or an air washer. The dry-bulb temperature measured leaving

the coil or air washer is actually very close to the dew point temperature of

the leaving air. Air entering the spray or washer will he cooled down along

it 8 constant enthalpy line until it is nearly saturated. Thus the various

regions on the psychrometric chart for this type of system are somewhat

different than those discussed above. Region III in figure 9 is bounded on

the right by the constant enthalpy line passing through the saturation curve

at a dew point temperature equal to T^gp. Outdoor air in region III is mixed

with return air so that the thermodynamic state of the mixed air falls on this

constant enthalpy line. Actual control of the system is unchanged, since the

mixing is done to maintain a cooling coil discharge air temperature (or air

washer discharge air temperature) equal to T^gp. Since T^gp is usually

lower than the temperature required to satisfy the sensible cooling load in

order to maintain close humidity control in the conditioned space, this air

will typically be reheated to the desired supply air temperature. The minimum

amount of outside air is used in regions la and lb for the same reasons

discussed above for systems not employing sprayed coils or air washers. The

changeover temperature, T

Cq, is, however, likely to shift somewhat due to

changes in the shape of regions II and III and because the energy requirements

16
DRY-BULB TEMPERATURE

Figure 8. Operation of a dry-bulb economizer cycle on a single-zone

reheat system (refer to figures 2 and 6)

17

HUMIDITY

RATIO
DRY-BULB TEMPERATURE

Figure 9. Psychrometric chart of dry-bulb economizer cycle for systems

employing sprayed coils or air washers

18

HUMIDITY

RATIO
to condition the supply air in these regions is different for systems

employing sprayed coils or air washers. Since this changeover temperature

will vary with climate and the value chosen for T

CASp, each system should be

analyzed separately to determine the optimal value of T^q. Operation of a

dry-bulb economizer cycle in regions I, II, and III for systems employing

sprayed coils or air washers is shown in figure 10.

The variable air volume system, shown in figure 4, behaves like either the

single zone air handling unit in figure 1 or the reheat system in figure 2,

depending on whether a heating coil (not shown in figure 4) is placed upstream

or downstream of the cooling coil. The difference is that the variable volume

system provides multi-zone control with a single supply duct by varying the

quantity of air supplied to each zone. The psychrometric charts for a drybulb economizer cycle applied to this type of system are identical,

respectively, to the ones discussed above in figures 6 and 9 for VAV systems

which do not employ spray coils or air washers and for those that do.

As mentioned earlier, dual duct systems and multi-zone systems differ only in

the location where hot air and cold air mixing takes place. These systems may

use either non-sprayed or sprayed cooling coils. In a system using a sprayed

cooling coil (or an air washer), all of the air is usually passed through the

sprayed coil, and then part is reheated and mixed with the cooling air to

satisfy the various zone requirements. However, the most common system

configuration uses a non-sprayed cooling coil and such an arrangement (for a

19
DRY-BULB TEMPERATURE

Figure 10. Operation of a dry-bulb economizer cycle on an air

handling system with sprayed coils or air washers

(refer to figures 3 and 9)

20

HUMIDITY

RATIO
dual-duct system) was shown in figure 5. The psychrometric charts for the

typical application of dry-bulb economizer cycles to these systems are given

in figures 6 and 9 for systems with non-sprayed and sprayed coils,

respectively.

21
4. DRY-BULB ECONOMIZER ALGORITHMS

Although the presentation in the previous section on how a dry-bulb economizer

cycle works tends to be somewhat complex because of the different types of

coils and systems involved, the actual dry-bulb economizer algorithm is very

simple. In addition, it is usually implemented on the various HVAC systems

described above in almost an identical manner. The algorithm may be expressed

in words as follows:

(a) If the measured outdoor air temperature, TQA, is greater than or equal to

the preselected changeover temperature, Tqq, then the outdoor, return and

relief air dampers should be positioned to admit the minimum amount of

outside air to the building that is necessary to satisfy the fresh air

requirements.

(b) For HVAC systems not using sprayed coils (or air washers), the dampers

should be positioned to admit the maximum amount of outdoor air when the

outdoor temperature, TQA, is greater than or equal to TCAgp but less than

the changeover temperature, Tqq. For systems employing sprayed cooling

coils (or air washers) the dampers should be positioned to admit the

maximum possible outdoor air as long as Tq^ is below Tqq and mechanical

cooling is required to maintain the cooling coil discharge air

temperature, Tq^, at its set point, T

CASp.

22
(c) The outdoor, return, and relief air dampers should be positioned to mix

outdoor air and return air when Tq^ is less than Tg^gp for systems not

using sprayed coils (or air washers) or when is less than Tgg and

mechanical cooling is not required to maintain TCA at T

CASp

for systems

with sprayed coils (or air washers). In both cases, the mixing should be

controlled so as to maintain the cooling air discharge temperature, TgA,

at its set point, Tg^gp.

(d) For systems with preheating coils, the mixed damper control should be

overridden and the dampers positioned to admit the minimum amount of

outdoor air when the outdoor air temperature, Tq^, is equal to or below

some assigned value, T^p^. The minimum amount of outdoor air should be

that required to meet the minimum fresh air requirements of the

conditioned space.

The actual sequencing of the dampers and the cooling coil valve can be

accomplished in a number of ways. If pneumatic actuators are used, the

dampers and coil actuators can be assigned different pressure ranges so that

it is physically impossible for controlled mixing and cooling coil operation

to occur at the same time. For direct digital control systems, the same thing

can be accomplished by introducing a small control differential or dead zone,

A, about the set point temperature of the cooling coil discharge air. This is

to prevent excessive switching between control of the cooling coil and control

of the dampers when the outside air temperature is very close to Tg^gp. This

approach is implemented by checking to see if Tg^ is greater than or equal to

23
TCASP + ^ before switching from mixing control of the dampers to control of

the cooling coil. Likewise, TCA must be less than or equal t0 TCASP

"

before switching from cooling coil valve control to control of the dampers for

mixing purposes.

When the outside air temperature, TqA

is at or close to the changeover

temperature, Tc0 , the use of control differential, A, can minimize the

frequency of switching between the outdoor air damper position to admit the

maximum amount of outside air (Region II) and that to admit the minimum amount

of outdoor air (Region I). This floating region associated with A is bounded

by T co + AActual control of the outdoor, return, and relief air dampers when operating

in the mixing mode and control of the cooling coil valve will depend upon the

type of control system used. A dedicated pneumatic or electronic controller

could be employed or direct digital control can be provided by the energy

management and control system. It will be pointed out, however, that direct

digital control of valves and dampers can be accomplished either using

position algorithms which require feedback on the position of a valve or a

damper, or using velocity algorithms which do not require feedback.

A logic flow diagram representing the operation of the dry-bulb economizer

algorithm is shown in figure 11. The input variables to this algorithm are:

24
A

u - mrec Jtdoor air dry-bulb temperature

Tco specified changeover temperature (TCHG is used in the computer

prof x- am)

tca measured cooling coil discharge air dry-bulb temperature

t

casp

set point value of cooling coil discharge air dry-bulb

temperature

A control differential (or dead zone) to minimize the switching

frequency of damper or coil valve opening. For simplicity, a

single control differential is used for all floating regions in

the dry-bulb economizer cycle algorithm in this report.

tmin specified outdoor air dry-bulb temperature where the minimum

position of the outside air damper is reached

SPRAY logical variable indicating type of cooling coil, SPRAY = TRUE

if system uses a sprayed coil or air washer, FALSE otherwise

The output variables of the algorithm are:

CCLV logical variable indicating whether cooling coil valve is to be

controlled to maintain TCA equal to TCAgp (CCLV = TRUE) or

closed (CCLV = FALSE)

DAMPOA integer variable for controlling dampers

= 0 for admitting minimum outdoor air

= 1 for controlling damper to mix outdoor and return air

to maintain TCA equal to TCASp

= 2 for admitting maximum outdoor air

25
Figure 11. Logic flow diagram of dry-bulb economizer cycle

26
'he abovp z ;;oritbm assumes that the building return air temperature, T^, 18

at or close to the set point temperature of the conditioned space. If the

HVAC system has recently been started up after the end of an unoccupied

period and Tj^ is not near the desired conditioned space temperature, then

this economizer algorithm should not be used or else it should be overridden

or supplemented with a building warm-up or cool down control algorithm.

