| Features
and
Benefits 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.
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