Diesel Effect Compressor Explosions

There have been a few rare but potentially fatal accidents involving exploding compressors due to an effect known as diesel effect. If the gas mixture being compressed contains enough air, the heat of compression can ignite the refrigeration oil in the cylinder, much the same way diesel oil is ignited in a diesel engine. The heat of compression plus the heat of combustion from the refrigeration oil then ignites the refrigerant in the cylinder, creating a dramatic increase in pressure which blows the compressor apart. Note that this is not just a phenomenon limited to flammable refrigerants, but can happen with A1 rated refrigerants such as 134a or 410A. How? 

Refrigerants are rated for flammability according to ASTM E681 at a temperature of 60°C. Many refrigerants that will not burn under ASTM E681 conditions will burn at higher pressures and temperatures, including R-22, R-134a and R-410A. It is worth noting that in tests where they were trying to create diesel explosions, the University of Tokyo found no significant difference between the behavior of A1 refrigerants (R-22, R-410A) compared to A2L refrigerants (R1234yf, R32). They also found that compressing refrigerant and air mixtures without refrigeration oil did not create a diesel explosion. The refrigeration oil had to be present. 

So how can we avoid compressor diesel effect explosions? Simply put: keep the air out. With no oxygen you cannot have an explosion. Air is never good for any refrigeration system anyway. It should not be news to anyone who has studied refrigeration at all that air does not belong in a refrigeration system. However, you may not realize that leaving air in the system not only hurts system performance and reduces the equipment life, it can create a real hazard to service technicians. Here are a few precautions you can take to avoid the specter of a diesel effect explosion in your refrigeration system. 

• Check new installations for leaks using nitrogen and repair any leaks in the system.

• Never use compressed air or oxygen for leak testing refrigeration systems. 

• Thoroughly evacuate the lines and coil of new split system installations AFTER verifying they don’t leak.

• Never pump a system down into a vacuum. Reducing the low side pressure to a vacuum can suck in air through leaks or incorrectly positioned service valves.

• Never jump out safety controls such a low-pressure switches. Forcing a system to run when it is low on refrigerant creates the possibility of sucking in air through leaks on the low side.

• Don’t simply add charge to systems that are low on refrigerant. This is especially true for systems that are significantly low. You should find and repair the leaks.

You can read more about the research into compressor diesel effect explosions here. https://hpc2017.org/wp-content/uploads/2017/06/o324.pdf

Bristol Compressors Closing

It is sad whenever a company in HVACR cannot make a go of it. Bristol Compressors has announced that they are closing after more than 40 years in business. They developed some innovative products, such as the Inertia Compressors with suction valves built into pistons that separated allowing suction gas to travel through the piston head, or the Twin-Single compressors that had a unique crankshaft which moved both pistons up and down in one direction and only one piston in the other direction.  More details on the closing here https://www.coolingpost.com/world-news/bristol-compressors-announces-closure/ 

https://www.youtube.com/watch?v=vChfXCWq9oM

Thermostatic Expansion Valve Failures

There have been a run of TXV problems in new units stemming from a chemical used as a rust inhibitor on some of the compressor bearings in Copeland Scrolls. A reaction between the rust inhibitor and the POE oil used in the compressor causes the POE to coagulate and collect in the TXV. This appears to be an industry wide problem, not limited to a single equipment manufacturer. The chemical causes the valves to get gunked up and stick, usually resulting in a restriction at the TXV. The symptoms can look very similar to an undercharged system – very low suction pressure, low head pressure, and high superheat. Normally, the way I differentiate low charge from a restriction is by subcooling. A restriction typically has a normal to high subcooling. However, this is often not the case if the compressor in the system is a newer Copeland scroll. The Copeland scroll compressors designed for R-410A unload when the suction pressure drops below 55 psig. This is to keep them from operating at pressures which will damage the compressor. If the restriction is severe enough, the suction pressure on an R-410a system can drop below 55 psig. The compressor unloads, limiting how much refrigerant is pumped and holding down the head pressure.  You have low suction pressure, low head pressure, high superheat, and low subcooling: classic undercharge symptoms. However, adding charge does nothing unless you really go whole hog and grossly overcharge the system. Then you can build some head pressure, but it won’t fix the restriction or make the system cool. If the compressor is unloading, you can often hear it and even feel it in the piping. If you have what looks like an undercharged system with no detectable leak and adding refrigerant has no effect on the pressures, I bet you have a restriction. These days the restriction is often in the TXV. Several equipment manufacturers have service bulletins out regarding this issue. Below are links to a bulletin from Nordyne and a link to a YouTube video showing the bulleting from Emerson.