A computer program, DBE, for this dry-bulb algorithm, written in Fortran 77,

is contained in Appendix A. A calling routine, DBEMAIN, is also presented

there. Sample input and output of the program using a Sperry 1100/82

computer are provided.

27
5. ENTHALPY ECONOMIZER CYCLE

In an enthalpy economizer cycle, the specific enthalpy or heat content of both

the outdoor air and the return air are determined. This information is then

used to decide whether the minimum or maximum amount of outside air should be

used or if the outside and return air should be mixed to satisfy the sensible

and latent cooling requirements of the conditioned space. Since these

decisions are made in real time and can dynamically account for any changes in

the thermodynamic state of the outside and return air, the enthalpy economizer

tends, in general, to save more energy than the dry-bulb economizer cycle

[2,3]. The need for an assigned changeover temperature, T^q, to account for

typical outdoor and return air conditions is also eliminated.

The operation of the enthalpy economizer cycle can most easily be explained

using psychrometric charts similar to the ones discussed above. Figure 12 is

such a chart for a system which does not employ a sprayed cooling coil or an

air washer. In region la of this figure, the enthalpy of the outside air is

greater than that of the return air and thus the dampers should be set to

admit the minimum amount of outside air required to meet the fresh air needs

of the conditioned space. In region lb, the enthalpy of the outside air is

less than the return air enthalpy, but the amount of sensible cooling

necessary to reduce the temperature of the outside air to the set point of the

cooling coil discharge air is greater than the amount needed if return air is

used. Because of this and because the majority of HVAC systems will be doing

28
only sensible cooling when the outside temperature is in most of region lb,

the minimum amount of outside air should be used in this region. In region

II, the use of the maximum possible outside air will reduce the system's

energy consumption since both the enthalpy and dry-bulb temperature of the

outside air are less than the corresponding values for the return air.

When the outside temperature is in region III of figure 12, the outside and

return air streams should be mixed to obtain a mixed air temperature of T

c^gp

and the cooling coil valve should be closed (i.e., no mechanical cooling is

done). For HVAC systems with preheat coils, such as the one in figure 2, the

outdoor and relief air dampers should be reset to their minimum open position

whenever the outside air temperature falls below an assigned minimum value,

T MIN ( ReSi°n

The psychrometric chart for an enthalpy economizer cycle applied to HVAC

systems using a sprayed cooling coil or air washer is shown in figure 13. As

in the case of the dry-bulb economizer cycle applied to this type of system,

region III is larger and extends, on the right side, to the enthalpy line

passing through T

CAgp. This is because if outside air and return air streams

are mixed to obtain this enthalpy, the resulting mixed air will be cooled down

to approximately T^gp by means of evaporative cooling as it passes through

the sprayer or air washer without mechanical cooling being done. The region

corresponding to maximum outside air use, region II, is also extended, on the

right, to the enthalpy line of the return air. Thus, for systems employing

sprayed coils or air washers, it is desirable to use the maximum amount of

29
DRY-BULB TEMPERATURE

Figure 12. Psychrometric chart of enthalpy economizer cycle for systems

which do not employ sprayed cooling coils or air washers

30

HUMIDITY

RATIO
Figure 13. Psychrometric chart of enthalpy economizer cycle for systems

with sprayed cooling coils or air washers

31

HUMIDITY

RATIO
outside air whenever the outside air enthalpy is less than the enthalpy of the

return air but greater than the desired enthalpy of the cooling coil discharge

air.

32
6 . ENTHALPY ECONOMIZER ALGORITHM

The enthalpy economizer may be expressed as follows:

(a) For systems not employing sprayed coils (or air washers), the outdoor,

return and relief air dampers should be positioned to admit the minimum

amount of outside air (needed to meet the space's minimum fresh air

requirements) when either the enthalpy or the dry-bulb temperature of the

outside air is greater than or equal to the enthalpy or the dry-bulb

temperature of the return air, respectively. For systems using sprayed

coils or air washers, the minimum amount of outside air should be used

whenever the enthalpy of the outside air is greater than or equal to the

return air enthalpy.

(b) The maximum amount of outside air should be used with systems which do

not contain sprayed coils (or air washers) whenever the dry-bulb

temperature of the outside air is greater than or equal to Tg^gp and both

the dry-bulb temperature and the enthalpy of the outside air is less than

the dry-bulb temperature, and enthalpy of the return air, respectively.

For systems containing sprayed coils (or air washers), the maximum amount

of outside air should be used as long as mechanical cooling is required

to maintain the cooling coil discharge air temperature, Tg^, at its set

point, TCASp, and the enthalpy of the outside air is below the enthalpy

of the return air.

33
(c) Controlled mixing of the outside and return air should be done when toa

is less than TCASp for systems not employing sprayed coils (or air

washers) or when mechanical cooling is not required to maintain TCA at

Tcasp f°r systems with sprayed coils (or air washers). The mixing should

be done to maintain the cooling air discharge temperature, Tq^, at its

set point, ^CASP*

(d) For systems with preheating coils, controlled mixing should be overridden

and the dampers positioned to admit the minimum amount of outside air

when the outdoor air temperature, Tq^, is equal to or below some assigned

value T MIN. The minimum amount of outside air should be that required to

meet the minimum fresh air requirements of the conditioned space.

The same points, regarding sequencing and control of the dampers and cooling

coil, that were discussed in the section entitled "Dry-Bulb Economizer

Algorithm," also apply to the enthalpy economizer cycle. Briefly, if direct

digital control is used on a system with a sprayed cooling coil (or an air

washer), sequencing of the cooling coil and dampers will require the use of a

small control differential or dead zone. A» about the set point temperature

t

CA.SP t0 Prevent excessive switching between control of the cooling coil and

control of the dampers. The same A can also be applied to prevent excessive

switching between the minimum and maximum open positions of the outdoor

damper. For both sprayed coil and non-sprayed coil systems, controlled mixing

of the outdoor and return air streams, as well as control of the cooling coil,

may also be accomplished using dedicated pneumatic or electronic controllers.

34
A logic flow diagram for the enthalpy economizer algorithm described above is

presented in figure 14. The input variables to this algorithm are:

t

oa

outdoor air dry-bulb temperature

t

ra

return air dry-bulb temperature

tca measured cooling coil discharge air temperature

t

casp

set point value of cooling coil discharge air dry-bulb

temperature

tmin specified outdoor air dry-bulb temperature where the minimum

position of the outside air damper is reached.

A control differential (or dead zone) to minimize the switching

frequency of damper or coil valve opening. For simplicity, a

single control differential is used for all floating regions

in the dry-bulb economizer cycle algorithm in this report.

BP measured barometric pressure

SPRAY logic variable indicating type of cooling coil, SPRAY = TRUE,

if system uses a sprayed coil or air washer, otherwise

SPRAY = FALSE otherwise

METRIC logical variable indicating whether metric or English units are

used, FALSE if English units are used

DEWPT logical variable. Set TRUE if outside and return air dew

points are measured. Set FALSE if outside and return air

relative humidities are measured.

35
Figure 14. Logic flow diagram of the enthalpy economizer cycle

36
In addition, the following measured data are also required:

DPOA dev point of outside air

DPRA dew point of return air

or

RHOA relative humidity of outside air

RHRA relative humidity of return air

depending whether the value of DEWPT is set TRUE or FALSE, respectively.

Output variables are CCLV and DAMPOA, which have been described in section 4 of

this report.

If the HVAC system has recently been started up after the end of an unoccupied

period and TRA is not near the setpoint temperature of the condition space,

then the enthalpy economizer algorithm given above should not be used or

should be overridden or supplemented with a building warm-up or cool down

control algorithm.

A computer program, called ETC1 for this enthalpy economizer algorithm, is

presented in Appendix B. A calling routine, ETC1MAIN, and a routine for

calculating the required psychrometric properties of moist air (PSYCHR) [9,10]

are also listed. Sample input and output are included.

37
7. DUAL-DUCT SYSTEMS

Dual-duct systems utilize two ducts to distribute the conditioned air to

conditioned spaces. One duct delivers chilled air, and the other carries

heated air at all times. In order to satisfy the thermal load requirement of

the conditioned zone, the hot and cold air is mixed in proper proportion in

the mixing box, which is located at each zone. The damper of the mixing box

operates in response to the zone thermostat [6]. A basic dual-duct system is

shown in figure 5. As briefly mentioned in section 2, dual-duct systems are

similar to multi-zone systems. The difference between the two systems is the

location where the cold and hot air is mixed. In the multi-zone system, air

mixing takes place at the air handling unit and then the mixed air is ducted

to various conditioned spaces using a single-duct. In addition, the multizone system has a set of cold and hot deck dampers serving for each zone.

