MICROPROCESSOR-BASED/DDC FUNDAMENTALS
139ENGINEERING MANUAL OF AUTOMATIC CONTROL
Night Purge
The night purge program uses cool, night outdoor air to
precool the building before the mechanical cooling is turned
on. Digital control systems often reduce fan capacity of VAV
AHU systems during Night Purge to reduce energy usage.
Outdoor temperature, outdoor RH or dewpoint, and space
temperature are analyzed. One hundred percent outdoor air is
admitted under the following typical conditions:
1. Outdoor air above a summer-winter changeover point,
such as 10°C.
2. Outdoor temperature below space temperature by a
specified RH or determined differential.
3. Outdoor air dewpoint less than 16°C.
4. Space temperature above some minimum for night purge
such as 24°C.
Enthalpy
The enthalpy program (Fig. 8) selects the air source that
requires the least total heat (enthalpy) removal to reach the
design cooling temperature. The selected air source is either
the return air with a selectable minimum amount of outdoor air
or a mixture of outdoor and return air as determined by local
control from discharge-air or space temperature measurement.
Measurements of return-air enthalpy and return-air dry bulb
are compared to outdoor air conditions and used as criteria for
the air source selection. A variation of this, although not
recommended, is comparing the outside air enthalpy to a
constant (such as 63.96 kilojoules per kilogram of dry air) since
the controlled return air enthalpy is rather stable.
Load Reset
The load reset program (Fig. 9) assures that only the minimum
amount of heating or cooling energy is used to satisfy load
requirements. Samples of zone demands are taken and the zone
with the greatest load is used to reset the temperature of the
heating or cooling source.
Fig. 9. Typical Load Reset Schedules.
Load reset used with digital controllers is the application that
most sets digital control apart from pneumatic and traditional
control. The application uses VAV box loadings to determine
VAV AHU requirements for air static pressure and temperature,
uses reheat valve loadings to determine hot water plant
requirements for temperature and pressure, and uses chilled
water valve positions to determine chilled water plant
requirements for temperature and pressure. These adjustments
to the various requirements reduce energy costs when all VAV
dampers are less than 70 percent open and AHU design
conditions (for example, 13°C at 0.5 kPa static pressure) are
not required.
Without knowledge of the actual instantaneous demands of
the loads that load reset controls, systems must run at the
theoretical worst-case setpoints for temperature and pressure.
As with most powerful application programs, load reset
requires a great depth of knowledge of the HVAC and control
process. The problem is that if load demands indicate that a
higher chilled water temperature would be appropriate and the
temperature increase is made, all loads are upset because of
the chilled water temperature change. The loads must adjust
and stabilize before another load reset adjustment is made.
Monitoring and tuning is required to assure high performance
control and loop stability. Three parameters determine the
performance and stability of the system: the magnitude of the
incremental corrections, the time interval between correction,
and the magnitude of the hysteresis between raising and
lowering the temperature.
C2439
NO NO
SELECT
MINIMUM
OA
OPTIONAL CONSIDERATIONS
YES YES
OADB
>
RADB
LEGEND:
OAh = OUTDOOR AIR ENTHALPY
RAh= RETURN AIR ENTHALPY
OA = OUTDOOR AIR
OADB = OUTDOOR AIR DRY BULB
RADB = RETURN AIR DRY BULB
OAh > RAh
1
1
SELECT
MINIMUM
OA
SELECT
LOCAL
CONTROL
Fig. 8. Enthalpy Decision Ladder.
ZONE WITH GREATEST
DEMAND FOR HEATING
HEATING LOAD RESET COOLING LOAD RESET
C4315
ZONE WITH GREATEST
DEMAND FOR COOLING
FULL
HEATING
UPPER
BOUNDARY
LOWER
BOUNDARY
40
15
HOT DECK
TEMPERATURE
(°C)
NO
HEATING
UPPER
BOUNDARY
LOWER
BOUNDARY
15
10
COLD DECK
TEMPERATURE
(°C)
FULL
COOLING
NO
COOLING
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