Year-Round System Control Processes. Honeywell AUTOMATIC CONTROL SI Edition

AIR HANDLING SYSTEM CONTROL APPLICATIONS

HEATING SYSTEM CONTROL PROCESS

SPACE CONTROL OF HEATING, ECONOMIZER (FREE COOLING), AND HUMIDIFICATION

Functional Description

SPACE

4

3

45

52

RA

CONTROL

PROGRAM

8

7

6

88

52

28

NC

5

SA

13

12

25

15

9

23.5

22

10

COOL

HEAT

Item

No.

1,2

OA

83

17

ECONOMIZER DECISION.

REFER TO PREVIOUS

ECONOMIZER OPTIONS

18

N.O.

N.C.

1

CONTROL

PROGRAM

20

13

ON

NORMAL

2

SUPPLY AIR

TEMPERATURE

SETPOINT

41

00

14

CONTROL

PROGRAM

13

19

22

SUPPLY AIR

TEMPERATURE

SETPOINT

13

OA MINIMUM SETPOINT

(NOTE: THE TEST AND BALANCE INITIAL

VALUE FOR PROPER VENTILATION IS 22)

27

SPACE

HEATING

DEMAND

100

0

11

SPACE

COOLING

DEMAND

100

0

16

M15175

3-5

6,7

8

9-11

12

Function

Control system energizes when fan is turned on (See FAN SYSTEM START-STOP

CONTROL).

Space humidity PI loop controls humidifier valve if space humidity falls to setpoint.

SA humidity high limit PI loop throttles humidifier valve if duct humidity rises above setpoint.

Control program coordinates space and humidifier leaving air humidity control and fan interlock.

Space temperature heating PI loop inputs to

SA temperature heating reset schedule.

SA temperature PI loop inputs to heating and mixed air control programs.

13

14

15,16

17

18

19

20

Hot water valve modulates to maintain SA temperature setpoint.

Control program coordinates space and SA heating control and fan interlock.

Space temperature cooling PI loop inputs to SA temperature cooling reset schedule.

Mixing dampers modulate to maintain SA temperature setpoint.

Economizer decision program determines when

OA is suitable to assist with cooling demand.

Mixing dampers minimum ventilation value.

Control program coordinates space and supply cooling control, ventilation control, and fan interlock.

246 ENGINEERING MANUAL OF AUTOMATIC CONTROL


FEATURES

1. The outdoor air quantity is modulated from a minimum to take advantage of free cooling based on space temperature demand.

2. The SA temperature will not fall below a set minimum.

3. Air is supplied at the temperature necessary to make up the sensible heat loss of the space.

4. Space relative humidity is maintained at a minimum value by a space humidity controller controlling the humidifier.

5. Separate setpoints for heating and cooling.

2. SA condition at design load is 32

°

C DB and 7.0 grams of moisture per kilogram of dry air.

3. Light load outdoor air condition is 13

°

C DB and 11.5

°

C

WB.

4. SA condition at light load is 24

°


5. Minimum outdoor air is 25 percent.

6. RA condition is 23.5

°

C DB and 35% RH.

Design Load

CONDITIONS FOR SUCCESSFUL OPERATION

See FAN SYSTEM START-STOP CONTROL.

1. An appropriate schedule of settings.

2. The low temperature limit controller located to respond to the lowest temperature in the duct.

OA

-18

°

C DB,

50% RH

1

MA 13

°

C DB,

7.5

°

C WB

RA 23.5

°

C DB,

35% RH

MIX

2 HEAT

HUMIDIFY

DESIGN LOAD

3

4

SA 32

°

C DB,

7.0g H

2

O/kg DRY AIR

32

°

C DB,

16

°

C WB

C3256

LIMITATIONS

This application used in applications that do not require mechanical cooling.

SPECIFICATIONS

Anytime the supply fan runs, control system shall be enabled.

Anytime the space relative humidity drops below the setpoint, the space humidity PI loop shall modulate the humidifier, subject to an SA humidity high limit override set at 88%.

As the space heating load varies from 0 to 100%, the SA heating PID loop setpoint shall be reset from 13

°

C to 41

°

C.

