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I’ve heard EPR (Evaporator pressure regulator) valves called suction regulators or hold back valves. In essence they hold back against the suction line to maintain a set evaporator evaporation or boiling temperature.
In refrigeration rack systems EPR valves play a vital role in ensuring that the product is cooled consistently and nearly constantly.
In an A/C system we have a TXV that maintains a constant superheat at the evaporator outlet. The evaporator temperature itself will fluctuate up and down depending on load.
In a refrigeration case you must first ensure you have full line of liquid using a sight glass or by checking subcooling. Then you make sure the case has proper airflow etc… then you set the EPR to maintain the proper coil evaporation temperature (by holding back pressure as needed) and then you check and / or set the TXV to the proper superheat. This ensures BOTH proper coil feeding as well as proper coil temperature.
Pretty cool right? (Pun intended)
We have discussed DTD (Design Temperature Difference) quite a bit for air conditioning applications, but what about refrigeration? Let’s start by defining our terms again
Suction Saturation Temperature
Saturation temperature is the temperature the refrigerant will be at a given pressure if it is in the process of changing state. This change of state would be from liquid to vapor (boiling) in the case of the low side (evaporator / suction line). When we look at saturation temperatures instead of pressures we can use similar rules and we will see similar saturation temperatures across all refrigerants when the application is the same. Experienced HVAC and refrigeration techs pay far closer attention to the saturation temperatures than they do pressures.
Evaporator TD and DTD
Evaporator TD (temperature difference) is the measured difference between the suction saturation temperature (evaporator boiling temperature) and the box temperature. DTD (design temperature difference) is the designed or expected TD.
Many A/C techs will confuse TD with Delta T. Delta T is the difference between the evaporator AIR temperature entering the coil to the air temperature leaving the coil. The Delta T will vary based on the humidity in the box where TD will not.
Target Box Temperature
The temperature the refrigeration box should maintain when the system is operating properly
The increase in temperature between the suction saturation temperature and the suction line temperature leaving the evaporator. Superheat is the temperature (sensible heat) gained between the point that all of the liquid boiled off in the evaporator coil and the suction line at the outlet of the coil. in refrigeration, like HVAC 10°F(5.5°K) of superheat is average with a range from 3°F to 12°F(1.65°K – 6.6°K) depending on the equipment type (10°F(5.5°K) for med temp, 5°F(2.75°K) for low temp, 3°F(1.65°K) for ice machines ).
Hot Pull Down
Refrigeration equipment is unlike HVAC equipment in that the evaporator will spend most of its life running in a very stable environment with minimal fluctuation in the box temperature.
On occasion a refrigeration system will see a huge change in load in cases where it was off and needs to “pull down” the temperature, or when doors are left open or when a large quantity of warm product is placed in the box. When a piece of refrigeration equipment is in hot pull down it cannot be expected to abide by the typical DTD or superheat rules and must be allowed to get near the design box temperature before fine adjustments are made to the charge, TXV superheat settings or to the EPR (Evaporator Pressure Regulator) if there is one.
Design Temperature Difference (DTD)
In air conditioning applications a 35°F(1.66°C) DTD is a good guideline for systems that run 400 CFM(679.6 m3/h) of air per ton of cooling (12,000 btu/hr). In refrigeration the DTD is much lower than in air conditioning.
There are several reasons for this but one big reason is the desire to maintain relatively high relative humidity levels in refrigeration to keep from drying out and damaging product. Keep in mind that NOTHING is a substitute from manufacturer’s data but here are some good DTD guidelines for traditional / older refrigeration equipment. Keep in min dthat the trend is toward lower evaporator TD on newer equipment.
