Keepin' 'er cool

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July 7, 2016 12:00 AM

Science is great isn’t it?
There you are on a 30-C summer day, you’ve packed up the car with items for the weekend, you’ve got the family loaded up, the dog, the trailer, and you fired up the engine.
Now to just get that air conditioning going so you can cool off and get on your way.
No dice.
Your family is hot and annoyed, sweaty, pawing at their cotton shirts, while your dog is the real victim, wearing a full fur coat in July and sweating profusely from the mouth.
Yes, air conditioning is just amazing to have on demand.
We all know the AC system takes that hot air and puts it outside, but for clarity, it is in fact only the heat that gets put outside, otherwise you’d be in a car with no air at all, and engineers cleverly decided this wouldn’t work.
So how then does it work?
How do we move only heat and not air itself?
Well, picture getting into a cool shower and letting the water run down your hot sweaty body.
Stay with me here OK, its about air conditioning and it’s the middle of the day and I’m about half way through the week, so focus.
Now, as that cool water trickles down over the skin, the heat of your skin is absorbed and is carried off by the water.
All because, as an unchangeable law of nature, heat travels from a point of high pressure to a point of low pressure.
So then,  if it’s hot outside and inside the car, how do you trick the heat into moving?
This is where refrigerant plays its role.
Your compressor squeezes it until it becomes a pressurized and hot gas, and then it enters the condenser. The condenser is mounted with the radiator, so incoming air or the engine fan always passes through it, causing that refrigerant gas to become a hot liquid.
It’s under high pressure until it hits the expansion valve, which lets a metered amount of refrigerant through.
Because the expansion valve is a restriction, there is a lower pressure in the loop upon exit, and that lower pressure allows the liquid to rapidly expand back into a gas state in the evaporator.
Your evaporator is like a mini-radiator with all its metal fins, and it’s through these fins your cabin air passes when you hit the AC button.
Your cabin air passes through these fins that contain a very low pressure, very cold gas, so that the heat in the air from your cabin is absorbed.
Once that refrigerant has left, the evaporator as a warmer gas, it is sucked back into the compressor where the process repeats.
When an AC system fails, there are a number of reasons.
In random order, the first thing to mention is the electromagnetic clutch. It’s duty to engage the compressor when it receives power, and to disengage when the power is cut. Over time this clutch can fail, and the result is a compressor that no longer engages, stopping the flow of refrigerant.
It’s likely your technician will replace the compressor unit entirely should this clutch fail.
Compressors themselves can seize if the small amount of oil in them is too degraded, and although a little more rare, it to requires complete replacement.
Any time a compressor is replaced, manufactures advise that to retain warranty you must also replace the receiver dryer, as it can easily become contaminated and cause more difficulty and possibly damage to the new compressor.
Leaks seem to be the most common cause of poor performance or complete failure.
We have a condition under the hood though where it gets extremely hot, then extremely cold, where there are vibrations and jarring bumps, so it’s no wonder that over time even a clean system can leak.
Most shops are equipped with a machine that draws out the refrigerant and oil from the system and holds a vacuum on it. It then watches for a pressure drop, indicating a leak.
When it come to the price of air conditioning diagnosis and repair, even in a good honest shop, forget the idea of cheap.
Not only is the refrigerant price dictated by the manufacture, the AC machines also cost shops anywhere from five to 10 thousand dollars.
There is always a fee to look at or diagnose an AC system because it takes time and shop resources.
So is there a way to avoid all this hassle?
Yes there is—stop ignoring the AC system for maintenance. 
Although the conditions in a compressor are different from in the engine, both still require good clean oil. You change your engine oil faithfully because it breaks down, it holds debris from wear and contamination. Why leave the same oil in your expensive compressor then for years on end when it can be changed every few years.
It’s a simple procedure, and for an average cost of about a hundred bucks, it’s also about 10 times cheaper than compressor replacement. A receiver dryer is also a hugely overlooked item. The receiver dryer holds debris, and traps moisture from the system in a desiccant. Its a common point of restrictions, and could also be replaced every few years.
They are, after all, substantially cheaper than replacing a compressor.
There isn’t really a way for the average driver to diagnose a failed AC system, but you can go into a shop at least knowing what your symptom means.
For example, if it seems only slightly cold, ask if your car is outfitted with a cabin air filter.
A clogged filter will restrict air flow through the evaporator and reduce its efficiency.
It could also mean you have a leak or refrigerant, or a restriction in the refrigerant loop. Generally, these will be cheaper problems to fix than when you have no cold air at all.
There are many different types of control systems in a modern vehicle, and of course this same technology is used in homes and offices everywhere.
I didn’t scratch the surface of why refrigerants give us this cooling benefit, but I think the geeks and science nerds out there know why it’s so amazing, and I think we really do owe a debt of gratitude to those chemists and engineers out there who experiment and conceive and build these gadgets for us.
Now if those folks could only make the automobile’s refrigeration system last as long as your home’s refrigerator, I’d be out of work, but impressed.

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