You need to know these principles.

It’s that time again — the height of the air conditioning service season. Are you truly ready? Sure, you’ve pulled out the recycling equipment, checked the filters, priced refrigerant, ordered PAG oil, O-rings, suction screen kits, dye, etc. But have you dusted off the basic knowledge that allows you to both troubleshoot accurately and do the job in a craftsmanlike way that avoids comebacks? No? Well, you’re just sitting there reading anyway, so take a few minutes to reinforce your skills with the following look at the refrigeration principles that Subaru vehicles, and indeed all cars, rely on.

Blend

In order for any HVAC system to work, it has to have a source of hot air and a source of cold air, a means to draw fresh air from outside and a way to recirculate cabin air (to minimize the smell of a squashed skunk or the “fresh” country aroma of a pig farm, while improving heating/cooling efficiency under extreme conditions). The hot side comes from the engine via the heater core, and the cold side from the air conditioning system via the evaporator core, both located in the plenum under the dash. Manual or ATC, we blend hot and cold to get the duct temperature required for passenger comfort. 

In order to make a humid 99-degree day tolerable inside that greenhouse we call an automobile, we have to enlist the laws of physics. This is accomplished by taking advantage of the fact that as a refrigerant changes state from liquid to vapor it absorbs heat — LOTS of heat — allowing us to redirect this heat to the outside via the compressor and condenser. 

Latent Heat Miracle

There are two kinds of heat, sensible and latent. Sensible heat is what you can feel and measure. Latent heat is hidden, heat that is absorbed or given off without changing the measurable temperature of the medium involved. We’re all familiar with water, right? Start with a pound of water at 70 deg. F. It takes one British Thermal Unit (BTU) to raise the temperature of one pound of water one degree Fahrenheit. If we add 142 BTUs to our pound of water, we’ll raise its temperature to 212 degrees. If we continue adding heat, we’ll eventually boil the water away, changing its state from liquid to vapor. It takes an additional 970 BTUs to convert that one pound of water to one pound of saturated steam once the water is at 212, but the whole time the water is changing state from liquid to vapor its temperature remains at 212 degrees. 

That 970 BTUs of energy is called the latent heat of vaporization, and, unlike the first 142 BTUs we added, it never raises the temperature of the water. It’s the amount of heat needed to force the water molecules to jump from the surface of the boiling liquid with enough energy to escape. That’s all heat is, anyway — a measure of molecular energy, of how rapidly the molecules of a substance are vibrating or moving.

All substances can exist in one of three states — solid, liquid or vapor — and all substances have latent or hidden heats of vaporization, condensation, liquefaction or solidification (sometimes called fusion). Anytime a substance changes state, hidden heat is released or absorbed in association with the change of state. The fact that steel normally exists as a solid at room temperature doesn’t meant it can’t exist as a liquid or vapor. It just takes a lot more heat to liquefy and subsequently vaporize steel than it does water. Water, the most abundant compound on earth, exists as liquid, solid and vapor within the normal range of temperature most of us experience in life, thus we’ve all seen it in its three states of ice, water and steam.

The Medium 

Everything in our surroundings is subject to these rules, including refrigerants. In order for a compound to be useful as a refrigerant it has to have certain characteristics, cold enough boiling points for example. Even though we mainly deal with R-12 and R-134a, you’d be amazed at the number of refrigerants available — we know of at least 86. 

Even water has a refrigerant number: R718.

We need a refrigerant that’s stable, changes state at temperatures that are useful to us (different applications use different refrigerants with different boiling points) and is non-corrosive. It should also be non-flammable, non-toxic and safe to handle. For all modern automotive applications, we use R-134a. It boils at about -16 deg. F. (for comparison, R-12 boils at -21.7), and has no chlorine to damage the ozone layer. 

Expansion and Flash Gas

Obviously, air conditioning systems are designed to take advantage of both physics and the characteristics of the refrigerant. Let’s start the process at the expansion device (you know, an open tank of refrigerant would make a fine refrigeration system if R-134a weren’t so expensive — you could just plumb a big tank to the expansion device at the evaporator inlet and dump the outlet of the evaporator into atmosphere, and you wouldn’t need a compressor or evaporator). By holding the refrigerant in the system under pressure, we keep it from boiling until it’s delivered to a heat exchanger located where it can cool the cabin of the car. As the refrigerant passes the expansion device, it’s atomized into small droplets, and in the low-pressure area of the evaporator the outer layer of each droplet boils away, lowering the temperature of the remaining liquid to -16 deg. F. This initial boil-off is called the “flash gas.” The center of the droplet remaining after the flash gas boils off, now cooled to -16 degrees, enters the tubes of the evaporator. On the opposite side of the tubes the fins are attached, increasing the surface area of the core. The blower pushes air from the cabin through the fins. In doing so, the remaining portion of the droplet boils away to a low-pressure, low-temperature vapor, absorbing 84 BTUs of heat per pound as it changes state, and the cabin air is cooled and dehumidified. 

