Catalogue

12 WSHP-PRC020D-EN
Application Considerations
Heat Rejection through a Closed Circuit CoolingTower
Cooling towers serve to reject heat from the condenser water loop to the atmosphere.Two types
of cooling towers are used with water-source heat pump systems: open or closed-circuit.The
towers themselves are different, but when an open tower is used in conjunction with a water-to-
water heat exchanger, the control of the two tower types is essentially the same.
Control for the closed-circuit cooling towers may be made with aTrane®Tracer™ Loop Controller
(TLC). With theTLC, up to four stages of cooling tower control are possible.
When the loop supply temperature is 4°F below the loop supply high setpoint, the first stage of
cooling is initiated by opening the closure dampers on the cooling tower.
At 2° F below the setpoint the next stage of cooling is initiated which is the starting of the tower’s
circulating pump. If the amount of heat rejected by the first two stages is not enough, the loop
temperature will continue to rise.When the temperature reaches the loop supply high setpoint, the
next stage of cooling is initiated.This is the first stage of cooling tower fans.
The differential between the stages now become 3°F and the temperature must remain above the
differential for three minutes. Up to three individual fan stages may be sequenced or the second
stage of fan can be the high speed of a multi-speed motor.
There are five different fan arrangements that theTLC supports: A single fan with a single motor,
a single fan with dual motors (pony motors), a maximum of three fans with a maximum of three
motors, a variable speed fan with a field supplied variable frequency drive, and a single multi-
speed motor.
Multiple cooling towers can be supported only if the cooling tower stages are controlled in parallel.
Boiler Operation
TheTLC will operate a boiler and the mixing valve respectively. Boiler control is traditionally
controlled by a separate boiler controller, provided by the boiler manufacturer.The boiler mixing
valve will control the mixture of the boiler water into the main loop to achieve the desired loop
supply water.
When the loop temperature falls below the low loop-supply setpoint, theTLC enables the boiler.
The ideal arrangement is for the boiler to have its own bypass loop so the boiler pump can circulate
water through the heat exchanger.The boiler will maintain the temperature of the water to the
desired setting in the packaged boiler control.
The three-way mixing valve is controlled by theTLC to add heat to the main loop by mixing in water
from the boiler loop. A proportional-integral-derivative algorithm controls the valve.The boiler is
not disable until the main loop temperature is 5°F greater than the low loop supply setpoint for
more than 5 minutes.
TheTLC will also monitor the boiler loop temperature and provide an alarm if the temperature is
below the boiler loop low limit after 30 minutes of run time.TheTLC will provide an alarm if the
boiler loop temperature exceeds the boiler loop high limit after 30 minutes continually.
Facilities Management
Water-source heat pump systems are naturally decentralized; thus they inherently provide
individual zone control.Typical installations use mechanical thermostats to provide localized
control. Central plant control is typically handled by a control panel located in the main mechanical
room. Minimal coordination is usually required between the central plant and the individual water-
source heat pumps for successful operation of the system. A direct digital control system is
recommended to help support coordination efforts between the central plant and the individual
water-source heat pumps.This enhanced coordination can result in reductions in operating cost
of the entire system.The following items are typical of the additional coordination: Night setback
and setup; After hour usage for tracking and billing; Pump cycling for occupied/unoccupied control;
Zone scheduling; Maintenance reporting for monitoring unit fault conditions;Trend logging of the
system water temperatures; Monitoring of system levels for items such as waterflow, temperature,
faults, heat rejector status, heat adder status and circulating pump status.