A differential valve de-energizes to create a pressure differential to allow flow in reverse. The biggest difference is that now, with a timer, instead of being subject to the unknown of when the system will cycle off and how long it will take to melt the frost, assuming the time of day is set correctly, you can reliably predict the defrost times. Without the pressure differential, we wouldn’t be able to push the liquid out of the coil because a pressure differential is required for anything to flow. Of course many new ones have their ranges in metric form now. In most cases, the manufacturer will give guidelines to set your defrost control system up. Let’s talk for a minute about how long a defrost needs to last… obviously, until the coil is completely clear of frost and ice, but we need to know when that is…. That’s why we want to give you an easy checklist to refer to before each trip. This relies on appropriate over-sizing and can lead to issues when the heat or moisture load is high, especially when the door are opened a lot for loading and unloading. Refrigeration is off; heaters are on, frost is melting away. One drawback to hot gas defrost is the expansion and contraction of refrigerant lines due to temperature swings can be extreme if the lines run far enough. Even when things go right, mechanical failures are always a risk. 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. 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. The latest victim in my appliance repair home self study program is a Frigidaire top freezer refrigerator model number FRT21NNGD1. 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. Previous Page; Page 1 of 8; Next Page; Grid View; List View; View as: Temperature Sensor . Warm, saturated vapor is sent to the evaporator where it condenses and is subcooled and forced back into the liquid line. Taking a step up from the idea that every off cycle is a defrost, we’re going to just add a timer to the circuit. It will spell out frequency or interval (time between defrosts) and duration of the defrosts. Don’t Forget to Report Any Irregularities or Alarms. Could also cause a nice fire if left unattended. The DTFD is normally attached to one side of the evaporator coil in a position that takes the longest to get warm during defrost. Manual defrost should only be used when the trailer reaches a temp of 40 degrees Fahrenheit or below. Non-hardening, non-drying fluid which bonds tenaciously to many different substrates. 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. 4.7 out of 5 stars 109. If the differential valve fails, we won’t have an adequate flow of refrigerant to get enough heat for a complete defrost. So instead we use a combination of time to set the number of defrost occurrences per 24hr period. This will likely cause a flooded start at the end of each defrost when the liquid refrigerant rushes into the compressor on startup. The temperature will hold stable for a couple minutes. This temperature will then start to rise as there is less and less frost to absorb heat from the gas. It depends on the design of the refrig. However, you’ll have to keep the engine and reefer unit running at all times to keep the current temperature setting. 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. I’m going to try to summarize it in a paragraph or two and give it a more thorough treatment in the future. Hot gas defrost where superheated discharge gas is directed into the evaporator and “Kool gas” a trademarked name for a defrost that directs saturated vapor from the top of the receiver unto the evaporator. This computer device can be found inside the cabin. The diagram below shows first how it is currently connected and then how we rectified the situation. The easiest way to correct this problem is to relocate the defrost timer into the evaporator, and break power to the liquid line solenoid valve during defrost. tested the bi-metal defrost thermostat, as per the manual, " the ohmmeter should show ZERO resistance (continuity). Defrost When Necessary. A heat pump needs to be able to switch between heat and cool and defrost which brings in the necessity for more control conductors and complexity. © 2017 HVAC School All Rights Reserved. You would then setup a length of time for the defrost as a best guess with too long wasting energy and warming the product or too short resulting in incomplete defrost and gradually freezing the coil. So the defrost starts on a scheduled time of 2 – 6 times per 24 hour period and terminates once the coil defrost termination thermostat ends the defrost. $55.94 $ 55. The most common are referred to as a DTFD control (Defrost Termination Fan Delay) or 3 wire control. It’s easy when your cargo doesn’t have restrictive temperature requirements. The evaporator coil cannot go above 32ºF so long as there is still ice in that area, so it stands to reason that if heaters are running on the coil and the coil is still at 32ºF or lower then there is still ice. For simple off cycle defrost, the fans continue to run to keep moving air across the coil and accelerate heat transfer. 3. I looked for the bimetal defrost thermostat clipped on the evaporator. When one goes in defrost the rest continue to run in refrigeration. :) Thank you, in advance for your support! For refrigeration techs, this isn’t surprising, but A/C coils ice over a lot faster than refrigeration coils do. This is often set to 30º or lower before the fan can come back on. If your load can tolerate 5-10 degree fluctuations from your set temperature, then your auto start-stop mode is the best way to go. I've seen ranges all the way from 90 degrees F down to 35. One that stops suction gas flow to the compressor and another that dumps hot discharge gas into that suction line, sending superheated discharge gas out to the evaporator where it rejects its heat to the frost on the lines and is condensed just like in a heat pump. 6. Make a note of the temperature in your notebook and go start a defrost. One that stops flow from the suction line into the compressors and one that directs gas into the suction line and back towards the evaporator. Defrost is necessary when the coil temperature drops below 32°F. This mode maintains a fixed temperature setting for loads that spoil under the smallest difference in temperature. In this case, because this particular sensor closes in colder than 32-degree temps you can’t even use a (freshwater) ice bath to test it. You’ll see this type of control on stuff like prep tables and smaller, under counter type refrigerator units. If your fail safe time is 30 minutes this means that once a defrost cycle begins the LONGEST it will remain in defrost in 30 minutes regardless of the defrost termination thermostat. One of the simplest and most common automatic defrost control strategies is commonly referred to as a “cold control,” more properly called a coil temperature sensing thermostat. Get Questions Answered Instantly. Defrost can be as simple as turning the compressor off for a period of time or as elaborate as reversing the flow of refrigerant for the whole system or for just parts of the system.