Compressors generate significant heat from compression work, motor losses, and friction.
This heat must be removed to prevent overheating, oil breakdown, and motor winding damage.
This lesson covers the five cooling methods used across different compressor types and
applications: suction gas, air, water, liquid injection, and oil cooling.
Superheat and subcooling describe the thermal state of the refrigerant at two critical
points in the vapour-compression cycle. Both are measured in degrees of temperature
difference from the saturation point at the prevailing pressure, and both are essential
diagnostic and commissioning parameters for every refrigeration technician.
Superheat
Superheat is the temperature increase of refrigerant vapour above its saturation
temperature at a given pressure. If the saturation temperature at suction pressure is
5°C (41°F) and the actual suction line temperature is 15°C (59°F),
the suction superheat is 10°C (18°F).
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Suction Superheat
Measured at the compressor suction port (or at the outlet of the evaporator).
The recommended range for most applications is 10–20°F (5.5–11°C).
Adequate suction superheat confirms that all liquid has evaporated before reaching
the compressor, protecting against liquid slugging. Excessive superheat indicates
a starved evaporator or refrigerant undercharge.
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Discharge Superheat
The superheat of the refrigerant vapour leaving the compressor discharge port.
A useful indicator of compressor health: if discharge superheat is very low, liquid
may be reaching the compressor; if it is excessively high, the suction gas is too hot
when it enters (often due to high suction superheat or high compression ratio), stressing
the compressor oil and valves.
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Low Suction Superheat — Risk
Suction superheat below 5°F (2.8°C) means liquid refrigerant may be present
in the suction line. Even small amounts of liquid entering the compressor at high
velocity cause valve damage (hydraulic shock) and wash the oil film from cylinder
walls, leading to accelerated wear. Correct immediately by adjusting the metering
device or addressing the cause of liquid floodback.
Subcooling
Subcooling is the temperature reduction of liquid refrigerant below its
saturation temperature at a given pressure. If the saturation temperature at liquid
line (condensing) pressure is 40°C (104°F) and the actual liquid line
temperature is 35°C (95°F), the subcooling is 5°C (9°F).
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Why Subcooling Is Important
Prevents flash gas at the metering device: if subcooling is insufficient, the pressure drop along the liquid line reduces the pressure below the saturation point, and some liquid flashes to vapour before it reaches the metering device — this reduces system capacity and can cause erratic metering device operation
Indicates adequate refrigerant charge: subcooling is one of the primary indicators used during charging; a system with correct subcooling (typically 10–20°F / 5.5–11°C for most systems) is presumed to have adequate refrigerant in the condenser
Increases refrigeration effect: subcooled liquid enters the evaporator with more heat absorption capacity because it must first warm to saturation temperature before evaporating; this marginally improves system COP
Excessive subcooling can indicate refrigerant overcharge, a restriction in the liquid line, or a liquid-line heat exchanger in the system; verify by checking system pressures and design specifications
1.3.2 — Discharge Temperature
The discharge temperature is the temperature of the refrigerant gas leaving the
compressor at the discharge port. It is one of the most useful indicators of overall
system and compressor health and is routinely measured during commissioning,
service calls, and performance verification.
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Maximum Safe Discharge Temperature
Most manufacturers specify a maximum continuous discharge temperature of
107°C (225°F). Above this threshold, refrigeration
oil begins to break down, carbonizes on valve plates and discharge passages,
and loses its lubricating properties. Sustained high discharge temperatures
are a primary cause of early compressor failure.
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What Raises Discharge Temperature
High compression ratio (high head pressure, low suction pressure); high suction
superheat (overheated gas entering the compressor); low volumetric efficiency; and
high ambient temperature around the compressor. Each of these factors adds heat to
the gas during compression, raising the discharge temperature independently of
the others.
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Measuring Discharge Temperature
Measure with a thermocouple or electronic thermometer as close to the compressor
discharge port as possible — ideally within 150 mm (6 in). The temperature
drops rapidly along the discharge line as the gas gives up heat. A reading taken
farther down the discharge line significantly underestimates the true compressor
discharge temperature.
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Consequences of Excessive Discharge Temperature
Oil carbonization: oil deposits form as hard carbon particles on valve plates, discharge ports, and discharge mufflers; particles circulate in the system and can restrict small-bore components
Acid formation: at high temperatures, oil and refrigerant react to form acids that attack copper plating, motor windings, and bearing surfaces
Valve plate damage: carbon deposits on valve seats prevent valves from sealing fully, reducing volumetric efficiency and raising discharge temperature further — a self-reinforcing failure cycle
Reduced oil viscosity: oil thins at high temperature, reducing film strength at bearing surfaces; bearing wear accelerates
Always investigate and correct the root cause of high discharge temperature — do not simply monitor it and hope it resolves
1.3.3 — Suction Gas Cooling
Most hermetic and semi-hermetic compressors use returning suction gas to cool the motor
and compressor. Cool, low-pressure vapour from the evaporator passes over the motor
windings before entering the compression chamber. This method is simple and requires no
external cooling system, but depends on adequate suction superheat to prevent liquid
refrigerant from reaching motor windings.
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Design Considerations
Motor heat load raises suction gas temperature before it enters the compression chamber, contributing to higher discharge temperature
Excessive suction superheat reduces refrigerant vapour density, lowering mass flow rate and capacity
Low suction pressure applications (low-temperature refrigeration) provide less cooling because lower vapour density carries less heat per unit volume
Some designs route suction gas directly to the cylinders, bypassing the motor, when lower discharge temperatures are required
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Field Practice
Maintain suction superheat within the manufacturer’s specified range —
typically 10–20°F (5.5–11°C) — to ensure adequate motor
cooling while preventing liquid refrigerant damage. Monitor suction and discharge
temperatures to verify proper operation. Inadequate refrigerant charge or a
malfunctioning metering device are the most common causes of suction gas cooling
problems.
