Tag: refrigeration

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.

Delta T

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 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 +/- 3°F

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 DT + 10°F superheat = 35°F suction line temperature +/- 3°F 

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 DT + 5°F superheat = -25°F suction line temperature +/- 5°F 

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.

— Bryan


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 are 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 

This article is written by Jeremy Smith CM, experience refrigeration tech and all around great dude. Thanks, Jeremy

A very common means of control seen on refrigeration equipment is the pump down control. Why do we use this rather than just cycling the compressor off and on like a residential HVAC unit?

Since most refrigeration equipment tends to be located outdoors, it comes down to ambient temperatures and the basic properties of refrigerant we all understand about temperature and pressure and how they can conspire to kill a compressor.

During periods of low ambient temperatures, if we were to just cycle the compressor off, it can easily get colder at the compressor than it is inside the space.   If the compressor cycles off for long enough as it would during a defrost cycle, refrigerant vapor will start to condense within the crankcase.  If we are lucky, the extent of this problem will be a unit that doesn’t start because the pressure of the refrigerant is lower than the cut in setting of the pressure control.  What typically happens, though, is that enough refrigerant will condense to start to settle under the lubricating oil causing a lack of lubrication on restart leading to bearing wear and premature failure.  If enough refrigerant condenses within the compressor housing, the resulting damage could cause valves, pistons and other internal parts to break if liquid gets into the cylinders.

How can we prevent this?

One thing that is applied across almost all sectors of our industry is crankcase heaters.   These small heaters, either immersion style heaters or wrap around style heaters add a small amount of heat to help keep the compressor oil warm and help to prevent vapor from condensing there. The effectiveness of these are limited by the wattage of the heater, the ambient temperature and the size of the compressor.   Too low an ambient or too large a compressor and they start to lose some effectiveness.

So, how else can we prevent condensation within the compressor?  Let’s look to the pressure/temperature relationship of refrigerant for the answer.   If we lower the pressure in the crankcase to a point where the saturation temperature of the refrigerant is below the ambient temperature the compressor is in, refrigerant cannot condense.   This is why we use a “pump down” type system.

In operation, a pump down control consists of little more than a liquid line solenoid valve, a thermostat control and a low-pressure control.   When the thermostat or defrost control opens, the solenoid de-energizes, stopping the refrigerant flow and allowing the system to pump the suction pressure down before the low-pressure control turns the compressor off.

How low should we set that cut-out?   The Heatcraft installation manual has us setting the cut out as low as 1” Hg vacuum, depending on the minimum expected ambient.  I like to set the cut in just below the lowest expected ambient temperature so that you don’t wind up in a situation like I mentioned earlier.   If the ambient gets too low and the cut in is too high, your unit won’t cycle on until it warms up enough resulting in a preventable service call.

Combining a pump down control with a crankcase heater and ensuring that all controls work properly at all times can save your compressor from damage in cold weather.


Jeremy Smith, CM

When we say that there is “flash gas” at a particular point in the system it can either be a bad thing or a good thing depending on where it is occurring.

Flash gas is just another term for boiling.

It is perfectly normal (and required) that refrigerant “flashes” or begins boiling directly after the metering device and as it moves through the evaporator coil. In order for us to transfer heat from the air into the refrigerant in large quantities we leverage the “latent heat transfer of vaporization”. In other words we transfer heat into the boiling refrigerant, or “flash gas”.

In a boiling pot of water we create flash gas by increasing the temperature of the water until it hits the boiling temperature at atmospheric pressure.

Inside of a refrigeration circuit we get flash gas when the pressure on the liquid refrigerant drops below the temperature / pressure saturation point or if the temperature of the refrigerant increases above the same point.

This “flashing” can occur in the liquid line when the liquid line is long or too small and also in cases with line kinks and clogged filter/driers. All of these instances result in a pressure drop and a drop in the saturation temperature.

It can also occur in the liquid line if it is run uninsulated through a space that is hotter than the liquid saturation temperature like on a hot roof or in an unconditioned attic. This is more rare and will generally only cause flashing in conjunction with another issue.

