HVAC/R Glossary

A
Absolute pressure - The sum of gage pressure and atmospheric pressure.
Accumulator- A storage chamber for low-side liquid refrigerant, also known as a surge
drum or surge header.
Azeotropic mixture- A mixture of liquids whose vapor and liquid phases in equilibrium
have identical compositions (the boiling point is constant).
Azeotropic point- The temperature at which a liquid mixture boils and produces a vapor
having the same composition as the liquid.

B
Bubble Point (refrigerant)-The pressure at which the liquid mixture starts to vaporize.

C
Capillary tube- A tube of small internal diameter used as a refrigerant pressure and flow
control between the high and low sides of a system.
Carbonization- The formation of carbonaceous deposits which may be produced by
decomposition of lubricating oil or other organic materials.
Cascade refrigeration system- A system with two or more refrigerant circuits, each
with a pressure imposing element.
Centrifugal compressor- nonpositive displacement compressor which utilizes
centrifugal forces to obtain pressure rises, used with low pressure refrigerants.
Change of state- change from one form of the three phases, solid, liquid, or gas, to
another.
Coefficient of performance, (COP) -The ratio of the refrigerating capacity to the work
absorbed by the compressor per unit time.
Compound compression- Compression accomplished by stages as in two or more
cylinders or machines.
Compression- a process that increases the pressure of a gaseous refrigerant.
Compressor – a device for mechanically increasing the pressure of a refrigerant.
Booster compressor - a compressor for raising the pressure of a gas delivered
to a following compressor
Compound compressor - a compressor in which compression is accomplished
by stages, as in two or more cylinders
Open type compressor - a refrigerant compressor with a shaft or other moving
part extending through its casing to be driven by an outside source of power,
thus requiring a shaft seal
Positive displacement compressor - a compressor that obtains compression
by reduction of the internal volume of a compression chamber by a piston
Reciprocating compressor - a positive displacement compressor that changes
internal volume of the compression chamber(s) by the reciprocating motion of
one or more pistons
Refrigerant compressor - that component of a refrigerating system which
increased the pressure of a compressible refrigerant fluid and simultaneously
reduces its volume while moving the fluid through the device
Semi-hermetic compressor- a hermetic refrigerant compressor whose housing
is sealed against entry of air by one or more gasketed joints and is provided with
means of access for servicing internal parts in the field
Welded hermetic compressor – a hermetic compressor whose housing is
permanently sealed by welding or brazing and is not provided with a means of
access for servicing internal parts in the field
Rotary compressor - a positive displacement compressor that changes internal
volume of the compression chambers by the rotary motion of a positive
displacement member
Scroll Compressor - a scroll compressor uses two interleaved scrolls to pump,
compress, or pressurize fluids such as liquids and gases
Swash-Plate Compressor - a compressor in which the pistons move parallel to
the crankshaft. The pistons are actuated by a cam or by a plate mounted axially
on the shaft and inclined to it
Compressor Capacity (refrigerating) – The heat removal by the refrigerant assigned
to the compressor in a refrigerating system. This rate equals the product of the mass
rate of refrigerant flow produced by the compressor and the difference in specific
enthalpies of the refrigerant vapor at its thermodynamic state entering the compressor
and the refrigerant liquid at the saturation temperature corresponding to the pressure of
the vapor leaving the compressor
Compressor displacement- Actual volume of gas or vapor, at compressor inlet
conditions, moved by a compressor per revolution or per unit of time
Compressor unloader- a device on or in a compressor for equalizing the high and low
side pressures for a brief period during starting in order to decrease the starting load on
the motor: also a device for controlling compressor capacity by rendering one or more
cylinders ineffective
Condenser- A vessel or arrangement of pipe or tubing that liquefies vapor by removal of
heat
Aircooled condenser- a refrigerant condenser that removes heat ientirely by
heat absorption of ambient air flowing over the condensing surfaces
Evaporative condenser- a condenser that removes heat by the evaporation of
water induced by the forced circulation of air over it
Water cooled condenser- A condenser that removes the heat of the refrigerant
by water flowing over condensing surfaces
Condensing Pressure- The pressure of a gas at which it condenses
Condensing Unit-An assembly of refrigerating compressor, receiver, condenser and
necessary accessories attached to one base
Cooling tower- A structure over which water is circulated to cool the water evaporatively
by contact with air
Critical Point- The state of a substance at which the corresponding physical properties
of liquid and gas are identical
Critical Pressure- for a specific fluid. The vapor pressure at which the liquid and vapor
have identical properties
Critical Temperature- Saturation temperature corresponding to the critical state of the
substance at which the properties of the liquid and vapor are identical

D
Dew point (refrigerant) – The pressure at which the vapors of a refrigerant mixture start
to condense
Discharge Pressure- An operating pressure in a refrigerating system measured at the
outlet of the compressor
Distributor- A device for dividing flow of fluids between parallel paths. In a refrigeration
system this device is commonly found on the evaporator after the thermal expansion
device
Drier- A manufactured device containing a desiccant placed in a refrigeration system
Dry bulb temperature- The temperature of air indicated by an ordinary thermometer

