To further reduce the system energy requirements, the following design considerations should be incorporated into any heat recovery system.

System Design Considerations

Heating Water Temperatures and Control — It is always desirable to use as low a heating water temperature as the application allows. Experience has shown that a design heating water temperature of 105 to 110 F can satisfy most heating requirements. Lower heating water temperatures increase the chiller operating efficiency both in the heating mode and in the cooling mode. In general, the heat recovery power consumption will increase 7 to 14 percent for every 10 F increase in the design heating water temperature. A consideration which is just as important as the design heating water temperature is how that temperature is controlled. In most cases, the heating water temperature control should be designed to maintain the return heating water temperature. By allowing the supply water temperature to float, the mean water temperature in the system drops as the chiller load decreases and less heat is rejected to the condenser. As the mean heating water temperature drops, so does the refrigerant condensing temperature and pressure difference which the compressor is required to produce at part load. This increases the unloading range of the compressor.

When the supply heating water temperature to the building system is maintained and the return heating water temperature to the condenser is allowed to float, the mean heating water temperature actually rises as the chiller load decreases and less heat is rejected to the condenser. As Figure O-8 illustrates, when the compressor unloads, the pressure difference that it must oppose to prevent surging remains essentially the same, while the compressor’s capability to handle the pressure difference decreases. Therefore, the unit’s capability to unload without the use of hot gas bypass is reduced.

Hot gas bypass artificially increases the load on the compressor (cfm of refrigerant gas) by diverting refrigerant gas from the condenser back to the compressor. Although hot gas bypass increases the unit’s power consumption by forcing the compressor to pump more refrigerant gas, it will increase the heat available to recover for those applications where significant heating loads remain as the cooling load decreases.

Figure O-8 — Refrigerant Pressure Difference

Figure O-9 — Heating Water Control Figure O-10 —

Chilled Water Reset

Chilled Water Reset — Chilled water reset is often a practical means of reducing energy consumption during periods of the year when heating loads are high but cooling loads are reduced. Resetting the chilled water temperature increases the evaporator refrigerant pressure. This increased evaporator pressure reduces the pressure differential the compressor must generate while in the heat recovery mode. A secondary benefit of chilled water reset is that it enables the chiller to produce higher heating water temperature than would normally be possible.