These dampers are located at the air handling unit and are controlled by each

zone thermostat. Due to similarity of the two systems, only the dual-duct

system is discussed in this section.

As shown in figure 5, the outside air and the return air by-pass dampers and

are mixed prior to the supply fan. The supply fan delivers the supply air to

the hot and cold decks respectively. In the hot deck, the air passing through

the heating coil is heated, while the air passing through the cooling coil is

chilled in the cold deck. Usually no mechanical cooling is needed during the

heating season (winter) and no heating is required during the cooling season

38
(summer). The temperature of the mixed air satisfies adequately the zone

requirement [6]. But at certain conditions, both heating and cooling

operations are required.

Figure 15 depicts the psychrometric chart of a dual-duct system without

sprayed coil or air washer. This figure shows the thermodynamic conditions of

the air during the cooling season with both heating and cooling coils in

operation. The mixed air (MA) temperature is slightly raised to that of the

supply air (SA) due to the fan heat gain. The two air streams after the

supply air fan are then chilled and heated to the conditions of CA and HA,

respectively. In the mixing box, they are mixed to the conditions of zone air

(ZA).

When an economizer cycle is applied to the dual-duct system, the cooling

energy is reduced by the free cooling of the outside air. However, the

overall energy cost of the system may be more wasteful than the system without

the economizer cycle, depending on the difference in the unit costs of the

cooling and the heating energy, the mass flow ratio of the cold and hot air

streams, and the temperature of the hot air (HA). Therefore, an enhancement

algorithm for the dual-duct system was developed and is described in the next

section.

39
DRY-BULB TEMPERATURE

Figure 15. Psychrometric charts of the dual-duct system without

sprayed coil or air washer

40

HUMIDITY

RATIO
8. ALGORITHM ENHANCEMENT FOR DUAL-DUCT AND MULTI-ZONE SYSTEMS

The enthalpy economizer algorithm presented in section 6 assumes that the

heating cost is the same as the cooling cost for the same amount of energy

used. The correctness of this assumption has no effect on single-zone

systems. But a more complex evaluation of the system operation is needed for

the dual-duct and multi-zone systems to determine the overall energy effect of

the enthalpy economizer operation. The enhancement algorithm described in

this section is an optional procedure which supplements the enthalpy

economizer algorithm presented previously. Application of the optional

enhancement requires the input of additional information which will be

described later.

As seen in figure 12, when the enthalpy economizer is employed to a singlezone system, the use of a minimum amount of outdoor air is economical if the

thermodynamic state of outdoor air is in region I, and admitting the maximum

amount of outdoor air is cost-saving if the psychrometric property of the

outdoor air is in region II. This statement is not always true for dual-duct

and multi-zone systems. The optional procedure introduces a dimensionless

factor, X , to be used to compare the energy costs of the system in these two

regions. The factor X is defined as the ratio of the total energy cost

(cooling and heating) of the air-handling system with the outside air damper

at the minimum opening position to that with the damper at its maximum opening

position. The derivation of X may be found in Appendix C.

41
In regions I and II, a decision based on the cost evaluation of X replaces the

decision made by the enthalpy algorithm described previously. If X is greater

than 1.0, the maximum amount of outside air is allowed to enter the system.

Otherwise, the minimum amount of outside air is admitted. When the

psychrometric condition of the outside air is in regions III and IV, no cost

evaluation is performed and the algorithm described previously applies. The

logic flow diagram of the enhancement routine is shown in figure 16. This

cost comparison routine incorporates the enthalpy economizer algorithm, ETC1,

and requires the following additional data:

COST

%A

X0AMIN

HGTOCL

DPCA

DPHA

or

RHCA

RHHA

logical variable indicating whether cost evaluation is desired

(COST = TRUE), or not (COST - FALSE).

dry-bulb temperature of heating coil discharge air

ratio of cooling coil air flow rate to supply air flow rate

ratio of heating coil air flow rate to supply air flow rate

ratio of minimum outside air flow rate

to supply air flow

cost ratio of heating energy to cooling energy for the same

amount of energy

$ of heating per Btu or KJ

$ of cooling per Btu or KJ

dew point of cooling coil discharge air

dew point of heating coil discharge air

relative humidity of cooling coil discharge air

relative humidity of heating coil discharge air

42
Figure 16. Logic flow diagram of the enhancement routine for

dual-duct system

43
A computer program, ETC2, of the enthalpy economizer algorithm incorporating

the optional enhancement reoutine (KCCOST) is given in Appendix D. Sample

input and output are included. When the logical value of COST is false, the

program ETC2 performs the same role as the program ETC1, and the logical flow

becomes identical.
9 . SUMMARY

Commonly used air handling systems were briefly reviewed in this report.

These systems included single-zone, reheat, variable air volume, and dual-duct

types having either dry cooling coils or sprayed coils. Energy Management and

Control Systems often incorporate dry-bulb or enthalpy economizer cycles in

order to reduce energy consumption. In this study, the psychrometric chart of

atmospheric air was divided into various regions, using the psychrometric

states of the return air, cooling coil discharge air, supply air, and other

pertinent parameters. Algorithms were developed for an EMCS to determine the

amount of outside air to be admitted into the air handling systems for outside

air conditions in the different regions and the types of coils used (dry or

sprayed)

.

For the economizer cycles based upon measured dry-bulb temperatures, the

algorithms require values of the outside air temperature, a specified

changeover temperature and its control differential, the actual and setpoint

temperatures of the cooling coil discharge air, a specified outdoor air

temperture below which the minimum position of the outside air damper is to be

used, and knowledge of the type of cooling coil (dry or sprayed). For the

enthalpy economizer cycle, the changeover and outside air temperatures in the

above list are replaced by the return air temperature and outside air

enthalpy, respectively. In addition, the return air humidity is also

required

.

45
Since the energy savings of dual-duct and multi-zone systems are complicated

by the relative costs of heating energy and cooling energy, an enhancement

algorithm for these systems was also developed vhich utilized the relative

costs of heating and cooling energies and the mass flow rates of the heating

and cooling air.

Computer programs, written in Fortran 77, were presented in the appendices to

assist the reader in implementing these algorithms.

46
REFERENCES

[1] Shavit, G., "Enthalpy Control Systems Increased Energy Conservation,"

Handbook of Energy Conservation for Mechanical Systems in Buildings,

edited by R. W. Roose, Van Nostrand Reinhold Co., 1978, pp. 449-460.

[2] Parken, W.H., Kao, J.Y., and Kelly, G. E., "Strategies for Energy

Conservation in Small Office Buildings," Natl. Bur. of Standards, NBSIR

82-2489, June 1982.

[3] Kao, J.Y., Parken, W.H., and Pierce, E.T., "Strategies for Energy

Conservation for a Large Retail Store," Natl. Bur. of Standards, NBSIR

82-2580, Sept. 1982.

[4] Kao, J.Y., and Pierce, E.T., "A Study of Sensor Errors on Building Energy

Consumption", 7th Energy Management and Control Soc. Conf., Salt Lake

City, Utah, Nov. 14-17, 1982.

[5] ASHRAE, System Handbook, Chapter 3, 1980.

[6] Haines, R.W., Control Systems for Heating, Ventilating and Air

Conditioning, 3rd ed., Van Nostrand Reinhold, 1983.

[7] Croome, D.J., and Roberts, B.M., Air Conditioning and Ventilation of

Buildings, 2nd ed., Vol 1, Pergamon Press, 1981.

[8] Honeywell, Energy Conservation with Comfort, 2nd ed., 1979

[9] Brokaw, R.S., "Calculation of Flue Losses for High-Efficiency Furnaces

and Appliances," ASHRAE Journal, Jan. 1979, pp. 49-51.

[10] ASHRAE, Fundamentals Handbook, Chapter 5, 1981.

47

APPENDIX A. COMPUTER PROGRAM LISTING OF THE DRY BULB ECONOMIZER

ALGORITHM (DBE) , AND SAMPLE INPUT AND OUTPUT

Q$Q$Q$* ETC ( 1 )

.

