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NBSIR 88-3739

Comparison of Direct Digitai

Controi and Pneumatic Controi

Systems in A Large Office Buiiding

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Steven T. Bushby

George E. Kelly

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Environment Division

Gaithersburg, MD 20899

March 1988

Sponsored by:

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

•Research Iiih ‘Nalioiial bur^

iormalion Center

NBSIR 88-3739

- COMPARISON OF DIRECT DIGITAL

CONTROL AND PNEUMATIC CONTROL

SYSTEMS IN A LARGE OFFICE BUILDING

Steven T. Bushby

George E. Kelly

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Environment Division

Gaithersburg, MD 20899

March 1988

Sponsored by;

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

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ABSTRACT

A distributed, microprocessor-based direct digital control (DDC) energy

management and control system was developed and installed in an eleven story

office building in Gaithersburg, Maryland. Over a period of one year the

performance of this system under various control strategies was investigated

along with the performance of a conventional pneumatic control system. The

quality of control and performance characteristics for the two control systems

were compared.

Overall the DDC system was found to maintain better control of the supply air

temperature and to follow planned reset schedules more closely than the

pneumatic system. One particular air handling unit performed as well under

pneumatic control as it did under DDC in maintaining a supply air setpoint,

but it did not precisely follow the planned reset schedule. Each air handling

unit had different performance characteristics while under pneumatic control,

but all units behaved essentially the same under DDC.

The results indicate that a pneumatic system can perform as well as a DDC

system but more effort is required to maintain system performance. The DDC

system was found to be more flexible and it is easier to insure that multiple

air handling units follow the same control strategy with the DDC system. The

instriomentation associated with the DDC system proved to be very useful in

troubleshooting problems with the pneumatic system because of the information

readily available to the operators.

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CONTENTS

Page

ABSTRACT ii

ACKNOWLEDGMENTS iv

LIST OF FIGURES v

1. INTRODUCTION 1

2. EMCS DESCRIPTION 2 2.1 Multiplexer Hardware 4

2.2 Multiplexer Firmware 5 2.2.1 Reporting by Exception 6 2.2.2 Custom Safety Interlocks 7 2.3 FID Hardware 7 2.4 FID Software 8 2.5 The Central Control Unit 9 2.6 The Pneumatic Control System 9

3. INSTRUMENTATION 10

4. SUPPLY AIR TEMPERATURE CONTROL DURING COOLING SEASON 14

4.1 Direct Digital Control 14

4.2 Pneumatic Control 16

5. SUPPLY AIR TEMPERATURE CONTROL DURING HEATING SEASON 18

5.1 Direct Digital Control 18

5.2 Pneumatic control 20

6. ENERGY PERFORMANCE TRENDS FOR STRATEGIES TESTED 28

6.1 Performance of economizer cycles 28

6.1.1 Verification of DDC dry bulb economizer 28

6.1.2 Verification of DDC enthalpy economizer 30

6.1.3 Comparison of DDC and pneumatic economizer control 32

6.2 Chilled water valve control 38

6.3 Steam preheat valve control 41

7. SUMMARY AND CONCLUSIONS 44

8. REFERENCES 47

iii
ACKNOWLEDGEMENTS

The authors of this report would like to acknowledge the valuable assistance

and cooperation of a number of people whose contributions helped make this

report possible. The work was funded by the National Bureau of Standards and

the U.S. Department of Energy. C. Warren Hurley provided assistance with

instrumentation and troubleshooting electronic problems in the early phases of

the project. Dr. Cheol Park provided assistance in modifying the control

system software to expand the flexibility of the DDC system. George Garvis and

his staff in the NBS plant air conditioning shop patiently allowed testing of

the DDC system on air handling units for which they were responsible. They

also provided assistance with installing instrumentation and connecting the DDC

system to the air handling units.

iv
LIST OF FIGURES

Figure

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Figure

Figure

1 . 1 The NBS Administration Building

1.2 HVAC service zones in NBS Administration Building

2.1 Schematic diagram of NBS laboratory EMCS processors,.

3 . 1 Schematic diagram of AHU instrumentation

3.2 Heat exchanger box for measuring supply air RH

3 . 3 Solenoid valve digital to pneumatic interface

4.1 Zone air reset schedule performance for all air handling

units under DDC

4.2 Zone air reset performance for AHU- 3

‘ under pneumatic control..

4.3 Zone air reset performance for air handling units under

pneumatic control

5 . 1 DDC outdoor air reset schedule

5.2 Outdoor air reset performance for all four air handling

units under DDC

5.3 Outdoor air reset performance under pneumatic control

5.4 Damper and preheat valve swinging on AHU- 5 under pneumatic

control

I 5.5 Influence of damper and preheat swinging on mixed and supply

air temperatures , pneumatic control

5.6 Influence of transition from pneumatic control to DDC on swinging problem

I 5.7 Supply air and mixed air temperature response to transition

from pneumatic to DDC

5.8 Outdoor air reset performance of AHU- 6 under pneumatic

control after repairs

6.1 Histogram of damper positions for AHU-3 under DDC with dry

bulb economizer

6.2 Histogram of damper positions for AHU-3 under DDC with dry

bulb economizer

2

2

3

10

11

13

15

16

17

19

19

21

22

24

25

26

27

29

31

V
Figure 6.3 Histogram of damper positions for AHU-3 under DDC with

enthalpy economizer 33

Figure 6.4 Histogram of damper positions for AHU-3 under DDC with

enthalpy economizer 34

Figure 6.5 Outdoor air damper position as a function of outdoor

temperature for each air handling unit under DDC 35

Figure 6.6 Outdoor air damper position as a function of outdoor

temperature for each AHU under pneumatic control 37

Figure 6.7 Chilled water valve position as a function of outdoor

temperature for each AHU under DDC 39

Figure 6.8 Chilled water valve position as a function of outdoor

temperature for each AHU under pneumatic control 40

Figure 6.9 Preheat valve position as a function of outdoor temperature

for each AHU under DDC 42

Figure 6.10 Preheat valve position as a function of outdoor temperature

for each AHU under pneumatic control 43

vi
1. INTRODUCTION

A nonproprietary microprocessor based energy management and control system

(EMCS) was built and installed in a large office building to investigate the

performance of previously developed direct digital control (DDC) algorithms in

a typical office building. The microprocessor-based EMCS was constructed

entirely from off the shelf, commercially available components and all communication and control software was written in house. Data were collected on the

performance of the EMCS over one year using a variety of control strategies.

The results of these tests were compared with each other and also with data

collected while the building equipment was being controlled by a conventional

pneumatic control system.

This paper presents performance characteristics and compares the quality of

control for the DDC and pneumatic systems. Some of the data collected were

also used to validate a computer model of this building which was used to

investigate the effects of different control schemes on energy consumption. A

description of the model and the simulation results are presented elsewhere [1].

The building used for this project, shown in figure 1.1, is an eleven story

office building which serves as the Administration Building for the National

Bureau of Standards in Gaithersburg, Maryland. The building is a reinforced

concrete structure, 49.7m (163 ft) high (excluding the' bulkhead on the roof),

built in the late 1960's. The width of the building is 15m (49 ft) and it is

67m (220 ft) long. The structure is enclosed by gray face bricks, insulated

porcelain steel panels, and glass windows. Extended aluminum enclosures,

located on both the north and south walls, cover concrete columns and the

supply air ducts which originate from the air handling units located on the

mezzanine floor of the building.

Each floor area is 867 m^ (9,330 ft^)and the ceiling height is 3.1m (10.2 ft).

Floors two through eleven are served by four air handling units. The four

service zones are shown in figure 1.2. Two return air ducts serve floors two

through six and two ducts serve floors seven through eleven. Thus only two

return air ducts serve each floor. This creates a complicated air distribution

pattern because each air handling unit supplies air to one zone on all floors,

but the return air to any particular air handling unit is a mixture of air from

all four zones in either the top half or bottom half of the building.

The four air handling units which serve these zones are all located on the

mezzanine level of the building. Each unit provides approximately 32,400 m3/hr

(19,500 cfm) of primary air to the zone. Two fluid to air heat exchange coils

are located in each unit, one providing preheat with steam, and the other

providing cooling to the air using chilled water. Air entering the units is a mixture of return and outdoor air controlled by dampers with pneumatic actuators.

The flow of steam and chilled water through the coils is controlled by pneumatically

actuated valves

.

The offices in the building contain induction terminal reheat units with a hot

water heating coil under the control of a local thermostat. Conditioned air

from the air handling unit is supplied through the terminal reheat unit.

1
Figure 1.1. The NBS Administration Building

North wal

Figure 1.2. HVAC service zones in NBS Administration Building

2
Return air is brought to the air handling unit through ducts located in the

ceiling of the hails in the building. Each office has grills over its door to

allow air to flow from the room to the return air duct when the door is closed.

2. EMCS DESCRIPTION

The experimental EMCS used for this project was developed by NBS staff. The

EMCS utilizes distributed computer processors arranged in a hierarchy as shown

in figure 2.1. The highest level is the central control unit (CCU) , the middle

level is the field interface device (FID) , and the lowest level is the multiplexer

(MUX). There is no interaction between the nodes of the system at each level,

that is one MUX cannot communicate with other MUXes and one FID cannot

commimicate with another FID.