The hot water valve shall be modulated as required to maintain the SA temperature setpoint.

Anytime the fan runs, the mixing dampers shall position to a minimum ventilation setting.

As the space cooling load varies from 0 to 100%, the SA cooling

PI loop setpoint shall be reset from 27

°

C to 13

°

C. The outdoor and return (and relief if applicable) air dampers shall be modulated as required to maintain the SA temperature setpoint.

Cooling damper control shall be returned to minimum position anytime the economizer mode is disabled.

Separate space heating and cooling setpoints shall be provided.

PSYCHROMETRIC ASPECTS

In the following charts it is assumed that:

1. Design outdoor air condition is –18

°

C DB and 50 percent relative humidity.

The following results are obtained:

Item

No.

Explanation

1

2

3

4

Design outdoor air condition is –18

°

C DB and

50 percent relative humidity.

Mixed air condition is 12.5

°

C DB and

7.5

°

C WB.

Heated air condition is 32

°

C DB and 16

°

C WB.

SA condition is 32

°


Light Sensible Load

MA 15 ° C DB, 11.5

° C WB

6

SA 24 ° C DB, 15.5

° C WB

7.5g H

2

O/kg DRY AIR

RA 22 ° C DB, 13 ° C WB

OA

13

°

C DB,

11.5

° C WB

5

LIGHT SENSIBLE LOAD C3257


Item

No.

Explanation

5

6

Mixed air condition is 15

°

C DB and 11.5

°

C WB.

SA condition is 24

°

C DB, and 7.5 grams of moisture per kilogram of dry air.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

247


YEAR-ROUND SYSTEM CONTROL PROCESSES

HEATING, COOLING, AND ECONOMIZER


EA

100

5

OA

NC

NC

NO

3 ON

2

NORMAL

RA

SPACE TEMPERATURE

CHILLED WATER COOLING

SETPOINT = FREE COOLING

SETPOINT PLUS

10 1.1

(0.8 MINIMUM)

FREE COOLING

SETPOINT

HEATING SETPOINT = FREE

COOLING SETPOINT MINUS

12 1.1

(0.8 MINIMUM)

8

24.5

SETPOINT

24.5

11

9

23.5

SETPOINT

22

13

SUPPLY

FAN

1

SA

ON

15

15

14

SUPPLY AIR

TEMPERATURE

SETPOINT

SPACE

HEATING

DEMAND

41 100

13 0

18

21

100

PERCENT

OPEN

4

20

18

19 16

36 0

17

SUPPLY AIR

TEMPERATURE

SETPOINT

FREE

COOLING

DEMAND

24 0

14 100

22


REFER TO PREVIOUS

ECONOMIZER OPTIONS

7

6

22

CONTROL

PROGRAM

OA MINIMUM SETPOINT

(NOTE: THE TEST AND BALANCE

INITIAL VALUE FOR PROPER

VENTILATION IS 22)

18

SUPPLY AIR

TEMPERATURE

SETPOINT

CHW

COOLING

DEMAND

24 0

13 100

0

100

96

M15176

Item

No.

1-3

4-6

7

Function


CONTROL).

Mixing dampers modulate to maintain minimum ventilation value and free cooling

SA temperature setpoint.

Economizer enables free cooling when OA is appropriate.

8

9-13

14

Space temperature dictates heat and cool demands.

Free cooling setpoint and heat/cool deadband values determine the SA temperature setpoint for hot and chilled water control setpoints. Software has a minimum 0.8

kelvins heating and cooling deadband.

Heat demand varies SA temperature setpoint.



15,16

17

18

19

20

21

22

FEATURES

Hot water valve modulates to maintain SA temperature setpoint.

Free cooling demand varies SA temperature setpoint.

Chilled water cooling demand varies SA temperature setpoint.

Chilled water valve modulates to maintain SA temperature cooling setpoint.

MA temperature sensor for operator information.

OA temperature sensor for operator information.

Control program coordinates space and supply cooling, heating, and ventilation control, and fan interlock.

1. Use of space control resetting SA temperature control adds stability. The MA sensor is required to prevent freeze-up if the free cooling setpoint is lowered in freezing weather.

