SENSIBLE AND LATENT HEAT TRANSFER
As heat is added or removed from a substance its temperature corresponding increases and decreases.
This holds true until the substance in question reaches a temperature where in cannot maintain it state
and it changes such as boiling and becoming a gas or freezing becoming a solid. As a substance
changes state it temperature remains constant.
The temperature at which the change of state occurs is both a property of the substance in question and
the temperature and pressure at which the substance is held.
The boiling point a substance increases with an increase in pressure and conversely decreases as
pressure is reduced. These two principles are fundamental to the operation of a refrigeration circuit.
OPERATION
The basis refrigeration circuit consists of a compressor, condenser, thermal expansion valve, evaporator
and receiver.
The compressor takes cool gas from the receiver and compresses the gas to become a hot high
pressure gas. The gas enters the condenser where it cools and condensers to become a liquid. As the
liquid condensers it gives up heat as it changes state. The liquid passes from the condenser to the
thermal expansion valve. As the liquid enters the Tx valve it undergoes a rapid pressure drop. The liquid
flashes off or boils. As the boiling liquid/gas enters the evaporator it absorbs heat in the process. The
gas, on leaving the condenser enters the receiver where any remaining liquid is removed before the gas
once again enters the compressor.
Because of the pressure difference between the two sides of the circuit the boiling point and
condensation point of the refrigerant are conversely different. The boiling point of the refrigerant is low
when the pressure is low as is the condition in the evaporator. The boiling point of refrigerants in these
conditions is as low as – 20 degrees. Once the resulting gas is compressed it boiling point and therefore
it condensation or dew point is also increased as is the condition in the condenser. As the hot gas cools
it again becomes a liquid. In the case of refrigerants the dew point is in the order of 15 degrees.
By changing the pressure of the refrigerant in various parts of the circuit, heat can be transferred from
one area to another.