The multiplexers shown in figure 2.1 are hardware oriented microcomputer signal

conditioning units performing low level tasks, with software fixed in EPROMs

(firmware) . A MUX interrogates all of the sensors and controls all of the

actuators tied to the system. Analog sensor inputs are converted into digitally

coded numbers for transmission to higher levels of the EMCS.

The FIDs shown in figure 2.1 are also microcomputers. They represent more of a balance between hardware and software than the MUXes . FIDs contain a mixture

of firmware and software and are intended to be flexible and easily reconfigured

to a new application. They contain software to communicate with the multiplexers

and make control decisions based on information from sensors connected to the

multiplexers. The FID software contains implementations of EMCS algorithms and

provides DDC of the valves and dampers on an air handling unit to maintain the

supply air temperature at a setpoint. FID processors also contain software for

communication with the CCU level. A FID is capable of controlling an air

handling unit without interaction with the higher level CCU.

CCU

FID

Multiplexer

F D

Multiplexer

F D

Multiplexer

F D

Multiplexer

• Figure 2.1. Schematic diagram of NBS laboratory EMCS processors

3
The central control unit, shown in figure 2.1, provides supervisory control

functions and is connected by serial communication lines to each of the FIDs

.

The ecu is used to collect and store historical data from EMCS sensors, detect

and display alarm conditions, and allow EMCS algorithm parameters used by field

processors to be changed. The CCU provides the operator interface with the EMCS.

In the event of a power failure, the FID and MUX are designed to restart

automatically. The FID loads a new copy of the software into RAM from the

onboard bubble memory and commences execution. Manual restart of the CCU is

required.

The experimental EMCS was installed in a manner which permits the continued

operation of the pneumatic control system which previously controlled the air

handling units in this building. Control of the air handling units can easily

be switched back and forth between the two systems. This provides a complete

and independent backup to the experimental EMCS

.

2,1 Multiplexer Hardware

Each multiplexer in the building EMCS has the following capabilities:

32 Analog input channels

4 Analog output channels

8 pulse accumulator or counter channels

8 relay- isolated binary input channels

16 optically isolated binary input channels

16 optically isolated binary output channels

The MUX hardware consists of six specialized circuit boards linked together by

a parallel communication bus and provided with appropriate power supply and

connectors for the serial communication to higher levels of the distributed

system. The bus used in the MUX is the IEEE-696 (S-100) bus designed for

microprocessor based systems. This bus has a 16 bit, bidirectional data bus

and a 24 bit, unidirectional address bus. The bus is based on a 100- line

parallel bus with 100-pin edge connectors along the bus for the insertion of

circuit cards. The bus-edge connector is often referred to as the "motherboard".

The IEEE-696 motherboard assembly is placed within a card cage which supports

the cards. This card cage and power supply are mounted in a NEIMA cabinet

together with signal conditioning hardware for the analog inputs and cooling

fans. The six specialized circuit boards placed in the motherboard are:

1. Single card computer (SCC) board.

2. Analog- to-digital board (two).

3. 4 Parallel input/output board.

4. 8 Parallel input/output board.

5. Digital- to-analog board.

The SCC board contains the master microprocessor for the MUX and is the

IEEE-696 bus master. This card uses the Z-80A microprocessor which runs at a speed of 4 MHz. Also on the board are 1 K byte of 4045 static RAM, sockets for

8 K bytes of 2716 EPROMs, three 8-bit parallel I/O ports, and an asynchronous

4
RS-232 serial port for communication with the field processor. Four EPROMS are installed to provide firmware for the MUX. These will be discussed in the MUX

firmware section.

Two analog-to-digital converter boards are installed to take readings from 32

analog sensors. The sensor outputs first pass through signal conditioning

cards which convert the various sensor outputs into 0-5 volt DC signals. The

sensors used and the signal conditioning circuits are discussed in the

instrumentation section.

The 4 parallel input/output board provides the capability for the MUX to

actuate digital outputs and determine digital input states. This board has 24

optically isolated binary inputs, 16 optically isolated binary outputs, and 8 magnet relay isolated binary outputs rated at 1 amp @ 32 volts DC.

The 8 port input/output board is used in the MUX primarily to allow a simple

switch register input to the MUX and a set of output LEDs. The input switches

are used to permit simple reconfiguration of the MUX without firmware changes

and the output LEDs are used to display MUX operational status. This board

actually has 8 parallel bidirectional I/O ports, but these are not used in the

present MUX configuration except to read 11 bits of switch register input and

set 8 bits of LED output.

The digital- to-analog board provides a means for the MUX to send analog voltage

signals to analog type controllers. This board has four independent digitalto- analog converters , each of which can be driven with a resolution of 12

bits. The board makes use of eight I/O ports, two for each digital-to-analog

converter. The range of each analog output can be selected independently of

the other outputs

2,2 Multiplexer Firmware

All multiplexer firmware is contained in four EPROMs. The four EPROMs and

their functions are:

MONITOR - used for start-up, diagnostic operations, and downloading

new software (not used in normal operations)

COMM - used for communication with the field processor, handles

all protocol and error checking.

ANALOG - sets analog outputs and reads analog inputs.

DIGITAL - sets digital outputs and reads digital inputs.

The activities of the MUX can be divided into two broad categories,

communication with the FID and scanning of the sensors attached to the

MUX, Most of the time the MUX will be scanning sensors, repeating the scan

of all points over and over without regard to any time schedule.

Execution of the point scanning software begins at the start of the ANALOG

module. The first instruction in the ANALOG module is a subroutine call to the

5
DIGITAL module, and, in fact, the entire DIGITAL module is written as a single

large subroutine to the ANALOG module. The DIGITAL module sets digital

outputs according to the values stored in the internal data base, reads digital

inputs and stores the values in the internal data base, and then checks the

digital inputs associated with pulse counters, incrementing counters in the data

base if the counter inputs have changed. When the execution of the DIGITAL

module is complete, a return to the ANALOG module occurs. The ANALOG module begins with setting analog outputs in accordance with the

analog output data base. Next, the analog input channels are read in sequence

and stored in the data base. Finally, a jump is made back to the start of the

ANALOG module

If the FID sends a communication to the MUX, an interrupt causes an immediate

jump to the COMM module. This module accepts the FID message (or rejects it if

there is an error) and determines what the FID is commanding the MUX to do . A

return is then made to the point scanning routine, which is allowed to complete

one full scan. Following this scan, a jump is made back to the appropriate

place in the COMM module, depending on what function the FID has commanded.

The command function will either be a transmission of requested data to the FID

or a reception of data from the FID to set analog or digital outputs. After

the command function is complete, a return is made to continuous scanning.

2.2.1. Reporting by exception

One of the most important functions of the MUX is to transmit to the FID data

for only those points which have changed by a certain amount since the last

transmission of data to the FID. This is termed reporting by exception. This

function is accomplished most efficiently by having the MUX check for data

change since last transmission as it scans the data points. If a point has

changed, the program will write the encoded point into a special message

buffer which will be ready for transmission when the FID requests it (thus

speeding up response to the command)

The three critical elements of the exception reporting are the filter increment

or differential, the structure of the exception message buffer, and the baseline for comparison with the current analog value to see if a change has

occurred. The filter increment is the amount that a data point must change

from the baseline before it is registered as a change. A filter increment of

zero means that any change is registered. However, if there is noise in the

readings, excessive exception reports will occur even when the actual physical

quantity measured may not be changing.

The exception buffer must always be current in the intervals between data

transmission. Since the data values will change at different times and rates,

the buffer must have a random access character. In the NBS MUX, the following

approach is used: every time the exception buffer is transmitted to the FID,

the buffer is subsequently filled with ASCII nulls. The buffer is sliced up

into sections, one for each data point. A data point is always associated with

the same buffer section. If a data point changes value, the point update

6
routine will also write the value of the point into the proper buffer section.

If the point changes again before buffer transmission, the buffer section is

overwritten with the new value. If a point never changes between transmission

times, the buffer section will contain nulls. When the buffer is transmitted

to the FID, if a null character is encountered, it is not transmitted. Thus

the message contains only exceptions , and the values are always current and

transmitted in the same order.

The most simple baseline that can be used to compare the current data value

with previous readings is the value in the RAM data storage area before it is

updated with newly acquired information. If this baseline is used, the system

will work but some changes will not be reported. If a change occurs at a rate

such that the change between scans is smaller than the filter increment used

for comparison, then since the scan rate is high frequency (0.1 kHz) the change

will never be reported by exception.

A second baseline possibility is to set aside RAM for a copy of all the analog

values. When an exception transmission is made, the copy is made. Thus the

baseline is the set of analog values at the time the last exception report

transmission was made. Still this is not perfect. If a slow change occurs,

with a change in an analog value less than the differential between exception

reports, the change will not be reported. The solution to this problem is to

use as a baseline the analog value that was most recently transmitted in the

exception buffer for that particular point. This is implemented by examining

the exception buffer after transmission and if a particular slot of the

buffer is not nulls, then the data value associated with that slot is transferred

to the storage for data values at last transmission.

