How Often Do My Evaporator Fans Need to Run?

evaporator fanWalk-in cooler owners have often asked if it is really a good idea to switch off the evaporator fans inside their coolers.  Sometimes, this is after a refrigeration serviceperson has flatly declared that the evaporators should never be turned off. Understandably, people are concerned with two key issues. First, they want to make sure the system works and keeps the cooler and its valuable contents properly cold. Second, they do not want to risk damaging their system.  As I’ll discuss below, intelligently shutting down your evaporators does not compromise the system’s ability to keep the cooler at the desired temperature, nor does it create potential to damage the compressor system. First, to get some context, let’s look at how things currently work. Since the earliest commercial mechanical refrigeration systems were installed, evaporator fans have been kept running constantly, 24/7/365.  The main reason for this is simple: evaporator coils can become clogged with ice.  An iced or frosted evaporator does not have sufficient airflow through it, so it does not cool properly and creates the risk that a cooler will get too warm.  It is essential to completely defrost the evaporator coils periodically to provide effective and efficient refrigeration. A traditional system relies on an oversized compressor and a time clock to ensure that the compressor stays off long and often enough when the compressor is not running to melt all the ice. In the old days, before the advent of computer controls, it was safer and easier to simply keep the evaporator fans running than to risk not having them on when needed.  Determining when they can safely and efficiently be switched off is too much for old-fashioned mechanical controls. A modern computerized control like the Freeaire® Cooler Controller can handle that job. When it determines that it is time for a defrost cycle, it will shut down the compressor so that no refrigerant is coming into the evaporator and will run the evaporator fans to warm evaporator coils until they reach a target temperature that indicates that there is no ice remaining. In other words, the Controller will run the fans only long enough to completely defrost the system, but no longer. Some suggest that it is necessary to constantly run the evaporator fans during the compressor’s off cycle to provide adequate circulation to prevent temperature stratification or temperature variations within the cooler. That job can be easily done with much less energy by installing a low wattage, but high airflow, circulating fan that operates whenever the evaporator fans are off or by switching on the evaporator fans for a few seconds every few minutes. The second issue that people bring up is whether turning the evaporator fans on and off is somehow dangerous for the system. An “old school” refrigeration guy that brings this up is probably concerned with “slugging” the compressor. In English this means that refrigerant would be sent back from the evaporator to the compressor as a liquid rather than as a gas. This could happen if the compressor is on, sending refrigerant to the evaporator, but the evaporator fans are not running. Our Cooler Controller is designed so that whenever the compressor is running, so are the evaporator fans. This helps to ensure that the refrigerant evaporates and returns to the compressor as a gas. To prevent slugging of the compressor, however, it is even more essential that two common compressor safeguards be used, especially in winter in a cold climate. The first is a pumpdown solenoid that causes almost all the refrigerant to be pumped out of the evaporator and the return pipe and to be safely stored on the output side of the compressor before it is switched off. The second safeguard is a crankcase heater that makes sure that any refrigerant that does remain behind stays as a gas that cannot harm the compressor when it resumes operation. These two measures are particularly important with a Freeaire system using outside air, since there could be many hours, weeks, or even months between when a compressor shuts off and when it finally switches on again. All Freeaire systems should have these safety features installed if they aren’t already. Another concern comes from some people’s perception that repeatedly switching the evaporator fan motors and relays will harm the motors. In fact, it is better to have them off than on. Like most mechanical devices, an electrical motor, including a modern ECM evaporator fan motor, is subjected to less wear and tear when at rest than when running constantly. Solid-state relays can switch evaporator fans off and on many times a day for decades without failing. A Freeaire® system saves energy by not running the evaporator fans unnecessarily. However, this in no way compromises proper refrigeration or does harm to any part of the system.