Nordyne Link

YouTube Link

Cut it Out!

The safest way to remove a refrigeration component is to cut it out, rather than de-brazing it. In the case of filter driers you don’t want to heat the drier and drive moisture and contaminants back into the system. With all components, there is the hazard of fire caused by vaporizing the refrigeration oil in the system. Heating the joint enough to render all the brazing material molten increases the temperature and pressure. When you pull the part loose, a stream of gasses comes out and is ignited by the torch flames and/or the red metal. Even if the refrigerant is non-flammable, the oil will burn. People have been seriously injured when large amounts of flaming oil discharged on them after de-brazing a refrigeration component. The most serious cases involved people de-brazing a system that was still fully charged. Accidentally cutting a line on a charged system would not be nearly as hazardous as de-brazing one.

In addition to the flame hazard of the oil, all fluorinated refrigerants create nasty, toxic fumes when exposed to flames even if they don’t burn. If you have ever gotten a strong snort of this stuff you know it is an experience to be avoided.

One more reason to be cautious now is the presence of so many refrigerant blends with small quantities of hydrocarbons in them. Although the quantity of hydrocarbons is small, the hydrocarbons can concentrate in the compressor oil when charged into a system with mineral oil. Even after recovery, there can still be volatile hydrocarbons in the compressor oil. You heat it to de-braze the compressor, and a flammable gas comes out when you pull the pipe loose. I have heard one story where the technician was de-brazing a compressor and was engulfed in a ball of flames for a short period of time after pulling the line out of the compressor. It was a brief flash fire that burned off his eyebrows and gave him a sunburn, nothing too serious – but definitely scary. The refrigerant was an R22 replacement rated A1/A1. Also, consider that with so many replacement refrigerants you really don’t know exactly what is in the system. Although the highly flammable R22 replacement refrigerants are illegal, they are widely available on the internet and they are sold to anyone with a credit card. Just on the off chance that the system you are working on was topped off with propane, you should avoid de-brazing. Make sure and purge the system with nitrogen before brazing the new compressor in. Not only will you avoid oxides inside the lines, you might also avoid fires in the area you are working in. I believe refrigerants with some degree of flammability will become more common. To be safe, we need to start treating all systems as if the refrigerant could burn. So the next time you have to remove a refrigeration component – CUT IT OUT!

Compressor Amp Ratings

Trying to figure out what a compressor’s amp draw should be can be a bit confusing. Most hermetic compressors only have the LRA (locked rotor amps) on the compressor data plate. The locked rotor amps tell you what the compressor will draw if the rotor cannot turn when the compressor is energized. This typically is 5 to 7 times as high as the operating current. But the operating current is not a fixed quantity. Changes in condenser and evaporator pressures change the work load of the compressor, which changes the actual operating current. Equipment manufacturers label their equipment with the compressor RLA (rated load amps). On complete systems submitted to UL, the rated load amp value will be determined by actual system tests. The same model compressor in two different applications can have a different RLA in each application. However, the RLA is not really there to tell you what the compressor amp draw should be. It is there for the purpose of sizing the wire, controls, and overcurrent devices for UL listed equipment.

Underwriter’s Laboratories requires that the compressor motor protection system will not permit a continuous current in excess of 156% of the rated load current. In commercial refrigeration, compressors and condensing units are often sold as separate component parts. Since there is no system to test, the compressor manufacturer must come up with an RLA. Compressor manufacturers work backwards to figure this out. They operate the compressor at different loading conditions to determine the MCC (maximum continuous current.) This is the highest amp draw the compressor can operate at continuously without tripping the overload. Since UL says the overload must trip no higher than 156% of the RLA (rated load amps), the RLA is calculated by dividing the MCC by 1.56. Some compressor manufacturers use lower safety factors of 1.4 to calculate the RLA. Either way, the RLA does not tell you what the amp draw should be for the particular operating condition in which you find the compressor in the field. In the case of individual compressors or condensing units, it does not even tell you what the amp draw should be at design point, because there is not a complete system, and thus, no system design point. To know what a compressor amp draw should be at any particular operating condition, you need a chart or table supplied by the compressor manufacturer. Some now have this data online. Bristol compressors will let you enter a specific operating condition and their web site will spit out all the relevant data – including the operating current.  

