Energy Efficiency: The energy efficiency of any process cooling system depends on the system’s heat balance. The exact amount of heat generated by the process has to be removed by the chiller. If too much or too little heat is removed, production may drop and quality may be affected or, alternatively, too much energy may be consumed for a given amount of output.

Applied Energy and its partners can measure and compare existing performance of refrigeration systems against theoretical benchmarks. Differences between them can be analyzed and changes made to improve performance. If necessary, modifications to the system design can be made to achieve optimum energy efficiency relative to production requirements.

Heat Balance: The main issue in process cooling is heat balance and there are 5 key areas that can be adjusted for optimum heat balance and control of refrigeration capacity:

• Compressor cycling
• Multiple compressors
• Cylinder unloading
• Gas bypass
• Condenser fan cycling & Water flow control

Compressor Cycling: Compressor cycling is one method for controlling the amount of cooling in a chiller. Most systems use a thermostat with a predetermined refrigeration temperature - the ‘set point’. When the amount of cooling reaches the set point, the thermostat stops the compressor from circulating refrigerant through the system, and while the water circulation pump continues pumping, the temperature of the process water rises. The thermostat detects this rise and turns the compressor back on.

The drawback of compressor cycling is the additional stress on the compressor motor windings when the compressor is started repeatedly. With the inrush current spiking up to 600-800% of its normal full load amps, compressor cycling over an extended period can cause premature failure unless a soft start is built-in to the system.

Multiple Compressors: There are some applications where refrigeration equipment failure could result in serious financial loss, e.g. frozen food, above and beyond the repair expense. In such cases, it is advisable to consider a multiple compressor chiller system.

For part-loads, one or more compressors may be started or stopped as required. This also provides redundancy should one of the compressors fail. Although the system operates at a lower capacity if a compressor fails, it should not be allowed to run in this condition for long periods of time to prevent potential damage to the other compressors.

Cylinder Unloading: Capacity can also be controlled through compressor cylinder unloading. A thermostat controls the solenoids (assuming the compressor has multiple cylinders) that force the discharge valve to stay open. Since the cylinder is now open to the discharge manifold, no refrigerant gas compression occurs. The result is a drop in refrigeration capacity in direct proportion to the number of cylinders being “unloaded.” The torque on the electric motor falls and results in lower power consumption. Cylinder unloading is the most desirable method of controlling since it balances a chiller’s refrigeration capacity with the process load and saves power consumption.

The cylinders typically unload from six to four and then two. This capacity reduction is then followed by gas bypass. For screw compressors, partial unloading can be accomplished because they have two stages.

Gas Bypass: As the compressor satisfies the process load, the chilled water from the evaporator begins to over-cool. A thermostat senses this drop in water temperature, and, at a preset temperature, the thermostat opens a solenoid hot gas bypass valve, allowing part of the refrigerant to bypass the condenser and the expansion valve.

Without heat removal by the refrigerant, the water in the chiller begins to warm up and, at a pre-established temperature, the thermostat closes the bypass valve when the chilled water reaches the set temperature. The hot gas bypass prevents the compressor from rapid cycling when the chiller operates under partial loads. This valve is particularly important when operating a semi-hermetic compressor since the compressor must receive a full amount of refrigerant for the motor windings to remain cool.

Condenser Fan Cycling and Water Flow Control: Condenser water flow control for water-cooled chillers, or condenser fan cycling for air-cooled chillers are other methods of capacity control. Both methods can only partly control a chiller’s capacity.

If the incoming air or water temperature drops seasonally, the colder water (or air) will increase a chiller’s capacity. If the process load remains constant during this change, throttling the condenser water flow or cycling the condenser fans will reduce the chiller’s capacity to a certain extent.