2. SA temperature is maintained only as high or low as required to meet space requirements.

3. Free cooling cycle selection determined by the economizer control program to minimize load on mechanical cooling system.

4. Optimum comfort temperature provided during free cooling cycle with energy conserving deadbands for heating and cooling.

The space temperature shall have a free cooling PI loop setpoint selected to provide optimum occupant comfort temperature. The space temperature shall have a chilled water cooling PI loop setpoint adjustable to no lower than 0.8 kelvins (minimum) above the free cooling setpoint. The space temperature shall have a heating PI loop setpoint adjustable to no higher than 0.8 kelvins

(minimum) below the free cooling setpoint.

As the space heating load varies from 100 to 0%, an SA heating PI control loop setpoint shall vary from 41

°

C to 13

°

C.

The hot water valve shall modulate to maintain the heating SA temperature setpoint, except that anytime the space temperature is greater than one degree above the free cooling space temperature setpoint, the hot water valve control PI setpoint shall be 11

°

C.

As the space free cooling load varies from 0 to 100%, an SA free cooling PI control loop setpoint shall vary from 24

°

C to

14

°

C. The mixing dampers shall modulate to maintain the free cooling SA temperature setpoint.

As the space chilled water cooling load varies from 0 to 100%, an SA chilled water cooling PI control loop setpoint shall vary from 24

°

C to 13

°

C. The chilled water valve shall modulate to maintain this cooling SA temperature setpoint.


In the following chart it is assumed that:

1. OA condition is 18

°

C and 53% RH.

2. System is in the economizer mode.


1. Sensor locations must be selected to measure representative air conditions.

2. SA temperature control loops should provide PI control to assure stability with minimum offset.

NOTE: In cold climates this unit would most often have the heating coil ahead of the cooling coil and the low temperature switch after the heating coil. Control would not change. In the configuration shown, it would be possible to add a dehumidification cycle with reheat (software) at a later time.

SPECIFICATIONS


Anytime the supply fan runs, control system shall be enabled, and mixing dampers shall position to minimum ventilation position during scheduled occupancy periods.

Anytime the economizer decision program determines that

OA is suitable to assist in cooling, the temperature controls shall be enabled to override the dampers minimum ventilation position for free cooling as required.

COOLING COIL

DISCHARGE 15

°

C

2

COIL

COOLING

LOAD

3

1

100% OA

18

°

C DB, 53% RH

SPACE

24.5

°

C DB, 38% RH

M15177


Item

No.

Explanation

1

2

3

100% economizer air is entering the cooling coil.

The chilled water coil cools the entering air to its 15

°

C setpoint (removing little moisture because the water flow is low and the OA moisture content is not high).

Space temperature is 24.5

°

C and 38% RH.


249


MULTIZONE UNIT


THE ZONE WITH THE GREATEST HEATING DEMAND

RESETS THE HOT DECK TEMPERATURE FROM

MAINTAIN ITS SPACE TEMPERATURE

THE ZONE WITH THE GREATEST COOLING DEMAND

RESETS THE COLD DECK TEMPERATURE FROM

MAINTAIN ITS SPACE TEMPERATURE

6

46

5

23.5

7

23.5

62

23.5

23.5

HALL

LOBBY

EAST

OFFICE

23.5

24.5

100

PERCENT

OPEN TO

COLD

DECK

SETPOINT

SPACE

TEMPERATURE

WEST

OFFICE

25

11

PERCENT

OPEN

22

OA

78

3

RA

00

10

2

NORMAL

ON

1


REFER TO PREVIOUS

ECONOMIZER OPTIONS

OA MINIMUM SETPOINT

(NOTE: THE TEST AND

BALANCE INITIAL

VALUE FOR PROPER

VENTILATION IS 22)

22

4

14

25 15

00

CONTROL

PROGRAM

16

12

Item

No.

1,2

3,4

5-8

9

Function


CONTROL).

Mixing dampers position to minimum position during occupied fan operation and modulate for cooling.

Zone mixing dampers modulate to maintain space temperature setpoint.

Zone with greatest cooling demand determines cold deck temperature setpoint, and zone with greatest heating demand determines hot deck temperature setpoint.

