As with any complex system, there are multiple points of failure. We have placed cookies on your device to help make this website better. FAQs | Repair Videos | Academy | Newsletter | Contact, By The reason for this is because the areas they control are kept so cold a good amount of liquid refrigerant would migrate to the evaporator in the off cycle. Refer to Command Controller for Priming Your Reefer Unit. Even when things go right, mechanical failures are always a risk. Simple, right? Warm, saturated vapor is sent to the evaporator where it condenses and is subcooled and forced back into the liquid line. It returns to the system through a check valve piped around the TEV, same as with a heat pump. In its simplest form, this timer just opens the control circuit to the compressor or the control valve for the set duration of the defrost. I will normally see a start temperature in the low ‘teens’ here and expect within 2-4 minutes to see it dropping and it will hit a low of -2°F to -6°F. As far as the system is concerned, the same thing is happening here that was happening before when we used a cold control or a low-pressure switch. Once it hits 65°F on newer equipment and 75°F or so on older equipment, you can be sure that there is no frost left on the equipment and that any further defrost is just wasting time and is detrimental to equipment operation and possibly to product shelf life. Moderate fin spacing medium temp coil with 6 fins per inch, Wide fin spacing on a freezer coil four fins per inch. Whether the coil has a lot of ice on it or very little it will still be cold enough for ice to form so using the temperature of the coil alone is a poor indicator of when a defrost is needed (initiation of defrost). The fail safe time can be a wide range of times depending on the application and frequency of defrost but 20 to 40 minutes is common. With this type of system, you set the cut in of the control to a saturation pressure equal to the same 37°F to 40°F, remembering this is saturation temp, not air temp, and adjust the cut out to maintain the temperature desired. To create a section of reversed gas flow, two valves actuate. A defrost “sensor” is generally a thermistor. Lowest price in 30 days . To prevent any risk of spoilage, perform a pre-trip test on your command controller to ensure it works properly. Regardless of the strategy the goal is the same. So what if we used time alone? A valve is installed in the discharge line that, when activated, creates a pressure differential. They aren’t running. As we were all taught in school, frost buildup is an insulator and prevents heat transfer, also airflow through a coil is a big factor. I usually check the Defrost T-stat when the unit's evap. I ran the compressor for about 20 minutes to try to get the t-stat cold enough to close, but either that wasn't enough time to get it cold enough to close or the bi-metal is stuck open. Quick get a meter and a ladder…. Slam the fans on right now, and you’ll have a wintery Wonderland in your freezer with icicles and snow all over in a week or so. In most cases, the manufacturer will give guidelines to set your defrost control system up. This is a rush of liquid that has condensed in the evaporator and has rejected so much heat that it is very subcooled. That’s why we want to give you an easy checklist to refer to before each trip. Time of day initiates a defrost, so say 6 AM, the timer switches to defrost mode. I’ve seen this as low as 9°F. Timed Defrost – In medium temperature you can use a defrost timer to simply pump down or cycle off the compressor at particular times while keeping the evaporator fan running to force a defrost  few times per day. Much of the timing depends on the length of the suction line and the amount of frost buildup on the coil. A thermistor changes resistance based on the temperature it is exposed to.