2.2.2. Custom Safety Interlocks

In order to provide special reliability to a research system such as the NBS

controls laboratory monitoring and control system, it is necessary to be able

to build in custom code to provide immediate actions if a measurement indicates

an undesirable situation has been reached. A typical action would be to

deactivate digital outputs which switch control from the digital control system

to a backup system. Thus if a temperature climbed out of a desirable range,

the digital control system would go off line, allowing a backup, which in this

case is the original, pneumatic control system to take control. The custom

code is executed at the end of the DIGITAL module.

2 . 3 FID Hardware

The FID hardware consists of seven specialized circuit boards linked together

by an IEEE-696 (S-100) parallel bus, described in the multiplexer hardware

section. The seven circuit boards used are:

1. CPU card

2. Real-time clock

3. EPROM memory board

4. RAM memory board

5. Bubble memory board

6. Bubble memory controller

7. Serial/Parallel interface card

7
The Central Processing Unit (CPU) card contains the master processor and is the

IEEE-696 bus master. The card used in the NBS FID is a Z-80A microprocessor

running at a clock speed of 4MHz and capable of performing dynamic memory refresh.

The real time clock card allows the FID to know the absolute time and thus keep

in synchronization with the rest of the system and the real world. A clock

also allows the FID to time software tasks that must be performed at regular

intervals and permit the use of time of day control actions such as night

setback without interaction from the CCU. The clock used in this system has a battery backup to allow the clock to keep time during short power outages

The EPROM memory board contains firmware to hold monitor/diagnostic routines

and software to "boot" the system from a cold start. Bubble memory utility

routines are included to load a new copy of software into RAM when power is

restored after a failure or after a reset.

The RAM board provides 64K bytes of dynamic memory for FID software and scratch

space for program execution. The board used has a memory access time of 240

nanoseconds and on board memory refresh when the CPU does not refresh the memory.

A bubble memory card was included in the NBS field processor hardware to

provide a means to recover from power failures. A copy of the processor

software is loaded into the nonvolatile bubble memory and stored for later use. When power is restored after a failure, the booting procedure loads a new copy

of software from the bubble memory into RAM. The bubble memory also allows

reloading of clean software if the software in RAM fails, gets hung, or picks up

an error. The bubble memory is theoretically insensitive to problems which

might inflict a floppy disc drive used for the same purpose.

The bubble memory controller card reads and writes to the bubble memory and

checks for errors in loading and saving data. It contains a separate Z-80

microprocessor chip which manages all bubble memory operations.

The serial/parallel interface card provides two serial communications ports to

allow the FID to communicate with the other levels in the EMCS. Asynchronous

RS-232 communication is used at both levels.

2.4 FID Software

The FID contains software for communication with both the MUX level and the CCU

level of the EMCS. It also contains the software implementations of the EMCS

control algorithms. The control algorithms used in this project were public

domain algorithms developed at NBS. A brief description of the algorithms used

is presented below.

ECONOMIZER algorithms are used to reduce the need for mechanical cooling in a building when the outside conditions permit cooling by bringing in outside air.

Two general types are used, dry bulb economizers, which control air handler

damper positions based on the difference between return air and outdoor air dry

bulb temperatures, and enthalpy economizers, which control dampers based on the

enthalpy difference between return air and outside air. Both types were

8
implemented in this project. The details of the algorithms used and their

verification have been discussed in previous reports [2,3].

TIME OF DAY control is used to schedule tasks to be executed at particular

times on particular days. Examples of these tasks include morning start up,

nighttime shutdown, weekend scheduling, and holiday scheduling. The details of

the time of day algorithm used in this project may be found in reference [4]

.

RESET SCHEDULES are used to vary the supply air setpoint temperature as a function of some measure of the load. In this project supply air setpoint

temperature was reset based on outdoor air temperature during the heating

season and zone air temperature during the cooling season. Details of the

algorithm used have been previously reported [3,5].

2.5 Central Control Unit

A minicomputer was used for the CCU in this project. Each FID was connected to

the CCU by a dedicated line connected to a logical port on the minicomputer.

The minicomputer disk drives were used to log data and alarms from the field

units

.

The CCU sioftware contains communication routines to continuously monitor and

log incoming alarms, collect data from field units, and provide the operator

interface. The operator can check the status and sensor outputs from any FID,

change parameters for control algorithms, manually start and stop tasks, change

time of day control parameters, and download new software to the FID.

The CCU software actually consists of up to thirteen tasks running simultaneously

on the CCU computer. Four FID I/O tasks are used to handle communication with

the FIDs , one task for each FID. All of the other CCU tasks interact with

these FID I/O tasks to send information to or obtain information from a FID.

Four CCU tasks handle data acquisition, again one for each FID. These data

acquisition tasks operate independently permitting customized data logging for

each FID. If data are not being collected from a particular FID then its

associated data collection task is not running. An alarm task periodically

polls each FID I/O task requesting an alarm report. Appropriate action is taken

to log any alarms which are found to a disk file and to send the information to

the alarm console task which handles output to a dedicated alarm console.

There is one task which serves as a watchdog to insure that conununications are maintained between the CCU tasks and takes appropriate action if a problem

develops. One task is used to provide the operator interface to the distributed

control system and lastly, one task serves as the master CCU task. The role of

the master task is to activate and deactivate the other tasks and serve as a controller of CCU intertask communication.

2.6 Pneumatic Control System

The pneumatic control system in this building was upgraded several years ago to

interface with an energy management system which can perform some supervisory

functions. It is primarily used to control nighttime shutdown and morning

start up times. The controller on each air handling unit is also capable of

implementing an economizer scheme, outdoor air reset, and zone air reset.

9
3. INSTRUMENTATION

Each of the four air handling units in the NBS Administration Building were

instriamented in a virtually identical manner. A schematic diagram of the

instrumentation used is shown in figure 3.1. Precision thermistors were used

for ail temperature measurements. An array of six thermistors was used to

determine an average mixed air temperature. Single thermistors were used to

measure supply air, return air, outdoor air, chilled water inlet, and chilled

water outlet temperatures.

Digital indicator (closed) T - temperature P - differential pressure

+ Digital indbator (open) RH = relative humidity All valves and dampers

DP « dew point position sensors

Figure 3.1. Schematic of AHU instrumentation

10
In order to implement the enthalpy economizer it was necessary to measure the

humidity (or dew point) of the air streams. Outdoor air dew point was measured

using chilled mirror devices. Supply air, return air, and mixed air humidities

were measured with thin film polymer capacitance type sensors. Special arrangements, described below, had to be made in the case of supply air RH measurement

because near saturated conditions are common during the cooling season in this

humid climate. The thin film polymer sensors are not satisfactory when exposed

for long periods to relative humidities above about 90%.

A heat exchanger box was built to raise the temperature of the supply air, thus

lowering the relative humidity before measurement. This was done by using a standard electrical box with holes drilled into it to accommodate a thermistor,

RH sensor, and air inlet and outlet. This box was placed several feet away

from the supply air duct and connected to the duct by a small copper tube. The

high pressure in the duct relative to the surround- ing room forces air through

the copper tube into the box, and eventually out the bleed hole into the room. The residence time in the feed line and box was long enough to raise the

temperature of the air to a point where reliable humidity measurements could be

made. This RH was then corrected for the temperature change assuming constant

moisture content heating. Details of the construction of these heat exchanger

boxes are shown in figure 3.2.

Figure 3.2. Heat exchanger box for measuring supply air RH.

11
This instrumentation for measuring the relative humidity of the various air

streams has some important limitations. Even with the special precautions

taken with the supply air RH measurement, accurate measurement of humidities

under high humidity conditions did not prove to be reliable. Even though the

heat exchanger box fulfilled its function of keeping the measured humidity

below 90%, energy balances around the cooling coil indicate that the humidity

measurements were sometimes inaccurate in predicting the latent cooling load.

This may be due to a combination of drift in the sensor calibration and uneven

mixing of air streams in the mixed air chamber giving false indications of the

true mixed air humidity. This implies that under some conditions it was not

possible to determine the amount of cooling provided by calculating enthalpy

changes on the air side. A small error in the measured value of RH results in

a large error in the estimate for the latent cooling load . Chilled water flow rate was measured with an orifice plate and a differential

pressure sensor. The orifice plates were part of the original HVAC system

design and it was only necessary to connect pressure sensors to the corner

taps. It was feared that the orifice plates may be eroded or corroded because

of their age. The orifice plate from one of the chilled water lines was removed

for inspection and found to be in very good condition.

Using the existing orifice plates for measuring chilled water flow also has

some important limitations. Orifice plates generally have a small turndown

ratio and are sensitive to disturbances in the flow stream. The piping in the

Administration Building does not provide adequate lengths of straight pipe to

insure smooth flow. The result of this is that the chilled water flow measurements

tend to become unreliable at low flow rates and also at high flow rates, making

a water side measurement of cooling load impossible when the chilled water

valve is open a small amount or a very large amount.

