Service Clinic: Compressor Replacement Tips Photo by Paul Perkins, Owens Companies

Service Clinic: Compressor Replacement Tips

The following 10 tips, if done and done correctly, will greatly reduce the chance of a compressor failing for almost any reason.

Every contractor knows that customer satisfaction can make or break his business. But, providing high levels of service goes beyond the initial equipment installation; it also requires a technician to be able to efficiently and accurately identify and resolve any system failures. Of all the technical support calls we receive at Danfoss related to compressors, well over 50% are for issues caused by application-related compressor failures, such as liquid return or a compressor operating outside of its intended envelope. And, a significant number of compressors are returned where no actual fault is found.

Understanding why a compressor failed, and that the cause has been remedied before you leave the job is paramount to ensuring customer satisfaction and minimizing callbacks. Replacing a compressor without knowing why it failed is like taking a shot in the dark; it will very likely fail again.

The majority of returned compressors check out in one of three ways:

1. There’s nothing wrong with them.
2. They over heated.
3. They failed due to liquid return. These are called “application-related failures.”

The following 10 tips, if done and done correctly, will greatly reduce the chance of a compressor failing for almost any reason.
1. Read the product literature. Installation guides — generally read as a last resort — are written for a reason. Issues such as under-tightened electrical terminal screws or compressor mounting bolts can unravel and make things go south pretty quick. Simply put, if you follow the instructions, you can generally avoid small issues turning into major ones.

2. Is it really dead? While poking it with a stick isn’t likely to do much, taking a few measurements can work wonders. On single phase compressors, before you remove it, measure the windings with an ohmmeter and refer to the manufacturer’s literature on your smartphone, if necessary. The important task is to verify that the internal overload, if present, is not open and tricking you. To double check, you can measure the resistance between the start and run terminals which will bypass the overload.

3. Verify model required. Before you replace the unit, verify the compressor model requirement with the system model — not the installed compressor. This may sound strange, but never assume that the compressor that was installed was the correct one to begin with. If it had been replaced previously, it’s possible that the correct compressor wasn’t available and a less suitable alternative was installed.

Understanding why a compressor failed, and that the cause has been remedied before you leave the job is paramount to ensuring customer satisfaction and minimizing callbacks.

4. Liquid flood back and migration. Pay attention to the saturated suction superheat (SST). Manufacturers have a minimum recommended superheat level at the compressor suction inlet based on the saturated temperature of the refrigerant at this location. The reason why the evaporator superheat is not used is because the suction line can pick up additional heat and there is a pressure drop between the evaporator and compressor suction. Always make sure the SST meets the minimum requirement for the temperature application of the system. Lower temperature systems can get away with lower SST values than higher temperature systems. The purpose of these values is to eliminate the return of liquid refrigerant to the compressor while it is operating, which can dilute oil and result in bearing failures.

RELATED ARTICLE: Commercial Compressor Replacement

Off-cycle migrations occur when refrigerant vapor migrate to the compressor because it is colder than the rest of the system. The oil will absorb the refrigerant which, upon start up, boils out and causes the oil to foam, greatly reducing lubrication. The remedy here is simple. Install a crank case heater to keep the compressor oil sump warm.

5. Voltage and current checks are a quick and easy insurance that the power that is being delivered to the compressor is what it actually needs. It is not uncommon for voltage imbalances or poor power factor to appear in commercial, food retail and industrial settings. Supplying 175 volts to a 230 volt compressor doesn’t mean you are taking it easy on it. Low voltage will increase current draw and excessively heat up the motor windings. If the customer wants to make sure this compressor lasts, he or she will need to ensure you have the correct power available to hook up to it.

Since current strongly affects winding temperature, excessive current over and above what is listed in the manufacturer’s specs is never a good thing. Evaporator load and condensing temperature strongly influence current draw so make sure you use the correct values for what the system is experiencing at the time you measure the current. Motor lifetime greatly diminishes with an increase in winding temperature.

6. Evacuation is important! Many options exist to reduce evacuation times, and it’s important to reinvestigate your options. Since your manifold gauge is essentially useless for accurately determining the level of vacuum, it would be better to take advantage of the new, smaller micron gauges and 3/8-in. or ½-in. vacuum rated hoses and do it right. Plus, you will likely cut down your evacuation times by half, if not more. If the system is properly dried and free of contaminates, the chance of something unexpected happening is greatly diminished. Refer to manufacturer’s specs for recommended evacuation levels. One  option is using dry nitrogen to break the vacuum, because it helps dry out the system. Purging before brazing ensures there’s minimal oxygen in the system to promote the formation of copper oxide.

7. Airflow checks. A critical but so often overlooked step. If airflow isn’t correct, there’s no reason to do anything else until it’s corrected. None of the readings you can take will represent reality unless the airflow is accurate. This includes both the indoor and outdoor airflows. You need to avoid dirty coils, and dirty air filters, and hedges growing around the outdoor unit or 8-foot fences for that matter.

8. Replace the electricals and drier with the compressor. The cost of opening the system and replacing a compressor is big, so why not ensure that the components that help it run properly also get replaced. Also, since it’s impossible to replace a compressor without opening the system, it only makes sense to replace the drier while it is still open.

9. According to some reports, about half of the units in the field are not being charged correctly. The easiest thing to do is follow the manufacturer’s recommended charging procedure. Usually there’s a table based on outdoor and indoor conditions combined with airflow values. This is why it’s important to take the time to properly measure the db and wb values and airflow volumes so that the values of superheat and subcooling can be placed into the correct context. In other words, there’s no fixed, perfect value of superheat or subcooling — only an appropriate value for how the system is operating right now. This is why it’s strongly recommended to have the values required in the charging table accurately measured when you charge or check the charge of the system.

10. Let the system cycle. This is another critical tip, but one that often seems daunting. It’s really quite simple. You’re already on the jobsite; you’ve gone through the effort to correct the failures. However, without having observed the system operate, you can never be sure there isn’t another issue that was overlooked or not found. Remember that some problems are more likely to occur during specific times in the cycle and can’t be identified until the system goes through a high to low load condition with all the changes involved.
Being able to accurately identify an issue the first time will not only minimize call-backs, but also save time and money. As new, more advanced technologies enter the market, education and training become increasingly important, to ensure proper installation techniques and service checks are done to ensure optimum equipment operation.

Jamie Kitchen is a trainer with Danfoss.

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