Tag: cold control

Why Defrost?

Let’s start with the basics and move on from there. Defrost is necessary when the coil temperature drops below 32°F. 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.

 

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. If the coil is iced up, the fans can’t move any air and without air movement, the equipment can’t do its job. This applies to all equipment 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? Because the fins on a refrigeration coil are much more widely spaced than those on an A/C coil. So, when an A/C coil starts to get cold and that little bit of frost starts to build on the tube surface and the fin, it affects airflow through the coil much faster than it would if the fins were spaced more widely apart.

Moderate fin spacing medium temp coil with 6 fins per inch

Wide fin spacing on a freezer coil four fins per inch

If we had refrigeration coils with fin spacing like an A/C unit, it would ice up too quickly, and we couldn’t get anything done. The wider fin spacing illustrated shows how refrigeration equipment can run longer between defrost cycles. The evaporator coils are built in such a way to accept a certain amount of frost before the performance starts to degrade.

 

So, how do we get the job of defrosting done?

 

The most basic defrost is one we all probably remember Granny taking everything out of the “icebox,” unplugging it and going after it with a screwdriver, hair dryer or an ice pick. Simple, right?

 

But there has to be a better way, doesn’t there?

 

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. You’ll sometimes hear it called a “constant cut in” control.

 

With either a little-coiled bulb on the end of the sensing tube or a tube that kind of embeds in the evaporator, this senses the temperature of the evaporator coil and cycles the compressor based on that. The sequence of events runs like this. Coil temperature rises above cut in which is typically in the upper 30s. I like to see about 37°F at the lowest. This setting is nonadjustable, hence 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 as 9°F. The cut out is what you’re adjusting when you adjust the control.

See what’s happening? Every single time it cycles off, the coil temperature has to rise above freezing by enough to ensure a good, complete defrost.

You’ll see this type of control on stuff like prep tables and smaller, under counter type refrigerator units.

 

Simple and easy.

 

A similar method for defrost control uses a pressure control to cycle the compressor. 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.

 

The big drawbacks of these controls are that they aren’t always predictable in that the defrost happens when the unit cycles rather than at a specific time (or times) every day and that the temperature can fluctuate over a pretty wide range. For some products, especially fresh meat, wide temperature swings are detrimental to product quality.

 

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. Now, we can set that timer up to shut the refrigeration off at regular intervals for a specific period. The interval and duration will be situation dependent as we’ll discuss.

Looking at this mechanical timer, the silver screws in the outer timer ring initiate defrost when they rotate past the pointer at the top left. The defrost ends when the copper-colored pointer on the inner ring rotates past the same pointer.

This digital timer has little black bars on the display indicating both time and duration of defrost. In that picture, the time of day isn’t indicated in the photo. 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.

 

So, what’s happening? 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. We’re shutting the refrigeration off and allowing the frost to melt naturally off of the coil. 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. Now, you can say that it defrost at, 6 AM and 6 PM for 45 minutes and the customer can note that and account for it when checking temps on their equipment.

 

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….

 

In most cases, the manufacturer will give guidelines to set your defrost control system up. It will spell out frequency or interval (time between defrosts) and duration of the defrosts. Because we’re trying to maintain proper product temperatures and we got away from the cold controls and low-pressure controls because they were fluctuating over a wide range of temperatures, we need to look for a way to limit that fluctuation.

 

For years this was only used on defrosts that added heat to the evaporator coil (which we will look at later) but in recent years with more stringent product temperatures requirements and temperature expectations from the customer, combined with government efficiency mandates, trimming even a couple minutes off of a defrost cycles improves both product holding quality and unit efficiency.

 

How does it work? The manufacturer will typically install either a thermostat or a temperature sensor on the coil or in the air stream leaving the coil. After experimentation in their labs, they determine just how warm that spot has to be to ensure the coil is free of frost. 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. Why?

 

We all learned about sensible and latent heat in school, right? Well, melting frost is just latent heat added to change the state, right? 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.