This flashing can be prevented by keeping line lengths and tight bends to a minimum, insulating the liquid line where it runs through very hot spaces and keeping the refrigerant dry and clean with one properly sized filter/drier.

It can also be prevented in most cases by maintaining the proper levels of subcooling. A typical system that has 10°+ of subcooling will not experience flashing in the liquid line under normal conditions.

When you walk up to a liquid line near the evaporator and you hear that hissing/surging noise or when you look in a sight glass and see bubbles you are seeing refrigerant that is at saturation, meaning it is a mix of vapor and liquid. This doesn’t necessarily mean it is “flash gas”, it could very well be that the refrigerant was never fully condensed to liquid in the condenser in the first place. This can be due to low refrigerant charge and in these cases the subcool will be at 0° Even when taken at the condenser.

True liquid line “flash gas” issues are cases where you have measurable subcooling at the condenser coil outlet but still see, hear or measure boiling/flashing refrigerant in the liquid line before the metering device.

— Bryan


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.

Discharge line

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.

Liquid line 

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 (1.24 bar – 1.47 bar) 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 backward, 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 winds 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°F(55°K) 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 an 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.


Testing 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 the 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°F to -6°F(-18.8°C to -21.1°C) ).  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°F (18.33°C) on newer equipment and 75°F (23.88°C) 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




I don’t do much in the way of “rack” refrigeration, but I recently had a conversation with experienced rack refrigeration tech Jeremy Smith and he got me thinking about EPR valves.

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)

— Bryan

Courtesy of Emerson

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.

Courtesy of Sporlan

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.


Say what???


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.

Courtesy of Sporlan


Let’s see if we can start to understand how these valves work in practice.


First, a few basics.


  1. Solenoids, like most valves, are directional. If you install it backwards, it isn’t going to work correctly.    This is why.
  2. Solenoids must be sized properly. You can’t just go buy a ½” solenoid valve and expect it to work because your line is ½”.   This is to ensure a small pressure drop across the valve which is what actually makes the valve work.


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





Photo Courtesy of Emerson

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.

— Bryan



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.


  1. Record model and serial of the condensing unit and the indoor equipment.
  2. Check phase rotation if possible. If not, you will check this during the initial startup. Remember not to energize equipment while under a deep vacuum.
  3. Check and tighten all electrical connections. I prefer to use a torque indicating device for this just to eliminate any chances I can for a problem down the road.
  4. If there are flanged or threaded connections on the refrigeration system, I’ll check and torque or tighten them at this time as well. Again, I prefer to use a torque indicating device when and where possible.
  5. Check that the metering device, condensing unit and oil type match the refrigerant being used. Make sure TEV bulbs are installed properly.
  6. If the unit has a headmaster and a fin/tube condenser, strip the panels off of the unit and measure the condenser for calculating a flooding charge. Go ahead and figure it and write that down, too.  Microchannel coils just have a lookup chart.
  7. Verify that any other trades involved have completed their work. It’s no fun to have a piece of equipment ready to run and not have power to it or to find out a day later that a condensate drain wasn’t  properly installed or heated if necessary.
  8. Check any doors on fixtures to make sure they close and seal properly. Make any adjustments needed.

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:

  1. Power to the unit on? Leave disconnect open for now.
  2. Power to the evaporator unit? Go ahead and turn that on.
  3. Power to any control valves like a liquid line solenoid?

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.

  1. Set the thermostat or temperature control and verify the setting with an accurate thermometer.
  2. Set the defrost timer to manufacturer’s specifications or customer’s specifications. Test operation of the timer as well and ensure that it not only keeps time but switches properly. Then set the timer to correct time of day.  If requested, provide customer with the defrost schedule.   Verify that any defrost heaters draw proper amperage and record.
  3. Many cases and freezers have mullion heaters to prevent frost and condensation on doors and frames. Check these for proper amperage and record.

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




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