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Why Defrost?

Let"s begin with the basics and move on from there. Defrost is crucial when the coil temperature drops listed below 32°F. Defrost can be as basic as transforming the compressor turn off for a duration of time or as fancy as reversing the circulation of refrigerant because that the whole system or simply parts of the system.

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As we were every taught in school, frost buildup is one insulator and prevents warm transfer. Also, airflow through a coil is a large factor. If the coil is iced up, the pan can"t move any kind of air, and also without wait movement, the devices can"t carry out its job. That uses to all devices with defrost, really.

Fin spacing

For refrigeration techs, this isn"t surprising, but A/C coils ice over a lot faster than refrigeration coils do. Why? since the fins on a refrigeration coil space much an ext widely spaced 보다 those on one A/C coil. So, as soon as an A/C coil beginning to obtain cold, and also that little bit that frost starts to develop on the tube surface and also the fin, the affects airflow with the coil much faster than it would if the fins were spaced more widely apart.

Moderate fin spacing medium-temp coil with six fins every inch

Wide fin spacing top top a freezer coil 4 fins every inch

If we had refrigeration coils with fin spacing favor an A/C unit, the coils would ice up also quickly, and we couldn"t acquire anything done. The broader fin spacing illustrated shows just how refrigeration tools can operation longer between defrost cycles. The evaporator coils are built in such a means to accept a certain amount of frost before the power starts to degrade.

So, just how do we gain the task of defrosting done?

The most an easy defrost is one we all most likely remember: Granny taking every little thing out that the “icebox,” unplugging it, and going after ~ it through a screwdriver, hairdryer, or an ice pick. Simple, right?

But there has to be a better way, doesn"t there?

One that the simplest and most usual automatic defrost manage strategies is frequently referred to as a “cold control,” much more properly called a coil temperature sensing thermostat. You"ll occasionally hear it referred to as a “constant cut-in” control.

With either a small coiled bulb on the finish of the sensing tube or a pipe that type of embeds in the evaporator, this senses the temperature the the evaporator coil and cycles the compressor based on that. The succession of occasions runs choose this. Coil temperature rises over cut-in, which is frequently in the top 30s. I like to see around 37°F at the lowest. This setup is nonadjustable, for this reason the name “constant cut-in.” control closes, bringing the compressor on. As the coil temperature drops, the control eventually reaches its cut-out point. I"ve seen this as low together 9°F. The cut-out is what you"re adjusting once you adjust the control.


See what"s happening? Every solitary time the cycles off, the coil temperature has to rise over freezing by enough to ensure a good, complete defrost.

You"ll watch this kind of control on stuff prefer prep tables and smaller, under-counter-type frozen fridge units.

Simple and easy.

A similar an approach for defrost regulate uses a pressure control for to ride bicycle the compressor. Through this form of system, you set the cut-in of the regulate to a saturation press equal to the very same 37°F come 40°F. Remember, this is saturation temp, no air temp. Readjust the cut-out to keep the temperature desired.

The huge drawbacks of these controls space that they aren"t always predictable. The defrost happens as soon as the unit cycles fairly than at a certain time (or times) every day. The temperature can likewise fluctuate end a pretty large range. For part products, specifically fresh meat, broad temperature swings space detrimental to product quality.

Taking a action up from the idea the every turn off cycle is a defrost, we will certainly just add a timer come the circuit. Now, we can set that timer as much as shut the refrigeration turn off at continuous intervals for a certain period. The interval and also duration will certainly be situation-dependent, together we"ll discuss.

Looking at this mechanical timer, the silver- screws in the outer timer ring begin defrost once they rotate previous the tip at the optimal left. The defrost ends when the copper-colored reminder on the within ring rotates previous the same pointer.

This digital timer has tiny black bars top top the display, denote both the time and duration the the defrost. However, the moment of job isn"t shown in the photo. In its most basic form, this timer simply opens the manage circuit to the compressor or the regulate valve for the collection duration of the defrost.

So, what"s happening? As much as the system is concerned, the exact same thing is happening right here that to be happening before when we offered a cold control or a low-pressure switch. We"re shutting the refrigeration turn off and permitting the frost come melt normally off of the coil. The biggest distinction is that with a timer, instead of being topic to the unknown of once the system will bicycle off and how lengthy it will take to melt the frost, assuming the moment of work is set correctly, you deserve to reliably guess the defrost times. You can say the it defrosts at 6 AM and also 6 PM for 45 minutes, and the customer deserve to note that and also account because that it when checking temps on your equipment.

Let"s talk for a minute around how lengthy a defrost demands to last. Obviously, until the coil is totally clear the frost and also ice, we require to understand when that is.

In most cases, the manufacturer will provide guidelines to set your defrost manage system up. It will certainly spell out the frequency or term (time between defrosts) and also duration the the defrosts. Since we"re trying to maintain proper product temperatures, and we acquired away native the cold controls and also low-pressure controls since they to be fluctuating end a wide variety of temperatures, we must look because that a method to limit the fluctuation.

For year this to be only used on defrosts that included heat come the evaporator coil (which we will look in ~ later) however in current years, with much more stringent product temperature requirements and temperature expectations from the customer, linked with government performance mandates, trimming even a couple of minutes turn off of a defrost cycles enhances both product-holding quality and also unit efficiency.

