W7100G DISCHARGE WATER TEMPERATURE CONTROL
63-4046—1
6
OPERATION
Chiller System Description (Fig. 5)
Chiller systems are used to provide cold water for many
types of end uses. The cold water may be piped to many air
handling units located throughout a building. These units then
provide cooling and ventilation for individual zones throughout
that building. Chillers are also used to supply cold fluid for a
variety of process applications. In either case, precise
discharge water temperature control is desired.
Fig. 5. Chiller system.
Discharge Water Temperature
Control Operation
The W7100G uses an integrating control band concept to
provide close, stable temperature control in discharge water
chiller systems. The control band concept matches required
operating capacity to chiller load, while the integral action
minimizes the offset from the control setpoint which normally
occurs in proportional-only type controllers.
The control band setting is centered on the discharge waters
setpoint (see Fig. 6). The control band setting is adjustable
from 0 to 10°F (0 to 6°C). This adjustment is used to stabilize
system operation.
Fig. 6. Discharge water control algorithm operation.
A description of how chiller control stages operate follows:
1.
Using Fig. 6 and beginning at point A, the discharge
water temperature is above the upper control band limit
and the minimum OFF time has elapsed (selectable
either 30 or 60 seconds, using fast response input).
2.
The W7100 first stage energizes.
3.
At point B, the discharge water temperature is still
above the upper control band limit.
4.
If the minimum time between stages has elapsed
(selectable), the next highest cooling stage energizes.
5.
At point C, the discharge water temperature has fallen
below the lower control band.
6.
If the minimum ON time has elapsed for that stage, the
highest stage that is on shuts off.
7.
This sequence continues until the cooling demand
increases (point D) at which time stages sequence ON
in the same manner as before.
8.
As a rule, any time the discharge water temperature:
a. is above the upper control band limit, a stage turns
on.
b. falls below the lower control band limit, a stage turns
off.
Reset of Discharge Water Temperature
Industrial Applications
Reset of discharge water temperature is not recommended in
a W7100G industrial application, because loads on industrial
applications typically require a constant temperature fluid for
cooling. Check industrial process manufacturers for specific
details.
Environmental Applications
In order to save additional energy under lighter loading
conditions, it may be desirable to change or reset the
discharge water temperature setpoint based on either space
or outside air temperatures. Reset has proved itself on hot
water boilers as an energy saving opportunity. These same
energy saving techniques can also be applied to chillers, to
more evenly match system capacity with system load.
Reset from Space Temperature
A chiller that is being used to chill water for single or multiple
air handling units can be made more efficient by resetting the
discharge water temperature up as the average space
temperature goes below a space temperature setpoint.
In order to accomplish this, space sensors (such as
T7047C1025) are strategically located in the building to
measure average space temperature. It is recommended that
at least 4 sensors be installed and wired to provide a signal
proportional to the average space temperature. The setpoint
of the reset potentiometer should be set at the lowest desired
cooling temperature of the building. See Fig. 7 for space reset
operation (algorithm). See Fig. 8 for wiring details.
As the average space temperature drops below the setpoint
of the reset sensors, the W7100G discharge temperature
setpoint is reset upward by an adjustable amount. Maximum
reset occurs when the average space temperature is 3°F
(1.7°C) below the setpoint of those sensors. At this time,
the W7100G setpoint will be fully reset.
In reset applications where high humidity can be a concern,
an outdoor air thermostat can be installed to override the reset
function. In lieu of outdoor air override an indoor space
humidistat can also be used. See Fig. 8 for wiring diagrams.
M17921
PUMP
RETURN
WATER
SUPPLY WATER
TO SYSTEM LOAD
REMOTE
SETPOINT
COMPRESSOR(S)
CHILLER
BARREL
W7100G
DISCHARGE
WATER CONTROL
ADJUSTABLE
CONTROL
BAND
0 TO 10 F
LOWER CONTROL BAND LIMIT
UPPER CONTROL BAND LIMIT
DISCHARGE WATER TEMPERATURE
W7100 SETPOINT
A
B
C
D
M17917
Summary of Contents for W7100
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