DBEALL <0

)

1 C

2 C

3 C DBE: Dry-bulb economizer cycle algorithm

4 C for building air handling systems

5 C

6 C JULY 29, 1983 C.P.

7

8 cc

9 c 10 c CCL V Logical variable for controlling a cooling

1

1

c coll valve. CCLV*T when the valve Is open and CCLV«*F

12 c when It Is closed.

13 c 14 c DAMPOA Integer variable for controlling dampers.

15 c DAMPOA * 0 for admitting minimum outdoor air

16 c 1 for modulating dampers

17 c * 2 for admitting maximum outdoor air

18 c 19 c DELTA Temperature difference of floating zone to minimize

20 c the switching frequency of damper or coll valve opening

21 c 22 c SPRAY Logical variable Indicating type of cooling coll.

23 c SPRAY * T If system uses a sprayed coll or air washer.

24 c SPRAY * F otherwise.

25 c 26 c TCA Measured cooling coll discharge air dry-bulb temperature

27 c 28 c TCASP Set point value of cooling coll discharge air dry-bulb

29 c temperature

30 c 31 c TCHG Specified changeover temperature

32 c 33 c TMIN Specified outside air dry bulb temperature below which

34 c minimum outside air Is admitted

35 c 36 c TOA Measured outdoor air dry-bulb temperature

37 c 38 c — Either English or metric unit can be used.

39 c 40 c 41 c 42 SUBROUTINE DBE

43 LOGICAL CCLV, SPRAY

44 INTEGER DAMPOA

45 COMMON /BK1 / TOA , TCHG , TCA , TCASP , TMI N .SPRAY .DELTA

46 & /BK2/ CCLV, DAMPOA

47 c 48 c Dry-bulb economizer

49 c 50 c Determine outdoor air damper opening and cooling coll valve

51 c oper at 1 on

52 c 53 I F { TOA .LE.TMIN) THEN

54 CCLV=. FALSE.

55 DAMPOA=0

56 ELSEIF(TOA.GE.TCHG+DELTA) THEN

57 CCLV*. TRUE.

A-l
56 DAMPOA-0

59 ELSEIF(TOA.LE.TCHG-DELTA) THEN

60 I F ( SPRAY ) THEN

61 I F ( TCA . GE . TCASP DELTA ) THEN

62 CCL V" . TRUE

63 DAMPOA-2

64 ELSEIFITCA.LE. TCASP- DELTA > THEN

65 CCLV-. FALSE.

66 DAMPOA-1

67 END I F 68 ELSE

69 IFtTOA.GE .TCASP+DELTA) THEN

70 CCLV" . TRUE

71 DAMPOA-2

72 ELSEIF(TOA.LE. TCASP -DELTA ) THEN

73 CCLV-. FALSE.

74 DAMPOA-1

75 END I

76 END I F 77 END I F 78 C 79 RETURN

80 END

81 C ** 82 C 83 C DBEMAIN « Main program to call the dry-bulb economizer

84 C algorithm for building air handling systems

85 C 86 c JULY 29, 1983 C.P.

87 c 88 c •« 89 c 90 CHARACTER TITLE<2>*80

91 LOGICAL CCLV, SPRAY

92 INTEGER DAMPOA

93 COMMON /BK 1 / TOA, TCHG, TCA, TCASP ,TMIN, SPRAY, DELTA

94 & /BK2/ CCLV, DAMPOA

95 NAMELIST /INPUT/ SPRAY , TCHG , TCASP , TMI N , DELTA

96 & /OUTPUT/ CCLV, DAMPOA

97 OPEN(7,FILE-’ DBEDATA ’ ) 98 REWIND 7 99 c 100 c Read Initial Input data

101 c 102 READ ( 7 , 1000 ) TITLE

103 READ(7, INPUT)

104 PRINT 1000, TITLE

105 PRINT INPUT

106 c 107 c Read measured Input data

108 c 109 10 I F ( SPRAY ) THEN

1 10 READ17,*, END-999) TOA , TCA

1 1 1 PRINT 2000, TOA, TCA

112 ELSE

113 READ(7,*,END=999) TOA

114 PRINT 2000, TOA

115 END I

A116 C

117 C Call the dry-bulb economizer algorithm

118 C 119 CALL DBE

120 C

121 PRINT OUTPUT

122 PRINT 3000

123 C 124 C Damper and cooling coll valve control routines should

125 C called here. These routines are machine-dependent.

126 C 127 GOTO 10

128 c 129 c Format statements

130 c 131 1000 FORMAT! A80/A80

)

132 2000 F ORMAT ( 8F 10 . 4

)

133 3000 FORMAT! IX, 79! ’-’ )

)

134 C 135 999 STOP

136 END

I A-3
Q$Q$QS"DBEDATA ( 1

)

1 INPUT DATA FOR DBE PRGOGRAM

2 DRY COIL

3 SINPUT SPRAY-F , TCHG-77 , TCASP-55 . .TMIN-30.0, DELTA-1 .0, SEND

4 95.0

5 75.0

6 67.0

7 40.0

8 80.0

9 74.0

10 50.0

153.0

12 55.0

13 55.5

14 55.0

15 57.0

16 55.5

17 55.0

18 54.0

19 55.0

2 * 50.0

9XQT ETC . DBES

INPUT DATA FOR DBE PRGOGRAM

DRY COIL

SINPUT

SPRAY - F.TCHG = . 77000000+002 , TCASP - . 55000000+002 , TM I N . 30000000+002 , DELTA

SEND

95.0000

SOUTPUT

CCLV = T , DAMPOA - 0

SEND

75.0000

SOUTPUT

CCLV = T, DAMPOA

SEND

s 2

67.0000

SOUTPUT

CCLV = T , DAMPOA

SEND

s 2

40.0000

SOUTPUT

CCLV = F, DAMPOA

SEND

a 1

80 . 0000

SOUTPUT

CCLV = T, DAMPOA

SEND

B 0

7 4 . 0000

SOUTPUT

CCLV = T, DAMPOA

SEND

-

2

A-4

. 10000000+001
50.0000

SOUTPUT

CCL V = F , DAMPOA - SEND

1

53.0000

SOUTPUT

CCLV = F, DAMPOA « SEND

1

55.0000

SOUTPUT

CCLV = F, DAMPOA - SEND

1

55 . 5000

SOUTPUT

CCLV = F, DAMPOA - SEND

1

55.0000

SOUTPUT

CCLV = F, DAMPOA - SEND

1

57 .0000

SOUTPUT

CCLV = T, DAMPOA - SEND2

55.5000

SOUTPUT

CCLV = T, DAMPOA = SEND

2

55.0000

SOUTPUT

CCLV = T, DAMPOA = SEND

2

54.0000

SOUTPUT

CCLV = F, DAMPOA - SEND

1

55.0000

SOUTPUT

CCLV = F, DAMPOA = SEND

1

50 . 0000

SOUTPUT

CCLV = F, DAMPOA = SEND

1

APPENDIX B. COMPUTER PROGRAM LISTING OF THE ENTHALPY ECONOMIZER

ALGORITHM (ETC1), AND SAMPLE INPUT AND OUTPUT

Q$Q$Q$*ETC

12

3

456

7

{ 1 ) . ETC 1ALL (0

)

c **««****•*'

ETC1 t Enthalpy economizer algorithm

for building air handling systems

VERSION I July 29, 1983 C.P.

Logical variable for controlling a cooling

coll valve. CCLV«*T when the valve Is open and CCLV*F

when It Is closed.

Integer variable for controlling dampers.

DAMPOA * 0 for admitting minimum outdoor air

1 for modulating dampers

2 for admitting maximum outdoor air

Temperature difference of floating zone to minimize

the switching frequency of damper or coll valve opening

Logical variable. Set TRUE If outside and return air

dew points are measured. Set FALSE If outside and return

air relative humidities are measured.

8

9 C --C 10 C CCL V

1

1

C 12 C 13 C 14 C DAMPOA

15 C

16 C

17 C 18 C 19 C DELTA

20 c 21 c 22 c DEVPT

23 c 24 c 25 c 26 c DPOA

27 c 28 c DPRA

29 c 30 c HOA

31 c 32 c HRA

33 c 34 c METRIC

35 c 36 c 37 c PB

38 c 39 c RHOA

40 c

41 c RHRA

42 c 43 c SPRAY

44 c 45 c 46 c 47 c TCA

48 c 49 c TCASP

50 c 51 c 52 c TMIN

53 c 54 c 55 c TOA

56 c 57 c TRA

Dew point temperature of outside air

Dew point temperature of return air

Enthalpy of outside air

Enthalpy of return air

(F) <C)

IF) ( C > (Btu/Lb) (KJ/kg)

( Btu/Lb ) (KJ/kg)

Logical variable. Set TRUE If metric unit Is used.

