Wheeler On HVACR: A Closer Look At The Condenser Coil
In my last article I discussed the basic air-conditioning cycle using the diagram shown on this page. Once you understand the basic cycles that are involved, it will be easier to focus on some of the components and discuss some common problems. So this month's article will focus on the condenser coil.
The condenser is that outside coil on an air conditioner through which warm air blows. Inside it, the hot refrigerant gas coming from the compressor is de-superheated, then it quickly reaches the saturation temperature where the gas is condensed into a liquid. Ideally, the hot gas enters the top of the coil and liquid refrigerant leaves the coil at the bottom, so no gas bubbles exit the coil to enter the metering device. Bubbles flowing through the metering orifice drastically reduce the system operating efficiency.
As the diagram shows, by the time the refrigerant exits the coil it should leave as 100% refrigerant, but it should be subcooled below the saturation temperature. The cooler the (liquid) line leaving, the higher the overall system efficiency. This lowers the pressure (lower temperatures = lower pressures in a saturated condition). So the idea is that the more heat you remove from the refrigerant, the more efficiently the system operates.
Since removing heat from the refrigerant raises efficiency, it's obvious that the larger the condenser coil, the more efficient the system becomes. However, there is a limit to how large you can make a condenser, both theoretically and practically.
First, realize that no matter how large you make the coil, you can't cool the refrigerant temperature any lower than the outdoor air flowing through it.
Second, the closer the refrigerant temperature comes to the outdoor-air temperature, the harder it becomes to remove more heat. This is because the lower the differential between the refrigerant temperature and that of the outdoor air, the slower the thermal transfer. In fact, it is virtually impossible to design a coil that will cool the refrigerant to the outdoor-air temperature. So there comes a point where increasing coil size isn't worth the extra cost.
Also, there are some alternatives to making the coil larger to remove more heat. Improving the thermal-transfer is another method of doing this. Improved fin designs and their surface contact to the coil tubing is one way. Adding "rifling" to the inside of the tubing is another way. Proper air-flow design and best use and sizing of the condenser fan are other important design factors.
Another problem with making a condenser coil too large is that this requires extra refrigerant to fill that coil, and this creates a problem with the refrigerant-to-oil ratio. Oil from the compressor travels through the system with the refrigerant, and when there is too much refrigerant (which is a solvent) in the system, the oil's lubricating qualities are reduced. So manufacturers design condenser coils to provide as much heat transfer as is practical, in as small a coil as possible.
Notice the temperatures that are shown in the diagram. These are fairly typical of a new high-efficiency air-conditioning system. On a 90 degrees F day the refrigerant saturation temperature will run 20-50 degrees F warmer (110-140 degrees F) and the liquid line leaving the coil will see 10-15 degrees F of subcooling. Of course, indoor cooling conditions will affect these temperatures. The warmer it is indoors, the more heat there is for the coil to reject.
Now, understand that manufacturers design their condenser coils to get the highest efficiency with the least refrigerant, so, anything that affects the heat transfer characteristics creates an imbalance, which drastically affects performance. Also, having the exact refrigerant charge is critical.
A dirty condenser coil cuts heat transfer. So the coil should be cleaned frequently (at least once per year). But if a corrosive (foaming) coil cleaner is used, this permanently damages heat transfer by destroying the fins and the critical fin-tube bond. So my suggestion is to just wash the coil with water (and soap) or vacuum it. However, just being outside naturally deteriorates the coil's efficiency, and this is especially true when the fin stock (aluminum) is bonded to a dissimilar coil metal (copper). Electrolysis destroys the fin/tube bond, resulting in a system-efficiency loss of about 2% per year.
Air in the system also greatly reduces system performance. Since air doesn't condense, it usually sits at the top of the condenser coil, effectively reducing the size of the coil by the compressed volume of the air, which reduces efficiency. And the flowing refrigerant drags some air bubbles through the metering orifice, which reduces efficiency there too.
Finally, there is the refrigerant charge. If you have too much refrigerant in the system, it backs up in the condenser coil, which reduces the coil's effective heat-transfer area. This raises the coil pressure, making the compressor work harder. On the other hand, too little refrigerant reduces subcooling. And reduced subcooling cuts heat transfer and efficiency. How critical is the charge on a normal residential air conditioner? It should be within 1/2-ounce of liquid measure! This is a goal seldom reached by most service technicians. Most residential air conditioners are grossly overcharged.
So if you put a digital thermometer on the liquid line leaving the condenser coil, you can learn a lot about the system's operating efficiency. As I've illustrated, its temperature should run 5-15 degrees F warmer than the temperature of the outdoor air. If the line feels quite warm, that's an indication of poor heat transfer due to a dirty or aging coil. Of course, proper airflow distribution through the coil is important too.
You can also watch the liquid-line temperature change as it reaches the proper charge. Its temperature will be the same as the outdoor-air as charge starts to be added. However, as the optimum charge is approached, the line temperature will rise several degrees. At this point the technician should stop adding refrigerant and allow the system to stabilize, because it could already be overcharged. When the proper charge is reached, the line temperature starts to drop as the liquid is subcooled. And too much subcooling means higher coil pressures and lowered efficiency.
Although very technical, I hope that I have somehow communicated to you how delicate the balance is when it comes to getting the highest efficiency out of an air conditioner. And while other portions of the system also play their role, when it comes to operating efficiency and the loss of efficiency over the life of an air conditioner, no part plays a more critical role than the condenser coil.