10,11

12,13

14

15

16

14

13

8

SA

M15178

Hot deck valve modulated to maintain hot deck temperature setpoint.

Cold deck valve modulated to maintain cold deck temperature setpoint.

Economizer enables free cooling when OA is suitable.

Fan leaving air temperature for operator information.

Control program coordinates cooling, heating, ventilation, and fan interlock control.



FEATURES

1. This application uses zone control of heating and cooling.

2. Deck temperatures dictated by zones with greatest heating and cooling demand and with a deadband.


All zones should be connected to load analyzer program to satisfy total load requirements. However, in larger systems it may be good practice to connect only selected diverse zone loads. Zones that may be allowed to go out of control (storage rooms, etc.) should not be connected to the load analyzer program.

In the winter, zone space temperature is maintained by mixing air from the cold deck with hot deck air (the temperature of which is dictated by the zone with the greatest demand for heating). The zone with the greatest demand for heating gets

100% hot deck air. The zone with the greatest demand for cooling (assuming a zone space temperature rises two degrees above setpoint and demands cooling) gets 100% cold deck air and dictates the cold deck temperature maintained via the economizer cycle.


1. Desired space condition is 24

°

C DB.

2. Outdoor air condition is 24

°

C DB and 80% RH.

3. The mixed air is 25 percent outdoor air during the cooling cycle.

4. Coil leaving air temperature is 13

°

C (at least one zone demands full cooling).

SPECIFICATIONS


Anytime the supply fan runs, control system shall be enabled, and mixing dampers shall position to minimum ventilation position during scheduled occupancy periods.

Each zone space temperature PID loop shall modulate its zone mixing dampers to maintain its space temperature setpoint.

The zone with the greatest temperature deviation below setpoint shall reset the hot deck temperature setpoint from 13

°

C to 36

°

C as required to maintain the zone space temperature 2 degrees below setpoint.

The zone with the greatest temperature deviation above setpoint shall reset the cold deck temperature setpoint from

25.5

°

C to 13

°

C as required to maintain the zone space temperature 1 degree above setpoint.

The hot deck PID loop shall modulate the hot deck hot water valve to maintain the hot deck temperature setpoint.

The cold deck PID loop shall modulate the OA/RA mixing dampers in sequence with the cold deck chilled water valve to maintain the cold deck temperature setpoint.

Anytime the economizer decision program determines that

OA is unsuitable to assist in cooling, the OA/RA mixing dampers shall be returned to their minimum ventilation position.


In the summer, zone space temperature is maintained by mixing air from the hot deck with cold deck air (the temperature of which is dictated by the zone with the greatest demand for cooling). The zone with the greatest demand for cooling gets

100% cold deck air.

4

COLD

DECK

13 ° C DB

1 OA 24

°

C DB, 80% RH

5

3

6

2

7

MA AND HOT DECK

RA 24 ° C DB, 60% RH


Item

No.

1

2

3

4

5

6

7

Explanation

M15179

OA temperature at example time.

Mixed air is 25% OA and 75% RA. This is also the hot deck air, assuming no zone temperature has dropped two degrees and demanded heating.

This line represents the cooling process of the cold deck air. The zone demanding the most cooling dictates how far the process goes from

Point 2 to Point 4.

13

°

C DB is the minimum cold deck setpoint set up in the program, set by a zone requiring full cooling.

Discharge air to a zone requiring half cold deck air and half mixed air.

The space cooling process line.

Return air is 24

°

C DB and 60% RH (humidity rises because humidity is high outdoors and only partial supply airflow is dehumidified).


251


4-6

7

8

9

10,27

11-16

HEATING, COOLING, HUMIDIFICATION, AND DEHUMIDIFICATION CONTROL

WITHOUT DEAD-BANDS


9

EA

83

5

OA

7

N.C.

N.C.

N.O.

