The preferred secondary refrigerant is water, if this can possibly satisfy the temperature requirement, i. e. if the load temperature is sufficiently above 0°C that water can be circulated without the risk of freezing.
The greatest demand for chilled water is in air-conditioning systems. For this duty, water is required at a temperature usually not lower than 5°C and, for this purpose, the evaporator operates with refrigerant temperatures close to freezing point. A very wide range of factory-built package chillers is available with either water or air-cooled condensers (see next chapter). Other types may have remote air-cooled or evaporative condensers, and so require refrigerant pipe connections on site to these condensers. Sizes range from 14 kW to 35 000kW, most installations being within the range 100-1500kW.
At water temperatures close to freezing, and with evaporators which are vulnerable to ice damage, it is important to have adequate safety controls, to check the calibration of these frequently and to avoid interference by unauthorized persons. Nearly all troubles from packaged water chillers arise from a failure of safety controls. Several types of controls are in use, frequently three or more on the same equipment, but there should never be less than two:
1. Water flow switch, to stop the machine if flow stops in the chilled water circuit
2. Refrigerant low-pressure cut-out to protect against loss of charge
3. Water outlet low-temperature cut-out
4. Evaporator pressure regulation valve
5. Hot gas bypass valve, to maintain the evaporating temperature above freezing point, not for normal capacity control.
Most packaged water chillers are large enough to have capacity control devices in the compressor. The main control thermostat will unload the compressor as the water temperature approaches a lower safe limit, so as to keep the water as cold as possible without the risk of freeze damage.
In all but the smallest installations, two or more chillers are used, or one chiller with two separate circuits. This arrangement gives some continuity of the service if one machine is off-line for maintenance or, another reason, gives better control and provides economy of running when loads are light.
If water is required below 5°C, the approach to freezing point brings considerable danger of ice formation and possible damage to the evaporator. Some closed systems are in use and have either oversize heat exchange surfaces or high-efficiency-type surfaces. In both of these, the object is to improve heat transfer so that the surface in contact with the water will never be cold enough to cause ice layers to accumulate.
Posted in Refrigeration and Air Conditioning