Zone terminal reheat is used in this building. It was not possible to instrument

all of the hot water risers feeding water to the various zones. Instead, the

hot water main supply line to the whole building was instrumented. An acoustic

"transit time" flow meter was used to measure flow rate and precision thermistors

mounted to the surface of the pipe and covered by insulation were used to

measure the total temperature change. With this information the total reheat

load to the building can be calculated.

Measuring air flow rates for each air handling unit was a difficult task. In

almost ail cases there was not sufficient straight duct work to establish

smooth flow conditions. Because of pressure drop constraints it was not

possible to place additional flow straighteners in the ducts. A series of

measurements were made in a grid pattern for each duct to determine the average

flow rate and to find a location to place a pitot tube to obtain estimates of

flow rate. Pitot tubes were mounted in these locations and connected to

differential pressure transducers.

All dampers and valves associated with the air handling units were instrumented

with slide resistors calibrated for a range of 0 - 100% open. A micro switch

was attached to the outdoor air dampers, chilled water valves, and steam

preheat valves to provide a digital signal to confirm that the device is

completely open (dampers) or completely closed (valves) . The digital signal is

12
necessary for sequencing control and eliminates the problem of determining how

close to 0% open is closed and how close to 100% open is really open. Although

generally reliable, there were times when a micro switch failed preventing an air handling unit from sequencing to the next control device when needed.

Each MUX contains eight signal conditioning cards with four channels each.

This permits building a custom signal conditioning circuit for each of the 32

analog input signals. In this way it was possible to filter and convert the

various voltage and current output signals from the sensors as needed to

produce a standard 0 - 5V full scale signal to the analog to digital converter.

The pneumatic control system controls device sequencing by changing pressure

from 3-15 psig. Each control device responds to a different part of that

pressure range. In order to control each device separately using DDC an interface between the digital control system and the pneumatic actuators was needed. Previous attempts to use a simple solenoid valve arrangement where

digital signals were used to open and close either a solenoid valve connected

to mains air pressure or a solenoid valve open to the atmosphere to increase or decrease control line pressure respectively resulted in difficulties due to air

leaks in the pneumatic lines [6]

Two different types of DDC/pneumatic interfaces were used in this experiment.

The first was an inexpensive modification of the solenoid valve arrangement

described above. In addition to the two solenoid valves, a commercially

available reference capacitor and 1:1 booster relay have been added. The

solenoid valves are used to control the pressure in the capacitor. The

capacitor is connected to one side of the diaphragm inside a 1:1 booster relay.

As the pressure on the control line side of the relay varies from the capacitor

reference the diaphragm moves, regulating a bleed and thus equalizing the

pressure on the two sides. A schematic diagram of this interface is shown in

figure 3.3.

pneumatic actuator

relay

Figure 3.3. Solenoid value digital-to-pneumatic interface

13
The second type of DDC pneumatic interface used is a motor-driven regulator.

The regulator is a special type that allows downstream pressure to be decreased

by exhausting air to the atmosphere. The regulator adjustment shaft is fitted

with two synchronous motors wired for opposite direction of rotation. Each

motor is connected to a separate digital output from the controller and turning

on one output allows 24 VAC to drive one of the motors in the desired direction.

Thus one output is used to increase pressure, and one output is used to

decrease pressure. This interface is much more expensive than the first type.

No difference in performance of the two interfaces was detected.

Switching control from the pneumatic controller to the digital controller was accomplished by the use of electronic to pressure (E/P) relays. The relays

were set up so that when energized the mains air pressure would be controlled

by the digital controller, bypassing the pneumatic controller. This arrangement

causes control to automatically switch to the pneumatic controller in the event

of a power failure. It also allows plant personnel to override the digital

controller in case of an emergency by disconnecting the power supply to the relays.

4. SUPPLY AIR TEMPERATURE CONTROL DURING COOLING SEASON

The plant personnel responsible for maintaining climate conditions in the

building follow two strategies for controlling building temperatures, one referred to as "winter control" and the other referred to as "summer control".

The criterion for deciding which conditions apply is the outdoor air temperature.

If the outdoor air temperature is 18.3"C (65°F) or warmer, then the "summer

control" strategy is u?ed. When outdoor air temperatures are lower the "winter

control" strategy is used.

For the Washington D.C. climate this corresponds roughly to a heating season

from November to March and a cooling season from April to October. The DDC

control system was implemented to approximate the summer and winter strategies

used by the pneumatic control system.

Under "summer control" conditions the supply air temperatures (Tsa) for each

air handling unit is determined by zone air temperature (Tza) according to the

following schedule:

For Tza < 26.6“C (78“F), maintain Tsa at 15.6“C (60"F)

.

For Tza > 26.6“C (78"F)

,

lower Tsa from 15.6“C (60“F) to 7.2°C (45°F)

linearly as Tza ranges from 26.6“C (78“F) to 26.7®C (80“F)

.

4.1 Direct Digital Control

The ability of the DDC system to follow the zone air reset schedule is shown in

figure 4.1. The data shown were selected to represent the full range of zone air temperatures which occurred while controlling the building in this mode.

Data from each FID are represented as well as both dry bulb and enthalpy

economizer cycles. The solid line indicates the expected reset schedule.

14
The figure shows that under DDC, supply air temperature was maintained within

approximately + 0.5°C (+ 1°F) of the reset schedule. This is consistent with

the tuning procedure used to maintain a setpoint temperature within + 0.5°C

(± 1°F), There was no difference in ability to maintain the reset schedule

detected between the four FIDs or the two economizer cycles. There was also no drift in performance between data collected in the late summer and fall

compared with data collected the following spring and summer.

Figure 4.1. Zone air reset schedule performance for

all air handling units under DDC.

15
4.2 Pneumatic Control

Data collected while the pneumatic system was in control show variation in

performance between air handling units and also variation in time for the same air handling unit (AHU) . Figure 4.2 shows the zone air reset history data from

AHU-3 for three different time periods. For each of these periods AHU- 3 maintained supply air temperature within a band of about + 0.4®C (— 0.7°F), which

is a tighter control than the DDC system, but the reset schedule being followed

drifted around the intended schedule. This is probably due to inadvertent

changes made to the control system during maintenance or drift. Evidence of

this drifting reset schedule was found in the other air handling units as well.

Figure 4.2. Zone air reset performance for AHU-3

under pneumatic control

Figure 4.3 shows a comparison of the performance of the four air handling units

under pneumatic control. In each case the data are selected from a group of

consecutive or nearly consecutive days which span a range of zone air temperatures.

It can be seen that each AHU is following a different reset schedule and there

is considerable variation in the spread of supply air temperatures for a given

zone air temperature. AHU-3 consistently maintained tighter control than the

other units with a spread of about + 0.4“C (+ 0.7°F). The worst spread was for

AHU- 6, which is about + 2“C (+ 3.6"F). The air handling units vary in design,

but the control devices are all of the same type. These data suggest that the

pneumatic control system is capable of maintaining control as well as the DDC

system but in practice it is often not the case because of difficulty in keeping

the controllers tuned and the desired reset schedule precisely adjusted.

16
a. AHU-3 b. AHU-4

c. AHU-5 d. AHU-6

Figure 4.3. Zone air reset performance for air handling

units under pneumatic control.

17
5. SUPPLY AIR TEMPERATURE CONTROL DURING HEATING SEASON

During the heating season supply air setpoint temperature is determined by the

outdoor air temperature. The reset schedule requested by plant management is a linear relationship between the supply air temperature and the outdoor air

temperature defined by the points:

Tsa = 18.3°C (65“F) when Toa = 18.3"C (65°F)

Tsa = 23.9“C (75“F) when Toa < 1.7“C (35“F)

As it turned out when the data was analyzed, the supply air temperature continued

to increase linearly as the outdoor air temperature dropped below 1.7°C (35°F).

The pneumatic control system switches automatically between "summer control"

and "winter control" when the outdoor air temperature is 18,3°C (65°F),

Because of the design of the two reset schedules, a step change in supply air

setpoint from 18.3“C (SS^F) to 15.6“C (60°F) takes place as control switches

from winter mode to summer mode

The DDC system requires the operator to change reset strategies from "winter

control" to "summer control". To avoid problems on days where the outdoor air

temperature was expected to be in the neighborhood of 18.3° C (65° F) and to

eliminate the step change in supply air temperatures, the DDC outdoor air reset

schedule was set up differently than the pneumatic schedule. The DDC reset

schedule followed the following format:

Tsa = 23.9°C (75°F) when Toa < 1.7°C (35°F)

Tsa decreases linearly from 23.9°C (75°F) to 19.7°C (67.5°F)

as Toa increases from 1.7°C (35°F) to 18.3°C (65°F).

Tsa decreases linearly from 19.7°C (67.5°F) to 15.6°C (60°F)

as Toa increases from 18.3°C (65°F) to 21.1°C (70°F)

Tsa = 15.6°C (60°F) when Toa ^ 21.1°C (70°F)

This reset schedule is shown graphically in figure 5.1.

