ASHRAE Terminology

(1) variable that cannot actually be measured directly but one which can be calculated by measuring other variables (e.g., measure wet-bulb temperature and measure dry-bulb temperature to determine enthalpy). (2) variable that is calculated from one or more inputs.

the cooling effect produced by the change in enthalpy between the refrigerant liquid entering the expansion valve and the vapor leaving the evaporator, generally measured in Btu per hour (kW or tons of refrigeration).

(1) (theoretical), enthalpy difference along an isentrope. (2) mechanical energy required by, or load imparted to, the piston of a compressor or shaft of a centrifugal compressor.

(1) portion of the total refrigerant heat-rejecting effect of a condenser, which is used for condensing the entering refrigerant vapor to a saturated liquid at the entering refrigerant pressure. (2) the portion of the total heat rejection of a condenser that is used for desuperheating and condensing the entering refrigerant vapor to a saturated liquid. This is the product of the mass rate of refrigerant flow and the difference between the enthalpy of the entering refrigerant vapor and that of the saturated refrigerant liquid at the leaving pressure.

(also known as heat content), thermodynamic quantity equal to the sum of the internal energy of a system plus the product of the pressure volume work done on the system. H = E + pv, where H = enthalpy or total heat content, E = internal energy of the system, p = pressure, and v = volume. (Compare to specific enthalpy.)

the heat released when a substance is combusted, determined as the difference in the enthalpy between the reactants (refrigerant[s] and air) and their products after combustion as defined in Section 6.1.3.5. The heat or enthalpy of combustion is often expressed as energy per mass (e.g., kJ/kg or Btu/lb).

ratio of increase of enthalpy of moist air to the rise of temperature expressed per unit mass of the dry air component under conditions of constant pressure and humidity ratio.

gas whose internal energy and enthalpy depend solely on temperature and, which is defined by pressure times specific volume divided by temperature, is a constant for a unit mass (pv/T = R). Also called perfect gas.

expansion of a fluid during which no heat or mechanical energy is exchanged with the surroundings, the enthalpy remains constant.

the change in enthalpy associated with a change in humidity ratio, caused by the addition or removal of moisture.

graph of enthalpy versus entropy of a vapor on which isobars, isothermals, and lines of equal dryness are plotted.

(1) (brine cooler) product of the mass rate of water or brine flow and the difference in enthalpy of the entering and leaving water or brine, expressed in heat units per unit of time. It is expressed also by the total refrigeration effect less the heat leakage losses. (2) (condensing unit capacity) rate at which heat is removed from outer media by a refrigerant in the low-pressure side or by the difference in total enthalpy between refrigerant liquid leaving the unit and the total enthalpy of the refrigerant vapor entering it. (3) (packaged air conditioners) rate at which heat is removed from the airstream, as measured entering the cooling coil and leaving the unit. (4) rate at which heat is removed by the primary refrigerant from the cooling medium (secondary coolant) that is used to transmit the refrigerating effect.

graphical representation of the properties of moist air, usually including wet- and dry-bulb temperatures, specific and relative humidities, enthalpy, and density.

enthalpy per unit mass of substance.

those data needed to calculate the equilibrium relations among pressure, volume, and temperature along with the enthalpy and entropy of the fluid in the liquid and vapor states.

(1) amount of sensible and latent heat removed from the conditioned space. (2) difference between the total enthalpy of the dry air and the water-vapor mixture entering and leaving the cooler.

see enthalpy.

(water or brine cooler), product of the mass rate of refrigerant flow and the difference in enthalpy of the entering and leaving refrigerant fluid, expressed in heat units per unit of time.

the product of the mass flow rate of refrigerant and the difference in enthalpy between the leaving and entering refrigerant, expressed in energy units per unit of time.

in a turbocompressor, the ratio of the variation of enthalpy of an isentropically compressed vapor to the work to be effectively supplied to the compressor (the enthalpy relating to the total pressure of the fluid at inlet and the static pressure at outlet).

(also known as stagnation efficiency), in a turbocompressor, the ratio of the variation of enthalpy of the vapor isentropically compressed from the total pressure at inlet to the total pressure at outlet to the work effectively supplied to the compressor.