Refrigeration and Air Conditioning Fundamentals
The Fundamentals domain is the theory the rest of the trade hangs on: thermodynamics, heat transfer, psychrometrics and the vapour-compression cycle. Questions ask you to predict what a gauge and a thermometer will read when something changes, and to calculate capacity, compression ratio and efficiency.
Heat and pressure basics
- Sensible heat changes temperature; latent heat changes state at constant temperature. One kilogram of ice at 0°C absorbs about 334 kJ to melt, and one kilogram of water absorbs about 2257 kJ to boil at 100°C at atmospheric pressure.
- Heat moves by conduction, convection and radiation, always from warmer to colder.
- At saturation, liquid and vapour coexist and pressure and temperature are locked together on the PT chart. Superheated vapour is above saturation temperature; subcooled liquid is below it.
- Compression ratio = absolute discharge pressure ÷ absolute suction pressure. Gauge readings must be converted first: add 101 kPa (14.7 psi).
The cycle
Evaporator absorbs heat at low pressure, compressor raises pressure and temperature, condenser rejects heat, metering device drops pressure. Subcooling at the condenser outlet and superheat at the evaporator outlet together tell you whether the metering device is feeding correctly.
Psychrometrics
Dry bulb is ordinary temperature; wet bulb includes evaporative cooling; dew point is where condensation begins. Enthalpy (kJ/kg of dry air) and humidity ratio (g/kg) are read from the chart. Coil capacity is the enthalpy difference of the air multiplied by the dry-air mass flow.
Worked example
Enthalpies are 400 kJ/kg at the evaporator inlet, 440 kJ/kg at compressor discharge, and 250 kJ/kg at the condenser outlet (and after the metering device). Refrigerating effect = 400 − 250 = 150 kJ/kg. Compressor work = 440 − 400 = 40 kJ/kg. COP = 150 ÷ 40 = 3.75. With a mass flow of 0.1 kg/s, capacity = 0.1 × 150 = 15 kW.
Traps
- Computing compression ratio from gauge pressures.
- Confusing saturation temperature with actual pipe temperature.
- Treating a zeotropic blend as if it had a single boiling point.
- Mixing up COP (dimensionless) with EER (Btu per watt-hour).