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Old 03-12-2006, 10:05 AM   #1 (permalink)
knkreb
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Reference Material: Truck A/C systems

This is technical article given to help all of us understand our mobile air conditioning systems better. This thread is not meant a general "reply to thread," but is open for posting if you have other technical material that you would like to add. If you have a question too, you may feel free to ask it, because it may add to the list of questions that others have. Thanks!
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As warm weather graces us, (well for must of us) it’s time to think about that all important thing: air conditioning.

First off, let’s dive into a few different aspects of mobile air conditioning systems. This is not your typical a/c system that you find in your home central air or window air conditioner. This system has an engine driven compressor. Engine driven compressors are slightly different than most.

When driving your vehicle down the road, your engine speed helps to increase the capacity of your compressor. This is because your engine is turning at a higher speed than just sitting at idle. Having a fan speed control, and temperature mixing capabilities makes this a very interesting system, but one that can make the cabin area comfortable under most conditions.

As our vehicles age, there are many different things that can contribute to loss of refrigeration capacity. Don’t think I’m talking about hanging meat here by saying “refrigeration capacity.” All air conditioning systems are refrigeration systems, they are just refrigerating at a higher temperature than what you associate with “refrigeration.”

Let’s talk basic refrigeration principle for just a moment here.

Refrigerant is a chemical that will has a temperature and pressure relationship. Think of your radiator system in your truck for moment. There is a radiator cap on your system. This is to increase the pressure in the system as the engine heats up. Ever wonder why? This is to help increase the boiling point of the coolant in the system. Water (by itself) boils at 212°F (100°C for you metric folks) As the pressure on the radiator system increases, the boiling point of water increases too. The same is true of refrigerant in the system. The higher the pressure, the higher the boiling point, and the lower the pressure, the lower the boiling point.

When you boil water on your stove, once it gets to boiling point, the temperature of the water stops at 212°F (or 100°C). It will not go over that temperature in a liquid state at atmospheric pressure. The pot is still absorbing heat. But it the temperature stays a rock solid 212°F (100°C). This is latent heat. There is a tremendous amount of heat that can be absorbed in a substance during change of state (latent heat).

Now we’ll do the same thing with a refrigerant. Except, we will change the temperature and the pressures throughout the system to get it to boil and condense back into a liquid at various parts in the system.

You will first have to get over your initial thought of “boil.” You think of boiling as “hot”, because it’s in reference to water. Other chemicals, such as refrigerants boil at low temperatures. Depending upon what refrigerant, depends upon what temperature and pressure it will boil at.

In the evaporator, you want the refrigerant to boil. The evaporator, is the low side of the system, the side that does the cooling. This is where liquid refrigerant is injected into the evaporator, but it is under a low pressure. When the pressure goes down, the boiling point does too. So, the refrigerant boils in the evaporator. This absorbs heat from the evaporator. Once it’s all boiled off, it’s in a vapor form, and goes to the compressor. The compressor pumps it up to a high pressure gas. The pressure increases, the temperature does too. I would not advise anyone to touch the discharge line of an operating compressor. They can be as high as 200+°F! Don’t believe me, you’ll have the scars to prove it.

That high pressure gas/vapor goes into the condenser to cool off. When it cools, that gas turns back into a liquid after giving up it’s heat. Once it’s returned to a liquid, it then goes through a “metering device.” That’s the dividing point between the high side (high pressure) and the low side (evaporator) of the system. Once the liquid refrigerant goes through that metering device and crosses into the evaporator, the cycle starts all over again.

This compressor is controlled by a low pressure switch. This pressure switch monitors the pressure on the low side or suction side of the refrigeration circuit. When the pressure falls below a certain pressure, the switch turns off the compressor. There are two different conditions when the low pressure switch turns off the compressor: lack of refrigerant, or lack of load on the evaporator.

Is it normal for the compressor to cycle on and off? Yes, depending upon why. If you have a cool dry day at idle and the cabin is near a comfortable temperature, it could be very normal to see the compressor cycle on and off. There are a great many factors that will effect the cycling rate of the compressor.

Can I time my compressor cycling time and know if it's properly charged? No, not really. With some experience, you may be able to determine if the refrigerant charge is to blame, or something else. Very quick cycling on/off, and then a long duration until the next on cycle *could* indicate a low on charge system. Cycling time is dependant upon: cabin temperature, fresh/re-circulated air, engine speed, relative humidity of air entering the evaporator, air temperature entering the evaporator, etc. etc. Way too many variables to tell by cycling time alone if the system is properly charged.

