6
The heat pump cycle
The Calorex heat pump provides thermodynamic heating by means of a vapour
compression cycle, (similar to that employed in a conventional refrigerator), in addition to
operating as an active solar collector.
Coefficient of Performance
The efficiency of a heat pump is usually called its ‘Coefficient of Performance’ – (C.O.P.) which is simply a ratio of heat output
to energy input, both being expressed in kW. Thus a heat pump absorbing 1 kW of electricity, collecting 4 kW of energy from
the air, and delivering 5 kW of heat to the water is said to have a C.O.P. of 5:1. Naturally this ratio will vary according to the
temperature of the ambient air.
COOL GAS
HOT GAS
HEAT
EXCHANGER
EVAPORATOR
COLD LIQUID
REFRIGERANT
HIGH PRESSURE SIDE
CONDENSED
REFRIGERANT
COMPRESSOR
EXPANSION
VALVE
AMBIENT
AIR
LOW PRESSURE SIDE
WATER OUT
WATER IN
THE HEAT PUMP CYCLE
The EVAPORATOR collects the heat from the air. This is either atmospheric
air, which has been heated by the sun, or internal air, which has been
heated by heat lost from another process. In Calorex heat pumps, high
volumes of air are drawn into the unit by the fan and passed across the
evaporator fins. The evaporator has liquid refrigerant passing through
it, which is at a considerably lower temperature than the ambient air.
Therefore the air gives up its heat to the refrigerant which then vaporises
to a gas. This pre-heated refrigerant gas then passes to:
The COMPRESSOR, where it is compressed,
which has the effect of upgrading the gas to a
much higher temperature. The electrical energy
that is used to drive the compressor is also
absorbed by the refrigerant gas. The hot gas
now enters:
The HEAT EXCHANGER (or CONDENSER), where it is surrounded
by the water to be heated in adjacent tubes. The refrigerant
vapour, at a higher temperature, gives up its heat to the water
which is returned to a storage tank. As the refrigerant cools it
returns back to its liquid state, i.e. it condenses, but remains under
high pressure from the compressor. The pressure is released by
passing the liquid refrigerant through:
The EXPANSION DEVICE, and from there, now at
normal pressure, it is returned to the evaporator and
the cycle starts again.