Walk-ins 10°F(5.5°K) DTD +/- 3°F(1.65°K)
Reach-ins 20°F(11°K) DTD +/- 5°F(2.75°K)
A/C 35°F(13.75°K)) DTD +/- 5°F(2.75°K)
You then subtract the DTD from your box temperature / return temperature to calculate your target suction saturation. You can then use this target saturation / DTD and compare it to your actual measured saturation and DT once the box is within 5°F – 10°F(2.75°K – 5.5°K) of it’s target temperature to help you set your charge, TXV and EPR as well as diagnose potential airflow issues when compared with suction superheat and subcooling / clear site glass.
For Example –
If you have a medium temp walk-in cooler with a 35°F(1.66°C) box temperature you would expect to see a suction saturation of 25°F(13.75°K) +/- 3°F(1.65°K)
When doing a quick inspection of a piece of refrigeration equipment without gauges you can use this data to do the following calculation –
35°F – 10°F(19.25°K – 5.5°K) DT + 10°F(5.5°K) superheat = 35°F(19.25°K) suction line temperature +/- 3°F(1.65°K)
In this particular case logic tells us that the suction line could be no WARMER than 35°F(1.66°C) because that is the temperature of the air the refrigerant is transferring its heat to. However by the time you factor in the the accuracy of your box thermometer and line thermometer and the assumed saturation temperature you would still expect a 35°F(1.66°C) suction line temperature +/- 3°F(1.65°K)
For a -10°(-23.33°C) box, low temp reach-in you would calculate it this way
-10°F- 20°F(-5.5°K to -11°K) DT + 5°F(1.65°K) superheat = -25°F(-13.75°K) suction line temperature +/- 5°F(1.65°K)
Clearly, this is NOT the way to commision a new piece of equipment or to benchmark a system you haven’t worked on before, but it can give you a quick glimpse at the operation of a piece of refrigeration equipment without attaching gauges, especially on critically charged or sealed systems.
The best practice is to know the equipment you are working on, read up on it and properly log benchmark data the first time you work on a piece of equipment or during commissioning.
It should also be noted as Jeremy Smith pointed out, in recent years TD’s have been decreasing as manufacturers seek higher efficiency through higher suction and lower compression ratios.
This means that TD’s as low as 5 can be designed into some units but keep in mind… the suction line can still be no warmer than the box so as DTD drops so does superheat and the critical nature of expansion valve operation.
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
Do you know how a solenoid valve works?
On the surface, I think we all understand how a solenoid valve works. The Coil energizes creating an electromagnet. That temporary magnetism lifts an iron plunger within the valve itself allowing refrigerant to flow.
But… is it really that simple?
Turns out, the answer isn’t as straightforward as you’d expect.
The simplest type of solenoid valves are direct acting solenoid valves. These are exactly what is described above. The iron plunger directly controls the flow of refrigerant through the valve. Every single solenoid valve you see incorporates a direct acting valve, but there is more than what meets the eye.
Direct acting solenoid valves have an inherent limitation. If the force created by the fluid flowing through the valve that is acting on the iron plunger is enough to lift that plunger, then it isn’t going to close regardless of what the electromagnetic coil tries to tell it to do. What this means is that direct acting solenoid valves are limited in size, and that size is pretty small.
So, how can we control the fluid flow in larger lines with solenoid valves?
We start to use the pressure within the system to actually force the valve closed.
These are called pilot operated or pilot actuated valves. The direct acting solenoid doesn’t try to control the entire flow, it only acts to control a small portion of the fluid which acts on a diaphragm or other device to open and close the valve.
Let’s see if we can start to understand how these valves work in practice.
First, a few basics.
Ok. Refrigerant flowing through an energized solenoid. Now, the coil de-energizes causing the iron plunger to drop and seal a tiny port. What this does it stop a small amount of flow from inlet to outlet, preventing that small flow from leaving the valve body. That small port being blocked causes pressure to build on top of the diaphragm or valve seat disc, forcing it down to seal the valve. The small iron plunger and spring don’t have the force required to force the valve closed but, by utilizing system pressure, we have a much larger amount of force available.
