When the timer initiates a defrost, a few things happen all at once. But one thing to need to be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost. Periodically the evaporator coil will need to defrost and the amount depends on how much moisture is brought into the box from opening doors and new product being added. If the coil is iced up, the fans can’t move any air and without air movement, the equipment can’t do its job. 3. Need Help? Typically, one drop of Nylog can be stretched about three feet before breaking. 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 most common are referred to as a DTFD control (Defrost Termination Fan Delay) or 3 wire control. 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. Defrost termination is really the standard for this type of system. You can see this same problem if the hot gas solenoid fails to close properly after a defrost. Once our DTFD control sees it’s high event temperature, usually about 55°F, it closes the part of the circuit to terminate defrost, same as before. Because this is a cooler (and not a freezer) it is set to defrost by simply shutting off the compressor four times per day while still running the evaporator fans. Meaning the T-stat will kick out at 50 degrees and kick back in at 30 degrees. A freezer thermostat controls the cycling of the compressor in conjunction with an Adaptive Defrost Control board. You will not pay more when buying a product through my link. 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. A valve is installed in the discharge line that, when activated, creates a pressure differential. How could we possibly tell when the coil needs to defrost by temperature alone? Well, melting frost is just latent heat added to change the state, right? 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. removed the the back panel to expose the evaporator. If the temperature sensor or thermostat isn’t working correctly it will still go back to refrigerating once the time is complete. Next we have the methods used for defrosting low temperature applications which are below freezing and generally 0ºF to -10ºF depending on what is being stored. A defrost termination thermostat is mounted onto the coil to detect when the coil is free of ice and will often be set to “terminate” or stop the defrost heat when the coil reaches around 55ºF – 60ºF to ensure the entire coil is ice free. If any of the 3 electrically activated valves fail to operate either because of a control system fault or a mechanical problem with the valve itself, we set ourselves up for trouble. Hence the -20. It depends on the design of the refrig. So, in the middle of summer in a hot, humid kitchen the defrost runs longer than it does in the middle of winter on outside access only cooler box. This valve will work for either but is really necessary for a Kool gas system. You’ll see this type of control on stuff like prep tables and smaller, under counter type refrigerator units. Electric Heat Defrost – On a set schedule (time) the compressor is pumped down or cycled, the evaporator fan is (generally) shut off and the electric heaters are turned on inside the evaporator coil. $55.94 $ 55. Lowest price in 30 days . So, since we’ll have more frost on a coil with a higher humidity than on one in a lower humidity environment, the coil with a higher frost buildup is going to take longer to melt off of that coil which means that it will take longer to reach that set temperature. Let’s take a look at how this adds some complexity to the defrost control system. Usually the range is more like 50 -20. When putting this thing through its paces, I couldn't get the defrost heater to kick on after advancing the timer to defrost. Driving Safe: The Good, The Bad and The Newbies. Manual defrost should only be used when the trailer reaches a temp of 40 degrees Fahrenheit or below. This is a quick, real-life diagnosis/find by Kalos Services senior commercial HVAC/R tech Eric Mele. At the same time the differential valve activates, both of these valves activate and start the defrost process. Defrost When Necessary. Always report alarm codes. 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. Now, we can set that timer up to shut the refrigeration off at regular intervals for a specific period. If you look at the timer you will see that the little pins are flipped four times per 24 hr period. Similarly, if the solenoid valve that opens to allow defrost gas into the suction fails to completely open, we won’t have enough flow. Hot Gas Defrost – On a set schedule the evaporator fan is shut off and hot discharge gas is pumped through the evaporator coil. I looked for the bimetal defrost thermostat clipped on the evaporator. What about the fans? It that line runs all the way back to the compressor unit, you can test it there although the further from the evaporator you measure the temperature, the less accurate the test becomes. Regardless of the strategy the goal is the same. However, you’ll have to keep the engine and reefer unit running at all times to keep the current temperature setting. This is why it is common to initiate defrost using time and terminate defrost using temperature and time.