Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 2 — Phase Transition of Water
2.4 Pressure, Boiling Point & Refrigerant Behaviour
The boiling point of any liquid changes with pressure. This single fact
is the entire basis of mechanical refrigeration — by controlling
pressure, a technician controls where and at what temperature a
refrigerant boils and condenses.
Jump to section
2.4.1 — Pressure Controls Boiling Point
Water boils at 212°F (100°C) at sea level because that is atmospheric
pressure. At the top of a mountain where pressure is lower, water boils at a
lower temperature — about 194°F (90°C) at 10,000 ft. In a
pressure cooker where pressure is higher, water boils above 250°F (121°C).
The liquid is the same; only the pressure changes.
Increase Pressure
- Boiling point rises
- Liquid stays liquid at higher temperatures
- More energy needed to boil
- Used in condensers
Decrease Pressure
- Boiling point falls
- Liquid boils at lower temperatures
- Less energy needed to boil
- Used in evaporators
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The Key Insight
A refrigeration system is essentially a pressure management system.
The compressor creates high pressure on one side (condenser) and low pressure
on the other (evaporator). These two pressures set the two temperatures at
which the refrigerant changes state — and those temperatures determine
what the system can heat or cool.
R-134a has a boiling point of −15°F (−26°C) at
atmospheric pressure. In a car AC system:
| Location |
Pressure |
Saturation Temp. |
What Happens |
| Evaporator (low side) |
~70 psi |
~40°F (4°C) |
Refrigerant boils, absorbing heat from cabin air |
| Condenser (high side) |
~250 psi |
~115°F (46°C) |
Refrigerant condenses, releasing heat to outside air |
The expansion valve is the dividing line between high and low pressure.
It drops the pressure from ~250 psi to ~70 psi, which drops the boiling
point from ~115°F to ~40°F, enabling the evaporator to absorb
heat from 75°F cabin air.
2.4.2 — Why Refrigerants Boil at Low Temperatures
Refrigerants are chosen specifically because they have very low boiling points
at atmospheric pressure — which means they are already vapour at room
temperature unless placed under pressure. This allows the system to create a
liquid that boils inside the evaporator at whatever low temperature the system
needs.
| Substance |
Boiling Point at 1 atm |
Typical Use |
| Water |
212°F (100°C) |
Not used as a refrigerant (boiling point too high) |
| Ammonia (R-717) |
−28°F (−33°C) |
Industrial refrigeration, food processing |
| R-134a |
−15°F (−26°C) |
Automotive AC, small appliances |
| R-410A |
−60°F (−51°C) |
Residential and light commercial AC |
| R-32 |
−52°F (−52°C) |
Modern residential systems (lower GWP) |
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Why Water Doesn’t Work as a Refrigerant
Water’s boiling point of 212°F at atmospheric pressure means
it would need to operate under a deep vacuum to boil at useful cooling
temperatures. System pressures would be dangerously low (below atmospheric),
making the equipment impractical and difficult to seal against air infiltration.
Refrigerants with naturally low boiling points operate at above
atmospheric pressure throughout the system, making leaks easier to detect
(gas escapes rather than air entering).
2.4.3 — Superheat
Superheat is the condition where a vapour has been heated
above its saturation temperature at a given pressure. A
superheated vapour contains no liquid droplets — it is fully in the
gas phase, just warmer than it needs to be to stay that way.
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How to Measure Superheat
Superheat = Actual vapour temperature − Saturation temperature
at that pressure.
Example: Evaporator pressure = 70 psi → saturation
temp = 40°F. Suction line temp = 50°F.
Superheat = 50 − 40 = 10°F
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Why Superheat Is Required
The compressor is designed to compress vapour only.
Liquid refrigerant cannot be compressed — it causes mechanical damage
known as slugging.
A small amount of superheat (typically 8–12°F / 4–7°C)
at the evaporator outlet ensures the refrigerant is completely vapour
before entering the compressor.
- Low superheat (below 5°F) — liquid refrigerant
may be reaching the compressor. Risk of slugging. System may be
overcharged or expansion device is open too wide.
- High superheat (above 15–20°F) — the
evaporator is not fully used; system is undercharged or expansion device
is too restrictive. Reduced cooling capacity.
- Target superheat — typically 8–12°F
for fixed-orifice systems; set at the expansion valve for TXV systems.
Always refer to the manufacturer’s specifications.
2.4.4 — Subcooling
Subcooling is the condition where a liquid has been cooled
below its saturation temperature at a given pressure. A
subcooled liquid contains no vapour bubbles — it is fully in the liquid
phase, just cooler than required for it to remain so.
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How to Measure Subcooling
Subcooling = Saturation temperature at condenser pressure − Actual
liquid temperature.
Example: Condenser pressure = 250 psi → saturation
temp = 115°F. Liquid line temp = 100°F.
Subcooling = 115 − 100 = 15°F
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Why Subcooling Matters
Subcooling ensures the refrigerant arrives at the expansion device as
100% liquid with no vapour bubbles (flash gas).
Flash gas in the expansion device causes erratic metering and reduces
system capacity. Subcooling prevents this and adds useful cooling capacity.
Typical target: 10–15°F (6–8°C).
Every refrigeration system has both superheat and subcooling occurring
simultaneously. Together they bookend the phase-change regions of the cycle:
- Subcooling ensures solid liquid enters the expansion
device. Occurs at the end of the condenser and in the liquid line.
- Expansion drops pressure and temperature; some liquid
flashes to vapour (this is normal and unavoidable).
- Evaporator boils the remaining liquid, absorbing
latent heat from the building. This is where useful cooling happens.
- Superheat at the evaporator outlet ensures all
liquid is gone before the vapour enters the compressor.
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Connecting Back to Section 2
Superheat is a sensible-heat region (temperature rising, no phase change).
Subcooling is also a sensible-heat region. The large middle zones —
condensing and evaporating — are the latent-heat regions where the
enormous energy exchanges that make refrigeration work are happening.