| Features
and
Benefits System Options — Heat
Recovery
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.
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. |