Set FALSE If English unit Is employed.

Barometric pressure

Relative humidity of outdoor air

Relative humidity of return air

(In.Hg) (Pa)

( percent

)

(percent)

Logical variable Indicating type of cooling coll,

SPRAY * T If system uses a sprayed coll or air washer,

SPRAY ® F otherwise.

Measured cooling coll discharge air dry-bulb temperature

Set point value of cooling coll discharge air dry-bulb

temperature

Specified outside air dry bulb temperture below which

minimum outside air Is admitted

Measured outdoor air dry-bulb temperature

Retrun air dry-bulb temperature

B-l
58 C 59 C

* 60 C 61 SUBROUTINE ETC1

62 INTEGER DAMPOA

63 LOGICAL CCLV, SPRAY, DEWPT, METRIC

64 COMMON /BK1 / TCA.TCASP.TMIN, SPRAY, DELTA

65 & /BK2/ DEWPT, METRIC

66 & /BK3/ CCLV, DAMPOA

67 i, /BK4/ TOA , DPOA , RHOA , TRA , DPRA , RHRA , PB

68 NAMELIST /OUT 1 / HOA.HRA

69 C 70 C Enthalpy economizer

71 C 72 C Compute enthalpies of outdoor and return air

73 C 74 CALL PSYCHRl TOA, DPOA, RHOA, PB,HOA>

75 CALL PSYCHRl TRA, DPRA, RHRA, PB.HRA)

76 C 77 C Determine outdoor air damper opening and cooling coll valve

78 C operation

79 C 80 IF(TOA.LE.TMIN) THEN

81 CCLV-. FALSE.

82 DAMPOA-0

83 ELSEIFIHOA.GE. HRA+DELTA ) THEN

84 CCLV-. TRUE.

85 DAMPOA-0

86 ELSE I F ( HOA. LE .HRA-DELTA) THEN

87 I F ( SPRAY > THEN

88 I F ( TCA . GE . TCASP + DELTA ) THEN

89 CCLV-. TRUE.

90 DAMPOA-2

91 ELSEIF1TCA.LE. TCASP -DELTA ) THEN

92 CCLV-. FALSE.

93 DAMPOA

94 ENDIF

95 ELSE

96 I F ( TOA . GE . TRA+DELTA ) THEN

97 CCLV-. TRUE.

98 DAMPOA-0

99 ELSEIF1TOA.LE. TRA-DELTA ) THEN

100 I F ( TOA . GE . TCASP + DELTA ) THEN

101 CCLV-. TRUE.

102 DAMPOA-2

103 ELSEIF1TOA.LE. TCASP -DELTA ) THEN

104 CCLV-. FALSE.

105 DAMPOA

106 ENDIF

107 ENDIF

108 ENDIF

109 ENDIF

1 10 PRINT OUT1

1 1

C 112 RETURN

113 END

114

115 C

B-2
116

1 17

118

119

120

121

122

123

C

CC

C

C

C

C

p _

PSYCHR t Psychrometr 1 c properties of moist air between

60 F (15.6 C) and 140 F (60 C) with estimated error

of 0.02 X Sources t ASHRAE 1981 Fundamentals Handbook, Chapter 5 ASHRAE Journal Jan. 1979 (R. S. Brokaw ) PP. 49-51

V 124 C 125 c . Engl 1 sh Metr 1 c 126

127

c

c — DB Dry-bulb air temperature F C 128 c DP Dew point temperature F C 129 c H Enthalpy Btu/Lb KJ/kg

130 c PB Barometric pressure 1 n . Hg Pa

131 c PSW Saturated water avpor pressure 1 n . Hg Pa

132 c PW Water vapor pressure 1 n . Hg Pa

133 c RH Relative humidity percent percent

134 c W Humidity ratio - - 135 c 136

137 c 138 SUBROUTINE PSYCHR ( DB , DP , RH , PB , H ) 139 LOGICAL DEWPT, METRIC

140 COMMON /BK2/ DEWPT, METRIC

141 c 142 c Dew point temp or relative humidity

143 c 144 I F ( DEWPT ) THEN

145 TEMP = DP 146 RH= 100.0

147 ELSE

148 TEMP = DB

149 END I

F

150 c 151 c Compute psychrometr 1 c properties using metric units

152 c 153 IF(METRIC) THEN

154 PSW= 3.37685E+3*EXP( 15.4636-7284. /< 1 .8 •TEMP+424 . )

)

155 PW=RH/ 100. *PSW

156 W=0. 62198*(PW/(PB-PW)

)

157 H=DB+W* ( 2501 . + 1 . 805* DB

)

158 I F ( . NOT. DEWPT) THEN

159 PWL0G=AL0G ( PW

)

160 DP =-35.957-1 . 8726*PWLOG+

1

. 1 689*PWL0G*PWL0G

161 END I

F

162 c 163 c Compute psychrometr 1 c properties using Eng 1 1 sh units

164 c 165 ELSE

166 PSW= EXP ( 15. 4636-7284. /(TEMP+392. ) ) 167 PW=RH/100.*PSW

168 W=0 . 62198*(PW/(PB-PW)

)

169 H=0 . 24*DB+W* ( 1061 .+0.444*DB)

170 I F ( . NOT. DEWPT) THEN

171 PWLOG=ALOG ( PW)

172 DP =79.047+30. 5790*PWLOG+1 . 8893*PWL0G*PWL0G

173 END I

F

B-3
174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

non

o

ooo

oooooooooooooooooooo

EN01F

Cc RETURN

END

ETC1MAIN i Main program to call the enthalpy economizer

algorithm for building air handling systems

VERSION I July 29, 1983 C.P.

TITLE Title of the Input data set

Y(I) Input data array

Y ( 1 ) PB

Y ( 2

)

TOA

Y( 3

)

DPOA If DEWPT-T, RHOA If DEWPT-F

Y( 4 ) TRA

Y( 5

)

DPRA If DEWPT-T, RHRA If DEWPT-F

Y( 6 > TCA

LOGICAL CCLV, SPRAY, DEWPT, METRIC

INTEGER DAMPOA

CHARACTER TITLE<2)*80

DIMENSION Y( 6

)

COMMON /BK1 / TCA.TCASP ,TMIN, SPRAY, DELTA

& /BK2/ DEWPT, METRIC

i, /BK3/ CCLV, DAMPOA

& /BK4/ TOA , DPOA , RHOA , TRA , DPRA ,RHRA , PB

NAMELIST /INPUT/ METR I C , DEWPT , SPRAY , TCASP , TMI N .DELTA

& /OUTPUT/ CCLV, DAMPOA

OPEN(7,FILE“’ ETC 1 DATA *

) REWIND 7 Read Inltall Input data

READ ( 7 , 1000 ) TITLE

READ(7, INPUT)

PRINT 1000, TITLE

PRINT INPUT

I F ( SPRAY ) THEN

N-6

ELSE

N =* 5 END I

Read measured Input data

10 READ17,*, END-999) <Y<I),I-1,N)

PRINT 2000, <Y( I ) , I

1 , N

)

PB -Y< 1

)

TOA-Y12)

TRA-YU)

B-4
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.-ooo

oooo

n

non

ooo

TCA«Y(6>

Dew point temp or relative humidity

IF(DEWPT) THEN

R HOA= 100 .

0

RHRA=100.0

DPOA=Y( 3

)

DPRA=Y ( 5

)

ELSE

DPOA-0.0

DPRA=0.0

RHOA=Y ( 3

)

RHRA=Y ( 5 END I F Call the enthalpy control algorithm

CALL ETC 1 PRINT OUTPUT

PRINT 3000

Damper and cooling coll valve control routines should be

called here. These routtnes are machine-dependent.