3

ON

2

NORMAL

83

9

PERCENT

OPEN

4

16

RA

0

10

18

15

100

13

27

11

20 52

52

24

SA

21 50

SPACE

11 23.5

88

1

ON

0

26

SUPPLY

FAN

13.5

25

16

12 23.5

13 35

REHEAT

SUPPLY AIR

TEMPERATURE

SETPOINT

COOLING

DEMAND

41

14

0

13 40

OA

CW

24

13

17

45

100

28


REFER TO PREVIOUS

ECONOMIZER OPTIONS

8

6

OA MINIMUM SETPOINT

(NOTE: THE TEST AND

BALANCE INITIAL

VALUE FOR PROPER

VENTILATION IS 22)

22

CONTROL

PROGRAM

COOL COIL

LEAVING AIR

TEMPERATURE

SETPOINT

13

19

HUMIDI-

FICATION

DEMAND

0

21 45

HUMIDIFIER

VALVE POSITION

0

23

100

55

100

22

50

M15180

Item

No.

1-3 17,18

Function


CONTROL).

Manual positioning value determines minimum ventilation mixing damper position.

Operator information, outdoor air temperature.


Operator information, MA temperature.

Heating coil valve modulates to keep reheat coil entering air from getting too low.

Space temperature PI loop resets setpoint of reheat coil SA PI loop to maintain constant space temperature.

19

20-26

27

Chilled water valve modulates in sequence with mixing dampers as required to maintain

SA PI setpoint.

Chilled water coil leaving air temperature lowered if required for dehumidification.

Space humidity PI control loop modulates humidifier valve to maintain space relative humidity, subject to an SA high limit humidity PI loop.

Control program coordinates ventilation, heating, cooling, humidification, dehumidification, and fan interlocks.



FEATURES

1. The system admits outdoor air for cooling based upon the economizer decision.

2. Space relative humidity is maintained by controlling both humidification and dehumidification.

3. Reheat prevents subcooling on dehumidification cycle.

4. Constant temperature and humidity control. (Do not use this where deadband temperature or humidity control is acceptable.)


For cooling conditions it is assumed that:

1. Design outdoor air condition is 35

°

C DB and 26

°

C WB.


°

C DB and 18

°

C WB.

3. System operates on 25 percent minimum outdoor air.

4. Space temperature setpoint is set at 23.5.

5. Space humidity control is set at 50 percent.

6. Coil leaving air temperature is at 13

°

C.


1. Heating is available during dehumidification cycle.

OA 35 ° C DB,

26

°

C WB

3

13

°

C DB

SPACE

23.5

°

C DB

2

1

MA

4

RA 24.5

° C DB,

17

°

C WB

1 KELVIN

SPECIFICATIONS


1. Anytime the supply fan runs, control system shall be enabled, and mixing dampers shall position to minimum ventilation position during scheduled occupancy periods.

2. Anytime the economizer decision program determines that OA is suitable to assist in cooling, the OA/RA mixing dampers shall be under control of the SA PI loop.

3. The space humidity PI loop shall modulate the humidifier, subject to a humidifier leaving air high limit humidity PI loop setpoint, to maintain the space humidity PI loop setpoint. Humidifying control shall be disabled anytime the chilled water valve is modulating or the fan is off.

4. The space humidity PI loop shall override the temperature controls to modulate the chilled water valve open for dehumidification if required to maintain the space humidity PI loop setpoint. The dehumidifying control loop shall be disabled anytime there is no hot water flow or temperature.

5. As the SA PI cooling demand varies from 100 to 45%, the cooling SA PI loop setpoint shall vary from 13

°

C to 24

°

C.

6. As the SA PI cooling demand varies from 40 to 0%, the reheat coil hot water valve SA PI loop setpoint (chilled water and economizer) shall vary from 11

°

C to 41

°

C.

7. The heating coil hot water valve shall modulate to prevent the cooling coil leaving air temperature from dropping below 11

°

C.


Item

No.

1

2

3

4

Explanation

COOLING C3263

Mixed air temperature at cooling design condition.

Air entering the coil is cooled along a line of constant moisture content until saturation is approached. Near saturation the moisture content is reduced as the air is cooled. This process involves both latent and sensible cooling.

Cooling coil leaving air temperature will be as low as required to satisfy either the space temperature controller or the space humidity controller whichever is calling for the greatest cooling. If dehumidification cools discharge temperature below setpoint, the heating coil provides reheat.

The space temperature is 23.5

°

C DB and the

RA temperature is 24.5

°

C DB. The 1 kelvin

DB rise is an example of sensible cooling load which may occur in ceiling, space, and RA ducts. The rise will be a function of system, building, and lighting arrangement.


