Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 1 — Fundamental Concepts
1.2 Heat & Temperature
Heat is energy in motion — always flowing from hot to cold. Understanding
the types of heat, temperature scales, and the three methods of heat transfer
is the foundation of all refrigeration theory.
Heat is a form of energy transferred between systems due to a temperature
difference. It always flows spontaneously from a higher-temperature body to a
lower-temperature body. Refrigeration systems use work to reverse this natural
direction of heat flow, moving heat from cold spaces to warmer surroundings.
Heat in HVAC/R is usually measured in kJ or Btus, or as a rate in kW or Btu/h.
Understanding the distinction between types of heat — latent and sensible
— is essential to analysing system performance.
Temperature is what a thermometer reads — it tells you
how hot something is. Heat is energy moving from a warmer
object to a cooler object; heat transfer is what HVAC equipment is built to control.
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The Key Distinction
You can add heat and raise temperature — that is straightforward.
But you can also add heat and not raise temperature, if the energy is being used to change state.
That second case is where most HVAC confusion happens — which is why heat
is separated into two types: sensible and latent.
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Latent Heat
Heat added or removed during a phase change at constant temperature.
During boiling or condensation, temperature remains constant while the substance
absorbs or releases latent heat.
Refrigerant absorbs latent heat during evaporation — providing the cooling effect.
Refrigerant releases latent heat during condensation.
Latent heat of vaporization is the dominant contributor to refrigeration capacity.
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Sensible Heat
Heat added or removed that results in a temperature change
without a phase change. In comfort cooling, sensible heat loads include warming
or cooling air, building materials, and equipment surfaces.
An adiabatic process may involve only sensible heat changes internal to the
fluid with no heat exchange with surroundings — for example, ideal gas
compression or expansion.
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Specific Heat
The amount of heat required to raise the temperature of 1 lb of a substance
by 1°F (or 1 kg by 1°C). Expressed in Btu/(lb·°F) or
kJ/(kg·°C).
Used in system design to compute heat transfer in fluids:
Q = m × cp × ΔT
Q = heat transfer • m = mass flow rate •
cp = specific heat at constant pressure •
ΔT = temperature change
1.2.2 — Temperature Scales
Temperature indicates the level of thermal energy of a substance and determines
the direction of heat flow. Temperature scales measure hotness or coldness, with
SI (metric) and Imperial (US customary) systems offering distinct approaches for
science, daily life, and engineering. In refrigeration work, Celsius (°C)
and Kelvin (K) are most common.
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Celsius (°C) & Kelvin (K)
Celsius sets water freezing at 0°C and boiling at
100°C — ideal for weather and everyday use worldwide.
Kelvin is the SI absolute scale,
starting at 0 K = absolute zero (−273.15°C), where all molecular
motion stops.
Fahrenheit is common in US HVAC — water freezes at
32°F and boils at 212°F. A comfortable room is 68–77°F.
Rankine is the absolute counterpart
used in thermodynamics: °R = °F + 459.67.
A hot day: 86°F = 30°C = 303 K
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Temperature Conversions
°F → °C: (°F − 32) × 5/9
°C → °F: (°C × 9/5) + 32
°C → K: K = °C + 273.15
°F → °R: °R = °F + 459.67
1.2.3 — Methods of Heat Transfer
Heat energy always flows spontaneously from higher temperature to lower
temperature regions. Refrigeration and heat pump systems use mechanical work
to move heat against this natural direction — from cold regions to hot.
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Conduction
Heat transfer through solid material or between materials in direct contact.
The rate depends on:
Temperature difference across the material.
Thermal conductivity of the material.
Thickness and area of the material.
In HVAC, conduction occurs through walls, roofs, floors, and equipment
casings — addressed through insulation and proper building design.
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Convection
Heat transfer between a solid surface and a moving fluid (liquid or gas).
Used in evaporator and condenser coils; coil design and airflow rate both strongly influence performance.
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Radiation
Heat transfer via electromagnetic waves (infrared) — requiring no
physical medium. All bodies above absolute zero emit radiant energy; the
net exchange depends on surface temperature, emissivity, and view factors.
In comfort applications, radiant heat from the sun, equipment, and building
surfaces contributes to internal loads and must be considered in equipment sizing.
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Factors Affecting Rate of Heat Transfer
Temperature difference between the two media.
Heat transfer area — e.g. coil surface area.
Thermal conductivity of materials and cleanliness of surfaces (fouling reduces transfer).
Fluid velocity on both sides of a heat exchanger.
Type of flow (laminar vs turbulent) and fluid properties (viscosity, specific heat).
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Field Practice
Technicians improve heat transfer by ensuring proper airflow, clean coils
and filters, correct refrigerant charge, and adequate water/glycol flow in
hydronic systems.
