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Operation
The Multistack chiller provides chilled water to an external load, based on return water temperature to the Multistack Master Control. When the entering
chilled water temperature sensor sends a signal to the master controller that cooling is needed, compressors will start and begin to produce chilled water.
A compressor start is determined by the entering chilled water (ECHW) upper set point and the variable set point setting in the Master Control System
Variables menu. When the ECHW sensor senses that the chilled water temperature has dropped below the set point, compressors will begin to cycle off.
Chiller modules equipped with modulating butterfly valves on the chilled water side are controlled from the leaving chilled water set point (lower set point)
in the System Variables menu. The module slave board(s) send a 2-10 VDC output to each actuator to modulate the water valve to maintain lower set point.
When a module is not running, valves close to prevent bypassing water at the operating chiller modules.
For variable flow applications one or more valves may remain open at all times to provide minimum flow through the chiller. The valve that stays open will
always be the lead compressor for that day, eliminating the need for an external bypass installed at the chiller. If modulating butterfly valves are used on
the condenser side of each module, the valves will control to a discharge pressure setting selected from the Factory Setup menu. This is also a 2-10 VDC
output each module board sends to the valve actuator. If the condenser pump cycles off while no modules are running, minimum flow doesn’t need to be
considered. If the condenser pump runs at minimum flow, the condenser bypass should be enabled in the Factory Setup menu.
Sequence of Operation, Leaving Chilled Water Temperature Control
The Multistack chiller is equipped with a microprocessor based supply water temperature controller. The chiller operates in response to leaving chilled
water (LCHW) set points.
AUTO (Toggle switch on machine)
In AUTO mode, the compressors operate as needed to maintain the LCHW set point. Cooling temperature control with CONTROL TO: set to LEAVING is based
on: LCHW SET POINT
LCHW OFFSET LCHW STAGE OFF
1. The Leaving Chilled Water (LCHW) set point default is 45 degrees F with a 35 to 80 degree ranges. The controller maintains this value within __ +/- of
the LCHW Offset.
2. The LCHW Offset default is 2 degrees F with a range of 0.5 to 10 degrees. This is the value above and below the LCHW Set point that compressors are
staged on and off.
3. The LCHW Stage Off delay is defaulted to 30 seconds with a 5 to 180 second range. This allows for a delay between the shutting off of compressors in
Leaving Chilled Water control. If the system leaving temperature drops too far too quickly (2 degrees below where the previous compressor was to
shut off) the controls will allow one more compressor to shut off within that 30 seconds. That should be enough to back off the leaving temperature
so the over shoot condition doesn’t continue.
Modulating valves on the modules will control to the following in AUTO MODE:
The SYSTEM VARIABLE of CONTROL TO: Set to a value of LEAVING.
Evaporator valve will control to the LCHW SET POINT.
Variable Flow Chilled Water and/or Variable Flow Condenser Water
Motorized actuating valves for varying water flow through the evaporators and or condensers are factory installed on each heat exchanger. To accomplish
VFD chilled water and/or condenser water flow the system must control the chilled water pump and/or condenser water pump based on pressure drop
across the chiller. As a compressor starts a valve is opened and pressure drop decreases. The pump should then speed up to match the desired pressure
drop. Control of the pump is external to the chiller. Additionally, the chilled water loop must have a minimum flow bypass at the end of the loop (by others)
to maintain minimum flow through the chiller.
The valves on a Multistack chiller are dual purpose. First, a user defined number of valves can be programed to remain open at all times as a minimum
flow bypass through the chiller. This is selectable at the master controller or through the BAS with an optional web-portal. Second, the motorized valves
modulate to maintain leaving chilled water temperature control out of each heat exchanger. The bypass valve(s) will follow the lead compressor and
remain locked in a bypass position and rotate with the lead compressor. Remaining valves will open when corresponding compressors start. The compres-
sor will then modulate for leaving water temperature control. Valves close when corresponding compressors shut off. When there is no demand for cooling
all valves are closed with only the pre-selected number of bypass valves remaining open for a minimum flow chiller bypass.
If all evaporators and condensers are equipped with variable flow valves the master controller, in conjunction with each corresponding slave controller, will
modulate the valves for simultaneous leaving chilled water and condenser head pressure control from each heat exchanger (with the exception of the heat
exchanger(s) programmed as the minimum flow chiller bypass).
