Thermodynamics, heat, temperature, pressure and psychrometry fundamentals
What this domain tests
Heat, temperature and pressure fundamentals plus psychrometry: unit conversions, sensible and latent heat, absolute versus gauge pressure, vacuum measurement, dry-bulb, wet-bulb and dew-point temperature, relative humidity, enthalpy and the comfort zone.
Core rules
- 1 kcal = 4.187 kJ; 1 TR = 3.517 kW = 3024 kcal/h = 12 000 Btu/h; 1 bar = 14.5 psi = 100 kPa; 1 kW = 860 kcal/h.
- Absolute pressure = gauge pressure + atmospheric pressure (1.013 bar). Refrigerant tables always use absolute pressure: a gauge reading of 60 psi at sea level is 74.7 psia.
- Vacuum: 1 atm = 760 mm Hg = 1013 mbar = 760 000 micron. A deep vacuum for a refrigeration circuit is around 500 micron.
- Sensible heat changes temperature (Q = m x cp x delta T). Latent heat changes state at constant temperature, for example 2257 kJ/kg to boil water at 100 °C.
- Psychrometry: DBT from an ordinary thermometer, WBT from a wet-bulb thermometer in moving air, DPT the temperature at which condensation begins. RH = vapour pressure / saturation pressure x 100.
- Comfort in Indian practice: about 22–27 °C DBT and 40–60 % RH. Always DPT ≤ WBT ≤ DBT, with equality at 100 % RH.
Worked example
A room load of 2 TR is 2 x 3.517 = 7.03 kW, that is 6047 kcal/h or 24 000 Btu/h. If supply air is 10 K below room air, mass flow = Q / (cp x delta T) = 7.03 / (1.02 x 10) = 0.69 kg/s. Dividing by air density 1.2 kg/m³ gives 0.57 m³/s, about 2070 m³/h.
Traps
Plotting gauge pressure on a P-h chart; forgetting the 860 kcal/h per kW step; claiming WBT can exceed DBT; treating latent heat as a temperature rise; mixing TR with kW in the same formula.