253


For heating conditions it is assumed that:

1. Design outdoor air condition is –18

°

C DB and 30 percent relative humidity.


°

C DB and 13.5

°

C WB.

3. System operates on 25 percent minimum outdoor air.

4. Space temperature is set at 23.5

°

C.

5. Space humidification control is set at 50 percent.


Item

No.

1

Explanation

2

3

4

Heating coil leaving air temperature will be as high as required to satisfy the space temperature controller.

Humidification will be provided to satisfy space humidification requirements.

The space heating and humidifying load varies with people and weather.

The 1 kelvin DB RA rise is discussed in the cooling example.

OA

-18

°

C DB

30% RH

4

1 KELVIN

SPACE

23.5

°

C DB

MA

RA 24

°

C DB, 17

°

C WB

3

2 HUMIDIFIER

DISCHARGE

1 HEATING COIL

DISCHARGE

HEATING C3264



VAV AHU, WATER-SIDE ECONOMIZER, OA AIRFLOW CONTROL


WARM-UP MODE INVOLVED AT OPTIMUM

START TIME IF PERIMETER SPACE

TEMPERATURE IS LESS THAN 21

WARM-UP ENDS WHEN RETURN

AIR REACHES 23.5

.

.

24

0.45

83

8

RA

14

21

ON

WARM-UP

MODE

OFF

PERIMETER

ZONE

23.5

12

SPACE

25

13

20

22

32 13

11

SUPPLY AIR

TEMPERATURE SETPOINT

17

NORMAL

2

ON

1

NORMAL

OA

CFM

9

2306

16.5

19

2300

10

16

00

18

76

4

63

PERCENT

LOAD 15

13.5

3

PROPELLER

EXHAUST

FAN

ON

SA TO VAV

BOXES

7

6

0.45

5

0.45

MAXIMUM

SETPOINT

CONTROL

PROGRAM

22

23

ANYTIME ALL VAV BOX DAMPERS ARE LESS THAN 90% OPEN, DECREMENT THE AHU

ANYTIME ANY VAV BOX DAMPER IS FULL OPEN, INCREMENT THE AHU DUCT STATIC

M15181

Item

No.

1-3

4-6

7-10

11-14

Function

Control system energizes when supply fan is turned on (See FAN SYSTEM START-STOP

CONTROL).

Supply fan loads to maintain duct static pressure.

During occupied periods, exhaust fan runs and

OA airflow is controlled.

When perimeter temperature is low at startup,

SA temperature setpoint is warm until RA temperature rises.

15-16

17-18

19-20

21

22


255

Heating valve maintains SA temperature setpoint during warmup.

Cooling valve maintains SA temperature setpoint during occupied periods.

OA and MA temperatures are operator information.

SA temperature setpoint switches from cooling to heating value during warm-up modes.

Control program coordinates temperature control, ventilation, and fan interlock.


23

24

Control program optimizes duct pressure setpoint.

OA shaft static pressure point shared from OA fan control system for this graphic (not a physical point as shown) and provided here for operator information.

NOTE: This system is often found on each floor of a building, and often includes an outdoor air fan (preferably with filtration to protect the AHU OA air flow elements) and shaft, and a water-side economizer. The waterside economizer provides chilled water year round (in cold weather the cooling tower provides chilled water via heat exchangers without the need for the chillers).

Dual equal sized chillers and boilers (non-redundant) are assumed.

FEATURES

1. Supplies constant temperature variable volume (energy conserving) air to VAV boxes.

2. Provides constant airflow of OA with varying supply airflow.

3. Provides 100% RA during warm-up periods

(preoccupancy purge may be required for IAQ).

4. Perimeter boxes provide space heat when required during occupied periods.

5. AHU provides heating during warm-up periods, and when

OA temperature effects cause low SA temperatures. If boxes have electric heat and the AHU hot water is from lower cost gas/oil-sourced heat, box heaters may be disabled during warm-up periods.

