A/C Primer - Jeep Commander Forums: Jeep Commander Forum
Regular Service / Maintenance This section contains discussion about regular service and maintenance (upkeep) of your Jeep Commander

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post #1 of 2 (permalink) Old 10-21-2016, 09:14 AM Thread Starter
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A/C Primer

This is for those that have little to no knowledge of how an A/C system works.

The very first thing to keep in mind when dealing with any refrigeration system is safety. The boiling point of R134a is -15.34*F. Refrigerants stored in cans, jugs and cylinders have the potential to rupture if over heated. Other than the small cans, which I'm not sure about, all bulk refrigerant containers have "pop off" safeties of some kind. The safety is to minimize the rupturing of the container when it is over pressurized. It does not protect you from flying debris or liquid refrigerant. -15*F can cause blindness, freeze skin and cause other unpleasant damage to your body. Please always wear PPE (Personal Protective Equipment) when working on and around any refrigeration systems.

The first statement of the 2nd law of thermodynamics - heat flows spontaneously from a hot to a cold body.

A good analogy is comparing an A/C system to the engine cooling system. Heat from the engine transfers to the colder coolant fluid. The now hot coolant circulates to the radiator where the heat is transferred to the colder ambient air.

The water pump = the compressor
The engine block = the evaporator
The radiator = the condenser

How does a refrigeration system actually work? Lets start at the outlet of the compressor.

High pressure, high temperature gas (discharge gas) exits the compressor. The discharge gas travels to the condenser. As the refrigerant looses it heat to the colder ambient air it reverts back to a liquid BUT is still high pressure. Upon exiting the outlet of the condenser the liquid high pressure refrigerant travels thru a drier and receiver. The drier removes small amounts of debris and moisture that may be present in the system. When the system is properly charged, the receiver will "hold" an amount of refrigerant. This allows a constant flow of liquid high pressure refrigerant to flow to the next device in the line, the Expansion valve (TXV, TEV, metering device). The metering device, having an adjustable orifice, creates a pressure drop across it. This pressure drop causes the refrigerant to expand and turn into a low pressure gas/liquid mix. As the refrigerant travels thru the evaporator it "picks up" the heat from the cabin and expands to be completely gas. The low pressure gas travels back to the compressor, is compressed and the cycle starts all over again.

As you can see there are a lot of changes of state of the refrigerant and many variable that will affect it's ability to pick up and reject heat. Air flow across the condenser and evaporator, ambient outside and inside temperature, over or under charge, air or other contaminates in the system.

A few things to remember.
1. Refrigerant NEVER wears out. It gets contaminated
2. Air in a system will cause the refrigerant oil to break down. This break down creates acids and sludge to form. The metering device is a very, very small orifice and can easily get clogged.
3. Air in the system will usually end up trapped in the condenser and effectively make the condenser SMALLER!
4.Good refrigeration practices and procedures go a long way to keeping your A/C system working well after servicing.

I am a 25 year journeyman refrigeration tech. I work on equipment from your local Family Dollar to large supermarkets and industrial buildings.

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post #2 of 2 (permalink) Old 10-22-2016, 10:54 AM
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Very nice write up - The biggest point is it removes heat, it doesn't make cold air.

06 Commander Limited Hemi - QDII,
Past -89 Comanche, 98 Grand Cherokee (2WD), 2000 Cherokee, 05 Grand Cherokee Limited 4.7
Had to replace the brakes - they opened more gas stations in town
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