The evaporator is an important part of your car's A/C system

The evaporator, also sometimes referred to as the evaporator core, is one of the two (maybe three) heat exchangers in a mobile A/C system. In a typical passenger car or pickup truck, the evaporator is usually located inside the passenger compartment, quite often deeply buried in or under the instrument panel. Some vehicles, usually vans or SUVs, have two evaporators; one under the instrument panel, or elsewhere at the front of the vehicle, and another one located in or toward the rear of the vehicle. The rear evaporator is often located behind a side panel or in the ceiling above the rear passengers.  

Evaporators are usually made of aluminum. They look like, and in fact are, similar to radiators, only thicker and smaller in overall size. Like radiators, evaporators consist of a series of internal tubes or flow paths with fins attached to them. Air can pass freely through the fins, just like a radiator. But unlike a radiator, where the internal tubes carry moving engine coolant, the passages in the evaporator carry moving refrigerant. When many people talk about refrigerant, they refer to it by its most popular brand name from years back, Freon or R-12. In the United States, Freon or R-12 was the type of refrigerant used in mobile A/C systems until about 1994, but it was replaced with a different refrigerant in all vehicles after the 1995 model year. The new refrigerant is HFC-134a (or R-134a).  

But getting back to the term heat exchanger, what does that mean?

In a mobile A/C system, cold, low-pressure liquid refrigerant enters the evaporator. Warm air from the interior of the vehicle passes through the evaporator by action of the blower fan. Since it's a fact of nature that heat always travels from a warmer area to a cooler area, the cooler refrigerant flowing inside the evaporator's absorbs heat from the warm air. At the same time, humidity in the air condenses on the cool evaporator's surface, then eventually drips out of a drain tube to outside the vehicle (think of how moisture forms on a cold bottle of soda pop on a humid day and forms a puddle on your kitchen counter). This is why you see water dripping underneath a car while the air conditioner is on. After the (now slightly warmer) refrigerant has completed its path through the evaporator, it moves on to the compressor.

So, as you can see, air conditioning does not actually cool the interior of the vehicle. What it really does is remove heat and humidity from it.

The evaporator can be somewhat compared to a heater core working in reverse:

• Both are located inside the passenger compartment, often in very close proximity, or even inside the same housing under the dashboard.

• The heater core has hot engine coolant flowing through it, bringing heat from the engine into the interior of the vehicle, where it is distributed by the blower fan.

• The evaporator has cool refrigerant flowing through it, which absorbs passenger compartment heat as the blower fan moves the warm air across it.

What could go wrong with your car's evaporator?

Evaporator failures can usually be summed up in one word: leakage. Leaks can occur for a few different reasons. Usually, either a seam or weld has gone bad, creating a leak point, or corrosion has occurred, causing an "outside-in" failure. This often happens because leaves or other organic material enter the evaporator case through the exterior air intake vents and come in contact with the evaporator's surface. This moist atmosphere causes decomposition of the organic materials, and can form caustic corrosive substances which can eventually perforate the surface of the evaporator. These same substances can also sometimes lead to an odor problem inside the vehicle, most noticeable when the system is first operated. Various deodorizing biocide treatments and drying modules are available to help with this problem. The evaporator fins are also susceptible to clogging from leaves and other debris.

Whichever the type of evaporator failure, it must be replaced. And while an evaporator in and of itself may not be (for most vehicles) a very costly part, depending upon its location (usually like heater cores, deeply buried beneath the instrument panel) this is often a very time consuming, and therefore costly repair.

When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.  

Sources

• Macs World Wide. http://macsworldwide.wordpress.com/2010/07/12/the-evaporator-is-an-important-part-of-your-car%E2%80%99s-ac-system/