Overview
Basic thermodynamics is the foundation of every refrigeration and air-conditioning system. F-Gas candidates must understand how energy moves through a system, how refrigerants change state, and how pressure and temperature interact within the vapour-compression cycle. This knowledge underpins safe handling, fault diagnosis, and efficient system design — all of which are assessed under EU Implementing Regulation 2015/2067.
SI Units and Measurement
Refrigeration professionals work with a core set of SI units every day. Temperature is measured in degrees Celsius (°C) or Kelvin (K), pressure in Pascal (Pa) or bar, and energy in Joules (J) or kilowatts (kW). Mass is expressed in kilograms (kg) and refrigerant charge is always recorded in kg for regulatory reporting. Ensure you can convert between absolute and gauge pressure: absolute pressure = gauge pressure + atmospheric pressure (≈ 1.013 bar).
Pressure and Temperature
- Gauge pressure reads zero at atmospheric pressure; absolute pressure reads zero at a perfect vacuum.
- The saturation temperature of a refrigerant is the boiling/condensing temperature at a given pressure. This relationship is unique to each refrigerant and is read from pressure-temperature (PT) charts.
- Superheat is the temperature rise above saturation in the evaporator outlet; subcooling is the temperature drop below saturation at the condenser outlet. Both are critical diagnostic measurements.
Heat Transfer
Heat always flows from a warmer body to a cooler body. The three modes are:
- Conduction — transfer through a solid material (e.g., copper pipe walls).
- Convection — transfer via fluid movement (e.g., air passing over evaporator fins).
- Radiation — transfer via electromagnetic waves (less significant in typical RACHP systems but still examined).
The Refrigeration Cycle
The vapour-compression cycle has four main stages: compression, condensation, expansion, and evaporation. At each stage the refrigerant’s pressure, temperature, and state change. Understanding the pressure-enthalpy (p-h) diagram allows technicians to visualise energy changes and identify faults such as low charge, overcharge, or restricted flow.
Latent and Sensible Heat
- Sensible heat causes a temperature change without a change of state.
- Latent heat causes a change of state (liquid ↔ vapour) at constant temperature.
- The high latent heat capacity of refrigerants is what makes the refrigeration cycle efficient.
Exam Tip: Be prepared to calculate superheat and subcooling from given pressure and temperature readings. Know how to use a PT chart for common refrigerants such as R-410A, R-32, and R-134a. Questions often test whether you understand the difference between gauge and absolute pressure.
