4.0 OPERATION
4.1 SYSTEM CAPACITY
An accurate load calculation is essential because selection of units is based on the required
total and sensible load at the specified conditions (i.e. Water temperature off condenser,
DB/WB F air on evaporator coil). Undersizing the unit might cause premature failure (Due to
overloading of compressor) and the room design condition will not be attained. Excessively
oversizing the unit will waste in terms of first and operating cost.
For a single compressor unit, capacity unloading is by on/off operation of the compressor in
response to a single stage thermostat.
For multiple compressors units, capacity control is by staging of multiple compressors
operation in response to a multi-stage thermostat (Maximum 4 steps unloading).
Supply air volume through the evaporator must be selected within the airflow limits for proper
operation. Too low air volume can cause the low pressure cut-out to trip at 35 psig, cause
freeze-up of evaporator coil, liquid slugging into compressor and thus causing damage to the
compressor.
In general, saturated suction pressure should not be below 58 psig (Equivalent to 32°F
saturated suction temperature). Operating the unit with excessively high airflow can create
noise and vibration, and cause moisture carry over. (Low-pressure trip may also indicate
system refrigerant leakage and lack of refrigerant charge). Consult factory if the selected unit
has to operate outside the airflow limitation.
4.2 SYSTEM WATER FLOW
One or more pumps must be selected to meet the required water flow rate and pressure loss in
the condenser water circuit. Low-pressure drop indicates insufficient water flow rate while too
high-pressure drop indicates the opposite. The condenser is designed for a fouling factor of
0.001. Excessive fouling and scaling in the condenser circuit can cause the system to operate
on high head pressure thus loss of cooling capacity, high power consumption and eventually
high pressure cutout. Too high water flow rate, on the other hand will cause excessive erosion
on the condenser inner water tubes.
4.3 TEMPERATURE CONTROL
Temperature control is via thermostat which should,
1.) be located about 5 ft. above the floor.
2.) be exposed to normal room air circulation.
3.) not be mounted on outside wall.
4.) not be exposed to radiant effects.
5.) be enclosed in protective cage and locked when exposed to tempering or damage.