How does that work? The manufacturer will typically install either a thermostat or a temperature sensor top top the coil or in the air stream leave the coil. After trial and error in their labs, they determine just how heat that spot has to be to ensure the coil is cost-free of frost. So, in a hot, humid kitchen in the middle of summer, the defrost runs longer than the does in the center of winter on one outside-access-only cooler box. Why?

We all learned about sensible and also latent heat in school, right? Well, melt frost is simply latent heat included to readjust the state, right? So, due to the fact that we"ll have an ext frost ~ above a coil with higher humidity than on one in a reduced humidity environment, higher frost buildup top top a coil will take longer to melt turn off of that coil, which means that it will take much longer to with that collection temperature.

In practice, here"s exactly how that timer handle defrost. The time of work initiates a defrost, so, let"s say 6 AM. At the time, the timer switches come defrost mode. Internally, that means that the contact in the timer open to de-energize one of two people the manage valve or the compressor. For an easy off bicycle defrost, the fans continue to run to keep moving air throughout the coil and accelerate warmth transfer. The defrost merely ends once the timer will the term pin, switching the timer contacts earlier to closed and energizing the load. If we have a discontinuation control, it"s a generally OPEN contact that close the door on the climb of temperature. So, as soon as that temperature reaches the termination suggest determined by the manufacturer, the call closes, energizing a little solenoid in the timer to press the contacts earlier to the regular position, nevertheless of the timer position. In an electronic control, this is just an additional signal input; either digital (NO/NC contact) or analog (sensor) tells the software program in the controller to switch the relay ago to refrigeration. A coil thermostat or sensor might be collection as low together 34°F, while an waiting sensing regulate will commonly be set between 48 and also 55°F.

Electric Defrost

Since some refrigeration tools runs in ~ temps significantly chillier than 32°F sometimes, we’re walking to need to include some heat due to the fact that there merely isn’t enough heat in the refrigerated space to acquire the frost melted without causing far-ranging damage come the product. The simplest means to include this warm is usually with an electrical heater. Let’s take a watch at how this to add some intricacy to the defrost manage system.

The straightforward timer-type defrost initiation manage doesn’t change. The same kind of timer is used, and when the defrost initiates, the refrigeration circuit de-energizes—same as before. The big difference now is that, in ~ the very same time, we’re energizing a heater that is walking to add heat come melt the frost turn off the evaporator coil. In the case of many pieces the equipment, we’re also going come de-energize the evaporator fan circuit. The is to store the heat concentrated where it is necessary to do the job in as small time together possible. We likewise don"t desire to blow hot, humid air approximately the refrigerated space.

Defrost discontinuation is yes, really the typical for this form of system. Practically all electric defrost equipment will have a kind of built-in defrost termination. The most typical controls are referred to as DTFD (defrost discontinuation fan delay) or three-wire controls. This dual-purpose manage handles both defrost termination, obviously, and post-defrost fan delay, which we"ll get to in a minute or two. The DTFD is typically attached to one next of the evaporator coil in a position that takes the longest to acquire warm throughout defrost. This way, the coil gets the best feasible defrost.

Refrigeration is off; heaters space on, frost is melting away. Every is well. As soon as our DTFD control sees the high occasion temperature, usually about 55°F, it close the door the part of the circuit to end defrost—same together before. The refrigeration machine starts ago up, and we"re moving warmth again, yet wait…

What around the fans? castle aren"t running. Quick! obtain a meter and a ladder…

We"re talking around the other fifty percent of the DTFD control. We"ve terminated defrost (DT). Now, we have to wait a couple of minutes till the coil temperature drops below freezing. We need to remember that the coil was just 55°F, and also there is part humid air quiet trapped in that sheet metal box increase there. Slam the fans on appropriate now, and also you"ll have a wintery Wonderland in your freezer v icicles and also snow everywhere in a week or so. Wait a minute or two, and the coil will certainly freeze that last little of moisture. When the coil temp autumn to around 30°F at the control, our fans will restart.

Gas defrost, particularly for big refrigeration systems, will require whole article in and of itself to covering in any kind of depth. I will try to summary it in a paragraph or two and also give the a much more thorough treatment in the future.

These, prefer all other defrosts, run on a time basis. The equipment where this is an ext common aren"t solitary systems yet multiplex equipment with multiple evaporators operating on various schedules. When one goes right into defrost, the rest continue to operation in refrigeration.

When the timer initiates a defrost, a couple of things happen all at once. A differential valve de-energizes to produce a press differential to permit flow in reverse. To develop a section of reversed gas flow, the device uses two actuating valves—one that stops suction gas circulation to the compressor and another the dumps hot discharge gas right into that suction line, sending out superheated discharge gas the end to the evaporator where it rejects its heat to the frost top top the lines and is condensed just like in a warm pump. It returns to the system through a examine valve piped roughly the TEV—the same just like a warmth pump. There is no the pressure differential, the warm gas cannot flow properly v the inspect valve.

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Once either the time limit or the discontinuation temperature is reached, all of those valves return to their regular positions, and also the refrigeration bicycle resumes normally.