Handbook of air conditioning and refrigeration / Shan K

28.30 CHAPTER TWENTY-EIGHT
Controls
Water-Loop Temperature Control. In a WSHP system with an open-circuit cooling tower and a
plate-and-frame heat-exchanger, the water temperature leaving the plate-and-frame heat exchanger
is controlled between 60 and 90°F (15.6 and 32.2°C) when the WSHP system is in operation. For a
typical WSHP system, a temperature sensor T1 may be located at the exit of the plate-and-frame
heat exchanger, as shown in Fig. 28.7. When the microprocessor-based DDC controller receives a
signal from T1, it actuates the following:
At 57°F (13.9°C)
At 60 to 68°F (15.6 to 20.0°C)
At 84°F (28.9°C)
At 86°F (30.0°C)
At 88°F (31.1°C)
At 90°F (32.2°C)
At 105°F
Alert by alarm. WSHP, cooling tower, water
heater, and pumps all shut down. Start the water-loop
water pumps.
Energize the water heater.
Start the cooling tower water circulating pumps.
Open the damper of the cooling tower.
Turn on the first-stage tower fans.
Turn on the second-stage tower fans.
Alert by alarm. System will shut down.
An outdoor air sensor T3 resets the water heater energizing temperature T2 in such a manner that
when the outdoor temperature T o drops from 60 to 0°F (15.6 to �17.8°C), T2 increases linearly
from 60 to 68°F (15.6 to 20.0°C).
To maintain space air at 75°F (23.9°C), 50 percent relative humidity, and a dew point of 55°F
(12.8°C) in summer, it is critical to maintain a minimum water-loop temperature T wl � 57°F
(13.9°C). If T wl � 57°F (13.9°C), condensate may form on the outer surface of uninsulated pipes in
the conditioned space and cause damage.
The actuating temperatures for turning off the second-stage and first-stage tower fans, shutting
off the cooling tower water pump, and closing the damper in the cooling tower as the water temperature
decreases should be successively 2°F (1.1°C) lower to prevent short cycling [e.g., turn off the
second-stage tower fans at 90°F (32.2°C), turn off the first-stage tower fans at 88°F (31.1°C)].
Capacity Control of Water Heater. If the water heater is a gas-fired hot water boiler or a hot water
heat exchanger, a temperature sensor T2 senses the water temperature leaving the water heater. The
heating capacity can be controlled by modulating the flow rate of gas or hot water by means of a
DDC controller. Because most hot water boilers contain a limited amount of water, if the temperature
sensor is located at the exit side of the boiler, the temperature response to modulation is fast.
Therefore, a broadband DDC controller should be used to prevent short cycling.
If an electric boiler is used, step control in appropriate stages can be performed through a sensor
located at the inlet to sense the temperature of entering water. Step control often causes temperature
fluctuation at the leaving side.
Safety Control. In addition to the high and low operating limit control of the water-loop temperature,
a flow switch or pressure differential sensor should be installed across the water-loop pump. In
case of pump failure, the entire system, including the water-source heat pumps, cooling tower, and
circulating pump, or water heater shuts down. After a delay of 10 to 15 s, the standby pump is energized.
At the same time, an alarm sounds with an indicating light. When water flow is resumed, the
cooling tower and circulating pumps, or the water heater and water-source heat pumps, are
restarted, with a delay between each stage to limit any sudden increase in starting current.
Water-Source Heat Pump Control. When the pushbutton is engaged, the electric circuit is energized
and the outdoor air damper is opened. When the zone temperature rises above the cooling
mode set point, the temperature sensor T4 (Fig. 28.7) calls for cooling. The DDC controller then