Enthalpy
Also known as: heat content
Short definitionThe amount of heat a substance carries per kilogram, measured in kJ/kg. What counts in refrigeration is the difference between two points: it tells how much heat the refrigerant or the air has picked up or given off in between.
When a customer asks about evaporator capacity, enthalpy is behind the answer. The heat the refrigerant carries entering the coil is subtracted from what it carries leaving; if the difference is 150 kJ per kilogram, a system circulating 0.1 kg a second cools 15 kW. Once the cycle is drawn on a pressure–enthalpy (p–h) diagram, compressor work and COP can be read from the same sheet.
On the air side, water vapour counts too; moist air carries far more heat than dry air. To work out the door load, outside air and room air are compared. In summer, a kilogram of outside air at 35 °C and 50% humidity holds around 81 kJ of heat; in a room at 0 °C and 90% humidity the figure is 8.5 kJ. Every kilogram of air that pours in through an open door leaves the 72 kJ in between as load on the equipment.