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The figure shows a typical vapor compression refrigeration cycle or the
basic refrigeration cycle. Whether the unit is built to produce four tons of cooling or 500 tons, all
of the fundamental components in the system and their function are the same. The term
“ton of cooling” is based historically upon the cooling effect one ton of melting ice could produce in 24 hours.
Before mechanical refrigeration could be produced conveniently and inexpensively locally, ice was delivered
regularly and used to produce the cooling effect. One ton of refrigeration is equivalent to 12,000BTU/hr or 3517W.
That is, this is the energy required to melt one ton (2000lbs) of ice at 32oF (0oC) in 24 hours.
The fluid in the system, referred to as the “working fluid”, continually
circulates through the system,
changing states as is needed to perform the various tasks in the cycle. These changes will be discussed
shortly. The working fluid can be natural occurring substances such as ammonia, carbon dioxide or methane,
man-made materials such as fluorocarbons, or distillates such as ethane, propane or butane. At one time
man-made chlorofluorocarbons (CFCs) were used in many refrigeration systems. However, their ozone depleting
effects in the atmosphere has caused them to be phased out over the next several years. Ozone is a natural
occurring gas in the upper atmosphere that protects objects and beings at the earth’s surface from harmful
ultra violet (UV) rays from the sun. For example, UV rays can cause skin cancer in humans. All of your
Hotchkiss Refrigeration technicians are specially trained and certified to properly handle and dispose of
these damaging CFCs.
Closed refrigeration systems like that shown in the figure is referred
to as a thermodynamic "cycle". This is called a cycle because the working fluid continually
circulates through the system returning to its original condition. That is as a liquid,
vapor or gas, and the thermodynamic properties such as temperature, pressure, etc., in
the approximate same location in each piece of process equipment for a given cooling load (e.g., in
the condenser or evaporator, or in the connecting piping). However, in the evaporator where the
actual cooling takes place, there may be more or less of the working fluid as a vapor
(or liquid), for example, depending upon the cooling load. The working fluid in the rest of the
system then compensates for the load in the evaporator. What takes place in each part of
the cycle will now be discussed.
Since this is a thermodynamic cycle, we may start our discussion at
any cycle location, and continue describing what takes place at each numbered location shown in the figure.
For convenience we will start our discussion at location no. 4. This is where the refrigerant or working
fluid enters the compressor as a low pressure gas or a vapor (the technical distinction between a gas and
a vapor is not needed to understand the concepts here). It is critical to the operation of the compressor
that there is no liquid entering the compressor and all our technicians know and understand how to handle
this situation.
As the compressor increases the pressure of the working fluid, the
temperature increases as well. Based upon the several working and load parameters set by the system and
the type of refrigerant used, the pressure and temperature of the working fluid at the discharge side
of the compressor are set. Hotchkiss Refrigeration technicians can perform diagnostics at the compressor
discharge to establish many of the system parameters and determine how well your refrigeration system is
performing. The working fluid leaves the compressor as a high pressure gas and enters the condenser
located at no. 5.
In the condenser, the working fluid changes state from a vapor into a
liquid. Heat is removed from the hot vapor or gas until the vapor condenses to a liquid. This is the heat
that is given off from the system, for example, when you walk past a window air conditioning unit on the
outside and you can feel warm or hot air blowing off the unit. The removal of this heat (by the motion of
the outdoor air over the coils of the condenser to produce the hot air) causes the refrigerant to change
from a vapor to a liquid. By measuring air temperatures coming off the condenser coil, your Hotchkiss
Refrigeration technician can determine the state of the system in the condenser.
From the condenser the working fluid in this system enters a receiver
tank which adds capacitance to the system or a place to store excess working fluid. Not all systems have
such a device but your Hotchkiss Refrigeration specialist will know of its existence or not and the purpose
for having it in the system, as well as the liquid line valve.
Enough heat has been removed from the refrigerant to condense it into
the liquid state, but the fluid is still hot, not much heat is removed to actually cool the liquid further
from the condensing temperature in the condenser. Therefore, in this system, the hot liquid is used in the
heat exchanger at no. 8 to warm the vapor that is returning to the compressor (through the accumulator
shown). This heat is added to ensure there is no liquid in the vapor returning to the compressor. The US
Department of Energy has shown that cooling the condenser liquid discharge (called sub cooling,
where the refrigerant is now a sub cooled liquid) can also improve cooling capacity and reduce energy
consumption. Hence the heat exchanger is located at position no. 8 in the cycle.
The working fluid now enters the strainer/drier located at position
no. 9 as shown. Here the refrigerant has removed from circulation any dirt or debris that may have entered
the system during the last servicing. Although all precautions are taken to prevent contamination when the
system must be serviced, it is a best preventative measure to use the strainer/drier in the line to maintain
the integrity of the working fluid. Hotchkiss Refrigeration technicians are well aware of the havoc such
contamination can cause for the system and are extremely diligent in preventing such occurrences.
The high pressure liquid refrigerant enters the expansion valve
(no. 10) where the pressure of the fluid drops significantly, with a corresponding drop in temperature.
This temperature drop with the accompanying pressure drop is a well known and understood physical
phenomenon. For example, when spraying the contents from a pressurized spraying can (filled with hair spray,
deodorant or air for cleaning dust from computer keyboards), as the material sprays from the can, the outside
of the can becomes cool to the touch. On particularly humid days with long spray times, atmospheric moisture
will actually start to condense on the outer surface of the spray can. This is the same physical phenomenon
that causes refrigeration systems to function – the working fluid changes suddenly from a high pressure
liquid to a low pressure liquid, with the corresponding temperature drop. Now the cooler fluid can be used
to cool other fluids or objects.
The refrigerant now enters the evaporator coil, no. 1, which is a
heat exchanger with the working fluid on the inside and the medium to be cooled on the outside, typically
air or water. The cooler refrigerant thus absorbs (removes) heat from the air, cooling it.
Thus the working fluid arrives as a vapor at location no. 2, the piping
just outside the evaporator. Note that the expansion valve has a “valve sensor bulb” on the line at this
location. This is a feedback based upon the temperature of the working fluid leaving the evaporator. As the
temperature of the exit line increases, the expansion valve will open further, allowing more refrigerant
into the evaporator, increasing the cooling effect.
The working fluid continues in the piping to the heat exchanger at no.8
then enters the accumulator at location no. 3. The accumulator is another place to allow liquid refrigerant
to be removed from the working fluid prior to entering the compressor where only vapor can be compressed.
Liquid refrigerant entering the compressor can cause severe damage to the compressor's mechanical components
and internal valves. The Hotchkiss Refrigeration technicians can assure this will not be a problem when
installing new equipment or retrofitting older equipment.
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