Air Conditioning

How does it work?

 
     
 
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Air Conditioning

 

Automotive Air Conditioning

 

 

How does it work? Where does the cold air come from? Modern refrigeration uses refrigerant to absorb heat and transfer heat and shed heat. Absorb, Transfer, Shed,- Absorb, Transfer Shed. Got it? It is a cycle of heat, or a heat cycle. The refrigerant absorbs heat from the interior of the vehicle in a component called the evaporator. Air is pushed (sometimes pulled) across the evaporator and since the evaporator is colder than the air coming across it, heat (from the air) is absorbed by the evaporator, and the resulting action is "cold" air being blown into the passenger compartment. However, this "cold" air is really just the same old air, but with the heat removed, or absorbed. Remember from science class-"heat moves from hot to cold". Now the refrigerant must move along to the next part of the A/C system, because the compressor is pulling it toward itself. Once inside the compressor, a tremendous amount of compression occurs, and the refrigerant travels at high pressure to the condenser. Remember all the heat we absorbed in the evaporator? The condenser is going to help us get rid of it. Since the compressor has increased our pressure, the result is an enormous amount of heat loss across the face of the condenser. As the refrigerant pushes on it's way out of the condenser, it has shed all the heat that was absorbed inside the vehicle. Still under high pressure, and with all it's heat removed, the refrigerant is now "hungry" for heat, so let's send it back inside to absorb more heat from inside the vehicle. This is accomplished simply by dropping the pressure with a small valve or fixed orifice. Once the pressure is reduced, the refrigerant enters the evaporator where it readily absorbs more heat. The heat cycle is complete. Of course there are more complexities involved with this heat cycle, such as the change of state that occurs in the refrigerant. It's bit larger than the scope of this article, as this is intended for the consumer, not the scientist.

 

These 5 Major Components are found on all modern A/C and refrigeration systems.

1) The Compressor is the hardest working piece of the A/C system. Rotated by the engine, it converts mechanical energy into refrigerant flow. The traditional design consists of pistons and valves. Others have vanes and some have scrolls. Considerable heat is generated by a compressor, and the cold refrigerant returning to it is vital to it's longevity. If an A/C system is not full to capacity, the compressor is not receiving enough cold refrigerant and oil, and overheating of moving parts and seals is occurring, and a shortened compressor life is inevitable.

In the compressor, the refrigerant is a GAS.

Without the compressor, we would have no way to move the heat from inside the vehicle, to outside. Just as the refrigerant absorbs the heat it must also shed the heat to complete the cycle. This is where the condenser comes in.

2) The Condenser receives the high pressure refrigerant from the compressor, and through a series of small tubes, allows the heat energy contained in the refrigerant to dissipate to the condenser, and onto the air as it rushes past. The condenser is strategically located to receive the coolest air possible (in front of the radiator, never behind). Air flow is also a very important concern, no air flow = no heat transfer.

In the condenser, the refrigerant is both a GAS and a LIQUID.

As the refrigerant enters the condenser, it is a very hot, high pressure gas. As it moves through the condenser, sufficient cooling is achieved to allow the hot gas to condense, turning it into a liquid by the time it has left the condenser. An enormous amount of heat is shed in this process, and is the real magic of modern refrigeration. As the liquid refrigerant leaves the condenser, it next goes to the expansion valve.

3) The Expansion Valve has one simple job. Drop the pressure. Pressures as high as 200-300 psi are quite common, but by the time the refrigerant sneaks through he valve, the pressure has dropped to 20-30 psi. This drastic pressure change allows the refrigerant to be very cold, and hungry for heat.

In the expansion valve the refrigerant is a LIQUID.

The expansion valve is located after the condenser, before the evaporator. Sometimes inside the vehicle, other times under the hood. The expansion valve readies the refrigerant for it's next task by atomizing the refrigerant to produce almost a fog so that maximum heat absorption can occur in the evaporator.

4) The Evaporator receives the cold, low pressure refrigerant from the expansion valve and allows it to flow from the "bottom-up". This enhances the heat absorption and allows evaporation to occur effortlessly. As the hot air from the passenger compartment is directed over the evaporator, there is enough heat (in the air) to boil the refrigerant. An enormous amount of heat energy is absorbed this way.

In the evaporator the refrigerant is a LIQUID and a GAS.

As the refrigerant leaves the evaporator, it has transformed almost completely to a gas, and heads off to the place it started- the compressor. Even though it has completed it's journey, there is one more important mission the refrigerant need to accomplish. On it's way to the compressor, the super cooled refrigerant brings cooling to the compressor, and brings cold lubrication with it. Without this cooling, the compressor would overheat and seize.

5) The Drier performs 3 important jobs. As the heat load for an A/C increases or decreases, the refrigerant needs to expand or contract. The receiver drier allows a little "head space" for this to occur. Similarly, when conditions are cool, the drier "gives back" so the system doesn't run out of refrigerant. Located inside the drier are fabric pouches that contain silicon beads that are able to absorb moisture and small levels of acid. The plumbing in the drier acts like a liquid/vapor separator to insure only liquid exits the drier.

The refrigerant in a receiver drier is a LIQUID.

The receiver drier is located after the condenser, and before the expansion valve. All expansion valve systems have receiver driers to help them. However, fixed orifice tube systems don't use a drier, instead they use an accumulator. The accumulator performs almost exactly the same functions as the drier, but does so after the evaporator (instead of before). The accumulator offers a better way to buffer the system for changing heat loads.

