“Compressors are the most expensive single piece of equipment in a refrigeration system, and an unplanned failure can cost tens of thousands of dollars.”

Compressor failures are a huge problem in the commercial refrigeration industry. Compressors are generally the most expensive single piece of equipment in a refrigeration system, and an unplanned failure can cause tens of thousands of dollars in new equipment and technician labor. Not to mention the potential food spoilage and lost sales if product temperatures get too high.

The good news is that the vast majority of compressor failures are preventable - with the right data and action plan. Based on all of the data Axiom Cloud has collected from supermarkets and cold storage facilities, combined with additional datasets and anecdotes from industry experts, we believe that over 90% of compressor failures can be prevented or significantly delayed. It’s because of this belief that you’ll often hear an Axiom team member say “compressors don’t die - they’re killed.”

To continue our series of educational blog posts that help improve supermarket refrigeration management, which has included posts like 6 Reasons to Improve Your Refrigerant Management, The Hidden Costs of Refrigeration Outages, and The Top 10 Reasons Floating Suction Isn’t Currently Saving You Money, we give you: The 7 Most Common Ways that Compressors are Killed (in no particular order).

1. Floodback

Compressor floodback is the continuous return of liquid refrigerant droplets into a compressor through the suction header. Floodback is most often a result of low superheat, which is caused by improper controller settings or low heat loads on evaporators from frosting or valve damage.

Floodback is particularly treacherous because it can be hard to detect without the right sensors, and it usually will not cause a compressor to fail immediately. Over time, floodback will wash oil off of a compressor’s lubricated surfaces, causing premature wear and overheating.

An effective (but invasive) way to avoid liquid floodback is to install suction accumulators on every suction line. Suction accumulators will prevent liquid refrigerant from reaching compressors until the accumulator is nearly full.

Today, it is also possible to avoid liquid floodback using real-time data analytics and automation. Suction temperature sensors and real-time superheat calculations (superheat is the difference between the refrigerant saturation temperature and the measured gas temperature) can be used to ensure adequate superheat in the suction gas at all times. If floodback is coming from one circuit in particular, superheat trends can often be correlated with defrost cycles to identify the culpable circuit, helping to solve the root cause of the problem before a compressor is killed.

Continuous liquid floodback caused by an expansion valve that was opened too far.

In the image above, Axiom’s apps were able to quickly identify floodback by calculating suction superheat in real-time and flagging a significant decrease. In addition, the software also noticed that the temperature of a specific display case dropped in temperature at the same time, implying that the issue was stemming from this case.

2. Liquid Slugging (Flooded Starts)

Another common cause of compressor failure is liquid slugging. Liquid slugging is the return of a mass of liquid into the cylinders of a compressor. Liquid refrigerant cannot be compressed, so its presence in a compressor can lead to extremely high pressures (up to 3,000 psi!).

Slugging almost always occurs on compressor startup, in what is known as a flooded start, but it can also be caused by a rapid change in operating conditions such as condenser fan cycling or hot gas defrost cycles. The resulting high temperatures and pressures result in immediate component failure in the worst case, or starve the compressor of oil in the best case.

During a prolonged or particularly cold off-cycle, gaseous refrigerant can condense in the compressor oil. Because liquid refrigerant is more dense than oil, it will then flow to the bottom of the compressor crankcase. Upon startup, the refrigerant will immediately boil into a gas, creating foam that can clog oil passages and damage cylinders.

“Liquid refrigerant cannot be compressed, so its presence in a compressor can lead to pressures of up to 3,000 psi!”

A couple popular but time consuming methods of avoiding flooded starts are installing crankcase heaters or completing a pump down cycle prior to prolonged off-cycles. Crankcase heaters reduce the risk of refrigerant condensing in a compressor, especially in low ambient temperatures, and pump down cycles remove most of the refrigerant from the low (suction) side of the system, avoiding excessive condensation.

Liquid slugging events that do not occur on startup are much harder to detect, and this is where intelligent data analytics can help. By integrating with existing refrigeration controllers and sensors, Axiom Cloud’s apps can identify conditions when liquid slugging is likely to occur and diagnose the root cause of the issue. In the image below, Axiom’s apps were able to pinpoint the root cause of liquid slugging by correlating spikes in suction temperature with the defrost cycles of a particular display case.

Liquid slugging caused by improper defrost termination.

3. Overheating

While overheating is often one result of floodback or liquid slugging, there are other potential causes of it. Any time a compressor overheats, its oil will lose the ability to lubricate, increasing the friction generated by moving parts. This will eventually result in a seized compressor.

Compressor overheating can be caused by high superheat, high discharge temperatures, inadequate airflow and high temperatures in the compressor room, low refrigerant charge, a fouled or damaged condenser, and a number of other conditions.

There are tried-and-true methods to avoid these issues - most of the time. For example, ensuring proper refrigerant charge, insulating suction lines so they do not absorb heat from the environment, oversizing condensers, regularly cleaning condensers, and using head cooling fans on compressors can all reduce the risk of compressors overheating.

However, overheating can also be prevented by closely monitoring relevant data (superheat, discharge temperatures, compressor room ambient temperature, refrigerant charge level, condenser TD, condenser subcool, and many others) carefully on a real-time basis. By monitoring and analyzing this data, Axiom’s apps can help to identify when conditions are trending in the wrong direction and diagnose the root cause of the problem - before threshold alarms or compressor failures.

