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Let’s take a deeper dive into the magic that is gas defrost..
Most techs who are familiar with heat pumps understand the basics of a gas defrost but when we apply this strategy to a larger system where we’re only reversing a small part of the system while we need to add some controls and valves to get the job done optimally.
Since we’re already familiar with the basics of defrost systems and controls, I’m not going to dwell on things like frequency or duration of defrost but we will get into some unique terminations methods and defrost efficacy testing that only work with reverse cycle defrosts.
There are 2 basic types of gas defrosts. 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. Each have advantages and disadvantages but both work essentially the same way.
So, defrost starts and a whole lot starts happening at once. 3 electrically actuated valves all have to work together to make this happen.
First, we need to create a pressure differential between the gas we’re sending into the evaporator and the liquid line. This is to allow that gas to flow through the evaporator and back into the liquid line. There are many different valves that are applied to do this and an in depth treatment of each valve isn’t really possible here, so we’ll just look at the 2 most common places they’re applied.
This is more common on hot gas defrost system as opposed to Kool gas systems. A valve is installed in the discharge line that, when activated, creates a pressure differential.
Same thing, really. This valve will work for either but is really necessary for a Kool gas system. A discharge differential won’t work for Kool gas.
Regardless of the location in the system, the valve is typically adjusted for an 18-20 PSI differential setting. If your equipment is significantly higher than your evaporator this may need to be set even higher. We’ll get into a method to test this and ensure that the defrost is working properly towards the end of the article.
Differential created, we now need to direct defrost gas to the evaporator. To do this, we have 2 valves. 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. At the same time the differential valve activates, both of these valves activate and start the defrost process.
Photo caption: the grey bodied valve, installed in the vertical line stops refrigerant flow to the compressors. The brass valve installed on the horizontal line opens to admit hot gas to the evaporator.
Out in the evaporator, we’ve got a check valve piped to bypass the TEV
not visible in this photo is the actual check valve. The line leaving the distributor allows condensed liquid to leave the coil, bypass the metering device and re-enter the liquid line through a check valve.
Last thing is that, with all this heat being forced into the evaporator we normally want to turn the evaporator fans off and sometimes turn on small heaters to prevent water running off the coil from freezing on a cold drain pan. Using either a pressure switch that cycles
Let’s “follow the gas” and try to visualize what’s happening during this defrost. So, we’re sending high pressure, superheated vapor into a cold suction line. That gas immediately starts rejecting heat into the surrounding pipe and any frost or ice that’s in contact with it. Remember, we’re going backwards, so we hit the outlet of the evaporator and we’re heating it up, melting that frost away and rejecting heat from the gas all the way. As we continue to pass through the evaporator, we’re going to reach a point where we’ve rejected enough heat to condense and possibly to even subcooling as a liquid. Eventually, we reach the metering device and are routed through a check valve that bypasses that and wind up in the liquid line. With a Kool gas defrost, we aren’t starting with superheated vapor, but the concept remains the same. Warm, saturated vapor is sent to the evaporator where it condenses and is subcooled and forced back into the liquid line.
As liquid is condensed and pushed through the check valve, more and more hot gas is allowed into the evaporator to provide more heat to completely defrost the coil. 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.
Is one ‘better’ than the other?
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. Remember that copper can expand over an inch per 100’ of pipe with a 100° change in temperature, so we have to consider the expansion and movement of the piping.
Using a Kool gas defrost helps with the pipe expansion problems but tends to have less heat available for defrost and, combined with a modern push to lower compression ratios for efficiencies sake, can have problems clearing the whole coil during colder weather.
So, what can go wrong??
Sounds like a great system. We’re reusing heat that would ultimately be wasted to melt frost from a coil. Economically and ecologically awesome, right?
As with any complex system, there are multiple points of failure. 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.
If the differential valve fails, we won’t have adequate flow of refrigerant to get enough heat for a complete defrost. Similarly, if the solenoid valve that opens to allow defrost gas into the suction fails to completely open, we won’t have enough flow.
If the suction stop solenoid fails to close, we’ll can see a range of problems from inadequate defrost from the amount of bleed through to a complete failure to close that allows all of the defrost gas to flow straight into the compressors. You can see this same problem if the hot gas solenoid fails to close properly after a defrost.
I promised earlier that I’d give a method to test gas defrosts to ensure that they’re working properly.
For this test to work properly, we need a coil that is free of large ice buildup but that has a ‘normal’ frost on it. If I’m troubleshooting a particularly difficult system, I’ll first clear all ice from the coil, then disable defrost overnight and return in the morning to ensure that I have the right conditions to test the defrost.
Now, I’ll connect a thermometer to the line that bypasses the TEV at the evaporator and allow that to stabilize. I really like to use a thermometer that record Min/Max readings for this job. You can also take temperature on the line leaving the evaporator or really anywhere along the liquid line that is dedicated 100% to that circuit. 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.
Make a note of the temperature in your notebook and go start a defrost. Monitor this temperature and a distinct pattern should emerge if defrost is functioning properly. The temperature will hold stable for a couple minutes. Typically this is already pretty cold because we’re in a refrigerated space, then it will start to drop. 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 to -6. This is a rush of liquid that has condensed in the evaporator and has rejected so much heat that it is very subcooled.
This temperature will then start to rise as there is less and less frost to absorb heat from the gas. Once all the frost is gone, this will start rising pretty rapidly. Once it hits 65° on newer equipment and 75° 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.
Much of the timing depends on the length of the suction line and the amount of frost buildup on the coil. A shorter suction line will result in a faster temperature drop while more frost on the coil will result in a slower but deeper dip in temperature before it starts back up.
This is also probably the best method to use to terminate this type of defrost. Monitor that temperature using whatever means available to you and, once the liquid temperature rises above either a manufacturer’s predetermined setting or one that you’ve field determined through testing, you can end defrost.
–Jeremy Smith CMS
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.
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 airstream 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 an 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.
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 defrosts, operate on a time basis. The systems where this is more common aren’t single system but multiplex systems with multiple evaporator 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 CM
When you work on a heat pump system and you want to test defrost there are so many different test procedures to follow to test the board and sensors. Most involve “forcing” a defrost by shorting out pins on the board or advancing the time on the defrost initiation by shorting our pins or installing a factory provided pin jumper.
Lots of pins and jumping involved.
But one thing to need to be be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost.
A thermostat is an open and closed switch, they are usually round in shape like the one shown above and they open within a set temp range and they close within a set temp range. The one shown above is a Carrier Defrost Thermostat and it closes at 30 degrees +/- 3 degrees and it opens at 65 degrees +/- 5 degrees. In this case because this particular sensor closes in colder than 32 degree temps you can’t even use an ice bath to test it. If it is below 32 outside it is easy to test (duh) otherwise you can just run it in heat mode with the cond fan off and see when it closes by using an Ohmmeter.
On a defrost thermostat you can also easily jump it out to test the board since it is just open an closed.
A defrost sensor is a thermistor. A thermistor changes resistance based on the temperature it is exposed to. In order to test you can measure the ambient temperature, make the the sensor is removed and acclimated, measure the Ohms of resistance and compare to the manufacturer chart.
You CANNOT jump out a thermistor.
P.S. – A new podcast about Heat pumps is out today HERE
In this episode of HVAC school Bryan covers
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In this episode of HVAC School Bryan talks with Jeremy Smith and they discuss
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