Thermal Migration in Desert Climates

The Situation

After several compressor failures were reported by a customer who was shipping equipment to the Sudan, the following information was gathered:

  • Most failures were immediate – within one hour of operation
  • Compressors that were examined were found to have little to no oil in them
  • There was no evidence of mechanical contact internally
  • All internal components were clean and undamaged

The Analysis

T/CCI engineers worked closely with the customer to evaluate the conditions, the protocols, and the failure details in order to conduct a thorough evaluation of the situation. T/CCI was able to simulate the reported conditions in their advanced testing facilities and recreate the same outcomes as the customer. The experienced engineers were able to confirm their original suspicions and offer solutions. Based on the investigation the following conditions were found to be consistent among all reported failures in this region.

  • Utilized high-side charging with liquid refrigerant (best practice)
  • Refrigerant charge level was determined to be 1.8 kg R134a

The Conclusion

Upon reviewing the climate of the region and the related failure analysis, the T/CCI engineers determined that the root cause of the failure is “thermal migration” which has been documented to cause premature failures in Automotive, Heavy Truck and Agriculture HVAC applications. Thermal Migration is a condition by which refrigerant moves statically throughout a refrigeration system due to changes in the climatic conditions. The nature of the refrigerant is such that it will always flow to the coldest area and since conditions are static and the refrigerant has density then gravity has a minor influence. In most instances some or all of the following conditions must be met to create this undesirable condition:

  • High Solar Load
  • Large glass surface area
  • High delta temperature between daytime and nighttime (low humidity ambient)
  • Significant elevation differences between system components
  • Vehicle is not operational for a period of time (days, weeks)

In most instances, Thermal Migration will not cause any issues with normal system operation nor compressor longevity. Under certain conditions it can be a very severe problem for a compressor. There are two significant problems.

  • Problem #1 – As refrigerant travels statically due to thermal migration it carries oil with it as the oil is miscible in the refrigerant. During thermal migration in the nighttime the refrigerant becomes condensed in the compressor but due to the warm mass of the road surface radiating heat upward in the cool night air causes the flow back through the suction line to the evaporator carrying “oil mist” in its vapor. This oil then condenses in the evaporator and pools in the passages and tubes. This night time flow occurs every night the vehicle is sitting. Over a period of time this results in a complete “washout” of all residual oil in the compressor.
  • Problem #2 – During the daytime the sun heats the interior of the cabin through the glass until internal temperatures reach 70-73 deg C around the evaporator. This causes the refrigerant to flow back (migrate) down the suction line to condense in the compressor. When started under this condition it’s possible for the compressor to undergo a hydraulic lock-up due to excessive liquid refrigerant in the cylinders.

The conditions in the Sudan met enough of these conditions to lead investigators to conclude that thermal migration was, indeed, the root cause of compressor failure.

The Solution

Knowing that the main causes of thermal migration are caused by climate conditions, a series of recommendations were developed.

Option 1:

Countermeasure: Check Valve in the suction line at the compressor

How it works: Check Valve will only allow refrigerant flow in one direction preventing migration through the suction line

Option 2:

Countermeasure: Oil Separator at the discharge line to reduce oil circulation in the system and maintain higher volume in the compressor

How it works: Uses an expansion chamber and separator passage that shunts a metered flow back to the compressor suction

Option 3:

Countermeasure: Eliminate “J” pipes of evaporator in and out, mount Block TXV horizontal and straighten 90 deg turns into block valve

How it works: By eliminating the “J” trap and creating a smooth transition through the TXV and plumbing it eliminates the oil trap and allows a natural gravity path for oil return

Option 4:

Countermeasure: Using logic module cycle the compressor 3-5 times rapidly at start up

How it works: This would prevent full slug of the compressor while allowing the initial pressure dynamics to open the TXV before the compressor pumping at 100%