GOTO 10

Format statements

000 F OR MAT ( A80/A80 > 2000 FORMAT! 8F 10. 4)

3000 FORMAT! IX, 79(

)

C

999 STOP

END

B-5
Q$Q$Q$*ETC 1 DATA ( 1 )

1 INPUT DATA FOR ETC1 PROGRAM

2 DRY COIL

3 $ INPUT METRIC-F , DEWPT-T .SPRAY-F .TCASP -55.0, TMIN-30.0, DELTA- 1 .0.SEND

4 29,,921 95 .0 60,,0 77,,0 55,,0

5 29,,921 75 .0 64.,0 77 .0 55,,0

6 29,,921 67 .0 57 , ,0 77 , ,0 55,,0

7 29,,921 40 . 0- 35,,0 77,.0 55.,0

8 29,,921 80 .0 57,,0 77,.0 55 , ,0

9 29,,921 74 .0 51 , ,0 77 .0 55,.0 10 29,,92* •50 .0 40,,0 77 .0 55,.0

1 1 29 ,921 53 .0 40,,0 77 .0 55 .0 12 29 ,921 55 .0 40,.0 77 .0 55,.0

13 29 ,921 55 .5 40,.0 77 .0 55,.0 14 29 ,921 55 .0 40,,0 77 .0 55,.0 15 29 .921 57 .0 40,,0 77 .0 55 .0 16 29,.921 55 .5 40,.0 77 .0 55 .0

17 29 .921 55 .0 40 .0 77 .0 55 .0 18 29,.921 54 .0 40 .0 77 .0 55 .0 19 29 . 921 55 .0 40,.0 77 .0 55 .0 20 29,.921 50 .0 40,.0 77 .0 55 .0

0XQT ETC . ETC 1 $

INPUT DATA FOR ETC1 PROGRAM

DRY COIL

SINPUT

METRIC = F.DEWPT = T, SPRAY « F, TCASP *= . 55000000 + 002 , TMI N . 30000000*002 , DELTA • SEND

29.3210 95.0000 60.0000 77.0000 55.0000

SOUT1

HOA = . 34966373+002 , HR A = .28536576+002

SEND

SOUTPUT

CCL V - T , DAMPOA = 0

SEND

29.9210 75.0000 64.0000 77.0000

$OUT 1 HOA = . 31938431+002, HRA = .28536576+002

SEND

SOUTPUT

CCL V = T, DAMPOA = 0

SEND

55.0000

29.9210 67.0000 57.0000 77.0000

SOUT1

HOA = .26861820+002, HRA = .28536576+002

SEND

SOUTPUT

CCL V = T, DAMPOA = 2 SEND

55.0000

29.9210 40.0000 35.0000 77.0000

SOUT1

HOA = . 14181454+002 , HRA = .28536576+002

SEND

SOUTPUT

55.0000

B-6

. 10000000+001
CCLV = F , DAMPOA - 1 SEND

29.9210 80.0000 ,57.0000 77.0000

SOUT1

HOA = .30038875+002, HRA « .28536576+002

SEND

SOUTPUT

CCLV - T, DAMPOA - 0

SEND

55.0000

29.9210 74.0000 51.0000 77.0000

SOUT1

HOA - .26412566+002, HRA *= .28536576+002

SEND

SOUTPUT

CCLV = T, DAMPOA = 2 SEND

55.0000

29.9210 50.0000 40.0000 77.0000

SOUT1

HOA = . 17612815+002, HRA = .28536576+002

SEND

SOUTPUT

CCLV * F, DAMPOA = 1 SEND

55.0000

29.9210 53.0000 40.0000 77.0000

SOUT1

HOA . 18339717+002, HRA - .28536576+002

SEND

SOUTPUT

CCLV - F, DAMPOA = 1 SEND

55.0000

29.9210 55.0000 40.0000 77.0000

SOUT1

HOA - . 18824318+002, HRA - .28536576+002

SEND

SOUTPUT

CCLV = F, DAMPOA = 1 SEND

55.0000

29.9210 55.5000 40.0000 77.0000

SOUT1

HOA = . 18945469+002, HRA = .28536576+002

SEND

SOUTPUT

CCLV = F, DAMPOA = 1 SEND

55.0000

29.9210 55.0000 40.0000 77.0000 55.0000

SOUT1

HOA = . 18824318+002, HRA = .28536576+002

SEND

SOUTPUT

CCLV = F, DAMPOA = 1 SEND

B-7
29.9210 57.0000 40.0000 77.0000

$OUT 1 HOA - .19 30 8920+002, HRA - .28536576+002

SEND

SOUTPUT

CCL V - T.DAMPOA - 2 SEND

55.0000

29.9210 55.5000 40.0000 77.0000

SOUT1

HOA « . 18945469+002, HRA - .28536576+002

SEND

SOUTPUT

CCL V • T.DAMPOA - 2 SEND

55.0000

29.9210 55.0000 40.0000 77.0000

SOUT1

HOA = . 18824318+002, HRA - .28536576+002

SEND

SOUTPUT

CCL V - T.DAMPOA - 2 SEND

55.0000

29.9210 54.0000 40.0000 77.0000

SOUT1

HOA = . 18582018+002 , HRA - .28536576+002

SEND

SOUTPUT

CCL V - F , DAMPOA - 1 SEND

55.0000

29.9210 55.0000 40.0000 77.0000

SOUT1

HOA = . 18824318+002, HRA - .28536576+002

SEND

SOUTPUT

CCL V = F, DAMPOA * 1 SEND

55.0000

29.9210 50.0000 40.0000 77.0000

SOUT1

HOA = . 17612815+002, HRA = .28536576+002

SEND

SOUTPUT

CCLV = F, DAMPOA = 1 SEND

55.0000

B-8
APPENDIX C. DERIVATION OF THE DIMENSIONLESS FACTOR, X

For dual-duct and multi-zone systems, the basic enthalpy economizer algorithm

described in section 6 is modified to take into account the differences in

the costs of heating energy and cooling energy. This is discussed in

sections 7 and 8. A dimensionless factor, X, is introduced as the ratio of

the total energy cost (cooling and heating) of the air-handling system with

the outside air damper at the minimum opening position to that with the damper

at the maximum opening position. The derivation of X is presented here (see

figures 5 and 14).

Energy equations for the system shown in figure 5 are as follows.

mOAh

OA + m

RA

h

RA

” "WW

'W'MA + qF

* m

SA

hSA

“HA

h

SA + qH

” “HAh

HA

m

CA

h

SA + qC = m

CA

h

CA

where m, h, and q represent mass flow rate, enthalpy, and heat flow rate,

respectively

Subscripts are:

OA: outdoor air

RA: return air

MA: mixed air

CA: the air leaving the cooling coil

( 1 )

( 2 )

(3)

(4)

C-l
HA: the air leaving the heating coil

SA: supply air

C: cooling coil

H: heating coil

F: supply fan

Assume that is negligible (q^^O) , and that the mass of added or removed

moisture is negligible with respect to air mass. Applying the mass conservation principle, heat flow rates for heating and cooling can be expressed, from

equations (1) through (4), as:

When the outside air damper is positioned to admit the minimum amount of

outdoor air,

(5)

q C m

CA

h

CA ^CA^SA) (mOA

h

OA + mRA

h

RA) ( 6 )

(7)

n> . = in OA OA,mm ( 8 )

where m^ 4 . is the mass OA , min

minimum opening position.

is the mass flow rate of the outside air with its damper at the

When the outside air damper is positioned at its maximum opening

m

RA

0 (9)

mOA

m

SA ( 10 )

C-2
Substituting equations (7) and (8) into (5) and (6), we obtain:

qH,min

= " <' m

HA

/mSA) £

m

OA,minh

OA + (mSA

" m

OA,min )hRAj (11)

q C,min m

CA^CA ^ mCA^

m

SA^ |TMOA,min^OA + ^ m

SA

mOA,min^

Similarly, with the damper at maximum position, the heat flow rate equations

become

( 12 )

qH,max

^

^HA " h0A^

qC,max

= m

CA

(hCA

" *W

We can normalize equations (11) through (14) by m^. From equations

and (12), we have

q H,min X

HA

h

IiA XHA

q C,min

= X

CA

h

CA

“ X

CA

xOA,minh

OA + (1 =

xOA,min^OA + ^ XOA,min^RA

(13)

(14)

(ID

(15)

(16)

where x = the ratio of heating coil air mass flow rate to supply HA

air mass flow rate

x^ = the ratio of cooling coil air volumetric flow rate to

supply air mass flow rate

x^ A . = the ratio of minimum outdoor air mass flow rate to OA,min

supply air mass flow rate

Assume that density variation due to temperature differences is negligible,

then xTTA , xOA , and xrtA . can be volumetric flow rate ratios instead of

HA CA OA,mxn mass flow ratios.