In truth, the large majority of solenoid valves a technician sees are pilot operated valves.
— Jeremy Smith CM
CO2 (R744) is naturally better suited for lower temperature refrigeration applications because of its low temperature saturated state at atmospheric pressure (-109.3F). You will notice I said “saturated state” because CO2 does not “boil” at atmospheric pressure. At any pressure below 60 psig CO2 goes straight from solid (dry ice) right to a vapor, This is why 60 psig is known as the “triple point” or the point that could be either solid, liquid or vapor.
Now go to the top of the range with CO2, when you apply 1055 psig the saturation temperature is 87.8F but go up even 1 more degree and CO2 CANNOT be liquified, this is known as the critical point of the substance. Whenever a substance is forced beyond it’s critical point it becomes what is known as a supercritical fluid and has properties that are unique to this state but it is certainly not a liquid. You can see more in this natural refrigerants PT chart.
In a transcritical (trans means beyond or through so transcritical means “beyond critical”) booster refrigeration system the low temp portion of the system operates using it’s own compressors that “boost” the refrigerant from the low temp side and discharge into the suction of the medium temp side. The high stage compressors then pressurize the CO2 (R744) above its critical pressure / temperature.
What is traditionally called a condenser becomes a gas cooler and decreases the temperature (rejects heat from) of the discharge without actually condensing it into liquid. The cooled supercritical fluid goes through a pressure reducing valve, where some of it condenses into liquid and the rest remains as gas. Liquid and gas are separated in a flash tank (receiver). Pressure in this tank are usually controlled to around 450 to 500psig.
It’s super critical that you understand all of this…
See what I did there.
Let’s take a walk through the startup and commissioning procedure of a conventional or “single” refrigeration condensing unit. We’re going to start with a unit that is fully piped in and has been pressurized for leak and strength testing. For brevity, we are going to assume a basic familiarity with industry standards, company and customer policies and requirements and the job site and any policies in place there.
Before we even swing a wrench at the machine, let’s familiarize ourselves with the job site and equipment. Take a walk around, check with the job supervisors, check in with other trades, etc. Find all the equipment you’re to be starting and make notes.
Step one is to make final leak tests. Typically, the installer records time, date and pressure data. If you put the pressure charge on the equipment, you should have done so. If you have had any temperature change, you MUST account for that. While nitrogen is chemically inert, all gasses respond to changes in temperature by changing pressure. The math is simple and I’ve addressed it in another article.
We’ll assume here the installers did their job properly and that there are no leaks to track down and fix. Blow that charge off and break out a nitrogen cylinder. Yeah, I know, you just blew off the nitrogen… That’s OK – you aren’t leak checking anymore.
Disconnect the pressure controls; you’re going to SET them. If they’re the little-encapsulated type, we’re at least going to check and record the operating pressures. If they’re of the brazed in variety, you’ll need to try to isolate parts of the system to pressurize them to test. I recommend referring to the manufacturer’s literature for proper control settings and, if they don’t offer a guideline, referring to the Heatcraft installation manual for guidance. Use your nitrogen regulator and manifold gauges to adjust each control to precisely the setting desired. The procedure that I typically use is to adjust the control to a setting that is near the top of the scale, set the applied pressure with the nitrogen to the desired pressure then adjust the control down until it closes. Some controls have a very audible ‘click’ when they open and close, others will require you to use an ohmmeter to determine when the contacts open or close.
Use a sharpie to record that setting right on the cover of the control or, if you prefer, inside the electrical control panel. Since many larger customers have specific commissioning paperwork they require, you might as well get your notebook out and record it there, too.
Once you’re done setting the controls, it’s time to evacuate the equipment. Make sure you’ve opened any and all service valves in the system and that any control valves are open or that you’re connected on both sides of the valve.