5.1 Direct digital control

The ability of the DDC system to follow the outdoor air reset schedule is shown

in figure 5.2. The data shown were selected to represent the full range of

outdoor air temperatures which occurred while controlling the building in this

mode. Data from each FID are represented as well as both dry bulb and enthalpy

economizer cycles. The solid line represents the expected reset schedule.

The figure shows that supply air temperature followed the reset schedule

closely and maintained supply air temperatures within a spread of approximately + 0.8°C (1.4°F) over most of the outdoor air temperature range. There

is a bulge in the data between 18 °C and 20 °C where the supply air temperature

rose above the setpoint. Examination of the data from those days showed that

the outdoor air dampers were wide open but not enough cooling was being

provided by the outdoor air. The chilled water valve should have opened to

provide further cooling but it did not.

18
SUPPLY

AIR

TEMPERATURE

(C)

Figure 5.1. DDC outdoor air reset schedule.

Figure 5.2. Outdoor air reset performance for all

four air handling units under DDC

19
Problems occasionally surfaced with the microswitches used to provide a digital

signal to notify the controller that a control device had reached the end of

its travel. This was particularly true for microswitches connected to the

outdoor air damper actuator because of dirt that was carried in by the air.

The chilled water valve probably failed to open because dirt prevented the

microswitch contact from closing and the controller never sequenced to the

chilled water valve. An open circuit indicates to the controller that dampers

are not completely open and the controller will continue to try to open the

dampers. Digital signals were not recorded along with the other data so it is

not possible to confirm this hypothesis.

The spread in the data is slightly wider for the outdoor air reset data than it

was the case for the zone air reset data. This is probably due to the fact

that over much of this range supply air temperature is controlled by damper

modulation. It is much harder to tune the dampers than the chilled water valve

to provide steady control because of the strong influence of the outdoor air

temperature on the sensitivity to damper position. There is no apparent

difference in quality of control between FIDs or between the dry bulb economizer

and the enthalpy economizer.

5.2 Pneumatic Control

Data illustrating the performance of the pneumatic controllers during "winter

mode" are shown in figure 5.3. As before, each air handling unit has a distinct performance pattern which does not match the expected reset schedule

depicted as a solid line in the figures.

AHU-3 showed the best performance (figure 5.3a), maintaining a tight temperature

band of about +1.0“C (1.8“F) and following the reset schedule closely for

outdoor air temperatures ranging from 2.0“C to 13.0*C (35.6“F to 55.4°F). This

is comparable to the performance of the DDC control system. At temperatures

below 2.0“C (35,6®F) the reset schedule continues its linear trend instead of

maintaining a constant setpoint as expected. This is probably a better control

strategy but it does not match the description given by plant personnel for

their planned reset schedule.

AHU-5 (figure 5.3.c) shows a pattern similar to AHU-3 except that there is much

more spread in the data. The spread in the data is due to sequencing trouble

with this air handling unit. Comparison with the steam preheat valve position

data in figure 6.10.C shows that the outdoor air dampers tend to fight against

the steam preheat valve in the temperature range - 5®C to 10“C (23-50®F). In

fact the outdoor air dampers and preheat valve had a tendency to swing open and

closed in alternating fashion in a very short time period.

An example of this swinging problem is shown in figure 5.4. This figure shows

the position of the steam preheat valve and outdoor air dampers for a time

period of about 20 minutes. The solid line indicates the position of the outdoor

air dampers and the dashed line represents the position of the steam preheat

valve. Both the steam preheat valve and the outdoor air dampers swing open and

closed in a cyclic fashion with a period of just less than 2 minutes. The two

cycles are about 180 degrees out of phase.

20
a. AHU-3 b. AHU-4

o -

Ce] kn,

p

i : Ed

Ois

12 o. ^ Oi «

<

a.p >o. CO

OUTDOOR AIR RESET HISTORY

^ r > 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1

1

1

-15 -10 -5

m i I I'l n 1 1 I I

1 1 I 1 1 I I 1

I 1 1 I I M n I I 1

1 I I 1 1 I 1

I I 0 5 10 15 20

OUTDOOR AIR TEMPERATURE (C)

cc 01.

Ed

PUS ' i2o: ^ 0*. « '

a.

OiP o.

CO

OUTDOOR AIR RESET HISTORY

^ I

I I I I I I I I I II I M I I I I I

I

I I I I II I I I

I

I I I I M I I I

I < I M I II II '< II I II I I

I I I I I I I I J

I -15 -10 -5 0 5 10 15 OUTDOOR AIR TEMPERATURE (C) 20

c. AHU-5 d. AHU-6

Figure 5.3. Outdoor air reset performance

under pneumatic control

21
o

Figure 5o4. Damper and preheat value swinging on AHU-5 under pneumatic control.

22
I:

!-

; The effect of this swinging action on the mixed air temperature and the supply

i air temperature is shown in figure 5.5. It can be observed that both of these

air temperatures swing up and down in conjunction with the steam valve and

dampers. The maximum peak to peak mixed air temperature swing about 15°C(27°F).

The supply air temperature swings are smaller, approaching 2.5°C (4.5°F).

The ability of the DDC system to control this air handling unit under similar

conditions is illustrated in figures 5.6 and 5.7. The first 30 minutes of data

in these figures shows the unit under pneumatic control. At that point the DDC

system comes on line and takes control of the air handling unit. Figure 5.6

shows that the DDC system immediately locks the outdoor air dampers closed and

damps out swings in the steam preheat valve over a period of about 20 minutes

.

The corresponding stabilization of the mixed air temperature and the supply air

temperature is shown in figure 5.7.

The ability of the DDC system to control this unit indicates that the pneumatic

system's problem was with the controller and not with the control devices or the air lines leading to their actuators. This is because the DDC system uses

the same pneumatic lines and actuators. Several attempts were made by the

plant maintenance staff to remedy this problem but they were never fully successful.

AHU-4 (figure 5.3.b ) showed a different type of problem. The data in this

figure fall into distinct regions. The time span represented is quite long but

changes due to drifting of minor adjustments made during routine maintenance

cannot explain these results. In many cases two distinct regions of operation

appear for the same day. The controller would be maintaining a particular

supply air temperature value for several hours and then suddenly a step change

in supply air temperature would take place, but without a corresponding change

in outdoor air temperature

.

AHU-6 (figure 5.3.d) also had considerable scatter in the supply air temperature

reset history data. There was a catastrophic failure in one of the pneumatic

components of this unit in March, causing the controller to loose control of

the steam preheat valve. The faulty part was isolated and replaced. After

this service the performance of the unit dramatically improved. Figure 5.8

shows the performance of AHU-6 for a two day period immediately after the

repairs were made. Although it still does not strictly follow the intended

reset schedule, the control is much tighter and the performance is comparable

to AHU-3.

23
TEMPERATURE

(C

LO

Figure 5.5. Influence of damper and preheat swinging on mixed

and supply air temperatures, pneumatic control

24
OPEN

I

I

0.5 1.0 1.5 2.0 2.5

TIME (hrs)

Figure 5.6. Transition from pneumatic control to

DDC stops swinging problem

25
TEMPERAUTRE

(C)

Figure 5.7. Supply air and mixed air temperature response

to transition from pneumatic to DDC

26
SUPPLY

AIR

TEMPERATURE

(C)

Figure 5.8. Outdoor air reset performance of AHU-6

under pneumatic control after repairs.

27
6. PERFORMANCE TRENDS FOR STRATEGIES TESTED

While it would be desirable to compare energy consumption for the different

data sets, it is a very difficult task to take performance data from selected

days and compare energy consumption with data associated with different

strategies different days, weather conditions, and internal loads. This

problem was compounded by measurement limitations in air flow rates, chilled

water flow rates, and relative humidities under some conditions, as discussed

previously. In an attempt to overcome these difficulties it was decided to use experimental data to validate a computer model and run simulations to investigate

energy performance. These results may be found in reference [1]. The

experimental data were also used to examine performance characteristics,

contrasting the DDC strategies with the performance of the pneumatic control

system. These characteristics were determined by examining the relative

positions of control devices as a function of building load. Outdoor air

temperature was selected as a measure of the building load. From this information

differences in the response of the controllers to similar loads can be seen.

6.1 Performance of economizer cycles

The performance of economizer cycles were examined in two ways, an analysis to

verify the correct performance of the DDC dry bulb and enthalpy economizer

algorithms and a comparison of the DDC control performance with the pneumatic

system control performance.

6.1.1 Verification of DDC dry bulb economizer

The dry bulb economizer algorithm selects one of four possible actions based on outdoor air temperature and two fixed parameters, changeover temperature and

minimum temperature. The changeover temperature is essentially a maximum

outdoor air temperature for which the dampers will be opened for cooling the

building. The minimum temperature is the temperature below which the dampers

will be forced closed to prevent freezing of the cooling coil. The four

possible actions taken by the economizer algorithm are:

1. Lock outdoor air dampers closed and use mechanical cooling.

2. Lock outdoor air dampers closed and do not use mechanical cooling.

3. Lock outdoor air dampers open and use mechanical cooling.

4. Allow outdoor air dampers to be modulated and do not use mechanical

cooling.

Further details of the dry bulb economizer algorithm can be found in

reference [2]

.

