Thursday, April 21, 2011

How much refrigerant does it take to make the fan blow harder?

Have you ever noticed that right after you buy a car you seem to see that particular model everywhere?  Similarly, once you solve a problem, every problem you see seems to be another iteration of that same problem. My theory is that we are programmed to repeat our successes. The brain says “that worked before, lets do that again!” We are all occasionally tempted by the same quick fix demon that whispers in our ear “this worked the last time!” The temptation is just to repeat our latest success without really looking carefully at the actual problem before us. We convince ourselves that this problem is just like the previous one. I think it is more difficult for younger technicians to resist the quick fix temptation. They have not seen as many problems, so their brain is less confused – it is certain the solution is whatever they have seen recently! Often the quick solution involves adding refrigerant. My brother Richard is a senior technician at his company. Having seen and experienced many things, other technicians often call him for help. Richard is also a joker, so they know they are going to have their tail twisted a bit when they call. Recently a young tech called Richard and said he could not get the suction pressure on an R22 unit above 50 psig even though he had added refrigerant three times and every time the pressure settled down to 50 psig. He didn’t understand why adding refrigerant did not raise his suction pressure. Richard asked him “how many pounds of refrigerant do you have to add to make the fan blow harder?” No reply. So he asks again “Say, I really don’t know, how many pounds of refrigerant DOES it take to make the fan blow harder?” Finally, the tech replies: “adding refrigerant won’t make the fan blow any harder no matter how much I add.” Richard then asks “so why are you trying to fix an airflow problem by adding refrigerant?” Of course there are several possibilities that could cause low suction pressure, but airflow is one of the most common.  He could also have some type of refrigerant restriction or a metering device issue. However, more information is needed to differentiate these from each other. A high superheat and low subcooling could indicate an undercharge. A low superheat could indicate an airflow problem. A high subcooling and high superheat would point to a refrigerant restriction or metering device issue. The problem is that the tech was only looking at the suction pressure and nothing else. He had already decided what the problem was and how to fix it before actually finding the problem. To his credit, he did recognize that he was pursuing the wrong “fix,” that is why he called for help. The next time you can’t understand why your fix is not working, ask yourself if you are trying to make the fan blow harder by adding refrigerant. Then call Richard.  

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