Level 1, Unit 3, Section 2 – 2.2 The Heating Curve & Phase Changes

Unit 3 — Refrigeration System Fundamentals & Maintenance

Section 2 — Phase Transition of Water

2.2 The Heating Curve & Phase Changes

Add heat to ice at a steady rate and water follows a precise sequence of temperature rises and plateaus — the heating curve. Every plateau is a phase change, and every phase change involves latent heat.

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📈The Heating Curve 🧮Specific Heat Values 🧊Latent Heat of Fusion 💨Latent Heat of Vaporization

2.2.1 — The Heating Curve of Water

Imagine starting with ice at −20°F (−29°C) and adding heat at a perfectly steady rate. The temperature does not rise continuously — it rises, pauses, rises again, pauses again. Each pause is a phase change consuming latent heat.

Stage 1

Ice Warms Up

Solid ice absorbs sensible heat. Temperature rises steadily from below freezing up to 32°F (0°C).

Specific heat of ice: 0.50 BTU/lb·°F

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Stage 2

Ice Melts

Temperature holds steady at 32°F (0°C) while the ice absorbs latent heat of fusion and converts to liquid water.

Latent heat: 144 BTU/lb (334 kJ/kg)

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Stage 3

Water Warms Up

Liquid water absorbs sensible heat. Temperature rises from 32°F to 212°F (0°C to 100°C).

Specific heat of water: 1.00 BTU/lb·°F

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Stage 4

Water Boils

Temperature holds steady at 212°F (100°C) while water absorbs latent heat of vaporization and converts to steam.

Latent heat: 970 BTU/lb (2,257 kJ/kg)

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Stage 5

Steam Superheats

Steam (vapour) absorbs sensible heat. Temperature rises above 212°F into the superheated range.

Specific heat of steam: 0.48 BTU/lb·°F

Heating curve diagram showing temperature vs heat added for water — The heating curve of water. Flat plateaus at 32°F and 212°F are the phase changes where latent heat is absorbed at constant temperature.

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Cooling Is the Same in Reverse

Start with superheated steam and remove heat: steam cools → condenses to liquid → liquid cools → freezes to ice. Each phase change releases the same latent heat that it absorbed going the other way.

2.2.2 — Specific Heat Across the Three Phases

Specific heat capacity tells you how much energy is needed to raise 1 lb of a substance by 1°F without a phase change. The three phases of water have very different values.

Substance	Imperial (BTU/lb·°F)	SI (kJ/kg·K)	Key Point
Liquid Water	1.00	4.19	Highest of the three — excellent heat storage
Ice	0.50	2.11	Half of liquid water; heats/cools faster per BTU
Steam (vapour)	0.48	2.01	Lowest of the three; superheat changes quickly

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Why Water’s High Specific Heat Matters

Water stores more heat per pound than almost any other common substance, making it ideal for hydronic heating and cooling loops.
A chilled-water system can carry large amounts of cooling with relatively small flow rates compared to air-based systems.
Ice has only half the specific heat of liquid water — so a pound of ice absorbs heat twice as fast per degree as liquid water does. This matters during defrost cycles.

2.2.3 — Latent Heat of Fusion — 144 BTU/lb

The latent heat of fusion is the energy involved in the solid↔liquid transition of water at 32°F (0°C). It is called “latent” because the temperature stays at 32°F throughout — the energy is hidden.

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The Numbers

To melt 1 lb of ice at 32°F → must add 144 BTU (temp stays at 32°F)
To freeze 1 lb of water at 32°F → must remove 144 BTU (temp stays at 32°F)
SI equivalent: 334 kJ/kg

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The Ton of Refrigeration Connection

The ton of refrigeration is defined from the latent heat of fusion. Melting 1 short ton (2,000 lb) of ice in 24 hours requires: 2,000 × 144 = 288,000 BTU ÷ 24 h = 12,000 BTU/hr = 1 ton of refrigeration.

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HVAC/R Applications of Fusion Latent Heat

Defrost cycles: Frost accumulates on heat pump evaporator coils. The defrost heater must supply latent heat of fusion to melt the ice before returning to normal operation.
Ice prevention: Evaporator coils in freezers must maintain surface temperatures cold enough to freeze any moisture that contacts them (the coil is operating below 32°F).
Sizing reference: 144 BTU/lb is a baseline technicians use to appreciate how much larger 970 BTU/lb (vaporization) really is — it’s about 6.7 times bigger.

2.2.4 — Latent Heat of Vaporization — 970 BTU/lb

The latent heat of vaporization is the energy involved in the liquid↔vapour transition at 212°F (100°C) at atmospheric pressure. It is the largest energy exchange in the water heating curve — by far.

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The Numbers

To boil 1 lb of water at 212°F → must add 970 BTU (temp stays at 212°F)
To condense 1 lb of steam at 212°F → must remove 970 BTU (temp stays at 212°F)
SI equivalent: 2,257 kJ/kg
This is 6.7× larger than the latent heat of fusion (144 BTU/lb)

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Why Is Vaporization So Much Larger Than Fusion?

In melting, water molecules move from a rigid solid lattice into a loose liquid structure where they are still close together and still attracting each other. The bonds are weakened but not broken.

In vaporization, the molecules must completely escape the liquid surface and become a gas, where they are far apart and barely interacting. This requires breaking all of the intermolecular attractive forces — a much more energy-intensive process.

This is why evaporation and condensation dominate heat transfer in refrigeration and HVAC systems. A small mass of refrigerant can carry enormous amounts of heat simply by changing between liquid and vapour states.

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Why This Is the Heart of Refrigeration

The entire vapour compression cycle is built around latent heat of vaporization. The refrigerant’s job is simply to:

Evaporator: Absorb latent heat by boiling at low pressure (low temperature) — pulling heat out of the building.
Condenser: Release that same latent heat by condensing at high pressure (high temperature) — dumping heat to the outdoors.
The compressor raises the pressure (and therefore the condensing temperature) so the heat can be rejected outside.

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Refrigerant Comparison

Water’s latent heat of vaporization is 970 BTU/lb. Common refrigerants are much lower — R-134a is about 168 BTU/lb, and R-410A is about 165 BTU/lb — but the principle is identical. The physics is the same; only the values differ.