How much refrigerant should be in my system? There is a name plate rating on your system to indicate how much refrigerant you need for the system to work properly.

How can I determine if my system has the proper refrigerant level in it? The absolute best way is to recover the refrigerant, and recharge the system by weight.

Why can I not just rely upon the pressures in my system? Pressures are only merely and indicator. You may be over or undercharged, but with so many variables in the system and the conditions that it is running under, pressures can be a bit misleading.

Why do local retailers sell recharge kits with gauges on them if pressures aren’t enough to go by? Pressures will get you in the ball park, but you run the risk of still being over/undercharged. Retails sell these, because… They sell.

What are these additives that can be put into my system to increase it’s capacity? These are just marketing ploys. There are some additives that my help with compressor lubrication improvement and help with oil return, by they don’t offer a sizable improvement in the system’s refrigeration capacity.

I see oil additives offered at retailers. Should I add oil to my system? There is a measured amount of oil that is to be in the compressor. Too much could end up causing you a situation that will cost you your compressor. Compressors compress vapor/gas. They do not compress liquids. Attempting to compress liquids leads to a sudden disassembly of parts with the compressor housing (buy a new compressor)

If I add more refrigerant to my system, will it cool better? NO! Adding too much refrigerant to a system is worse than not enough. As said before, compressors compress vapor/gas, not liquid. Increasing the refrigerant charge increases the chances of not boiling off all the refrigerant in the evaporator, and liquid entering the compressor. Not to mention your evaporator pressure increases. The higher the evaporator pressure, the higher the temperature of the evaporator. The warmer that evaporator is, the less “cooling” you get from it.

There are “alternative” refrigerants offered. Should I use them? Later model years use 134a refrigerant in the system. If you doubt what refrigerant your system uses, check under the hood for the labeling. DO NOT USE THE WRONG REFRIGERANT.

Older systems use R12 refrigerant. R12 is available for those who would like to second mortgage their homes. The price is very high per pound.

Can I covert an R12 system to R134a? Yes, but you may lose capacity on the system. It was designed to operate at certain pressures and temperatures. R134a needs a larger condenser. You may be disappointed with just merely retrofiting over to R134a from R12.

Can I just add a different refrigerant to the system if it’s low? Legal speaking? No. You need to have one type of refrigerant in the system. Thermodynamically, it may work, but you will not have any reference material available for pressures and temperatures.

I see refrigerants offered that say that you do not need to be a licensed certified technician to purchase it. What are they? These are different blend refrigerants. Some of these are not legal for use in automotive applications. Some of these refrigerants are flammable.

What is a certified technician? These are the techs that have taken a test to be approved to purchase and use refrigerants in systems. Depending upon local laws will depend upon what is required in your area. R12 you must be EPA certified national speaking. Generally speaking R134a you don’t need certification, but that varies from location to location. Check to see what is needed (if you are really interested in the legality of it )

Is there anything to improve the capacity of my system? Make sure that you have good air flow through the evaporator coil, condenser coil, and the proper refrigerant charge.

I seem to be losing capacity, what do I do? Proper refrigerant charge is one of the first stops. If the charge is correct, and your low side pressure is still too low, and not enough refrigeration effect is occurring, then the next part that should be investigated is the orifice tube. That is the metering device. The orifice tube can get plugged up with black debris. This is an indicator of “black death.” If that is the case, then either you are about to, or have had a compressor failure.

My compressor has failed before, does that the next will fail too? If you did have the proper clean up procedures done in the case of “black death,” then you should be okay. If you just plug-n-played a new compressor with out any further investigation, then I you may be at risk for a new compressor.

My system is low on refrigerant. Are there any common leak points? Compressor shaft seal is a common one. This happens a lot in between seasons. If the compressor is not cycled on every so often, the seal loses it’s oil and lubrication to help it seal better if it has aged. Mobile air conditioning systems can be very difficult, due to vibration, movement, and abuses that happen on the road.

Hope this helps everyone understand their systems a little bit better.
I bought a cheapy gauge from a retailer. It indicates HIGH or seek professional help for my system. What does that mean? Retailers offer little gauges that indicate the system's evaporator pressure. They color code between certain pressures to generally indicate: add, okay, over charged, or "danger." If your pressures are reading high on the gauge, There are a few things that you may have. Either you have an overcharged situation, excessive load on the evaporator, or an in-efficient compressor. For over-charged systems, reduce the charge. Excessive load, turn the fan down to low speed and see what the pressure does. If the pressure remains high (with the clutch engaged) your compressor may not be pumping as well as it should.

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Last edited by knkreb; 06-30-2006 at 11:39 PM.
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