6. Duct static pressure setpoint is lowered anytime all VAV box dampers are less than 90% open (If this strategy is used, the duct static pressure pickup need not be at the end of the longest run, but may be at any location where the air is not too turbulent. The maximum setpoint is higher than if the pickup had been at the end of the run because of the pickup location).

7. Reduced fan airflow is provided during warmup, night purge, and cool-down periods to reduce fan energy, which varies with the cube of the fan airflow.

8. Reduced fan airflow and staged AHU startup during cooldown periods requires only one chiller and keeps the chiller pull-down ahead of the AHU cooling demands such that AHUs get cold (not cool) water for effective cooling and dehumidification, and allows the chiller to operate at an efficient loading (less than 100%).


1. Airflow element and transducer must be kept clean and calibrated.

2. OA fan must provide adequate OA shaft pressure.

Alternatively, if there is no OA fan, the OA airflow setpoint may be maintained by modulating the RA damper which would have to be added.

3. A controller network and adequate software and programming provided to support communication between the box controllers and the fan controller to allow static pressure reset and to position the box dampers properly during night purge, warmup, and cool-down periods.

4. If any VAV box whose damper position is a program input can never satisfy its space cooling demand for any reason

(and its damper is always open), the duct static pressure reset program will not lower the duct static pressure setpoint.

The duct static pressure reset program works best when there are no more than thirty monitored VAV boxes per fan system (with great quantities of boxes, it is likely that at least one box damper will always be full open).

For example, if an interior zone is always under a full cooling load, static pressure reset will not occur unless that zone (and similar zones) is oversized. The oversized zone would then throttle back when the building is at full load when the duct static pressure is at design.

5. All AHUs must be near the same normal occupancy schedule or the cool-down start-up specification edited.

6. Boiler, chiller, pumping system, and OA fan controls carefully networked into the AHU control schemes to assure smooth and efficient building operations.

7. All specified values and setpoints are tuned to specific project requirements.

SPECIFICATIONS

NOTE: A set of 16 similar sized AHUs are assumed.


Anytime any AHU starts in the optimum start cool-down mode, three to four AHUs shall start and the remaining AHUs shall stage on at five minute intervals (unless they similarly start under their optimum start programs). Any time any AHU operates in the night purge, warmup, or cool-down modes of operation, all associated perimeter VAV boxes shall operate at

60% of their maximum airflow setpoint, and all associated interior VAV boxes shall operate at 25% of their maximum airflow setpoint, unless the OA temperature is less than –9.5

°

C in which case the perimeter VAV boxes shall operate at their maximum airflow setpoint.



During unoccupied periods, anytime the top floor west zone perimeter space temperature is greater than 25

°

C and the OA temperature is less than 22

°

C and the OA dew point is less than

16

°

C, the night purge program shall start. When the night purge program starts, AHUs 9 through 16 (provided their west zone space temperatures are greater than 23.5

°

C) shall start, and the

OA and exhaust fans shall start. When an AHU runs in the night purge mode, its OA damper shall position full open. When the OA fan runs in the night purge mode, its duct static pressure setpoint shall be reset to a value 50% above the normal maximum setpoint. AHU fans running in the night purge mode shall stop when their noted space temperature drops to 23.5

°

C.

Anytime the night purge program runs for one minute and any of AHUs 9 through 16 are off, AHU’s 8 through 1 shall start respectively on a one-for-one basis (provided their west zone space temperatures are greater than 23.5

°

C). Anytime all fans shut down in the night purge mode, the night purge program mode shall end.

Anytime the supply fan runs, the return fan shall start and the control system shall be enabled. Also, anytime the supply fan runs during scheduled occupancy periods the exhaust fan shall start.

At the scheduled occupancy time, each AHU OA damper control loop shall be enabled under EPID control with a start value of 50 and a ramp duration of 400 seconds. Each AHU

OA damper shall modulate to maintain its OA airflow setpoint.

The supply fan loading shall be under EPID control with a start value of 20% and ramp duration of 150 seconds. The supply fan shall load to maintain the duct static pressure setpoint.

The SA temperature shall be under EPID control with a start value of 50% (at which point the hot and chilled water valves are both closed) and a ramp duration of 120 seconds. The hot and chilled water valves shall be modulated in sequence to maintain the SA temperature setpoint.