1.3.4 — Air Cooling
Open-drive compressors and some semi-hermetic designs use ambient air for cooling.
Fans blow air over the compressor exterior, removing heat from cylinder heads, motor
housing, and crankcase. This is the simplest external cooling method and requires no
water supply or piping.
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Design Considerations
Requires adequate clear space around the compressor for unrestricted airflow; recirculation of hot discharge air raises ambient temperature and reduces cooling effectiveness
Effectiveness is limited by ambient temperature — in high-ambient environments, air cooling alone may be insufficient to maintain safe discharge temperatures
Cooling fins cast into cylinder heads increase surface area; fins must be kept clean to maintain heat transfer
Some installations use a fan and shroud to direct airflow across specific heat-generating surfaces
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Field Practice
Ensure adequate clearance around the compressor for airflow per manufacturer
specifications. Keep cooling fins clean and free of debris, oil film, and dust
accumulation. Verify that cooling fans operate at rated speed. In compressor
rooms, confirm ventilation is sufficient to prevent heat build-up — ambient
temperature at the compressor should not exceed the design maximum.
1.3.5 — Water Cooling
Larger compressors often use water-cooled heads, oil coolers, or jacketed cylinders
to remove heat. Water cooling is more effective than air cooling per unit of surface
area and allows reliable operation at high ambient temperatures where air cooling
would be marginal.
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Water-Cooled Heads
Water passages cast into cylinder heads remove heat of compression directly at
the source. This reduces discharge temperature, lowers oil temperature at the
discharge valves, and improves compressor efficiency. Water flow is regulated to
maintain proper head temperature — overcooling can cause refrigerant
condensation in the discharge passages.
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Water-Cooled Oil Coolers
Shell-and-tube or plate heat exchangers cool lubricating oil before it returns
to the compressor bearings. Water temperature must be controlled carefully —
overcooling the oil below its viscosity optimum reduces bearing film strength, while
undercooling allows oil to thin and lose lubrication effectiveness.
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Field Practice
Maintain water flow rates and supply temperatures per manufacturer specifications
— typically 85–95°F (29–35°C) supply water. Inspect
and clean water-side surfaces regularly to prevent fouling and scale buildup,
which sharply reduce heat transfer. Use treated water or a closed-loop system to
prevent corrosion and biological growth. Protect against freezing if the system
is exposed to ambient temperatures below 0°C (32°F).
1.3.6 — Liquid Injection Cooling
Some screw compressors inject liquid refrigerant directly into the compression chamber
to absorb heat of compression. This reduces discharge temperature and allows operation
at higher compression ratios than would otherwise cause excessive temperatures.
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Design Considerations
Liquid injection point is located partway through the compression process, where partial compression has already raised refrigerant temperature
Injection rate is modulated based on measured discharge temperature to maintain temperature within safe limits
Cooling the partially compressed vapour reduces the work done in the remaining compression, slightly reducing efficiency — the tradeoff for enabling high-compression-ratio operation
Enables operation at conditions (high ambient, low suction temperature) that would otherwise produce discharge temperatures beyond safe limits for oil and valves
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Economizer Cooling — A More Efficient Alternative
Some systems use an economizer port at an intermediate pressure point
in the compressor. Partially expanded refrigerant from the condenser — rather
than fully liquid refrigerant — provides cooling at the intermediate pressure
stage. This approach is more efficient than simple liquid injection because it also
increases system capacity: the additional refrigerant injected at the economizer port
contributes to the final compressed output.
1.3.7 — Oil Cooling
In oil-flooded screw compressors, the large volume of injected oil absorbs compression
heat directly. This hot oil must be cooled before reinjection into the compression
chamber. Four oil cooling methods are used, selected based on available utilities,
ambient conditions, and installation requirements.
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Air-Cooled Oil Cooler
Finned coils with fans reject oil heat to ambient air. Simple, requiring no water
supply or additional utilities. Effectiveness is limited by ambient temperature —
in high-ambient conditions, air cooling may not maintain oil temperature within
the required range.
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Water-Cooled Oil Cooler
Shell-and-tube heat exchangers cool oil using condenser water or a dedicated water
circuit. More effective than air cooling and maintains consistent oil temperature
regardless of ambient conditions. Common in larger industrial and commercial screw
compressor installations.
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Thermosyphon Oil Cooler
Uses the evaporation of liquid refrigerant drawn from the condenser to cool the
oil. Hot oil warms the refrigerant, which evaporates and returns to the condenser.
Self-regulating — cooling capacity automatically matches heat load. No pump or
external utilities required beyond the refrigerant circuit.
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Liquid-Injection Oil Cooler
Cool liquid refrigerant is mixed directly with hot oil before reinjection into
the compressor. Simple and compact. Because some of the refrigerant evaporates into
the oil stream, this method affects oil-refrigerant ratio and requires careful
control to avoid refrigerant dilution of the oil charge.
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Field Practice
Monitor oil temperature continuously — typical operating range is
130–180°F (54–82°C). Maintain proper cooling medium flow
and supply temperature. Clean oil coolers on a scheduled basis; fouled surfaces
reduce heat transfer and allow oil temperature to climb toward unsafe levels.
Verify that temperature control valves cycle correctly. If oil temperature cannot
be controlled within range, investigate cooling circuit flow, fouling, and
ambient conditions before concluding the oil cooler must be replaced.