We grab this vapor and recycle it over and over by attaching the suction side of a pump to the evaporator outlet, thus maintaining a low pressure area into which our refrigerant can expand. The pump will take a large volume of low-pressure, low-temperature gas and compress it into a high-pressure, high-temperature gas. We have to do this because the gas temp at the evaporator outlet is less than ambient, and in order for us to reject heat, the vapor temperature must be higher than ambient — heat only flows from a higher to a lower temperature, the first of those immutable laws of thermodynamics. We compress and concentrate not only the volume, but also the heat. Now that the vapor is warmer than ambient, we throw it into a condenser, where (because the vapor temp is now in the 130-165 degree range) it dumps heat to the air flowing through the condenser (90 looks pretty cool compared to 130-165). As it rejects its 84 BTUs per pound, the vapor is condensed into a high-pressure, high-temperature liquid, and as is routed once again to the expansion device, where it’s atomized, flash gas drops the temperature of the remaining liquid, and it changes state in the evaporator once again, absorbing another 84 BTUs per pound of heat from the cabin. 

So, those few ounces of R-134a (some late model Subaru systems hold only 16 to 18 ounces) change state over and over, pumping heat from the cabin to the atmosphere. The downside? As ambient temperatures rise, the system loses capacity because liquid line temperatures also rise, and more refrigerant is expended as flash gas to cool the increasingly hotter liquid coming from the condenser. Air conditioning would stop working altogether if the ambient temperatures and humidity rise to the point where insufficient heat is dumped and liquid line temperatures get high enough so that all of the refrigerant metered by the expansion device is consumed as flash gas, and no liquid makes it into the center of the evaporator to change state and absorb heat.

Practical Points

With that theory in mind, the following points will make more sense:

• Take a careful look at aftermarket condensers and evaporators to make sure they match the originals. A simple 3/8 in. tube-and-fin type won’t work in a modern multi-flow application. Buy O.E., or only the best brand names.
• Apropos of the above, R-134a is commonly blamed for poor performance after retrofit when the real reason is often that the replacement condenser or evaporator is not up to O.E. standards for heat rejection. 
• Another factor in the weak-cooling retrofit situation is air (NCGs — Non-Condensable Gases). We’ve seen the problem corrected by simply removing the charge and running the vacuum pump overnight. That’s really what you’re vacuuming, you know. You can’t get much moisture out no matter how long or hard you pull the system down. That’s the job of the dessicant.
• We used to have a large margin for error with compressors. Now, with speeds up to 14,000 rpm, terrifically tight internal tolerances, and lubrication issues, much more precision is needed in both remanufacturing and service.
• Compressor lubrication priorities:
  • #1 Have oil in there.
  • #2 Have PAG oil in there.
  • #3 Have the recommended viscosity PAG oil in there.
• The switch to 42-volt systems will bring electrically-powered compressors with it, but the numbers will be small for a long, long time.
• Speaking of compressors, whenever one has worn out or broken internally how do you protect the new unit from the shrapnel that’s probably been pumped into the refrigeration circuit?  Forget about flushing.  It’s just not going to do it.  While add-on filters that you splice into a line are available, there’s often not enough room, and you’ll introduce the possibility of a leak.  That’s what makes the in-line suction-side screen kit so popular.  It’s easy to install, and will keep any big pieces of metal from getting into the fresh compressor.
• A smelly car interior takes the fun out of motoring and can also be downright embarrassing.  Broad-spectrum disinfectant sprays and foams that you inject into the evap box are commonly used to kill the bacteria, mold, mildew, and fungi that grows in moisture, and some products also coat the fins with a water-shedding acrylic.  These are vastly superior to the type you spray into the outlet ducts.
• The EPA’s Stratospheric Ozone Information Hotline is 1-800-296-1996, and its website is www.epa.gov/ozone/609/609/html
• The automobile is down to just over one percent of manmade ozone-depleting emissions, but the Europeans are pushing for a switch to CO2 systems anyway.