Performance data from AHU-3 under DDC dry bulb economizer control were selected

for a wide range of outdoor air temperatures to validate the performance of

this algorithm. The positions of the outdoor air dampers were assumed to be in one of four possible states. These states were completely open, completely

closed, between half open and fully open, and between half open and fully

closed. Figure 6.1 contains four histograms to indicate the number of times

the outdoor air dampers were in each of the four states. The number of

occurrences of each damper state is plotted as a function of the outdoor air

temperature

.

28
NUMBER

OF

OCCURRENCES

100

150

200

250

300

OUTDOOR AIR TEMPERATURE (C)

Figure 6.1. Histogram of damper positions for AHU-3

under DDC with dry bulb economizer

29
Theoretically, if the algorithm were operating properly, the outside air damper

should always be closed above the changeover temperature (20°C or 68°F during

these tests) and open just below the changeover temperature. Figure 6,1

indicates that this was in fact the case for the days shown. It should be

noted however, that cases have been seen where there was some small overlap

between these two regions with the dampers being open above the changeover

temperature and being closed below the changeover temperature. This was due to

the fact that there is a dead band in the control algorithm between sequencing

changes to prevent rapid fluctuations between control modes when conditions are near the boundary between two modes. Figure 6.1 also shows that the outdoor

air dampers were not open at outdoor air temperatures below the minimum value

of 5.5“C (42“F)

.

These results indicate that the dry bulb economizer algorithm

functioned correctly during these tests.

These same histograms from AHU-3 under DDC dry bulb economizer control are plo tted against the enthalpy difference between return and outdoor air in

figure 6.2. This figure shows that the dry bulb economizer never opened the

outdoor air dampers under conditions where the outdoor air enthalpy was greater

than the return air enthalpy. It also shows that the dampers were not always

open when the outdoor air enthalpy was lower than the return air enthalpy.

Although the dry bulb economizer did reduce mechanical cooling load by using

outdoor air for cooling, the data indicate that some further improvement in

energy savings may be achieved by using an enthalpy economizer which could take

advantage of outdoor air cooling under conditions where the dry bulb economizer

did not.

6.1.2 Verification of DDC enthalpy economizer

The enthalpy economizer algorithm used four measured values from the EMCS in

order to determine the enthalpies of the outdoor and return air. The measured

values were the return air dry bulb temperature, the return air relative

humidity, the outdoor air dry bulb temperature, and the outdoor air dewpoint.

The algorithm must also be able to access the supply air setpoint temperature

and one parameter, the minimum temperature, must be set. The algorithm has

four possible output decision:

1. Lock outdoor air dampers closed and use mechanical cooling.

2. Lock outdoor air dampers closed and do not use mechanical cooling.

3. Lock outdoor air dampers open and use mechanical cooling.

4. Allow outdoor air dampers to be modulated and do not use mechanical

cooling.

If the outdoor air enthalpy is greater than the return air enthalpy by at least

a small differential, set at 1.2 kJ/kg dry air (0.5 BTU/lbm dry air), then

decision 1 above is made. If the outdoor temperature is less than the minimum

temperature than decision 2 is made. If the outdoor air enthalpy is less than

the return air enthalpy by at least a small differential, greater than both the

minimum temperature and the supply air setpoint, then decision 3 is made. If

the outdoor air enthalpy is less than the return air enthalpy by at least a small differential, the outdoor air temperature is both greater than the

minimum temperature and less than the supply air setpoint, then decision 4 is

made. A more detailed description of the enthalpy economizer can be found in

reference [2]

.

30
NUMBER

OF

OCCURRENCES

C'

Figure 6.2. Histogram of damper position for AHU-3

under DDC with dry bulb economizer.

31
Performance data from AHU-3 under DDC enthalpy economizer control were selected

for a wide range of outdoor air temperatures to verify the correct performance

of this algorithm. The positions of the outdoor air dampers were assumed to be

in one of four possible states. These states were cpmpletely open, completely

closed, between half open and fully open, and between half open and fully

closed. Figure 6.3 contains four histograms to indicate the number of times

the outdoor air dampers were in each of the four states. The number of

occurrences of each damper state is plotted as a function of the enthalpy

difference between return air and outside air.

Theoretically, if the algorithm were operating properly, the outdoor air

dampers would never be open when the outdoor air enthalpy is greater than the

return air enthalpy. Also, the outdoor air dampers would never be closed when

the outdoor air enthalpy is less than the return air enthalpy and the outside

air temperature is above the minimum. In figure 6.3 it may be observed that

the outdoor air dampers were in fact open at times when the outdoor air enthalpy

was greater than the return air enthalpy. All of these occurrences however,

fall within the 1.2 kJ/kg dry air differential allowed by the algorithm

indicating that the algorithm performed correctly.

To verify the performance of the dry bulb economizer, damper position histograms

were plotted as a function of outside air temperature. For completeness, a similar plot of the enthalpy economizer damper position histograms is shown in

figure 6.4. Notice that in the enthalpy economizer histograms there is no single temperature above which the dampers are always closed. Instead there is a spread of temperatures from about 20°C to 24°C (68-75“F) for which the

dampers may be either open or closed. This provides a further indication that

the enthalpy economizer can reduce energy consumption when compared to the dry

bulb economizer by providing outdoor air cooling when the dry bulb economizer

does not.

Enthaply economizers do not always outperform dry bulb economizers from the

standpoint of energy consumption. Several factors are important in determining

this issue. They include climate, selection of the parameters for the dry

bulb economizer, and sensor accuracy. See for example references [7- 9] . The

experimental data collected in this study does not conclusively show how much

energy savings may be possible from the use of enthalpy economizers in the

Washington D. C. area. Because of the high humidity found in this area it

would be expected that savings large enough to be significant could result.

6.1.3 Comparison of DDC and pneumatic economizer control

The outdoor air damper position for each air handling unit, while under DDC,

is plotted as a function of outdoor air temperature in figure 6.5. The points

in these plots represent hourly average values for temperature and position.

Both dry bulb economizer and enthalpy economizer data are shown. The solid

lines represent a best fit to the data. For AHU-5 and AHU-6 the best fit

curves are linear and for AHU-3 and AHU-4 the best fit curves are a power fit.

32
NUMBER

OF

OCCURRENCES

Figure 6.3. Histogram of damper positions for AHU-3

under DDC with enthalphy economizer

33
NUMBER

OF

OCCURRENCES

100

150

200

250

300

350

400

OUTDOOR AIR TEMPERATURE (C)

Figure 6.4„ Histogram of damper positions for AHU-3

under DDC with enthalphy economizer.

34
iTTT

m

r1

1

M

rm1

1(

>i

11

ma

m1

11

1

11

1n|

n

r

11

n

rr

a. AHU-3 b. AHU-4

c. AHU-5 d. AHU-6

Figure 6.5. Outdoor air damper position as a function of outdoor

temperature for each air handling unit under DDC

35
Each air handling unit shows somewhat different characteristics with regard to

outdoor air damper position. This result is expected because of variations in

the design of the units and differences in the load in the four zones being

served. The temperature range over which outdoor air is used for cooling is

similar for all four units, about 8 - 20°C (46.4 - 68°F).

The outdoor air damper position histories for pneumatic control are shown in

figure 6.6. The points in these plots are hourly average values for temperature

and position. The data for AHU-3 under pneumatic control closely matches the

data for AHU-3 under DDC , except for a few points indicating that the dampers

were opening at outdoor air temperatures below 0°C (32“F). The solid ‘line in

this figure is the fit to DDC data from figure 6. 4. a. It illustrates the good

agreement between data collected while under DDC with the data collected while

under pneumatic control. This unit comes the closest to behaving the same way

under pneumatic and DDC control.

The outdoor damper position history for AHU-4 shows random scatter for outdoor

air temperatures between 2°C and 18°C (35.6°F and 64.4“F). Comparison with the

position history of the steam preheat valve in section 7 will show that this is

due to sequencing trouble with this unit.

The outdoor air damper position plot for AHU-5 shows a linear relationship

between damper position and outdoor air temperature. The slope and intercept

of the best fit line are both different from the results of the DDC data.

Under pneumatic control, the dampers open sooner and more slowly than was the

case with DDC control. For outdoor air temperatures below 12“C (54°F), outdoor

air dampers are more open under pneumatic control than when under DDC. The

best fit lines suggest this trend will continue to outdoor air temperatures up

to 20“C (68“F), but there is no data to confirm this trend under pneumatic

control. The temperature at which the dampers begin to open under pneumatic

control is dangerously low because of the possibility of freezing the cooling coil.

The plot showing the outdoor air damper position history for AHU-6 under

pneumatic control (figure 6.6.d) shows two distinct curves. One curve represents

data collected before the pneumatic failure discussed in the section 5.2, and

the other curve represents data collected after repairs were made. Both curves

indicate outdoor air dampers opening at lower temperatures than the same unit

under DDC control, but the curve representing data taken after repairs is much

closer. For damper positions of 20% open or greater the two curves have

essentially the same slope, with the DDC curve shifted to the right about 2“C

(3,6“F). This indicates that for AHU-6 the dampers open sooner under pneumatic

control than under DDC and also for a given outdoor air temperature in the

range of damper modulation the dampers are open wider under pneumatic control.