Equations (13) and (14) yield:

q

' H,max

= X

HA

(hHA

" h0A)

q ’c,max

= XCA

(h CA

" h0A)

(17)

(18)

C-3
Defining 3 as a unit energy cost ratio in terras of dollars of heating to

cooling, the dimensionless factor, X, can be expressed as:

X

( Bql

H,rain ) + ( q C,rain )

^ Bq H,max^ + ^ q C,max^

C19)

In equation (19), q' . and q' should be zero or positive, and q' , rr y mm ti y nisx u y min

and q' should be zero or negative. As a result, X is alwavs Dositive and

u ,raax can be used as a criteria to decide the outside air damper position. When X

is greater than 1, it is more economical to have the maximum amount of

outside air entering the system than the minimum amount of outside air.

Therefore, the outside air damper should be opened fully. Otherwise, it should

be at its minimum opening position.

C-4
APPENDIX D. COMPUTER PROGRAM LISTING OF THE ENTHALPY ECONOMIZER

ALGORITHM WITH ENHANCEMENT (ETC2) , AND SAMPLE INPUT

AND OUTPUT

Q$Q$Q$«

12

3

45

6

78

9 10

1

1

12

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ETC < 1 > . ETC2ALL (0

)

C **********************************************************************

c

C ETC2 t Enhanced enthalpy economizer algorithm

C for building air handling systems

C The enhancement Is applicable to a dual-duct system.

C

C VERSION II July 29, 1983 C.P.

C

C

CC CCLV Logical variable for controlling a cooling

C coll valve. CCLV*T when the valve Is open and CCLV«F

C when It Is closed.

C

C COST Logical variable for economical cost analysis In

C determining the damper opening. Set TRUE If

C the cost analysis routine Is desired. FALSE otherwise.

C Only dual-duct systems are applicable In cost analysis.

CC DAMPOA Integer variable for controlling dampers.

C DAMPOA * 0 for admitting minimum outdoor air

C “1 for modulating dampers

C “2 for admitting maximum outdoor air

C

C DELTA Temperature difference of floating zone to minimize

C the switching frequency of damper or coll valve opening

C

C DEWPT Logical variable. Set TRUE If outside and return air

C dew points are measured. Set FALSE If outside and return

C air relative humidities are measured.

C

C DPOA Dew point temperature of outside air (F) (C)

C

C DPRA Dew point temperature of return air (F) (C)

CC HOA Enthalpy of outside air (Btu/Lb) (KJ/kg)

C

C HRA Enthalpy of return air (Btu/Lb) (KO/kg)

C

C METRIC Logical variable. Set TRUE If metric unit Is used.

C Set FALSE If English unit Is employed.

CC PB Barometric pressure (In.Hg) (Pa)

C

C RHOA Relative humidity of outdoor air (percent)

C

C RHRA Relative humidity of return air (percent)

C

C SPRAY Logical variable Indicating type of cooling coll,

C SPRAY = T If system uses a sprayed coll or air washer,

C SPRAY * F otherwise.

CC TCA Measured cooling coll discharge air dry-bulb temperature

CC TCASP Set point value of cooling coll discharge air dry-bulb

C temperature

C

D-l
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oooo

ooooo

oonoooono

TMIN Specified outside air dry bulb temperature below which

minimum outside air Is admitted

TOA Measured outdoor air dry-bulb temperature

TRA Retrun air dry-bulb temperature

SUBROUTINE ETC2

INTEGER DAMPOA

LOGICAL CCLV, SPRAY, DEWPT, METRIC, COST

COMMON /BK 1 / COST, TCA, TCASP , TMIN, SPRAY, DELTA

& /BK2/ DEWPT, METRIC

& /BK3/ CCLV, DAMPOA

& /BK4/ TOA , DPOA , RHOA , TRA , DPRA , RHRA , PB

NAMELIST /OUT 1 / HOA.HRA

Enthalpy economizer

Compute enthalpies of outdoor and return air

CALL PSYCHR( TOA, DPOA, RHOA, PB.HOA)

CALL PSYCHR < TRA , DPRA , RHRA , PB , HRA

)

Determine outdoor air damper opening and cooling coll valve

operat 1 on IF(TOA.LE.TMIN) THEN

CCLV- . FALSE

DAMPOA=0

ELSEIF(HOA.GE.HRA+DELTA) THEN

CCL V= . TR UE

DAMPOA=0

ELSEIF1H0A.LE.HRA -DELTA) THEN

I F ( SPRAY ) THEN

I F ( TCA . GE . TCASP DELTA ) THEN

CCLV-. TRUE.

DAMPOA-2

ELSEIF(TCA.LE. TCASP -DELTA ) THEN

CCLV-. FALSE.

DAMPOA

END I

ELSE

IF (TOA.GE.TRA+DELTA) THEN

CCLV-. TRUE.

DAMP OA-0

ELSEIF(TOA.LE.TRA-DELTA) THEN

I F ( TOA . GE . TCASP DELTA ) THEN

CCLV- .TRUE

DAMPOA-2

ELSEIF (TOA.LE. TCASP -DELTA ) THEN

CCLV-. FALSE.

DAMPOA

END I F END I

END I F END I

D-2
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non

non

oooooooooooooooonooooooo

o

oooooo

PRINT 0UT1

At this point, the outdoor air damper opening can be changed

from maximum to minimum, and vice versa depending on the result

of the cost analysis. This cost analysis Is only recommended

to a dual-duct system.

IF(COST.AND.CCLV.AND. (DAMPOA.EQ.0 .OR. DAMPOA . EQ . 2 > ) THEN

CALL HCCOST

PRINT OUT 1 ENDIF

RETURN

END

PSYCHR i Psychrometr 1c properties of moist air between

60 F (15.6 C) and 140 F (60 C) with estimated error

of 0.02 X Sources i ASHRAE 1981 Fundamentals Handbook, Chapter 5 ASHRAE Journal Jan. 1979 (R. S. Brokaw ) PP. 49-51

DB Dry-bulb air temperature

Engl 1 sh

F

Metr 1 c

C DP Dew point temperature F C

H Entha 1 py Btu/Lb KJ/kg

PB Barometric pressure 1 n . Hg Pa

PSW Saturated water avpor pressure 1 n . Hg Pa

PW Water vapor pressure 1 n . Hg Pa

RH Relative humidity percent percent

W Humidity ratio -

SUBROUTINE PSYCHR ( DB , DP ,RH , PB , H

)

LOGICAL DEWPT, METRIC

COMMON /BK2/ DEWPT, METRIC

Dew point temp or relative humidity

I F ( DEWPT ) THEN

TEMP-DP

RH- 100.0

ELSE

TEMP-DB

ENDIF

Compute psychrometr 1 c properties using metric units

IF (METRIC ) THEN

PSW=3.37685E+3*EXP( 15. 4636-7284. /( 1 . 8*TEMP+424 . )

)

PW-RH/ 100 . *PSW

W=0.62198MPW/(PB-PW> ) H = DB+W* (2501 . + 1 . 805*DB >

D-3
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o

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n

oo

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n

o

o

n

r>

o

o

o

o

o

o

o

on

o

oo

n

o

r>o

oo

o

o

o

no

ooo

I F ( . NOT . DEWPT ) THEN

PWLOG-ALOG(PV)

DP --35. 957-1 .8726*PWL0G+1 . 1 689*PVL0G*PV10G

END I

Compute psychrometr 1 c properties using English units

ELSE

PSW-EXP ( 15. 4636-7284. /(TEMP +392. )

PW-RH/100. *PSW

W-0 . 62 1 98* ( PW/ ( P B-PW )

H-0.24 + DB+WM 1061 .+0.444*DB)

I F ( .NOT. DEWPT) THEN

PWLOG-ALOG < PW>

DP-79.047+30. 5790*PWLOG+ 1 . 8893*PWL0G*PWL0G

END I

ENDIF

RETURN

END

HCCOST i Outside air damper opening based upon the economic

decision on heating and cooling costs for dual duct

systems

July 29, 1983 C.P.

