Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 4 — Vapour Compression Cycle
4.3 — Components of the Vapour Compression Cycle
Four major components, three refrigerant lines, and a set of accessories make up
every vapour compression system. Knowing what each one does — and what
condition the refrigerant should be in at each point — is the foundation
of all service and commissioning work.
Jump to
4.3.1 — Compressor — The Heart of the System
The compressor is the only component that adds energy to the refrigerant. It
maintains the pressure difference between the low and high sides, and its
discharge creates the high temperature needed to reject heat in the condenser.
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Reciprocating
Piston-and-cylinder design. Reliable, field-serviceable on open types. Common in residential
and light commercial equipment. Vulnerable to liquid slugging.
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Scroll
Two interlocking spiral scrolls compress vapour. Quieter and more efficient
than reciprocating for most A/C applications. Dominant in residential split systems.
Sensitive to incorrect rotation direction.
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Rotary
A rolling piston compresses vapour against a cylinder wall. Compact and quiet.
Common in small window A/C units and dehumidifiers. Limited to smaller capacities.
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Screw
Twin helical rotors compress large volumes of vapour. Used in commercial
chillers and industrial refrigeration. High capacity, good part-load efficiency
with slide valve capacity control.
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Compressor Protection Devices
Compressors are the most expensive component to replace. They are protected by:
High-pressure cut-out (HPCO): Shuts off the compressor if discharge pressure exceeds a safe limit (dirty condenser, blocked airflow, overcharge).
Low-pressure cut-out (LPCO): Shuts off the compressor if suction pressure drops too low (low charge, blocked filter, frosted evaporator).
Internal overload / motor protector: Protects motor windings from heat and current overload.
Crankcase heater: Prevents refrigerant migration and liquid slugging on start-up in cold ambient conditions.
Accumulator: Traps liquid in the suction line before it reaches the compressor.
4.3.2 — Condenser — Where Heat Is Rejected
The condenser receives hot, high-pressure discharge vapour from the compressor
and rejects heat to the surroundings until the refrigerant condenses to a
subcooled liquid. The total heat rejected equals the evaporator load
plus the heat of compression.
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Air-Cooled
Refrigerant-to-air heat exchanger with fan(s). Simple, low cost, widely used
in residential and light commercial. Capacity falls on hot days as outdoor
ambient temperature rises.
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Water-Cooled
Water flows through a shell-and-tube or brazed-plate heat exchanger,
absorbing heat. Paired with a cooling tower. More efficient than air-cooled
because water temperature is lower and more stable than outdoor air.
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Evaporative
Combines water spray and airflow over the coil surface. Evaporation of water
provides additional cooling effect. Very efficient in hot, dry climates.
Requires water treatment and scale management.
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Dirty condenser = high discharge pressure
A condenser coil fouled with dirt, cottonwood, or debris forces the
refrigerant to condense at a higher temperature and pressure. High discharge
pressure increases compression ratio, reduces compressor efficiency, raises
discharge temperature, and can trip the high-pressure cut-out. Coil cleaning
is one of the highest-value preventive maintenance tasks.
4.3.3 — Metering Device — The Pressure Regulator
The metering device creates and maintains the pressure difference between the
high and low sides of the system. It controls how much refrigerant enters the
evaporator — and therefore controls evaporator superheat.
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Thermostatic Expansion Valve (TXV)
Modulates refrigerant flow to maintain a set evaporator superheat (typically
8–12°F). A sensing bulb attached to the suction line signals the valve
to open or close. Adapts to varying load conditions automatically.
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Electronic Expansion Valve (EEV)
A stepper-motor-driven valve controlled by the system electronics. Can
respond faster and more precisely than a TXV. Standard in modern variable-speed
systems and heat pumps where conditions change rapidly.
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Capillary Tube / Fixed Orifice
A fixed restriction with no moving parts. Simple, inexpensive, and reliable.
Cannot adapt to changing loads. Used in small equipment with relatively stable
operating conditions (window units, small commercial systems).
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TXV position and superheat
A TXV set too high (too little refrigerant) produces high evaporator superheat
and starves the evaporator. Set too low (too much refrigerant) produces low
superheat and risks liquid reaching the compressor. Field adjustment or
replacement is required when superheat deviates persistently from target.
4.3.4 — Evaporator — Where Useful Cooling Happens
The evaporator absorbs heat from the medium being cooled — air, water,
brine, or product. Low-pressure liquid/vapour mixture from the metering device
enters and boils, absorbing latent heat at nearly constant temperature.
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Direct Expansion (DX)
Refrigerant flows directly through the coil. Air or another fluid passes
over the outside. Compact and efficient. Used in virtually all residential
and most commercial systems.
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Flooded Evaporator
The shell side is filled with liquid refrigerant and the fluid to be chilled
flows through tubes. Very high heat transfer efficiency. Used in large chillers
for buildings and industrial processes.
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Airflow — The Evaporator’s Most Critical Variable
Evaporator capacity depends directly on airflow across the coil. Reduced
airflow (dirty filter, blocked return, failed blower motor) causes:
Lower evaporator load → lower suction pressure and temperature
Coil frost or icing, further reducing airflow
Low suction pressure can trip the LPCO
Increased evaporator superheat (or paradoxically, flooding if the TXV
responds to low superheat incorrectly)
Always verify airflow before
diagnosing a refrigerant charge issue.
4.3.5 — Refrigerant Lines
Three refrigerant lines connect the major components. Each carries a different
condition of refrigerant and has different design requirements.
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Discharge Line
Compressor → Condenser
High pressure, high temperature superheated vapour. Typically the hottest
pipe in the system (can exceed 200°F / 93°C). Must be sized to minimise
pressure drop and must pitch toward the condenser to assist oil return.
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Liquid Line
Condenser → Metering Device
High pressure subcooled liquid. Warm to the touch (typically 80–110°F
/ 27–43°C). Houses the filter-drier and sight glass. Must remain
fully liquid — no flash gas.
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Suction Line
Evaporator → Compressor
Low pressure superheated vapour. Cold and often sweating with condensation
(normal). Must be insulated to limit heat gain and sized for proper oil return
velocity (0.5–1.0 m/s in risers).
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Suction line insulation is not optional
Every degree of heat gained by the suction vapour before reaching the
compressor raises the compressor suction temperature, increases discharge
temperature, and reduces efficiency. Uninsulated suction lines in warm
mechanical rooms can raise total superheat by 5–15°F —
enough to cause compressor overheating over time.
4.3.6 — System Accessories
Accessories improve reliability, serviceability, and safety. A technician must
know the purpose of each accessory and what its condition indicates about system
health.
Accessory
Location
Function
What it tells a technician
Receiver
Liquid line (after condenser)
Stores excess liquid refrigerant; ensures liquid seal to metering device