The capillary tube is a very popular metering device used on many fractional horsepower refrigeration systems. It is basically a length of very small diameter copper tubing soldered or brazed between the system’s liquid line and the inlet to the evaporator. A common problem encountered with this metering device is that it can become restricted, causing a reduced amount of refrigerant to flow into the evaporator. Although a capillary tube can become totally restricted, a far more common problem is when it becomes partially restricted, allowing some refrigerant to flow into the evaporator but not enough to satisfy the requirements of the system.

This seemingly easy problem to diagnose is actually rather tricky at times. This is because a restricted capillary can resemble a system with a low refrigerant charge. Both problems can present similar symptoms. A system with a restricted capillary tube will operate with a lower than normal suction pressure, as will a system with a low refrigerant charge. Both system problems will also operate with a lower than normal discharge pressure. Depending on the severity of either problem, the pressures may not be exactly equal, but close enough to cause any technician difficulty in determining the true problem.

This may lead a technician to guess at the problem and tell the customer that the system has low refrigerant charge due to a system leak - a reasonable conclusion. The customer okays the technician to search for the leak, repair it, and recharge the system with refrigerant. After much wasted time, the technician is unable to find the leak and tells the customer that the leak must be so small that he cannot locate it. It is best to just go ahead and recharge the system with some refrigerant and see how long it takes to leak out - again a somewhat reasonable conclusion.

As the technician adds refrigerant to the system, he notices the low side pressure does not change much, but the high side pressure starts to climb well above normal levels. He realizes the problem is not a system leak as he first suspected, but a restricted capillary tube. Now he is faced with telling the customer that his original diagnosis was incorrect and that now the system has a more serious and more expensive repair. This leads to a very unhappy customer and an embarrassed technician.

Since most capillary tube systems have a relatively small refrigerant charge, one practical approach to determining the difference is to remove the refrigerant charge and weigh in the correct amount of refrigerant. If this resolves the problem, the issue was a low refrigerant charge more than likely due to a system leak. Now it makes sense to go ahead and spend the time to search for the system leak. If the problem remains after adding the correct amount of refrigerant, the problem is most likely a restricted capillary tube. 

This method is a little time-consuming, but it will allow a technician to diagnose the true cause of the problem the first time without guessing and potentially misinforming the customer.

Publication date:09/06/2010