 

In practice, here’s how that timer handles defrost. Time of day initiates a defrost, so say 6 AM, the timer switches to defrost mode. Internally, that means that the contacts in the timer open to de-energize either the control valve or the compressor. For simple off cycle defrost, the fans continue to run to keep moving air across the coil and accelerate heat transfer. The defrost ends, in the simplest form, when the timer reaches the duration pin, switching the timer contacts back to closed and energizing the load. If we have a termination control, it’s a normally OPEN contact that closes on the rise of temperature. So, when that temperature reaches the termination point determined by the manufacturer, the contact closes energizing a small solenoid in the timer to push the contacts back to normal position regardless of the timer position. In an electronic control, this is just another signal input, either digital (NO\NC contact) or analog (sensor) that tells the software in the controller to switch the relay back to refrigeration. A coil thermostat or sensor might be set as low as 34°F while an air sensing control will typically be set between 48 and 55°F.

 

Electric Defrost

Since some refrigeration equipment runs at temps significantly colder than 32°F sometimes, we’re going to need to add some heat because there simply isn’t enough heat in the refrigerated space to get the frost melted without causing significant damage to the product. The simplest way to add this heat is usually with an electric heater. Let’s take a look at how this adds some complexity to the defrost control system.

 

The basic timer type defrost initiation control doesn’t change. The same type of timer is used and when defrost initiates, the refrigeration circuit de-energizes the same as before. The big difference now is that, at the same time, we’re energizing a heater that is going to add heat to melt frost of the evaporator coil. In the case of most pieces of equipment, we’re also going to de-energize the evaporator fan circuit. This is to keep the heat concentrated where it is needed to do the job in as little time as possible. We also don’t want to blow hot, humid air around the refrigerated space.

 

Defrost termination is really the standard for this type of system. Almost all electric defrost systems will have a type of defrost termination built in. The most common are referred to as a DTFD control (Defrost Termination Fan Delay) or 3 wire control. This dual-purpose control handles both termination of defrost obviously and post-defrost fan delay which we’ll get to in a minute or two. The DTFD is normally attached to one side of the evaporator coil in a position that takes the longest to get warm during defrost. This way, the coil gets the best possible defrost.

 

Refrigeration is off; heaters are on, frost is melting away. All is well. 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. Refrigeration machine starts back up and we’re moving heat again, but wait…. What about the fans? They aren’t running. Quick get a meter and a ladder…

This is the other half of the DTFD control. We’ve terminated defrost (DT) now we have to wait a couple of minutes until the coil temperature drops below freezing. We have to remember that coil was just 55°F and there is some humid air still trapped in that sheet metal box up there. 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. Wait a minute or two and the coil will freeze that last bit of moisture. When the coil temp drops to around 30°F at the control, our fans will restart.

 

Gas defrost, particularly for large refrigeration systems is going to require an entire article in and of itself to cover in any depth. I’m going to try to summarize it in a paragraph or two and give it a more thorough treatment in the future.

 

These, like all other defrost, operate on a time basis. The systems where this is more common aren’t single system but multiplex systems with multiple evaporators operating on different schedules. When one goes in defrost the rest continue to run in refrigeration.

 

When the timer initiates a defrost, a few things happen all at once. A differential valve de-energizes to create a pressure differential to allow flow in reverse. To create a section of reversed gas flow, two valves actuate. 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. It returns to the system through a check valve piped around the TEV, same as with a heat pump. Without the pressure differential, the hot gas cannot flow properly through the check valve.

 

Once either the time limit is reached, or the termination temperature is reached, all of those valves return to their normal positions, and the refrigeration cycle resumes normally.

 

— Jeremy Smith CMS

 

This article is written by Christopher Stephens of JVS Refrigeration in California with just a few additions by me (Bryan) in italics. Thanks, Chris!


Reach in refrigerators are an interesting side of our industry, often looked at as frustrating and troublesome. Often working in kitchens or convenience stores the refrigerators are never located in a convenient place to work on them, and that tends to lead to frustration on the technician’s part. Please understand my article pertains to medium temperature refrigerators. I also advise you to use manufacturers OEM parts when possible as the unit was designed to work with them. One of the more misunderstood and misdiagnosed parts is the temperature controller.

