Oil management as applied to supermarket parallel compressor
rack systems is probably one of the least understood systems 

BY STEVE ESSLINGER

       Many service techs believe that an efficient oil separator/management system will reduce the overall oil charge on a properly operating refrigeration system. The fact is a refrigeration system oil charge is greater with a highly efficient oil management system as opposed to no oil management system.

       This assertion is based on the refrigeration system being properly installed, balanced and maintained. Properly installed and maintained refrigeration systems have adequate refrigerant velocities in the piping to return oil. To offer an analogy, stand in drizzling rain for one hour and you’ll get soaked. Stand in a downpour for five minutes and you’ll get soaked. In either scenario, you are equally wet.

The total oil charge is equal to the total of:

• The oil in the field piping.

• The oil in the compressor crankcases.

• The oil in the oil management components.

With correct piping practices and given a properly balanced refrigeration system, the oil charge in the piping reaches a point where refrigerant velocities sweep excess oil back to the compressors. This piping saturation point is the same in a refrigeration system with or without an oil management system.

Therefore, given equal time and a system problem the difference is the amount of oil put into the piping during the circuit run cycle. Less oil is logged in the system with an efficient separator in a given time period as opposed to a system with no separator when refrigerant velocities fall off.

It is important to note that an oil management system works like a mechanical time delay to limit the amount of oil movement into the piping between defrost cycles.  When defrost is terminated, the pressure/velocity is high enough to return any excess oil that has been trapped in the system.

Oil is trapped in the piping when common maintenance problems reduce heat transfer and reduce refrigerant velocity. These include:

• Thermostatic expansion valve (TEV) with high superheat.

• Honeycomb diffuser plugged with dirt limiting air movement.

• Stopped drains and ice impacted evaporators.

• Store humidity is higher than 55 percent, causing heavily frosted evaporators and minimal heat transfer.

• Evaporator fans are out.

According to one compressor manufacturer, nine out of 10 compressor mechanical failures are caused by oil slugs.  Oil levels that rise significantly after defrost termination are an indicator of system abnormalities.  The problem should be identified and corrected.            
 

Understanding recirculation rates

Compressor oil recirculation rates are 1 percent to 3 percent of the number of pounds of refrigerant for which a compressor is rated. As an example, an average 100-horsepower, low­temperature, R-404A parallel supermarket rack system will have a minimum of 90 pounds of oil in recirculation an hour at design conditions (1 percent oil recirculation). Note that worn compressors can and do have much higher oil recirculation rates.

Oil recirculation rates are dependent on three things:

• The compressor running.

• The size of the compressor.

• The wear on the compressor (up to 10 percent oil recirculation when severely worn).

Maintaining correct compressor crankcase oil levels on a parallel refrigeration rack with differing size compressors (multiplexed) can be challenging at best. As an example, a 15-horsepower compressor on a medium-temperature application will have an oil recirculation rate of 18 pounds an hour at 1 percent.

On the same rack a 5-horsepower compressor that is more worn will have an oil recirculation rate of 5 percent and will discharge 25 pounds of oil an hour when running. The smaller compressor has a higher oil recirculation rate compared to the larger compressor. While unexpected, it is a real world oil management problem.

With a common suction line it is easy to see why some compressor crankcases are overfilled when the float assemblies are working perfectly. Oil returns to the suction header at an average rate of the combined operating compressors. The return rate may exceed the discharge rate of one or more compressors leading to compressor oil overfilling.

Oil-float assemblies are designed to control low oil levels, not high oil levels. Additionally, compressors that are on extended off cycles will overfill with oil. This is because of the minimal but normal discharge vapor leakage past the valves into the compressor. The vapor moves to suction and the oil drops out into the compressor body through impingement.

Oil management systems have fallen short in several areas:

• High oil levels in compressor crankcases occur when oil returning through the suction gas is greater than the compressor discharge.

• Oil slugs from the evaporator can return directly into the compressor body causing mechanical failure.

• Oil slug builds up in the compressor crankcase due to extended periods in the off cycle, causing mechanical failure when compressor starts.

• Do not offer external pumping to continuously move and filter lubricant to eliminate the need for changes (due to suspended particulate).
   

New technology addresses problems

One refrigeration company introduced an oil management system in 2004 with a patent pending that addresses these common problems. The cost is about the same as a polyolester (POE) oil change on an average supermarket rack system.

The improved parallel compressor oil management system:

• Corrects high compressor oil levels when oil in the suction gas exceeds the amount of oil leaving in the dis­charge gas.

• Traps oil slugs in the suction header for removal before compressor damage occurs.

• Continuously adjusts oil level to 3/8-inch of a sightglass in operating and non-operating compressors.

• Continuously pumps compressor crankcase oil through filter, eliminating the need for oil changes (due to suspended particulate).

Parts in the new system include (numbers correspond to diagram above):

  1. Oil float. The oil level float is designed with a special port to remove oil from the compressor. This level is set at 3/8-inch of a sightglass as recommended by most compressor vendors.

  2. Header drain tube. Suction headers are designed with an elbow or "dip pocket" to collect excess refrigerant and oil coming back to the suction side of the system. The drain connection is connected to the suction side of the maximizer pump.

  3. Check valve. Check valves are used to prevent back flow into any compressor crankcase or suction line if there are any compressor problems.

  4. Pump. Capable of pumping oil, gas and liquid refrigerant.

  5. Oil filter. Special filter removes 99 percent of 3 micron particulates from the oil.

  6. Differential check valve. Differential check valve to vent high side pressure down to a predetermined pressure over suction pressure to ensure oil flow to compressor floats.

  7. Sightglass. Used to verify oil flow in the system.

  8. Isolation valve. Used for servicing components.

  9. Oil separator. Removes oil from discharge gas.

10. Oil reservoir. Used as a storage vessel for oil.

11. Solenoid valve (suction header). The solenoid is closed under low superheat conditions to prevent liquid refrigerant from being pumped to the oil reservoir.

Steve Esslinger is vice president of operations at Zero Zone Refrigeration and a member of the Manufacturers Service Advisory Council (MSAC).