Background information to accompany Air Conditioner/Refrigeration Cycle presentation.

1. Kelsey Radabaugh
Cameron Calhoun
Nathan Braun
Ralph El Sayah
Air Conditioner PowerPoint Notes:
Intro:
An air conditioner is essentially a reverse Carnot cycle. The parts of a typical air conditioning
unit usually form a closed system consisting of a compressor, a condenser, an expansion valve
and a evaporator. Motorized fans help to circulate the conditioned air, while thin metal fins allow
heat to dissipate quickly. Air conditioning units contain a special chemical called a refrigerant,
which has the unique ability to change from a gas to a liquid in a short amount of time. Contrary
to popular belief, air conditioning is not about adding cool air to the room, but more about
drawing heat away from it. The widow unit itself does not create heat, it just transfers heat. For
heat to transfer there has to be a temperature and pressure difference. In the refrigeration process
there are two sections which produce a pressure difference: a high-pressure, high temperature
section (the condenser) and a low-pressure, low temperature section (the evaporator).
Compressor:
Air conditioning begins with the refrigerant entering the compressor as a cool, low pressure gas
near room temperature. The gas is then compressed by the piston system and becomes very hot
under high pressure. The compressor does work on the gas creating entropy.
Evaporator:
This is the part of the process right after the expansion valve. The refrigerant runs into the
evaporator which acts as a heat exchanger and absorbs some of the heat from the area that the air
conditioner is trying to cool. By absorbing this heat it cools its surroundings. During this process
the refrigerant goes from a cool, low pressure liquid to a slightly warmer low pressure gas. The
refrigerant changes states by breaking the bonds between the compounds using the thermal
energy that is absorbed in the heat. This is why this part in the process is called the evaporator.
After the refrigerant has done this it returns to the compressor stage to start over again.
Expansion Valve:
The liquid then moves to the expansion valve under high pressure. This valve restricts the flow
of the fluid, and lowers its pressure as it leaves the expansion valve.
Condenser
The condenser, located on the outside of the AC unit, takes the hot gas from the compressor and
converts it into liquid by creating bonds and releasing thermal energy. The condenser which is
made up of groupings of long hollow coils filled with hot liquid allows heat to escape from the
refrigerant via multiple fin-like vents in the condenser’s casing. When the refrigerant reaches the
end of these coils, it is markedly cooler and in liquid form but still under high pressure.

2. Intro to calculations:
The first law of Thermodynamics explains that energy can be neither created nor destroyed, but
can be changed from one form to another. The following representation shows a basic
refrigeration system with typical values. Since no mass flow rate of the refrigerant has been
provided, the entire analysis is done in terms of specific energy values. In this example, using
refrigerant 134-a, we show the calculations and solutions for the Heat absorbed by the evaporator
(Qevap), the heat rejected by the condenser (Qcond), and the work done to drive the compressor
(Qcomp).
Step 1
Saturated vapor enters a compressor with a temperature -20 degrees Celsius. Starting with an
energy balance you find that the process is adiabatic therefore q=0 and there is no change in
kinetic or potential energy. This gives the equation that the negative work= the change in
enthalpy. H1 found in a table for saturated vapor at -20 degrees Celsius and h2 found in a table
for saturated vapor that has a pressure of 1MPa and a temperature of 70 degrees Celsius.
Therefore the work done by the compressor is equal to 65.5 KJ/Kg.
Step 2
The vapor leaves the compressor at 70 degrees Celsius and enters the condenser. Again use an
energy balance to solve for the heat dissipated by the condenser. This process is isometric
meaning no work is done and the potential and kinetic energy are negligible. You are then left
with heat equals the change in enthalpy. H2 has the same value as in step 1 and h3 is found in a
liquid table for a liquid that is at 30 degrees Celsius and has a pressure of 1MPa and a
temperature of 30 degrees Celsius. The heat dissipated by the condenser equals -210 KJ/Kg.
Step 3
The liquid leaves the condenser at 30C and enters the expansion valve. This is and adiabatic and
isometric process, therefore no heat or work is done. The change in enthalpy is equal to zero.
However, the temperature and pressure are lowered to -20C and 132.7KPa respectively.
Step 4
The last phase of the refrigeration cycle is the evaporator, which is basically the reverse process
of the condenser. So the heat absorbed is equal to h1-h4. The refrigerant returns to a saturated
vapor and the cycle begins again.
P-H diagram
This is a pressure vs. enthalpy diagram of an ideal air conditioning cycle for refrigerant R134-a,
which is a common refrigerant, found in air conditioners.