Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 4 — Vapour Compression Cycle
4.1 — Temperature Ranges
Not every refrigeration system operates the same way. Matching a system to its
application starts with the evaporating temperature — the temperature
at which the refrigerant boils inside the evaporator. Three broad ranges cover
everything from a home air conditioner to a blast freezer.
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4.1.1 — Evaporating Temperature — The Key Classification Parameter
The evaporating temperature is the saturation temperature of the
refrigerant inside the evaporator coil at the operating suction pressure. It is
the temperature at which the refrigerant boils — absorbing latent heat from
the surrounding air, fluid, or product.
The evaporating temperature must always be lower than the temperature of
the medium being cooled. The greater the difference, the faster heat flows —
but efficiency drops. System designers balance this trade-off when selecting the
target evaporating temperature for any given application.
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Evaporating temperature and suction pressure are linked
You never measure evaporating temperature directly — you read suction
pressure on your gauge set and convert it to saturation temperature using the
P–T chart. High suction pressure = high evaporating temperature;
low suction pressure = low evaporating temperature.
High-temperature systems maintain spaces or products above freezing.
Comfort air conditioning is the largest category, but beverage coolers, water
chillers, and some process cooling also fall here.
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Evaporating Temperature Range
+35°F to +55°F (+2°C to +13°C)
Evaporator operates a few degrees below the desired supply air or
product temperature to achieve consistent control.
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System Characteristics
Moderate condenser pressure. High compressor volumetric efficiency.
Relatively low pressure ratios — making these systems the most energy
efficient of the three categories.
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Common Refrigerants
R‑410A, R‑32, R‑134a
Selected for good performance at higher evaporating temperatures, low toxicity,
and (for R‑32) lower global warming potential than older blends.
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Example — Residential Split Air Conditioner
A residential A/C system delivers supply air at approximately 55°F (13°C)
while maintaining an evaporator saturation temperature near 40°F
(4.5°C). The 15°F temperature difference between the evaporator
and supply air provides effective heat absorption without risking coil frost.
Using R‑410A, the corresponding low-side pressure is approximately
118 psig — a value every residential service technician
quickly memorises.
4.1.3 — Medium-Temperature Refrigeration — Commercial Cooling (+32°F to +10°F)
Medium-temperature systems keep products cold but not frozen. They
bridge the gap between comfort cooling and deep-freeze applications, and they
dominate the commercial food service and retail sectors.
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Evaporating Temperature Range
+25°F to +10°F (−4°C to −12°C)
Low enough to maintain product temperatures slightly above 32°F (0°C)
without freezing perishables.
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System Challenges
Frost management is critical — the coil operates below freezing and must
be defrosted regularly. TXV superheat control is essential for efficiency as
loads vary through the day.
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Common Refrigerants
R‑404A, R‑407A, R‑448A, CO₂ (R‑744)
CO₂ transcritical and cascade systems are increasingly common as
HFC phase-down regulations take effect in many jurisdictions.
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Example — Supermarket Dairy Case
A supermarket dairy display case must maintain products near 36°F
(2°C). The evaporator operates at approximately 20°F
(−7°C) to provide adequate temperature difference for heat
transfer through the case’s glass door and product load.
These cases are often connected to a centralised
rack refrigeration system in the store’s machine room, with multiple
cases sharing one or more compressors.
Low-temperature systems are the most demanding of all. They maintain products
well below freezing — from standard chest freezers to industrial blast
chillers that rapidly freeze product to prevent ice crystal damage.
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Evaporating Temperature Range
0°F to −40°F (−18°C to −40°C)
Specialised cryogenic applications can go significantly lower, requiring
cascade or multi-stage compression systems.
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System Challenges
Very low suction pressures produce high compression ratios, stressing
the compressor. Oil management becomes critical as lubricant viscosity changes
dramatically. Pressure drops in suction lines must be minimised.
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Common Refrigerants
R‑404A, R‑448A, R‑449A, R‑717 (ammonia), CO₂
Ammonia is preferred in large industrial freezers for its excellent thermodynamic
properties and zero GWP, despite its toxicity classification.
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Example — Frozen Food Display Case
A frozen food display case maintains product at −5°F
(−21°C). The evaporator runs at approximately
−20°F (−29°C) to maintain the required
temperature difference for heat transfer.
Because single-stage compression becomes increasingly
inefficient below −20°F, larger systems often use two-stage
compression or an economiser cycle to improve
efficiency and reduce discharge temperatures.
HFC refrigerants such as R‑404A and R‑410A are being phased down under
the Kigali Amendment and equivalent national regulations. When selecting
refrigerants for new installations, always verify current regulatory status
in your jurisdiction and consult the equipment manufacturer’s recommendations.