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| 4 Basic Parts of Vapor Compression Refrigeration System |
Vapor compression is just one task done by the mechanical parts to complete the heat extraction process. Heat absorption has nothing to do with it, but the exact opposite. To successfully absorb and remove heat, there are 4 major processes that should occur in the system. This could be expressed in terms of what happen on the refrigerant, and what mechanical processes that causes it. To better understand this concept, let us study the processes that occur in the system as the refrigerant circulates.
Compression or Decreasing the Volume of Vapor
When the heat absorbing agent called refrigerant is injected
into the system, the first process that it undergoes is compression. This happens inside the compressor when
refrigerant that must be in vapor form is being induced by the suction pressure
of the compressor. As vapor refrigerant enters into the compression chamber, it
is about to be subjected to sudden decrease in volume (compression action). This occurs when the
piston moves up and compresses the vapor within the cylinder. Boyle's Law
This law introduces the relationship of pressure to the volume of fluid. It states that the pressure of vapor changes if the
volume also changes at constant temperature. This is the perfect description of
the condition of vapor refrigerant inside the compression chamber before piston
moves upward to accomplish compression stroke (in the case of reciprocating
compressors). According to this law, change in pressure is inversely proportional to the change in volume or the space of a container to which the gas is being held. When the volume increases, the pressure decreases. So, to increase the pressure
of that vapor refrigerant, volume must be reduced. Boyle's law is expressed in the following formula.
Gay Lussacs's Law
This gas law tells us the relationship between pressure and temperature if the volume of gas is held constant. It says that, temperature and pressure of vapor or any fluid are directly proportional to each other at constant volume. This is based from the idea that when we increase the amount of heat on a fluid, its molecular activity increases. The increase in molecular movement can be expressed as a pressure of fluid. Gay Lussac's law can be expressed in the following formula.
Vapor compression is based on the idea that has been brought by these two gas laws. The ultimate purpose is to release the heat that vapor holds so that vapor will liquefy or condense and can be used again. So, to increase the temperature of fluid without burning anything or
without producing a flame, it can be done by increasing the pressure or
reducing the cylinder of fluid.
Condensation or Heat Extraction
Condensation is just an after effect of compression. In
short, if compression is not properly done due to mechanical failure,
condensation will never take place. When temperature of compressed vapor is not
high enough against the ambient temperature, heat will not move effectively
from the compressed vapor. Condensation is a result of increasing the temperature of vapor
refrigerant to create temperature difference from the air surrounding the
condenser coil.
As heat transfer occurs and leaves the vapor refrigerant
within the condenser tube, vapor turns into liquid at high temperature. With this idea, it is important to note the
fact that, when refrigerant is in liquid state, it means it does not hold heat
in that body. This is the reason why heat
can be felt when touching condenser coil of any refrigerating equipment. The heat that we can sense at the condenser is the proof that heat is being rejected outside in that specific point of the system. From there, liquid refrigerant continues
to move to the next stage.
Expansion or Increasing the Volume of Liquid
Heat absorption that creates cooling effect is due to the
expansion of liquid refrigerant. If compression
is a process if decreasing the volume of vapor to increase its pressure and
temperature, to create a temperature different between the vapor refrigerant
and the surrounding air, expansion is the perfect reverse of the process.
Expansion is a method of increasing the volume of metered liquid
refrigerant, to decrease its pressure and temperature. This is to create
temperature difference between the surrounding air (outside the evaporator
tube) against the partially vapor refrigerant inside. Again,
from the second principle of thermodynamics mentioned above, heat always
travels from warmer body to colder body. In this particular spot in the system,
the colder body is the refrigerant that has been subjected to low
pressure.
As being said, pressure and temperature are directly proportional
to each other, and in inverse proportion to the change in volume. When
increasing the volume (expansion), pressure of the liquid refrigerant decreases
as well as the temperature. This makes
the heat transfers from outside surface of the tube into the refrigerant, which
is partially in vapor state. When heat leaves a certain space, it produces
cooling effect.
This makes the refrigerator cabinet becomes cooled. The point is, the heat that must be removed must be properly isolated and no external heat leakage should occur.
Evaporation or Absorption and Handling of Heat
If condensation of vapor refrigerant is an after effect of
compression, similarly, evaporation of liquid is an after effect of expansion.
By subjecting liquid refrigerant at high temperature into sudden increase in
volume, its pressure and temperature decreases. As the temperature of refrigerant decreases
lower than the ambient temperature, heat moves from warmer body (air outside) to
the colder body (refrigerant inside the tube).
As heat penetrates into the inner part of the evaporator to find that
colder body inside, partially liquid refrigerant at low pressure and
temperature completely evaporates. With this in mind, it is important to note
that, when refrigerant is in vapor state, it means it holds a certain amount of
heat. Just like what happen when water evaporates into the atmosphere when
heated up to 100 degrees Celsius.
One thing that must be understood here is that, heat moves
into the refrigerant body because refrigerant itself is a substance that is
highly reactive to heat. It evaporates
at negative temperatures lower than freezing point of water. This makes the liquid refrigerant evaporates or
boil even at lower temperatures or even when cold is already produced sensed by
our bare hands. This characteristic of refrigerant leaves no heat behind from
its surrounding area.
When liquid refrigerant completely turns into vapor, it then
moves into the suction line of the compressor at low pressure and temperature. Another
factor that makes the vapor refrigerant stays in low pressure aside from the expansion
of liquid is the suction pressure by the compressor. At this stage, heat that has been absorbed is being properly
handled by the refrigerant. It then transported into the compressor to be extracted
outside when it reaches condenser,
where heat can do no harm. After the condensation process the liquid refrigerant is now again ready for another cycle.
To know the specific spots of the refrigeration system where
this processes took place, it is necessary to mention again the 4 parts of
mechanical refrigeration system.
Compressor –
Where compression or sudden decrease in volume of vapor occurs to increase its pressure and
temperature.
Condenser – Where
heat leaves the vapor body of refrigerant and move into the outside air due to
mandatory heat transfer when there is a temperature difference between 2
bodies. Heat transfer is also aided by used of forced air or induced air by the
used of fans and blowers. On larger units heat transfer is sometimes aided with
water.
Expansion Valve –Where
expansion or sudden increase in volume of metered liquid refrigerant occurs.
Evaporator –
Where heat absorption occurs and where cooling effect is produced. In actual
unit, since refrigeration system is nothing but just a loop of tubes separated
by the compressor and the expansion valve, expansion process occurs at the
inlet of evaporator. It is the point in the system where refrigeration cycle
starts, where heat absorption started to transpire.
