Troubleshooting: High Discharge Temperature

A compressor that runs hot or trips on high temperature is telling you it cannot get rid of its heat. Remember that most of the electricity a compressor uses becomes heat. The Compressed Air Challenge puts it at as much as 80 to 93 percent. When that heat has nowhere to go, the machine shuts itself down, or the oil and parts break down early.

Symptoms the customer will describe

• Rotary screw trips on high temperature, often in afternoons or summer
• High-temp warning on the controller
• Oil darkens fast, smells burnt, or varnishes
• Hot, wet air going downstream, overloading the dryer
• Piston unit runs very hot, discolored head or discharge line

Likely causes, most common first

1. Hot room or poor ventilation. The compressor is breathing its own exhaust air, louvers are blocked, or the room was closed in. The most common cause by far.
2. Dirty coolers. Oil cooler and aftercooler fins plugged with dust and oil mist. Air cannot pass through.
3. Low oil level or wrong oil (lubricated screw). Oil carries heat away from the compression. Too little, or the wrong grade, and temperature climbs.
4. Plugged oil filter or stuck thermostatic valve. Oil bypasses the cooler or flow is restricted.
5. Cooling fan problem. Broken belt, failed motor, or fan running backward after electrical work.
6. Running above rated pressure. Higher pressure means more heat of compression.
7. Water-cooled units: low water flow, high water temperature, or scaled heat exchangers.
8. Piston units: leaking valves or running past the unit’s duty rating.

Quick checks a rep can do on the phone

1. “What is the room temperature next to the compressor while it runs?” Compare to the maximum ambient on the compressor’s data sheet.
2. “Is hot air from the compressor ducted outside, or does it stay in the room?”
3. “Can you see light through the cooler fins?”
4. “What does the oil level sight glass show? When was the last service?”
5. “Did it start after a service or electrical work?” Wrong oil or fan rotation are suspects.
6. “What pressure is it set to? Has anyone raised it?”
7. “Does it only trip on hot days?”

Fixes

• Fix ventilation: duct hot exhaust out of the room, bring in cool outside air, clear the louvers.
• Clean the coolers. Follow the maker’s cleaning method so fins are not damaged.
• Bring oil to the correct level with the maker’s specified lubricant. Do not mix oil types.
• Replace the oil filter. Service or replace the thermostatic valve.
• Repair the fan or correct rotation.
• Lower the set pressure to what the plant actually needs.
• For water-cooled units, restore water flow and descale the heat exchanger.

What to quote

• Compressor maintenance kits: oil filter, air intake filter, separator element
• Compressor lubricant to the maker’s specification
• Thermostatic valve kits and cooling fan parts
• Ventilation fans and ducting for the compressor room
• Cooler cleaning or a service visit

Safety note

Never open an oil fill cap or service a lubricated compressor while it is hot or under pressure. Shut down, lock out, and bleed pressure first, per the maker’s manual.

Sources

1. Compressed Air Challenge, Fact Sheet 10, Heat Recovery with Compressed Air Systems (as much as 80 to 93 percent of electrical energy becomes heat).
2. DOE/Compressed Air Challenge, Improving Compressed Air System Performance, 3rd ed. (compressor room ventilation and heat recovery; cooling system maintenance).
3. Compressor maker operation and service manuals (high discharge temperature causes: cooler fouling, oil level, thermostatic valve, fan, ambient limits). Trip temperatures and maximum ambient vary by model; use the specific maker’s figures.