Heat Pumps; General; Operation - Honeywell AUTOMATIC CONTROL Engineering Manual

HEAT PUMPS

GENERAL

A heat pump is a refrigeration system that provides both
heating and cooling within the same unit. In the heating mode,
the pump delivers heat from a heat source to the conditioned
space. In the cooling mode, the pump removes heat from a
space and transfers it to a heat sink. Heat pumps use standard
refrigeration components (compressor, expansion valve,
evaporator, and condenser) and a reversing valve to reverse
refrigerant flow through the coils. A refrigerant reversing valve
switches at changeover to convert the condenser to an evaporator
and vice versa.
Figure 36 shows heat pump cycles. In the cooling cycle,
refrigerant flow uses the outdoor heat exchanger coil as the
condenser to reject heat from the space, and the indoor coil as
the evaporator. In the heating cycle, flow of air and refrigerant
is reversed making the outdoor coil the source of heat. The
indoor coil becomes the condenser and provides heat for the
space. When outdoor air temperatures are too cold to provide
enough heat transfer, electric resistance heating elements can
be used to provide supplemental heat. An alternative method is
to reverse indoor and outdoor airflow across the coils and
eliminate the reversing valve.
COMPRESSOR
INDOOR
COIL
EXPANSION
VALVE FOR
COOLING
COOLING CYCLE
COMPRESSOR
INDOOR
COIL
HEATING CYCLE
Fig. 36. Refrigerant Flow in Heat Pump
Cooling and Heating Cycles.
REFRIGERANT
REVERSING
VALVE
OUTDOOR
COIL
CHECK
VALVE
REFRIGERANT
REVERSING
VALVE
OUTDOOR
COIL
EXPANSION
VALVE FOR
HEATING
CHECK
VALVE
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INDIVIDUAL ROOM CONTROL APPLICATIONS
Heat pumps are typically classified by the heat source at the
"outdoor" coil. The common air-to-air heat pump uses outdoor
air as its heat source during the heating cycle. A water-to-air
heat pump uses water as the heat source during the heating
cycle. The water supply may be a well or a lake. In the cooling
mode, the outdoor coil rejects heat and the air or water becomes
the heat sink.
In commercial applications, a closed-loop or runaround water
supply may serve multiple units (Fig. 37). This system relies on
load diversification. Some units may be cooling while other zones
operate in the heating cycle. In this case, the water loop is a source
to heating units and a sink to cooling units, transferring heat from
one to the other. Water-loop temperatures are maintained between
70 and 90F to provide an adequate heat source or heat sink. A
central boiler together with a chiller and/or cooling tower temper
the water in the loop during peak heating and cooling periods.
For a discussion of central regulation and control of water pump
hydronic loops, refer to Chiller, Boiler, and Distribution System
Control Applications section.
HEAT
PUMP
CHILLER OR
COOLING
TOWER
BOILER
Fig. 37. Heat Pump Closed-Loop System.

OPERATION

Small and medium-sized heat pumps usually heat and cool
indoor air and are controlled from a space thermostat. Large
heat pumps usually provide warm and chilled water and are
controlled by a chilled water temperature control.
Space thermostats are two-position and usually multistage.
They typically provide automatic changeover, switching the
heat pump between heating and cooling as required to maintain
space temperature. The first stage of the thermostat cycles the
compressor. If the system is in a heating mode, additional
thermostat heating stages will bring on supplementary resistance
heat if the heat pump cannot meet the load. In the cooling mode,
there is only one stage that cycles the compressor. If desired,
changeover between heating and cooling can be manual rather
than automatic. The thermostat can have separate heating and
cooling settings or a single setting with a fixed deadband
between heating and cooling.
425 ENGINEERING MANULA OF AUTOMATIC CONTROL
THERMOSTAT
PUMP
C3022