The refrigerant in the accumulator is a GAS.

The accumulators is several times larger than the drier, and many times the accumulator is hidden away in a fender, and may not be easily accessible.



The rest of the system is made up of hoses and pipes, wiring, switches, modules, motors, fans, doors, actuators and of course the refrigerant. All vehicles have most of these additional items, and they will vary widely from vehicle to vehicle.
In addition, many vehicles now have electronics that position blower levels, temperature, and door positions. These systems are called Automatic Temperature Control systems and can add an additional layer of complexity.
On large vehicles, SUV's and busses, dual evaporator systems are quite common, and have additional layers on controls and plumbing that must be check and repaired.
Airflow is critical to good performance of an HVAC (heating, ventilating, air conditioning) system. Without good air flow, heat transfer is disrupted.

Installation of air conditioning is possible in most newer cars and trucks. Factory A/C look is simulated, and in some cases factory components are used. Older muscle cars are also popular installation projects.

Contamination
Air des not belong in an A/C system, and will take up space required by the refrigerant. Additionally, the pressure characteristics of air and refrigerant are far different, and air will disrupt the performance drastically especially on hot days.

Moisture is the enemy of refrigeration. Chemically incompatible, allowing the formation of sulfuric acid. Nothing good can happen when sulfuric acid is circulation inside your vehicle at 250 psi. Nothing.

Refrigerant contamination almost always occurs by sloppy service habits from a previous A/C service. Completely avoidable, but adds an expensive layer of additional cost to an ailing system.

Debris from previous compressor failures is a common problem. Only a thorough system flush can restore the conditions that will allow a compressor to survive.

Black Death is a phenomena that describes the appearance of the debris found during cleanup. It is generally found when the compressor experiences low refrigerant and oil levels, and high heat at the same time. Teflon particle are melted and ground away from the compressor and pushed in the direction of refrigerant flow. As this debris moves to the condenser, the liquefied Teflon cools, hardens, and plugs the passages. As this cycle repeats itself, the result is utter compressor failure, and an expensive mess that requires additional component replacement.

Evaporator smells can be very intense. The evaporator offers an environment that is dark, moist, quiet, and has a supply of "food" being pumped into it. If mold or mildew gets a foothold, it can be difficult to get rid of. New products now emerging on the market have finally given some relief to this problem. Depending on the smell, sources for the problem can sometimes be located. Synthetic products can "brew-up" really nasty smells if they are leaking in the evaporator. Other times dead rodents are extracted from inside. Lots of smells. Lots of solutions.

The Inspection is a mandated (by State of California) list of items to check on an A/C system before any A/C work can be completed. We take it seriously. Others do not. This is the inspection form we use.

Should I Retrofit My A/C? Absolutely. But only when the time is right. When a compressor is replaced with a receiver drier or accumulator, the time for retrofit analysis has arrived. Some older cars may not exhibit good performance after retrofit. Others may need additional fans or other component upgrades. The newer the car, the better the retrofit will be. However, retrofitting an A/C on a car that has 80,000 miles on it, the original compressor, and a worn fan clutch means: 1) poor A/C performance 2) compressor failure within a year. I've seen it over and over. I DO NOT RECOMMEND RETROFITTING AN OLD COMPRESSOR TO A NEW REFRIGERANT! EVER!


Dye was once a dark red syrup that was injected into an A/C. Not very easy to see and quite messy to use. These days dyes have come a long way. Modern dye is lime green in color, and is easily injected into an A/C system. No mess (usually). After several hours or days, the dye can be spotted and a leak repair made. But that's not the best part- the dye "fluoresces" in the presence of ultra violet light. So it literally shines and shows itself even around dark and greasy dirty areas. Dye can be very useful in finding stubborn leaks that occur over time, or during certain conditions such as only during high heat loads, only during cold weather, only when a fitting experiences contraction etc. Simply invaluable. But to find a leak instantaneously, the "sniffer" must be used...

Heated Diode leak detector is an very accurate "sniffer" type detector that alerts the technician to a leak that is occurring at that very second. No waiting, no dyes, no rechecking after several hours. Instant results. Fantastic for finding and fixing leaks, and for QC'ing a repair before releasing a vehicle back to the customer. However, when the leak occurs only at night, or only at freeway speeds, or only during winter inactivity, the dye is the leak finding tool of choice.

Stop Leak for an A/C? Polymers have been developed that have allowable characteristics to remain in liquid form in the refrigeration cycle. Upon contact with moisture, the polymer will harden, and form a "scab" , slowing or stopping the leak. The problem is, if your system is old, has been serviced a few times, has leaked down a few times, it probably has moisture in it. Since the polymer reacts with moisture, if you install the stop leak, you've now filled your A/C with plastic! No more leaks. No more A/C! Do not use stop leak. It will not save you Additionally, professional refrigerant recovery equipment is vulnerable to damage from this polymer as well, and your A/C shop may ask you to take your car elsewhere if stop leak is found to be in your system.

Enhancing Products are available. Some allow greater heat transfer by reducing surface tension of the oil. Some target specific smells that accumulate in the evaporator. If it's on the market, have know about it, have read the research, and have it in stock.

 

 

 

 

 

 

  
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