Anytime the optimum start perimeter zone space temperature sensor is less than 21

°

C at start-up time, the SA temperature setpoint shall be 32

°

C until the RA temperature rises to

23

°

C, at which time the SA temperature setpoint shall be lowered to 13

°

C. The EPID shall be invoked at the switching of the setpoint to 13

°

C with a start value of 50 and a ramp duration of 180 seconds.

EXPLANATION:

With VAV fan systems, operation during unoccupied periods should be based on minimum energy cost (not minimum ontime). Ideally, for a dual chiller building, the VAV box airflow would be regulated to run one chiller at its most efficient operating point. At this point the AHU fan would draw a small portion of its full-load amperage, good dehumidification would occur, and pumping energy may be reduced. In the night purge mode of operation, the objective is to supply a maximum amount of OA to the AHUs, and to direct it to the warm areas of the building. Reducing the AHU airflow and increasing the OA airflow should result in the supply airflow being a significant proportion OA. If IAQ requirements dictate a prepurge cycle, this operational mode should suffice then also, but would be staged by time rather than temperature.



1. Outdoor air condition is 35

°

C DB and 26

°

C WB.


°

C DB and 14.5

°

C WB.

3. Coil leaving air temperature 10

°

C.

4. 80% RA.

OA 35 ° C DB,

26

°

C WB

SPACE LOAD

3

COIL DISCHARGE

10

°

C DB, 8

°

C WB

2 COOL

1

15.8

°

C DB

4

RA 26

°

C DB,

14

°

C WB

C3266


Item

No.

Explanation

1

2

3

4

RA mixes with 20 percent (minimum position) outdoor air to obtain mixed air condition.

Air entering the coil is cooled along a line of constant moisture content until saturation is approached. Near saturation the moisture content is reduced as the air is cooled. This process involves both latent and sensible cooling.

Mixed air is cooled and dehumidified by cooling coil to obtain cooling coil leaving air condition.

Reheat coils and/or space internal load heats air to 25.5

°

C DB and 14.5

°

C WB.


257


VAV AHU WITH RETURN FAN AND FLOW TRACKING CONTROL


EA

83

20

22

17

24.5

30

RA

4

ON 58

9

PERCENT

LOAD

4.530

10

EXHAUST

FAN

ON

5

EXHAUST

FAN "ON"

DIFFERENTIAL m

3

/s

1.274

13

ZERO

CALIBRATION

360

14

SPACE

PRESSURIZATION

DIFFERENTIAL m

3

/s

0.613

12

CONTROL

PROGRAM

31

18

14.5

83

21

NORMAL

2

ON

1

NORMAL

3

6.423

11 m 3 /s

SA

OA

16

9

00

6

63

PERCENT

LOAD

0.060

23

24

0.060

25

26

76

13

27

7

8

0.45

0.45

Item

No.

1-5

6-8

9-14

15

17


REFER TO PREVIOUS

ECONOMIZER OPTIONS

CONTROL

PROGRAM

32

ON

WARM-UP

MODE

OFF

29

32 13

28

19

SA

TEMPERATURE

SETPOINT

OA

TEMPERATURE

13 4

16.5

-21

22

15

OA MINIMUM SETPOINT

(NOTE: THE TEST AND BALANCE INITIAL

VALUE FOR PROPER VENTILATION IS 22)

M15182

Function

Control system energizes when supply fan is turned on (See FAN SYSTEM START-STOP

CONTROL).

Supply fan loads to maintain duct static pressure at the end of the longest run.

Return fan loads to track supply airflow minus exhaust airflow minus air for pressurization.

Manual positioning value determines minimum summer ventilation mixing damper position. This position is fixed for the OA and

16

17

18 relief air dampers; but is a minimum position for the return air damper, the value of which will be overriden to maintain a constant mixing box negative static pressure and thus a constant

OA airflow.

OA determines SA temperature setpoint.


Mixed air temperature is for operator information.


Year-Round System Control Processes. Honeywell AUTOMATIC CONTROL SI Edition

HEATING SYSTEM CONTROL PROCESS

SPACE CONTROL OF HEATING, ECONOMIZER (FREE COOLING), AND HUMIDIFICATION