A first look at these data would suggest that the pneumatic control system took

better advantage of outdoor air for cooling because for a given outside air

temperature, the dampers were open wider under pneumatic control than they were

under DDC. It will be shown in section 6.3 that this is not the case because

the dampers are opening at low enough temperatures to require steam preheat to

maintain supply air temperatures. Note also that AHU-5 and AHU-6 tend to

maintain supply air temperatures lower than the reset schedule in the outdoor

air temperature range for damper modulation (see figures 5.3.c and 5.3.d).

36
milimimilniii

i

tjiliiiim

ii

a. AHU-3 • b. AHU-4

OUTDOOR AIR DAMPER POSITION

0 10 OUTDOOR AIR TTliPSRATURS (C) SO 30

c. AHU-5 d. AHU-6

Figure 6.6. Outdoor air damper position as a function of outdoor

temperature for each AHU under pneumatic control.

37
Under DDC the lowest temperature at which the dampers were allowed to be open

was 5.6"C (42°F). This value was chosen to prevent any possibility of freezing

the cooling coil. This particular temperature is supposed to be the temperature

at which the freeze protection logic of the pneumatic control system is to

become active, opening the steam preheat valve.

Under pneijimatic control, all of the units had outdoor air dampers open with

outdoor air temperatures below 5.6"C (42®F) at least once. This was particularly

a problem with AHU-4 and AHU-5. There were no cases where this problem

occurred while the units were being controlled by the DDC system.

6.2 Chilled water valve control

Chilled water valve position as a function of outside air temperature under DDC

is shown in figure 6.7. The results for AHU-3, AHU-4, and AHU-5 all agree

closely in the temperature range 20 “C to 32 °C (68 to 90®F) . Data are scarce at

temperatures above 32“C (90“F) for all units except AHU-5. The trend of the

data for AHU-5 is an interesting shape. As outside air temperature climbs from

20“C to 32®C (68 to 90"F) the chilled water valve opens quickly at first, then

slows down and the curve flattens out. The valve modulates around 40% open

until at temperatures above 32®C (90“F) it opens rapidly to about 80%.

This trend makes sense when the reset schedule is considered. The controller

is modulating the valve to maintain a setpoint of 15.6"C (60°F) until the

outdoor air temperature reaches about 32 ®C (90“F) . As the outdoor air temperature

climbs higher the zone air temperature starts to climb above 25.6°C (78°F) and

the reset schedule sharply decreases the supply air temperature. This shows up

as the steep climb in chilled water valve position.

The data from AHU-6 indicate that the chilled water valve tended to be open

less for a given outside air temperature than the other units. This is a surprising result because AHU-6 is a smaller unit and it was expected that more

chilled water would be required to maintain supply air setpoint.

The data for chilled water valve position while under pneumatic control are shown in figure 6.8. The results for AHU-3 are comparable with the DDC data

for the same unit. This is consistent with the fact that the zone air reset

history as shown in figures 4.1 and 4.3a also agree (the data in figure 6. 8.

a

correspond with the early June data of figure 4.2).

The data in figures 6.8.b and 6.8.C show the chilled water valve opening more

quickly than was the case under DDC. This also agrees with the zone air reset

data shown in figure 4.3. Both of these units began to sharply reduce supply

air temperature before the zone air temperature reached the 25.6“C (78° F) mark.

The data for AHU-6 show more scatter than was present under DDC, but the amount

of valve opening is in the same general area. This also agrees with the zone air reset history data in figure 4.3. The zone air reset data for AHU-6 had

considerable spread but approximately followed the expected reset schedule.

38
%

OPEN

%

OPEN

60

80

100

0

20

40

60

00

100 CHILLED WATER VALVE POSITION ^ CHILLED WATER VALVE POSITION

LEGEND

0 DRY BULB ECONOMIZER

A ENTHALPY ECONOMIZER

A

I

i?i i

| i i i i i i i i ii i i i i i i i i rn 0 10 20 30 40

OUTDOOR AIR TEMPERATURE (C)

LEGEND

0 DRY BULB ECONOMIZER

A ENTHALPY ECONOMIZER

o:

'<S*.

o;

C\2.

i

i 'wnB89»- 10 20

I I I

I I I 30

OUTDOOR AIR TEMPEPATXJRE (C)

a. AHU-3 b. AHU-4

CHILLED WATER VALVE POSITION CHILLED WATER VALVE POSITION

o

- LEGEND

= oDRY BULB ECONOMIZER a

: A ENTHALPY ECONOMIZER

o:

GO. LEGEND

oDRY BULB ECONOMIZER

A ENTHALPY ECONOMIZER

&

A

02

o:

N. &

8

i nnoi I ttt

10 20 30

OUTDOOR AIR TEMPERATURE (C) n

40

oi

(M aaaA4V\ aoaso^

00<30^r n i

i n-i' I I I I I I

I I I I I I I I I

I 0 10 20 30 40

OUTDOOR AIR TEI.IPERATURE (C)

c. AHU-5 d. AHU-6

Figure 6.7. Chilled water valve position as a function of

outdoor temperature for each AHU under DDC

39
CHILLED WATER VALVE POSITION ^ CHILLED WATER VALVE POSITION

o:

03 .

I [ I I I I I

I t 0 10 20 30

OUTDOOR AER TE1.IFERATURE (C)

a. AHU-3 b. AHU-4

^ CHILLED WATER VALVE POSITION O ^ CHILLED WATER VALVE POSITION

LEGEND

0 DRY BULB ECONOMIZER

A ENTHALPY ECONOMIZER

o:

o::

c«.

0-Pi*WHI1MO<?GF>i fTfWl> t I TT'I I I I I I' I l~l T~|- 10 20 30

OUTDOOR AIR TEMPERATURE (C)

c. AHU-5 d. AHU-6

Figure 6.8. Chilled water valve position as a function of outdoor

temperature for each AHU under pneumatic control

40

n-i40

n

40
6.3 Steam preheat valve control

The position of the preheat valve under DDC control as a function of outdoor

air temperature is shown for each air handling unit in figure 6.9. The results

for AHU-3 and AHU-4 are very close. The slope for AHU-6 is steeper. Some

sequencing problems developed with AHU-5 resulting in the scattered data shown

in figure 6.9.c.

It is significant to note that for three of the four air handling units under

DDC, there is no overlap in the outdoor air temperature regions where the

dampers and preheat valve are being used to control supply air temperature.

There is no conflict between trying to cool with outdoor air and heat with

steam at the same time. The one exception to this occurred with AHU-5.

Comparison of figures 6.5.c and 6.9.c shows that in a few cases the outdoor air

dampers were about 8% open for outdoor air temperatures between 9‘’C and 11 °C.

At these same temperatures this preheat valve was about 8% open.

The preheat valve position data for pneumatic control are presented in figure

6.10. Comparing the preheat valve position data for AHU-3 under pneumatic

control with the data from DDC shows that the preheat valve opens at higher

outdoor air temperatures and opens more quickly under pneumatic control than it

does under DDC. This is consistent with the differences in the supply air

reset pattern shown in figures 5.2 and 5. 3. a. Instead of maintaining a constant supply air setpoint when outdoor air temperature drops below 1.7°C

(35“F)

,

the pneumatic controller continues to raise the setpoint. This of course

requires more steam and accounts for the difference in preheat valve position

curves. AHU-5 data follows the same pattern.

There is considerable scatter in the preheat valve position data for AHU-4

under pneumatic control. This is consistent with the scatter in supply air

temperatures and outdoor air damper positions. Comparison of figures 6.6.b and

6.10.

b clearly shows that the steam preheat valve and the outdoor air dampers

are fighting each other in the temperature range 0 - 10 °C (32 - 50°F) . This

not only wastes energy but also results in the poor control of supply air

temperature indicated in figure 5.3.b.

All of the data for AHU-6 in figure 6.10.d are from before the repairs were

made to the unit in March. The scattered data for outdoor air temperatures

above 7“C fail in a region where the steam preheat valve is fighting the

outdoor air dampers. The best fit line is a fit only to non zero data for

outdoor temperatures below 7®C. This data looks quite reasonable and falls in

a range where there is no indication of conflict with the outdoor air dampers

.

41
S*OPEW

»OPER

PREHEAT VALVE POSITION

oz

4". LEGEND

o DRY BULB ECONOMIZER

A ENTHALFT ECONOMIZER

•to 0 to zo OUTDOOR AIR TEMPERATURE (C)

a. AHU-3 b. AHU-4

„ PREHEAT VALVE POSITION

^ “

QO.

oLEGEND

o DEIY BULB ECONOMIZER

A EKTHALPT ECONOMIZER

1

1 I ~

-to 0 10 zo OUTDOOR AIR TEIOPERATURE (C)

c, AHU-5 d. AHU-6

Figure 6.9. Preheat valve position as a function of outdoor

temperature for each AHU under DDC

42
SSOPEN

»

.

