WheelerOnHVACR: Function of the Evaporator in the Air-Conditioning Cycle
Due to the typical dissimilar metal (copper/aluminum) construction, evaporator coils also lose efficiency over time. As with condenser coils, corrosives in the air and acids created by the breakdown of debris accumulating on the coil attack the copper/aluminum bond. However, this doesn't happen as quickly as with the outdoor coil because of filtration and because the indoor air is usually less contaminated. However, cleaning techniques that employ foaming acid or alkaline products definitely reduce the coil's efficiency and life. This is one reason why it is important to change the evaporator coil whenever the condensing unit is replaced. An old evaporator coil matched to a new high-efficiency condensing unit (split system) will never reach the condensing unit's maximum possible efficiency. And built-up dirt accumulation always impedes airflow.
Another reason why old evaporator coils should be replaced is that older standard-efficiency systems employed smaller, less-efficient designs. Yes, both the indoor and outdoor portions were designed for low efficiency. So no matter how efficient you make the outdoor portion, there isn't much gain when the indoor coil is undersized, deteriorating, and dirty. By the way, there is just no cure for problems caused by improper selection of evaporator coils (too large or too small).
There are basically two different configurations of evaporator coils - the "I" or slab coil and the "A" coil. Their application usually dictates the configuration. Slab coils are mostly used in horizontal or multi-poise applications and "A" coils are primarily used in upflow and downflow applications. Why the difference? It has to do with proper airflow distribution and providing an effective means of draining water off the coil. Since the air flows (up or down) through the center of an "A" coil, it is conditioned evenly. Also, the shorter vertical surfaces provide a more reliable means of draining water that accumulates on the fins, with less chance of dripping. The water droplets tend to follow the slanted surface, where it is channeled into a pan and through a condensate drain.
Slab coils usually must be taller to achieve the same surface area as an "A" coil, and this creates a need for a larger housing. In addition, where slab coils are used in a slanted position, special care must be taken in design to prevent water from dripping from its longer surface. This becomes especially difficult when a low refrigerant charge causes the lower portion of the coil surface to be warm and dry.
An air-conditioning evaporator coil typically provides a 20 degrees F temperature drop, depending on the load conditions and the entering relative humidity. To be technical, an evaporator running at rated conditions should provide a 6.67-Btu heat absorption for every pound of air crossing it (about 10 degrees F on a wet bulb thermometer).
Since the refrigerant entering the evaporator is mostly liquid and at a lower pressure than in the condenser coil, it evaporates at a low temperature (about 40 degrees F). If the evaporator pressure is too high (due to improper compressor or metering-device selection, or due to a refrigerant overcharge), both cooling and dehumidification are impaired. If the evaporator pressure is too low (due to a refrigerant undercharge, insufficient loading or airflow or a refrigerant restriction), not only is there a loss of cooling efficiency, but condensed water will freeze on the coil whenever its temperature drops much below 32 degrees F. So, proper design and application are critical.
Typically, the refrigerant leaving a Direct Expansion (DX) evaporator coil is all superheated gas. If liquid (saturated) refrigerant leaves the coil, this can damage the compressor and cooling capacity is lost. If the leaving gas is too superheated, cooling capacity is also lost and a warm surface can cause the coil to drip. Maintaining the proper superheat leaving the evaporator is the primary function of the proper refrigerant charge and the metering device. The typical leaving superheat is 10-15 degrees F warmer than the saturation temperature.
What can go wrong with an evaporator? Well, other than the problems of dripping, tube-fin deterioration, and surface loading mentioned above, there are three or four more things that can go wrong. One of the most common of these is improper air distribution across the coil surface. This condition can be caused by poor design or deterioration of parts on the air-supply side. However, it is more commonly created by poor design of the air distribution on the leaving-air side of the coil. A hard turn in the ductwork very close to the coil or a branch takeoff too close to the coil will cause uneven airflow, so part of the coil sees too much load (too warm) and the rest of it has too little load (too cold).
Another often overlooked evaporator problem is uneven refrigerant distribution. Since almost all evaporator coils are constructed with multiple parallel circuits to reduce pressure drops, anything that causes more refrigerant to flow through one circuit than the rest greatly reduces the operating efficiency. A clogged circuit (factory defect) or poor distribution design (field retrofit) can cause this.
With slab coils, another common problem is condensate blowoff. This is where condensate droplets blow off the coil into the adjoining downstream ductwork. This creates a major source of indoor air quality pollution due to the resulting mold growth. Of course, the primary cause of this defect is just too much air (greater than 450-fpm) flowing across the coil surface.
And finally, evaporator coils are often blamed for bad odors. They aren't always really to blame, but they are often replaced for this reason. Mold or bacterial growth is usually the cause, but use of chemicals inside the structure (such as DMSO) can also cause this problem.
So yes, evaporators are just coils¿non-moving parts. But isn't it amazing what can go wrong with them.