HVAC Terms

Reference data and engineering information about hvac terms for hvac systems applications.

Overview

Engineering reference data for HVAC Terms in HVAC systems.

Key Formulas

Sensible Heat

$Q = \dot{m} c_p \Delta T$

Heat causing temperature change.

Latent Heat

$Q = \dot{m} h_{fg} \Delta\omega$

Heat causing moisture change.

COP (Cooling)

$COP = Q_c / W$

Coefficient of performance.

Variables

Symbol	Description	Unit
$Q$	Heat transfer	W
$\dot{m}$	Mass flow rate	kg/s
$c_p$	Specific heat of air	J/(kg·K)
$\Delta T$	Temperature difference	K

HVAC Term Definitions

Term	Definition
ABSOLUTE HUMIDITY	The weight of water vapor in a given amount of air.
ABSOLUTE PRESSURE	Pressure measured with the base of zero.
ABSOLUTE TEMPERATURE	A temperature scale expressed in degrees °F or °C using absolute zero as a base (Rankine or Kelvin scale).
ABSOLUTE ZERO	The temperature at which molecular activity theoretically ceases: -459.67°F or -273.15°C.
AIR CONDITIONING	The process of controlling the temperature, humidity, cleanliness, and distribution of the air.
AMBIENT	Refers to the temperature surrounding a body or unit under test.
ATMOSPHERIC PRESSURE	The weight of a 1 unit column of the earth's atmosphere.
BI-METAL	Two metals with different rates of expansion fastened together; warps when heated/cooled to open/close a switch or valve.
BOILING POINT	The temperature at which the addition of any heat will begin a change of state from a liquid to a vapor.
BRITISH THERMAL UNIT (BTU)	The amount of heat necessary to change the temperature of 1 pound of pure water 1 degree Fahrenheit (°F).
CAPILLARY TUBE	A refrigerant control consisting of a small diameter tube which controls flow by restriction, sized by inside diameter and length for each application.
CENTIGRADE	A temperature scale with the freezing point of water 0°C and the boiling point 100°C at sea level.
CHECK VALVE	A valve designed to permit flow in one direction only.
COMPRESSION	The reduction of volume of a vapor or gas by mechanical means.
COMPRESSION RATIO	The ratio determined by dividing the discharge pressure by the suction pressure.
COMPRESSOR	A mechanical device used to compress gases; main types: screw, scroll, reciprocating, centrifugal, and rotary.
CONDENSATION POINT	The temperature at which the removal of any heat will begin a change of state from a vapor to a liquid.
CONDENSING MEDIUM	The substance, usually air or water, to which the heat in a condenser is transferred.
CONDENSING UNIT	The portion of a refrigeration system where the compression and condensation of refrigerant is accomplished (the 'high side').
CONDUCTION	The transfer of heat from molecule to molecule within a substance.
CONTACTOR	An electromagnetic actuated relay, usually used to close the circuit to a compressor.
CONVECTION	The transfer of heat by a moving fluid.
COOLING ANTICIPATOR	A resistance heater in parallel with the cooling circuit that adds heat during the off cycle to shorten it.
COP	Ratio of work performed or accomplished as compared to the energy used.
CUBIC FEET PER MINUTE (CFM)	A common means of assigning quantitative values to volumes of air in transit.
CYCLE	The complete course of operation of a refrigerant back to a selected starting point in a system.
DENSITY	Mass or weight per unit of volume.
DISCHARGE LINE	A tube used to convey the compressed refrigerant vapor from the compressor to the condenser inlet.
DISCHARGE PRESSURE	The pressure read at the compressor outlet; also called head pressure or high side pressure.
DRY AIR	Air which contains no moisture vapor.
DRY BULB TEMPERATURE	Temperature read with an ordinary thermometer.
EFFECTIVE TEMPERATURE	An arbitrary concept combining temperature, humidity, and air movement into a single value as sensed by the human body.
ENTHALPY	Total amount of heat in one pound (kg) of a substance calculated from an accepted temperature base, expressed in BTU's per pound mass (J/kg).
EQUIVALENT LENGTH	That length of straight tubing with the same pressure drop as a fitting, valve, or accessory of the same nominal size.
EVAPORATIVE COOLING	The cooling effect of vaporization of a liquid in a moving air stream.
EVAPORATOR	A device in which a liquid refrigerant is vaporized, often with some super-heating.
EVAPORATOR SUPERHEAT	The actual temperature of the refrigerant vapor at the evaporator exit compared to the saturated vapor temperature indicated by the suction pressure.
EXTERNAL STATIC PRESSURE	The sum of the static and velocity pressures of a moving air system at the point of measurement.
FAHRENHEIT	A temperature scale with the freezing point of water 32°F and the boiling point 212°F at sea level.
FEET PER MINUTE (FPM)	A term assigned to the velocity of a moving air stream.
FILTER-DRIER	A device that removes moisture, acid, and foreign matter from the refrigerant.
FLASH GAS	Instantaneous evaporation of some liquid refrigerant at the metering device due to pressure drop, cooling the remaining liquid to the desired evaporation temperature.
FREEZING POINT	The temperature at which the removal of any heat will begin a change of state from a liquid to a solid.
SATURATED VAPOR	Vapor in contact with a liquid.
SATURATION	A condition of stable equilibrium of a vapor and a liquid.
SENSIBLE HEAT	Heat that can be measured or felt; always causes a temperature rise.
SIGHT GLASS	A glass installed in the liquid line for visual inspection of liquid refrigerant to detect vapor and monitor moisture content.
SLUGGING	A condition where liquid enters the compressor, causing hammering and possible damage.

Additional HVAC Formulas

Compression Ratio: $\text{Compression Ratio} = \frac{P_d}{P_s}$ where $P_d$ is discharge pressure and $P_s$ is suction pressure.
Coefficient of Performance (COP): $\text{COP} = \frac{\text{Useful Heating or Cooling Provided}}{\text{Work Input}}$ For refrigeration systems, this is often expressed as: $\text{COP}_{\text{cooling}} = \frac{Q_{\text{cooling}}}{W_{\text{input}}}$ where $Q_{\text{cooling}}$ is the cooling effect and $W_{\text{input}}$ is the work input.

References

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