Why you should get in love with thermodynamics
Here comes the 4 part series you all have been waiting for. The ultimate on how to fix the air condition system in your already too hot car. Bored of driving sweating and always going around with the windows open? Now it’s the time, you have been letting this job off for far too long. Brace yourselves, global warming is coming!
I will cover a standard fix of AC system in four parts. Before even beginning you need to know the basics and the theory behind the technology, this will be covered here in part 1. If you are an expert on AC systems then just head over to part 2.
Part 2 will cover how to change the AC compressor. Part 3 covers the changing of the receiver/drier and the last and fourth part covers how you change the expansion valve. These three components are usually the ones you need to care about if your AC system is not working
Automotive AC Theory
Air conditions are based on the thermodynamic heat pump cycle and if you are really into thermodynamics you can dig into this topic really geeky and deep, but this is not the purpose here. The automotive AC works just pretty much like any refrigerator, this technology is old and proven, no rocket science really. A heat pump moves heat from one source at a lower temperature to another location with a higher temperature with using mechanical work.
Either used for heating or refrigerating the operational principle is the same, but here we are interested in the refrigerating part.
Mechanical work is needed to shift the heat to flow from a colder location to a warmer one since this is opposite to the natural heat cycle in the universe. The mechanical work in a car is made by the car engine.
The vapor-compression cycle is most common for automotive uses. Here it uses a recirculating liquid called a refrigerant that undergoes phase changes to absorb and remove the heat. Many of you have heard about Freon, but this is just a brand name for some types of refrigerant that was common in the olden days, now it is prohibited due to the ozone depletion of such CFC gases. Newer refrigerants do not deplete the ozone layer, but have high global warming potential, so it’s important that you don’t leak these refrigerants out into the atmosphere.
Vapor-compression cycles all have 4 key components: a compressor, a condenser, a thermal expansion valve and an evaporator. For most uses you will also find a fifth component which is the receiver/drier, this is always found in automotive AC.
Since the system is closed and recirculating it doesn’t really have a start, but lets start with the refrigerant entering the compressor. The compressor is the part that circulates the refrigerant and is driven by the engine through a tension belt. The refrigerant enters in vapor form at low pressure, but gets compressed by the compressor and it will form a high pressure vapor with higher temperature, known as superheated vapor. It then enters the condenser.
The condenser is a radiator where the superheated vapor runs through tubes where a fan or water cools down the vapor so it condenses into a liquid. Here the refrigerant removes away the heat to the flowing air or water. This is the part where the heat pump can be used as an oven, but in the car refrigeration is wanted and the condenser is just radiating heat into the atmosphere. The colder high pressure liquid is then routed to the receiver/drier as a saturated liquid.
It is located usually before the expansion valve in the high pressure part of the system. They serve three important functions:
- Temporary storage container for refrigerant when system is not in use.
- Filtering debris inside system.
- Removes moisture that can have gotten into the system which can create corrosion and destroy the compressor.
The liquid then goes to the expansion valve where the liquid can drop a lot in temperature.
The Expansion valve (metering device)
The saturated liquid goes through the expansion valve where the pressure suddenly drops, the sudden drop in pressure lowers the temperature of the now liquid and vapor mixture to a much colder temperature than the temperature of the space which need to be refrigerated, like the inside of a car. The cold mixture of refrigerant then is routed to the evaporator.
Inside the evaporator, which is also like a radiator where tubes crisscross over metal fins to expand the possible area of which heat can be absorbed. A fan blower pushes warm air inside the car through the evaporator where the heat gets absorbed by the cold refrigerant and cold air comes out on the other side. This you will feel as the cold air conditioned air coming out of the vents. The liquid inside the evaporator tubes gets, you guessed it; evaporated into a vapor form due to the heating and carrying this heat back to the compressor completing the cycle.
I hope this was informative and covered the basics, so now you have the understanding of your car’s not so complicated AC system.
Cheers, Robs out