Arkansas HVACR NewsMagazine May 2021

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reasons why this conclusion may have been reached. The job of an air cooled condensing coil is to reject heat from the refrigerant to the air. The rate at which it does this is a function of contact time, temperature differential, the thermal conductivity of the material through which heat is being transferred and turbulence of both fluids (refrigerant on the inside of the tubing and the air on the outside). You may have noticed that modern condenser coils are larger than they used to be, the reason for this is simple. The larger the surface area of the coil, the more heat can be transferred from the refrigerant to the air resulting in a lower required condensing temperature and lower head pressure. In other words, by increasing the contact time we don’t need as great of temperature difference between the refrigerant in the tubing and air passing over it to accomplish the same amount of heat transfer. Engineers have also learned that by changing the design of coils we can get greater contact surface area with less refrigerant with coils such as micro- channel or they can get greater internal turbulence by adding grooves or rifling in the tubes of better external turbulence by adding little kinks to the fins of the coil. They do all of this to attempt to move heat from the refrigerant in the most efficient way possible. I applaud them for their efforts. So how could a “dirtier” coil ever be more efficient? It is at least theoretically

possible that certain types of surface fouling might act to create more air turbulence and actually increase heat transfer… and , if you tested 100 systems in field conditions, you may find a few that exhibit this undesigned behavior depending on the type of coil and the type of soil. How many of us who do small kitchen refrigeration have gone out to a freezer not keeping temp or an ice machine not making ice like it once did, only to clean the condenser and everything starts working properly again? In our minds we imagine that the dirt or grease is acting like an insulating “blanket” preventing heat transfer, and that is certainly one factor , but it isn’t the only thing going on. Condenser Fan Efficacy Condenser fans are prop fans, more technically known as “axial” fans as opposed to blower wheels which are known as radial or centrifugal. Axial fans are good at moving a lot of air against very low pressure, but as soon as the pressure starts to build their performance drops off REALLY quick. We have all walked up to a condenser fan where the air was just sort of beating out of the side instead of really pushing out the top like it’s supposed to. Once you clean the coil it starts moving air again and you can really tell the difference. So much of the decreased heat transfer comes from the fact that dirt In the field we know this isn’t normal…