Even though you’ve got a lot of work to do while the pump is running, I still prefer to use the faster no manifold, large hoses, core pulling method. That way, I’m spending more time in a deep vacuum and, if something goes wrong and I have to make a repair and evacuate the system a second time, I’m not spending a lot of time watching a vacuum pump run. I’m not even digging my micron gauge out just yet, just hook up the pump and let it run.
While the pump is running, you’ve got some details to attend to.
Get that micron gauge out now and let’s check the progress of our evacuation. Again, much has been written on this subject, so I don’t want to belabor the point of the how and the why here. Pull a proper deep vacuum on the equipment according to the customer’s standards, the manufacturer’s standards or industry standards and record times and evacuation levels here if the commissioning paperwork requires it.
Evacuation complete, here is where starting a refrigeration unit diverges from starting up a residential one. Residential equipment typically comes precharged for a specific amount of line set length. All you have to do is open the lines, start the equipment and check charge. Split refrigeration equipment doesn’t come precharged because the manufacturer can’t know how their machine is going to be installed. We will have to field charge it.
Final checks before charging:
Put the cylinder on a scale and start adding refrigerant to the equipment. Techniques vary somewhat here, but I start by adding liquid refrigerant straight into the receiver valve and liquid line while monitoring suction pressure. Suction pressure is rising, so we’ve got flow through the system. If we don’t see a suction rise, we need to stop and investigate. Maybe a valve is closed or not energized properly. For us, everything is going nicely, go ahead and close that disconnect to the condensing unit to energize the equipment. If you weren’t able to test phase rotation earlier, now is the time. Verify that the compressor and fan motors are rotating in the correct direction and make any corrections necessary before proceeding any further.
While adding gas, the compressor is going to short cycle a fair bit while you’re getting enough refrigerant into the system to keep it running. Some guys like to bypass the low-pressure safety and I’ve done that. I’m also not opposed to opening up some liquid to the suction line. Not full flow, but get some in there.
For right now, we’re going to charge this unit to a moderately cloudy/bubbly sight glass. We’ll come back and finalize the charge later and I’ve always found it easier to start low and add up to final charge than to add too much and have to remove some or be uncertain of our charge. It’ll work, sorta, even low on charge. Once the machine is running on its own, add just enough to get that sight glass in that cloudy state and stop. Record the amount of refrigerant added in your notebook. Monitor pressures and suction temperature for right now. If your superheat really starts to drop into the flooding range, it’s time to go check the evaporator to see why, but that’s pretty rare.
Continually monitor temperature in the box while monitoring the unit operation until box temp gets to within 5° or so of the desired temperature.
Now, we get to do some wrench twisting.
First things first, you MUST HAVE a solid column of liquid to continue, so add the rest of the charge. Clear the glass and add your flooding charge. Record that total amount of refrigerant added both on the unit and in our notebook
Now, let’s go check and set the superheat. Having a box that is close to temp and a solid column of liquid is important because without both conditions being present, a TEV cannot properly regulate superheat at the coil. Connect your gauges and temperature probes and monitor for a couple minutes. Again, record the information. Pressures, temperatures, superheat…. Write it down. Adjust the superheat to the manufacturer’s or customer’s specifications. Be aware as you’re doing this that the unit may cycle off and throw your readings off. You can adjust thermostats or bypass controllers to keep the unit running while doing this but be careful to not allow the unit to get too cold as this will affect the operation of the valve at normal conditions.
Final details and checkout.
Now our unit is running and we’ve got everything setup right where we need it, we need to turn our attention to details.
Now we can sit down, fill out the customer’s paperwork and submit that to them.
Before leaving the job site, it should go without saying that we need to clean up any debris left behind. I also like to present the customer with their case manuals, give them a quick run through of the equipment and answer any questions they have. Be sure to leave a business card because even though we’ve been diligent in starting and commissioning this equipment, they may have problems or just questions down the road.
— Jeremy Smith CM
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
In this episode of HVAC School Bryan talks with Jeremy Smith and they discuss
And Much more…