DPCA Measured dew po 1 nt of cool 1 ng coll discharge air

DPHA Measured dew po 1 nt of heating coll discharge air

HCA Entha 1 py of cool 1 ng coll discharge a 1 r HGTOCL Unit energy cost ratio In terms of dollars of

heating to cooling

={$ of heating per Btu or KJ ) / ( $ of cooling per Btu or

KO ) HHA Enthalpy of heating coll discharge air

LAMMDA Real variable Indicating the ratio of the cost with

minimum damper opening to that with maximum damper

open 1 ng

RHCA Relative humidity In percent of cooling coll discharge air

RHHA Relative humidity In percent of heating coll discharge air

TCA Dry-bulb temp, of cooling coll discharge air

THA Dry-bulb temp, of heating coll discharge air

XCA Ratio of cooling coll discharge air flow rate to supply

air flow rate

D-4
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onnn

non

oooooaooooo

XHA Ratio of heating coll discharge air flow rate to supply

air flow rate

XOAMIN Ratio of outside air flow rate thru the minimum damper

opening for fresh air requlrment to supply air flow rate — Either metric or English unit can be used.

SUBROUTINE HCCOST

LOGICAL CCLV, SPRAY, COST

INTEGER DAMPOA

REAL LAMMDA

COMMON /BK1 / COST, TCA, TCASP ,TMIN, SPRAY, DELTA

& /BK3/ CCLV, DAMPOA

& /BK4/ TOA , DPOA , RHOA , TRA , DPRA , RHRA , PB

& /BK5/ HGTOCL , DPCA , RHCA , THA , DP HA , RHHA , XOAMI N , XCA , XHA

Compute enthalpies

CALL PSYCHR ( TOA, DPOA, RHOA, PB.HOA)

CALL PSYCHR ( TRA , DPRA , RHRA , PB , HRA

)

CALL PSYCHR (TCA, DPCA, RHCA,PB,HCA>

CALL PSYCHR (THA, DPHA, RHHA, PB.HHA)

Compute the ratio of the cost with minimum damper

opening to the cost with maximum damper opening

QHMI N=XHA*HHA-XHA*< XOAMI N*HOA+< 1 -XOAMI N)*HRA)

QCMI N=XCA*HCA-XCA*( XOAMI N*HOA+ ( 1 -XOAMI N)*HRA)

QHMAX-XHA* ( HHA-HOA

)

QCMAX*XCA* ( HCA-HOA

C

I F ( QHMI N . LT .0) QHMIN=0.0

I F ( QHMAX . LT . 0 ) QHMAX=0.0

I F ( QCMI N . GT . 0 ) QCMIN=0.0

I F ( QCMAX . GT .0 > QCMAX=0.0

C

CMI N=HGTOCL*QHMI N-QCMI

CMAX = HGTOCL * QHMAX -QCMAX

I F ( CMAX . GT .0 .AND. CMIN.GT. 0) THEN

LAMMDA=CMI N/CMAX

END I

C

IF ( LAMMDA.GT. 1.0) THEN

DAMP0A=2

ELSE

DAMPOA=0

END I

PRINT 1000, LAMMDA

C

1000 F0RMAT(/5X, ’COST EVALUATION HAS BEEN MADE ’/

& 5X , ’LAMMDA = ’ ,F10.4/)

C

RETURN

END

D-5
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ETC2MAIN i Main program to call the enhanced version of the

enthalpy economizer algorithm for building air

hand 1 1 ng systems — The enhancement Is applicable to a dual-duct systems. ' VERSION II July 29, 1983 C.P.

301 C TITLE Title of the Input data set

302 C Y ( I ) Input data array

303 C Y ( 1 ) - PB 304 C Y( 2 ) - TOA

305 C Y( 3 > - DPOA If DEWPT-T, RHOA If DEWPT306 C Y ( 4 ) • TRA

307 C Y ( 5 ) - DPRA If DEWPT-T, RHRA If DEWPT308 c Y ( 6 > - TCA

309 c Y ( 7 ) - DPCA If DEWPT-T, RHCA If DEWPT310 c Y( 8 ) - THA

311 c Y ( 9 ) - DPHA If DEWPT-T, RHHA If DEWPT312 c Y ( 10) - XCA

313 c Y( 11 ) XHA

314 c 315 c

*

LOGICAL CCLV, SPRAY, DEWPT, METRIC, COST

CHARACTER TITLE<2>*80

INTEGER DAMPOA

DIMENSION Y< 11

)

COMMON /BK1 / COST, TCA, TCASP ,TMIN, SPRAY, DELTA

DEWPT, METRIC

CCLV, DAMPOA

TOA , DPOA , RHOA , TRA , DPRA , RHRA , P

HGTOCL , DPCA , RHCA , THA , DPHA , RHHA , XOAMI N , XCA , XHA

NAMELIST /INPUT/ METR IC , DEWPT , SPRAY , TCASP , TMI N , DELTA , COST

& /OUTPUT/ CCLV, DAMPOA

& /HCIN/ HGTOCL .XOAMIN

OPEN(7,FILE=’ ETC 2 DATA ’

)

REWIND 7

/BK2/

/BK3/

/BK4/

/BK5/

Read Initial Input data

READ ( 7 , 1000 ) TITLE

READ17, INPUT)

PRINT 1000, TITLE

PRINT INPUT

I F ( COST ) THEN

READ ( 7 , HC I N

)

PRINT HCIN

N= 1

ELSE I F ( SPRAY ) THEN

N = 6 ELSE

N = 5 END I

D-6
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CC READ MEASURED INPUT DATA

C

10 READ17,*, END-999) ! Y < I > , I » 1 , N > PRINT 2000, (Y( I ) , I “ 1 , N

)

PB =Y ( 1

)

TOA=Y ( 2 TRA=Y<4>

TCA=Y( 6

)

THA=Y ( 8

XCA=Y ( 10)

XHA=Y! 1 1

CC Dew point temperature or relative humidity

C

IF(DEWPT) THEN

RHOA= 100 .

RHRA-100.0

RHCA-100.0

RHHA-100.0

DPOA=Y( 3

)

DPRA=*Y ( 5

)

DPCA-Y17)

DPHA“Y( 9

)

ELSE

DPOA-0.0

DPRA-0.0

DPCA-0.0

DPHA=0.0

RH0A»Y!3>

RHRA=Y ( 5

)

RHCA“Y ( 7 RHHA=Y { 9 END I

C

c

C Call the enthalpy economizer algorithm

C

CALL ETC2

C

PRINT OUTPUT

PRINT 3000

C

C Damper and coll valve control routines should be

C called here. These routines are machine-dependent.

C

GOTO 10

C

C FORMAT STATEMENTS

C

1000 FORMAT! A80/A80)

2000 FORMAT! 8F 10. 4)

3000 FORMAT! IX, 79! ’-’)

C

999 STOP

END

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D-10
NBS-114A ' -"Q

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (iee instructions)

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

NBSIR 84-2832 MARCH 1984

4 TITLE AND SUBTITLE

Economizer Algorithms for Energy Management and Control Systems

5. AUTHOR(S)

Cheol Park, George E. Kelly, and James Y. Kao

6. PERFORMING ORGANIZATION (If joint or other than N BS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Office of Buildings & Community Systems U.S. Navy Civil Engineering Laboratory

U.S. Department of Energy U.S. Department of Defense

1000 Independence Avenue, SW Washington, DC 20310

Washington, DC 20585

10. SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here) Economizer cycles have been recognized as important energy conservation measures

for building air handling systems and have been included in most Energy Management

and Control Systems (EMCS). This report describes the psychrometric processes of

the most commonly used economizer cycles and presents algorithms for implementing

these cycles on a typical Energy Management and Control System.

Economizer cycles included in this study are dry-bulb and enthalpy types, as applied to both dry coils and sprayed coils. In addition, an enhancement to

the normal enthalpy economizer cycle algorithm is presented for dual-duct or multi-zone system which takes into account differences in the costs of heating

energy and cooling energy. Computer program listings of the algorithms and sample

input/output data are shown in the appendices. A brief discussion of common types

of air handling systems is also given to help the reader better understand the

application of the algorithms presented in this report.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) control strategies; cooling energy; dry-bulb economizer cycle; energy management and

control system; enthalpy economizer cycle; heating energy.

13. AVAILABILITY

|~Y~1 Uni imited

| | For Official Distribution. Do Not Release to NTIS

[—1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington,

20402.

D.C.

14. NO. OF

PRINTED PAGES

81

15. Price

[X] Order From National Technical Information Service (NTIS), Springfield. VA. 22161 $11.50

USCOMM-DC 6043-P80