Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 2 — Phase Transition of Water
2.1 Sensible Heat & Latent Heat
In HVAC/R, we don’t just move air and water — we move heat.
Understanding the difference between sensible and latent heat is the
foundation of everything from comfort control to refrigerant cycle analysis.
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2.1.1 — Water as an Energy Carrier
Water is one of the most effective materials for moving heat in existence. It can
store a large amount of energy per pound, and when it changes state
— from ice to liquid, or liquid to vapour — it absorbs or releases
enormous quantities of heat without any change in temperature.
This is why water shows up in almost every part of HVAC/R:
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Chilled-Water Systems
Large commercial buildings circulate chilled water through fan coil units to
deliver cooling. Water absorbs heat from each room and carries it back to the
central chiller plant.
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Steam & Hydronic Heating
Boilers convert water to steam or hot water, then distribute it to radiators
or fan coils. Steam in particular carries very large amounts of energy in a
small mass — exactly because of latent heat.
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Cooling Towers
A cooling tower removes heat from condenser water by evaporating a small
fraction of it into the air. This latent heat removal cools the remaining water
for recirculation to the chiller condenser.
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Dehumidification Coils
When warm humid air passes over a cold evaporator coil, moisture condenses
out of the air onto the coil surface. The water then drains away — this
is how air conditioning removes humidity.
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The Big Picture
Refrigerants behave the same way as water — they absorb heat by
boiling (evaporating) and release heat by condensing. Water is the teaching
model because its values are well-known and easy to visualize.
2.1.2 — Sensible Heat
Sensible heat is energy that you can sense — it
causes a change in temperature that you can measure with a thermometer. When you
add sensible heat to a substance, its temperature rises. When you remove sensible
heat, its temperature falls. The substance does not change state.
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Specific Heat Capacity
The amount of heat needed to raise 1 lb (or 1 kg) of a substance by 1°F
(or 1°C / 1 K) without changing its state.
Liquid water: 1.00 BTU/lb·°F (4.19 kJ/kg·K)
Ice: 0.50 BTU/lb·°F (2.11 kJ/kg·K)
Steam: 0.48 BTU/lb·°F (2.01 kJ/kg·K)
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Sensible Heat Formula
For liquid water (specific heat = 1 BTU/lb·°F), the formula
simplifies to:
Q = heat (BTU) • m = mass (lb) •
ΔT = temperature change (°F)
For other substances: Q = m × cp × ΔT
A hydronic heating system circulates 100 lb of water through a boiler. The
water enters at 120°F and leaves at 180°F.
How much heat did the boiler add?
The temperature rose from 120°F to 180°F, so this was purely
sensible heating — no phase change occurred.
2.1.3 — Latent Heat
Latent heat is “hidden” heat. When a substance changes
state, it absorbs or releases large amounts of energy without any change in
temperature. The energy goes into breaking or forming the molecular bonds that hold
the substance together in its current state, not into making it hotter or cooler.
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Latent Heat of Fusion
Heat involved in melting or freezing — the
solid↔liquid transition. For water at 32°F / 0°C:
144 BTU/lb (334 kJ/kg)
To melt 1 lb of ice: add 144 BTU (temp stays at 32°F).
To freeze 1 lb of water: remove 144 BTU (temp stays at 32°F).
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Latent Heat of Vaporization
Heat involved in boiling or condensing — the
liquid↔vapour transition. For water at 212°F / 100°C:
970 BTU/lb (2,257 kJ/kg)
To boil 1 lb of water: add 970 BTU (temp stays at 212°F).
To condense 1 lb of steam: remove 970 BTU (temp stays at 212°F).
During a phase change, a thermometer shows no temperature change
even though heat is actively being added or removed. This is because the energy
is used to rearrange molecular bonds, not to increase the kinetic energy of
the molecules (which is what temperature actually measures).
- Ice melting at 32°F — feels cold, but the temperature is
not falling any further while it melts.
- Water boiling at 212°F — temperature stays at 212°F no
matter how high the flame is turned; only the rate of boiling changes.
- Refrigerant boiling in an evaporator — temperature stays at the
saturation point while absorbing heat from the room air.
2.1.4 — Sensible vs. Latent — The Comfort Connection
In HVAC, a space can hit the temperature setpoint but still feel hot and
uncomfortable if the latent (moisture) load is not being handled. Sensible and
latent heat are always present together in the real world.
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Sensible Cooling Load
The energy removed to lower air temperature. This is what the
thermostat measures and controls. Formula for air:
Result in BTU/hr
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Latent Cooling Load
The energy removed to dehumidify the air — condensing
moisture out of it. Formula for air:
ΔW = moisture differential (gr/lb)
The Sensible Heat Ratio (SHR) is the fraction of the total cooling load that is
sensible. It tells you how much of the system’s work goes to temperature
reduction vs. dehumidification.
- SHR near 1.0 — almost all sensible; dry climate
(e.g. desert). System mostly lowers temperature.
- SHR near 0.7 — significant latent load; humid
climate (e.g. coastal or tropical). System must remove large amounts of
moisture.
- Oversized systems often have poor SHR performance —
they cool too quickly and shut off before removing enough moisture, leaving
the space feeling clammy.