_

»OPjaf

PREHEAT VALVE POSITION

Oo*

a. AHU-3 b. AHU-4

c. AHU-5 d. AHU-6

Figure 6.10. Preheat value position as a function of outdoor

temperature for each AHU under pneumatic control

43
7. SUMMARY AND CONCLUSIONS

A distributed, microprocessor-based EMCS was developed and installed in an eleven story office building in Gaithersburg, Maryland. Over a period of one year the performance of this system under various control strategies was investigated, along with the performance of a conventional pneumatic control system.

The pneumatic control system in this building is connected to a site wide

energy management system which has nighttime shutdown capabilities. The

controller on each air handling unit is also capable of implementing a dry bulb

economizer cycle to take advantage of outdoor air for cooling, zone air reset,

and outdoor air reset. The control strategies implemented in the DDC system

were an attempt to match the outdoor air reset schedule used in the winter

heating season by the pneumatic system and the zone air reset schedule used by

the pneumatic system in the cooling season. Both dry bulb and 'enthalpy

economizers were tested in the DDC system.

Comparison of the ability of the control systems to maintain supply air

temperature according to the planned reset schedules showed that the DDC system

performed much better. Under DDC all of the air handling units showed virtually

the same performance with respect to their ability to maintain the desired

reset schedule. Supply air temperature was maintained within + 0.5°C (1°F) of

the scheduled value. This applied to both outdoor air and zone air reset

schedules. No difference was detected between using the dry bulb economizer

and the enthalpy economizer in this regard.

The pneumatic control system showed different performance for each air handling

unit, AHU-3 maintained control as tightly or sometimes better than the DDC

controller. AHU-3 also came the closest to following the planned reset

schedules differing primarily in the supply air temperatures for very low

outdoor air temperatures (below 2°C or 36“F). The other three air handling

units varied widely in their ability to maintain the supply air temperature.

In the worst case a deviation of + 3.5“C (+ 6.3®F) was observed. Each unit

seemed to follow its own version of a reset schedule.

It was not possible to directly compare the energy consumption between the

pneumatic and DDC systems because exact weather conditions can not be duplicated.

General performance trends for both the pneumatic and DDC systems were investigated

by plotting the position of control devices as a function of outdoor air

temperature for each air handling unit

This analysis indicated that during the heating season the pneumatic control

system used more steam heating than the DDC system under similar loads. This

was at least in part due to the fact that for outdoor air temperatures below

0“C (32“F) the pneumatic controllers tried to maintain supply air temperatures

higher than the planned setpoint. Another contributing factor was the fact

that for outside air temperatures in the range 0 - lO^C (32 - 50°F) , AHU-4,

AHU-5, and AHU-6 all had instances where the outdoor air dampers were open to

provide cooling from outdoor air while the steam preheat valve was open to

provide heating.

44
The results from the cooling season showed that the pneumatic control system

provided more mechanical cooling as indicated by the relative positions of the

chilled water valve under DDC and pneumatic control. Here the difference is

most likely due to the fact that the pneumatic controllers maintained supply

air temperatures below the planned setpoint, thus using more chilled water.

Again AHU-3 was an exception to this rule. A comparison of the performance of

the DDC dry bulb economizer and enthalpy economizer did not show any apparent

difference in chilled water valve position as a function of outdoor air

temperature, indicating that there was little or no difference in chilled water

usage for these strategies.

Analysis of outdoor air damper position during the cooling season for the DDC

enthalpy and dry bulb economizer cycles did however, provide some evidence that

the enthalpy economizer utilized outdoor air cooling under conditions when the

dry bulb economizer could not. This is an expected result in the humid climate

found in the Washington D.C. area. The fact that this was not apparent as a reduction in mechanical cooling may be a result of the fact that valve position

as a function of outdoor air temperature is a very rough indicator of the

amount of mechanical cooling provided. It is also possible that data were collected on only a few days when the difference in performance between the two

economizer cycles would be apparent, providing little evidence of the true

potential. Computer simulation may be a more reliable indicator of the

benefits of using an enthalpy economizer.

AHU-3 serves the building zone which contains offices used by the top administration

officials, including the director of NBS . As a consequence this unit receives

higher priority maintenance and service from plant personnel. The performance

of AHU-3 suggests that if sufficient effort were put into tuning and maintaining

the other controllers the pneumatic system could perform as well as the DDC system.

The information obtained in this study suggests that one principle advantage of

the DDC system is that it is easier to set up the reset schedules and to see that they are maintained. One problem with the pneumatic control system is

that the tuning of the system drifted with time and was sensitive to inadvertent

changes made during routine maintenance. The DDC system did not suffer from

these shortcomings. This does not mean that it is not necessary to retune for

changes in operating conditions, it only means that the tuning parameters once set will not drift to different values or be effected by any hardware maintenance

performed on the controller.

Another advantage of the DDC control system is the increased ability to obtain

information about the performance of the controllers. This permits problems to

be discovered earlier and also provides much useful information for diagnosing

their cause. There were several times during the course of this experimental

work when our data collection process pointed out problems with the pneumatic

control system that were unknown to plant personnel. There were also occasions

where a problem was suspected and they came to us to collect data on a particular

unit to provide information which could be used to isolate probable causes or check that repairs made did in fact solve the problem.

In summary, the DDC system performed better and consumed less energy than the

pneumatic control system. These differences could be reduced considerably or

45
eliminated by careful maintenance of the pneumatic control system. DDC appears

to provide greater long term stability of performance and better information

about the performance of the system to assist in identifying problems early and

diagnosing their cause. The DDC system also provides greater flexibility to

change control strategies.

46
REFERENCES

[1] Park, C. , Kelly, G. E. , and Bushby, S. T. , "Simulation of a Large Office

Building System Using the HVACSIM Program"

.

[2] Park, C., Kelly, G. E. , and Kao, J. Y. , "Economizer Algorithms for Energy

Management and Control Systems," Nat. Bur. Stand. (U.S.) NBSIR 84-2832,

February 1984.

[3] May, W. B. and Kelly, G. E. , "Verification of Public Domain Control

Algorithms for Building Energy Management and Control Systems." Nat. Bur.

Stand. (U.S.) NBSIR 85-3285, December 1985.

[4] May, W. B. , "Time of Day Control and Duty Cycling Algorithms for Building

Management and Control Systems," Nat. Bur. Stand. (U.S.) NBSIR 83-2713,

June 1983.

[5] May, W. B. , "Control Algorithms for Building Management and Control

Systems - Hot Deck/Cold Deck/Supply Air Reset." Nat. Bur. Stand. (U.S.)

NBSIR 84-2846, March 1984.

[6] May, W. B. , Borresen, B. A., and Hurley, C. W. , "Direct Digital Control

of a Pneumatically Actuated Air - Handling Unit". ASHRAE Trans. Vol 88,

1982.

[7] Spitler, J. D. , Hittle, D.C., Johnson, D. L. Pederson, C. 0 ., "A Comparative

Study of the Performance of Temperature-Based and Enthalpy Based Ecomomy

Cycles". ASHRAE Trans. Vol 93, Pt 2, 1987.

[8] Kao, J. Y. , "Strategies for Energy Conservation for a Large Office Building" . Nat. Bur. Stand. (U.S.) NBSIR 83-2746, July 1983.

[9] Koa, J. Y. , and Pierie, T. E. , "A Study of Sensor Errors on Building Energy

Consumption". ASHRAE Journal Dec. 1983, p. 42-45.

47
NBS-T14A (REV. 2«8C) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 88^3739 MAY 1988

4. TITLE AND SUBTITLE

Comparison of Direct Digital Control and Pneumatic Control

Systems in a Large Office Building

5. AUTHOR(S)

Steven T. Bushby and George E, Kelly

6. PERFORMING ORGANIZATION (If joint or other than NBS, 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)

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-lBS, FIPS Software Summary, is attached.

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

bi bliography or literature survey, mention it here)

A distributed, microprocessor based direct digital control (DDC) energy management and

control system was developed and installed in an eleven story office building in

Gaithersburg, Maryland. Over a period of one year the performance of this system

under various control strategies was investigated along = with the performance of a conventional pneumatic control system. The quality of control and performance trends

for the two control systems were compared.

Overall, the DDC system was found to maintain better control of ,the supply air

temperature and to follow reset schedules more closely than the pneumatic system. One

air handling unit performed as well under pneumatic control as it did under DDC in

maintaining a supply air setpoint, but it did not precisely follow the reset schedule.

Each air handling unit was found to have different performance characteristics while

under pneumatic control, but all units behaved essentially the same under DDC.

The results indicate that a pneumatic system can perform as well as a DDC system but

more effort is required to maintain system performance. The DDC system was found to

be more flexible and it is easier to insure that multiple air handling units follow

the same control strategy with the DDC system. Instrumentation associated with the

DDC system proved to be very useful in troubleshooting problems with the pneumatic —sy

- stem^

^

^ 12. KEY WORDS fS/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) control; computer; DDC; maintenance; monitoring; office building;

pneumatic; trends

13. AVAILABILITY

rx)t Unlimited

I I For Official Distribution. Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

55

15. Price

$13.95

USCOMM-OC 8043-P80

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