“The result of an oil loss is excess friction, which leads to overheating and a seized compressor.”

4. Oil Loss or Blockage

Oil loss can also be caused by many different factors, but one that has not been discussed is a simple blockage or oil leak. Oil should leave the compressor crankcase, travel through the system, and return to the compressor at about the same rate.

Oil flow can be blocked in the evaporators due to low heat loads, in the suction line or suction accumulator due to a restriction, because of an oil leak in the compressor, or because of a clogged filter in the oil lines. The result of an oil loss is excess friction, which leads to overheating and a seized compressor.

Oil loss can be prevented by keeping compressors from short cycling, checking oil levels and filters regularly, and ensuring all piping is sloped toward the compressors. Oil levels and temperature data can also be actively monitored to prevent low oil levels from becoming a long-term issue that eventually causes compressor failure.

5. Short Cycling

Short cycling is when a compressor cycles on and off too frequently (greater than 40-50 times per day for semi-hermetic compressors found in grocery stores). Short cycling can be caused by low refrigerant charge, obstructions in the suction line, a mismatch between compressor sizing and system load, or control issues like a low suction pressure cut out or a high pressure cut in. As a typical rule of thumb, compressors can cycle approximately 500,000 times before they fail.

The main issue with short cycling is, once again, a loss of oil and lubrication in the compressor. Each time a compressor starts up, oil is pumped out, and without sufficient runtime (5-10 minutes) an equal amount of oil will not return through the suction line. This imbalance will lead to an oil shortage, which will result in a compressor failure. Short cycling is also a very inefficient way to meet the required cooling load, and will lead to significantly higher energy consumption than necessary.

Historically, the main ways to prevent short cycling have been to ensure that there is adequate refrigerant, a large enough suction pressure control band, and proper compressor sizing. Now, with intelligent data analytics, short cycling can be easily detected by monitoring cycle counts, and in most cases the root cause can also be diagnosed remotely.

In the image below, it should be obvious that these compressors are cycling much more than they should be! However, without a solution actively monitoring for this issue, short cycling often goes unnoticed until it is too late.

These compressors were cycling over 100 times per day!

6. Refrigerant or Oil Contamination

If it hasn’t been clear from the discussion of the previous five failure modes, the only things inside a compressor should be oil and gaseous refrigerant! Anything besides refrigerant and oil in the system is considered a contaminant, including air, water, dirt, noncondensables, and chips of piping or brazing.

Moisture in a refrigeration system will react with refrigerant to form acids, which can erode components in compressors and throughout the entire system. Noncondensables such as air or nitrogen can lead to compressor overheating. Solid contaminants should be trapped in strainers or filters, but they can do damage to compressors before being trapped.

The best prevention for contamination is to practice good installation and service habits, as contaminants are most likely to be introduced during service events. If it is suspected that a system has a high concentration of contaminants, check all the filters, and evacuate the system if necessary to remove air and moisture.

7. Corrosion or Electrical Issues

Finally, as with any other device that consumes electricity, compressors are susceptible to corrosion or other electrical issues. As previously mentioned, moisture in a refrigeration system will react to form acids, which can quickly corrode a compressor’s electrical system and other components. Power surges, improper wiring, or voltage spikes can also lead to failure of one phase of a compressor’s three-phase motor. A failure of one phase of the motor leads to an imbalance that will cause overheating.

ACHR News has a great article to help troubleshoot compressor electrical problems. The best prevention for electrical problems is to maintain a low-humidity environment around compressors, keep moisture out of the system, and ensure proper maintenance of a compressor’s wiring and electrical system (contactors, wires, terminals, fuses, etc.).

With high resolution power meters and data analytics, compressor electrical issues can be detected, and failures predicted, by monitoring a compressor’s power consumption and normalizing this signal to the rest of the system’s operating conditions. This is known as power signature analysis, and it is a powerful way to predict electrical problems before a compressor fails.

Axiom’s apps can help!

With proper refrigeration monitoring and maintenance, compressors can operate efficiently and effectively for 20-30 years in grocery stores and cold storage facilities. However, as we now know, there are many ways to end a compressor’s life early, which can lead to thousands of dollars in equipment and inventory damage and further erode a supermarket’s razor thin margins.

Axiom Cloud’s Facilities Analyzer and Virtual Technician apps were purpose-built to autonomously predict, and in many cases prevent, the compressor failure modes listed above, along with dozens of other refrigeration system issues. By integrating directly with leading refrigeration controller brands, the apps generate as much value and insight as possible from hundreds of existing sensors. If your refrigeration systems are in need of a data-driven upgrade, contact Axiom today.

Turner is a Senior Solutions Engineer at Axiom Cloud.

Axiom Cloud’s mission is to use software and automation to transform how the world’s cooling systems are powered, operated, and maintained, in order to generate significant climate and financial impact. Axiom’s team of refrigeration experts, data scientists, energy nerds, and software developers solves retail grocery’s biggest energy and maintenance challenges by layering intelligence onto their existing refrigeration systems. If you’re interested in learning more about our mission or our apps for commercial refrigeration, please contact us today.

Compressors Don't Die - They're Usually Killed