29-12-2012, 06:15 PM
THE VAPOUR COMPRESSION CYCLE
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INTRODUCTION
The basic vapour compression or mechanical refrigeration
cycle involves the circulation of refrigerant, which in the
process of boiling (evaporating) absorbs large amounts of
heat and gives up heat when condensing. This heat which
must be gained or lost during the change of state is called
latent heat of vaporisation. It is in general more than the
specific heat, that is the heat lost or gained during a one degree
change in temperature.
There are in principal, only two pressures within the refrigeration
system, which are relative to the required evaporating
and condensing temperatures but vary depending on the
refrigerant used, the temperature of the space to be cooled
and the type of application.
An example of a basic vapour compression circuit for an air
conditioning application is shown above.
1. The refrigeration cycle start with high pressure liquid (eg 9
BAR) in the liquid line passing through a restrictor device,
which typically could be a capilliary line or an expansion
valve. Here it is allowed to expand and its flow is regulated.
2. Next it passes into the evaporator where with its pressure
greatly reduced eg 1.5 BAR, it will be boiling at –5°. But
in order for it to boil it must absort heat, which it gets from
the metal of the coils which in turn absorbs the heat from
the room space which, in this example might be around 5°C.
Saturated Vapour
A vapour which has an element of its liquid form contained
in it and is sometimes described as “wet”.
Sensible Heat
Sensible heat is where a body or substance has become
hotter or colder to the extent that it can be perceived by
the senses (ie to the touch), to apreciate if it feels hotter or
colder or by sight by observing the change in degrees on a
thermometer. Sub cooling of a liquid refrigerant is the result
of a reduction of sensible heat.
Latent Heat
Latent heat is the heat that has been absorbed by or given up
from a substance where there is no apparent change in
temperature. This happens when liquids freeze and boil. For
example when water freezes it gives up heat but the thermometer
stops at 0°C and when it boils the thermometer stops
at 100°C at atmospheric pressure. Latent heat is absorbed
in the evaporator and dissipated in the condenser with no
difference in temperature thoughout the majority of either.
Condensing Temperature
Is the temperature at which vapour turns into a liquid at a
given pressure.
Boiling Point
Is the temperature at which a liquid turns into a vapour at
atmospheric pressure.
Enthalpy
Enthalpy is a measurement of the amount of energy,
measured in joules (J) or kilojoules (kJ) contained in 1kg of a
substance, and is determined by the temperature and
pressure of that substance, calculated from a base reference
temperature of 0°C for water and –40°C for refrigerants.
Pressure-Enthalpy Diagram
There is a pressure-enthalpy diagram for each refrigerant.
This diagram shows the amount of heat contained in one unit
of weight of the refrigerant in its saturated liquid state and
also its vapour state at different pressures, their
corresponding temperatures, in a scale called enthalpy. In the
next issue we will explain how to use pressure-enthalpy
diagrams to measure the efficiency of a refrigeration system