A well-designed screw refrigeration compressor is one of the most efficient commercial compressors on the market today. Developed to solve the engineering problems common with the earlier generations of commercial compressors, a screw compressor has many advantages.
A screw compressor is highly reliable. It is not subject to a high rate of vibration, which means it’s easier to find a place in your operating environment where it won’t cause a disruption. Since fewer things can go wrong, diagnosing issues is generally easier.
That said, a screw compressor still needs an effective maintenance routine behind it. And even though it has some great benefits, it may be less familiar than the reciprocating compressor. It’s important to know the ins and outs of your compressor so you can extend its service life.
Big questions about screw compressors arise when talking about refrigerant migration and oil loss.
Preventing Refrigerant Migration in Your Screw Compressor System
Screw compressors are most commonly seen in refrigeration systems, with some of the most impressive of all found in large commercial systems. These larger systems should be watched carefully for issues that might result in reduced cooling or system stoppage.
When you are trying to keep an industrial refrigeration system going, a small issue can quickly turn into a big one – and seconds count if you lose refrigeration completely. Anyone working with an industrial screw compressor must know about both refrigerant migration and oil loss.
Refrigerant migration is most likely to occur during the off cycle or a long shutdown. Here, refrigerants in either liquid or vapor form will have the tendency to move from areas of high pressure to areas of lower pressure. This leaves the unit’s suction line and crankcase vulnerable to intrusion.
Within a refrigeration system, oil needs to have a very low vapor pressure to avoid being vaporized as pressure drops within the crankcase. A crankcase heater prevents oil from gathering in the crankcase itself but will tend to force it into the suction line instead.
When the refrigerant condenses in the suction line, it results in “slugging” during the on cycle. This can be highly destructive to a compressor, potentially taking an otherwise sound and well-maintained unit into a crisis state within just a few minutes of operation.
What’s the solution? Unfortunately, preventing refrigerant migration is not easy.
Installing an automatic pump-down system may be the most efficient approach. This will help to ensure all of the refrigerant is completely removed from the evaporator, suction line, and crankcase before the off cycle takes place.
Oil Loss in Screw Compressors Is the Source of Some “Refrigeration System Myths”
Ask industrial refrigeration system and HVAC professionals about their screw compressor experiences, and oil loss is bound to come up. Many people believe the oil charge on a screw compressor needs to be topped off on a regular basis. Typically, this happens every few months in large refrigeration facilities.
However, it might not be as necessary as it was back in the old days.
Refrigeration technicians with a lot of experience – or those who were trained by a previous generation – are familiar with older compressors that bypassed large amounts of oil every year. More recently, the major compressor manufacturers have tackled this problem as a glaring example of inefficiency.
Over time, twin rotary screw compressors have entered service equipped with large separators. These use coalescing filters that can be highly efficient. In fact, some large screw compressors can go for many years without having any oil added to the system at all.
So, what’s the source of gradual oil loss you might notice in your system today?
The best thing to check up on here is whether or not your coalescing filters are correctly installed. Some manufacturers have done an excellent job integrating the coalescing filters. On the other hand, certain brands have developed a reputation for filters that pass oil from day one of service.
As a general rule, brands that have the best handle on this situation use guide bars to line up the coalescing filters during installation easily. Without these bars, even the most experienced team members can misalign the filters because they are heavy and awkward to access during the initial startup.
Oil Loss May Be an Early Sign of Compressor Failure
If you have not noticed any oil loss previously, but it seems like a constant thorn in your side today, your screw compressor may be aging out. Like most other commercial compressors, screw compressors last about eight to ten years on average when appropriate maintenance is performed.
A remanufactured industrial compressor gets you back on track by replacing your unit with an identical model fully engineered to meet all performance and environmental benchmarks. It’s faster, easier, and less expensive than sourcing an all-new model from an OEM wholesaler.