Evap Temp Effect on Compressor Capacity

I really appreciate the detailed data that some manufacturers have available to the public on the internet. For example, Bristol Compressors has detailed performance specifications for all of their compressors online for anyone to see. Not only do they have the specs for standard rating conditions, but you can also enter in your own. This allows you to see the effect that changing system conditions has on the compressor capacity, energy use, and EER. For example you can see what happens if the evaporator temperature drops from 45°F to 40°F. At first, that does not seem like a very big deal. But in the case of a Bristol H81J223ABC, that change drops the capacity from 21,800 Btuh to 18,800 Btuh and drops the EER from 9.8 to 8.9. Now ask yourself this question: “How easy would it be to have an evaporator temperature 5°F too cold because of a dirty air filter, dirty coil, restrictive duct, slight undercharge, partially clogged filter drier, incorrect piston (orifice), incorrectly adjusted TEV – and I am sure you can think of a few more.  In fact there are so many ways to be just a little off it seems likely to occur. Unfortunately, the effect on system performance is not little. The capacity drops by 14% (18,800/21,800 = 0.86 or 86%) and the EER drops by 9% (8.9 / 9.8 = 0.91 or 91%).  For a more dramatic drop in capacity, look at what happens to a commercial refrigeration compressor when the evaporator temperature drops from -10°F to -20°F. The Bristol L61J113ABC produces 7000 Btuh at its -10° rating point. At -20°F this drops to 3800 Btuh. At only 10°F colder, the compressor capacity drops to 54% of its rating! That extra 10°F colder costs dearly in electricity. With some creativity and exploration you can use comparisons of different operating points to illustrate many issues in HVACR systems.

You can find this interactive compressor playground at www.bristolcompressors.com Engineering Support > Compressor Search or just CLICK HERE  Enter whatever details you want to start the search. Once on the page for a particular compressor, click “Specific Point” towards the top right of the screen to enter your own operating conditions. Have fun!

Do You YouTube?

Electronic social networking has made it possible for people to enjoy global communication with millions of people who share common interests. HVAC/R is no exception. Take a look around the web and you can quickly find many discussion forums, web sites, and media aimed at HVAC/R technicians. You have to exercise some discipline when perusing web materials because it is easy to while away hours of time reading and viewing content on subjects that you are interested in. YouTube is especially addicting because they have figured out how to keep feeding you video clips that are related to what you are watching. If you start looking at clips of compressors, more and more compressor clips will keep coming up in the side bar for you to view. However, my discussion is not how to avoid internet addition, but how to employ internet addiction for teaching purposes. The easiest way is simply to give your students unfettered access to the web, point them to a list YouTube clips, and stand back. This approach does come with some amount of risk because everything is on YouTube. The very openness of the format which encourages sharing and communication also aids in the transmission of inaccurate or even offensive ideas. There are oscillating parts on YouTube videos that are not attached to compressors! For this reason, many schools filter or restrict online material so severely that it is not really possible to use YouTube live. Many schools block sites like YouTube entirely. If that is the case, you can still view and download the videos at home and then take them to school. This gives you control over what is seen and allows your students access to the material. Although YouTube does not provide point and click ability to download their files, many people provide sites or programs to do it. I use a free program called YouTube Downloader. It is available on CNET, a trustworthy site for downloadable computer files. I then download the files onto a jump drive to take to school.

There are literally hundreds of video clips on YouTube that are helpful in HVAC/R, but my favorites are clips that show things you can’t see elsewhere. A gentleman in Australia has several clips of compressors he has cut open. Many of them operate. Seeing the movable scroll on a scroll compressor orbit as the compressor is energized is way cool! I must admit that seeing him cutting open compressors with a grinder, sparks flying appeals to the kid in me! Before you run out and start sawing up compressors, why not take a look at what he has already done. To help you get started on your internet addiction, I have included a short list of YouTube compressor videos to whet your appetite. The titles are hyperlinked, just click on the title to go to the video.

Open Scroll 3

Off with its head

Daikin Relucance DC Swing Compressor

30 HP Carlyle Semi

Kirby Reciprocating Compressor