Trane ctv-prc007-en manual Heat Recovery, Cont, Simultaneous Heating and Cooling

System	Heat Recovery
Options	(Cont.)

Figure O-7 — Typical Operating Cycles

Simultaneous Heating and Cooling

The Trane Heat Recovery CenTraVac™ chiller is an excellent choice for applications requiring simultaneous heating and cooling. CenTraVac models save energy by recovering heat normally rejected to the atmosphere and putting that energy to use providing space heating, building hot water or process hot water. This heat is provided at a fraction of conventional heating systems cost. A heat recovery CenTraVac can provide 95 to 120°F hot water.

An advanced computer selection program chooses a heat recovery condenser to match your needs. Two separate condenser shells are used with the Heat Recovery CenTraVac chiller. The heating circuit and cooling tower circuit are separate, preventing cross

contamination. Refrigerant gas from the compressor flows into both condenser shells allowing heat rejection to one or both condenser water circuits.

The reliability of the Heat Recovery CenTraVac chiller has been proven in installations around the world. This option is completely factory packaged.

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

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