System Options — Heat Recovery

Use of the Heat Recovery CenTraVac^® can significantly reduce the energy operating costs of many buildings by using heat which normally would be rejected to the atmosphere. Typical uses for this heat are perimeter zone heating, reheat air conditioning systems and any hot water requirements. Any building with a simultaneous heating and cooling load is a potential candidate.

Most heating applications require water temperatures higher than the 85 F to 95 F typically sent to the cooling tower. Therefore, most heat recovery chillers are required to produce higher leaving condenser water temperatures, and thus will not duplicate the energy efficiencies of cooling-only machines. Figure O-7 illustrates the typical operating cycles of a cooling-only machine and a heat recovery machine. The most noticeable differences are:

1 The pressure differential provided by the compressor is much greater for the heat recovery cycle.

2 The amount of heat rejected from the heat recovery condenser is greater than that which would be rejected in cooling-only operation.

3 There is a decrease in the refrigeration effect. (RE) Higher condensing pressures increase the intermediate pressure in the economizer. Therefore, the liquid in the economizer has a higher enthalpy during the heat recovery mode than during standard chiller operation and the refrigeration effect is slightly decreased. Because of this decreased refrigeration effect, the compressor must pump more gas per ton of refrigeration.

The effect of this increased pressure differential and decreased refrigeration effect is a heat recovery machine which has a higher kW/ton energy consumption during heat recovery operation.

Typical catalog kW/ton for heat recovery machines operating in the heat recovery mode range from .64 to .84 kW/ton compared to a range of .61 to .79 for a cooling-only machine. Not only can there be an energy consumption penalty paid due to the inherent differences in operating cycles for heat recovery machines, but traditional machine design can add to that energy handicap. In the past, a heat recovery machine’s operating efficiency was normally penalized year- round by having the capability to produce high heating water temperatures. Impellers are selected to produce the maximum required refrigerant pressure difference between the evaporator and condenser, Figure O-8. Usually, that meant the impeller diameters were determined by the heat recovery operating conditions.

During cooling-only operation, the condensing pressures and temperatures are normally lower than during the heat recovery operation. So, in essence, the impeller diameters were oversized. This would result in a compressor efficiency during cooling- only season which was lower than if the impellers had been selected for a cooling-only application.

The multi-stage compressor and advanced impeller design on the CenTraVac^® chiller reduce this costly energy penalty. Neither the capacity nor the power consumption changes substantially as the heat recovery operating conditions divert from the cooling-only condition. The multi-stage compressor allows a closer match of impeller size to the operating condition. In addition, the computer designed impellers and crossover are designed to reduce losses as the kinetic energy of the refrigerant gas is converted to static pressure.

These advances make the Trane Heat Recovery CenTraVac^® chillers even more attractive now than in the past.

• The CenTraVac heat recovery chiller was designed for efficient operation with kW/ton efficiencies among the best in the industry for heat recovery chillers.
• The energy penalty paid in the past to operate a heat recovery machine in the cooling-only mode is essentially eliminated.