Keep in mind that some refrigeration temperature controls sense the evaporator coil temperature (not the desired box temperature) some use intake air sensors and some use supply air sensors. The medium being sensed (Coil, return air (intake) or supply air (discharge) will greatly impact how the controls function and what impacts them.

Personally, I break temperature controllers down into five different types, please understand that these are generic descriptions and you should always lean on the manufacturer if possible to understand their control strategies. 

  1. Standard Pressure Control – these work on the principle that at any given pressure saturated refrigerant is a constant temperature. This style of control is not used very much anymore as a means of temperature control because it is not very precise and to an untrained technician, it can be hard to set the temperature correctly.To use this control strategy, you need to understand what evaporator T.D. (Temperature differential) your reach-in was designed with, you will need a temperature pressure chart, you will need an accurate set of refrigeration gauges, and an accurate thermometer. With all these tools you can take your desired box temperature and find it on your pressure chart read across the pressure chart and find the corresponding pressure for your desired box temperature and that will be your cut-in pressure to set your control at. Now we need to find the cut-out setting typically we want the system to have about a 5-8 degree differential between the cut out and cut in to reduce system short cycling this will likely be about 8 degrees colder than the cut-in temperature, so take your desired box temperature subtract your differential of 5-8 then subtract your designed evaporator T.D. (specific to the equipment but likely 20 -30 degrees for reach ins) and find that number on the temperature pressure chart than read across the pressure chart and find the corresponding pressure and that will be your cut out setting.  Understand that pressure controls are never exact, so you will need to adjust accordingly in the field.
  2. Constant Cut in Control (electromechanical) – These are one of the most common temperature control’s that you will find in reach in refrigerators because they are the most economical for the manufacturers as they have an off cycle defrost built into them. They work by inserting the sensing bulb into the evaporator coil and they have a set temperature that they turn back on (cut in) at no matter how cold you turn the dial. They work very similar to the pressure control as they are designed with the evaporator T.D. (Temperature Differential) in mind, but instead of using pressure they sense the evaporator temp on the surface of the coil, they do have a knob to adjust the cut-out temperature, but you have no control over the cut in temperature that is why they are called constant cut in. By design they also have a built-in defrost as the cut in temperature is usually 37 to 41 degrees (for a cooler/refrigerator) depending on the manufacturer. They rely heavily on proper superheat and proper refrigerant charge. If the charge is incorrect or the superheat is not correct the coil could get too cold and the control could prematurely shut off. This could lead a technician to diagnose a bad control if they did not understand how they work.  If you come upon a reach in that is short cycling and shutting off too soon, make sure to check the charge and measure the evaporator superheat before you diagnose a bad control.
  1. Constant cut in (Digital) – These work the same as the electromechanical control, but they typically have two probes one to be located in the coil and one to be located in the return air stream. They tend to have more features that are available, such as an added defrost cycle based off time (every four hours, every six hours, etc.…..)  while still using box temperature as a fail-safe. For example, say the control has a defrost every four hours if the coil temperature comes above a pre-determined temp say 40 degrees the control will terminate the defrost. The controls can also shut off the evaporator fan motors during the off cycle to save energy and reduce warm is intrusion into the unit. These types of controls are on many HC (Hydrocarbon) units being built today.
  2. Universal electromechanical – these typically have one sensing bulb that you mount in the return air stream and they turn on and off via the temp setting.
  3. Universal digital – These are usually aftermarket controls and can have several different control strategies and can usually be customized to do anything, from heating to cooling to defrost depending on the manufacturer.

 

Something to understand is that reach in refrigerators is usually designed to perform in a certain environment and if something changes such as the ambient temp in that environment, or if doors are left open. The box will not perform correctly, I suggest you take a step back before you start throwing parts at a reach in and evaluate the environment you may find your problem there!

— Chris

As always I suggest “Commercial Refrigeration for Air Conditioning Technicians” by Dick Wirz as the bible for refrigeration training 

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