E
EER- the ratio of net cooling capacity in BTU/h t the total rate of electric input in watts,
under designated operating conditions
Enthalpy- a thermodynamic property of a substance defined as the sum of its internal
energy plus the quantity Pv/J, formerly called total heat and heat content
Enthalpy Chart - a graphical representation of thermal properties of a substance with
enthalpy as one of the coordinates
Entropy - the rate of the heat absorbed by a substance to the absolute temperature at
which it was added
Entropy chart - a graphical representation of thermal properties of a substance with
entropy as one of the coordinates
Equation of state - a thermodynamic expression relating the volume, pressure and
temperature of a given substance
Evaporating Temperature- the temperature at which a refrigerant vaporizes within an
evaporator
Evaporator – that part of a refrigeration system in which the refrigerant absorbs heat
from the contacting fluid by evaporation
Expansion valve (thermostatic) - a controlling device for regulating flow of refrigerant
into a cooling unit: it is actuated by changes in evaporator pressure and superheat of the
refrigerant leaving the cooling unit. The basic response is to superheat

F
Flash gas - that portion of the liquid refrigerant that is vaporized by sudden reduction of
pressure
Flooded Evaporator - an evaporator in which the total volume of refrigerant does not
evaporate. Refrigerants with zero to negligible glide are required for this application

G
Gage Pressure - pressure measured from atmospheric pressure as opposed to
absolute pressure
Glide- the range in temperature measured at constant pressure during the evaporation
or condensation of zeotropic refrigerant blends

H
Head Pressure - operating pressure measured in the discharge line of a compressor,
fan or pump
Head Pressure Control - a technique which uses electrical or mechanical devices or
schemes to control the condensing pressure of a refrigeration system to compensate for
the variances in the condensing medium
Heat - a form of energy that is exchanged between a system and its environment or
between parts of the system induced by temperature difference existing between them
Heat Capacity - the amount of heat necessary to raise the temperature of a given mass
one degree; numerically, the mass multiplied by the specific heat
Heat exchanger- a device to transfer heat between two physically separated fluids
Horsepower- unit of power in the I-P system; work done at the rate of 550 ft lb per sec
(745.7 W)
Hot Gas bypass regulator- in a refrigeration system, an automatic valve that maintains
suction pressure above a given value by diverting a certain quantity of highside vapor to
the low side of the system
Hygroscopic- absorbs and retains moisture

I
Ice storage system (ice bank) – a thermal storage system, usually applied for comfort
cooling, that uses the phase change properties of water/ice. Ice is formed during periods
of low refrigeration demand for use during periods of high refrigeration demand. This
system is also applied to shift compressor run time to “off peak” electrical hours
Ideal gas (perfect gas) – a gas whose internal energy and enthalpy depend solely on
temperature and that is defined by the perfect gas equation, pv = RT
Isentropic Process - a thermodynamic change at constant entropy

L
Liquid recirculation refrigerating system – a refrigeration system that flushes a
refrigerant liquid to saturated suction pressure and temperature in an accumulator and
them feeds it by a mechanical pump or by refrigerant vapor pressure to the evaporators.
This liquid is normally fed at a rate greater that the evaporation rate for the refrigerant to
insure wetting or the entire evaporator surface for improved heat transfer
Liquid, subcooled- a liquid whose temperature is lower that the condensation
temperature at its given pressure
Load- the amount of heat per unit time imposed on a refrigeration system by the
required rate of heat removal
Load (latent heat load) – the cooling load to remove latent heat
Load (sensible heat load) – the cooling load to remove the sensible heat
Low-pressure-stage – in a refrigeration system, the compression stage from a low to
an intermediate pressure level

M
MBTUH – I-P unit of power. One thousand Btu per hour.
Molecular sieve- an absorbent composed of porous alumina-silicates with pores of
uniform molecular dimensions that will selectively absorb molecules of the substance to
be gathered
Mollier diagram – a graphical representation of the properties of a refrigerant, generally
in terms of enthalpy and entropy

N
Non condensable gas- gas in a refrigeration system that does not condense at the
temperature and partial pressure at which it exists in the condenser, therefore imposing
a higher head pressure on the system

O
Oil return - migration of oil from the evaporator to the crankcase of the compressor.
Oil separator - a device for separating oil and oi vapor form the refrigerant, usually
installed in the compressor discharge line.
Oil trap – a device for separating and collecting oil at a given point in a refrigerating
circuit.

P
Pressure – thermodynamically, the normal force exerted by a homogeneous liquid or
gas, per unit of area on the wall of a container.
Psychrometric chart- a graphical representation of the properties of moist air, usually
including wet and dry bulb temperatures, specific and relative humidities, enthalpy, and
density.
Pump down – of refrigerant, the withdrawal of all refrigerant from the low side of a
system by pumping it to either the condenser or the liquid receiver.

R
R-value – in thermal insulation, the thermal resistance of insulation materials or
constructions
Rankine cycle - a theoretical thermodynamic cycle used in steam engines comprising
four principal stages: vaporization of liquid under high pressure; expansion of steam;
condensation of steam; pumping of the liquid back to initial pressure
Refrigerant - the fluid used for heat transfer in a refrigerating system; the refrigerant
absorbs heat a low temperature and low pressure and transfers heat a higher
temperature and a higher pressure, usually with changes of state
Refrigerant charge - the actual amount of refrigerant in a closed system
Reverse cycle defrosting - A method of hot gas defrosting where discharge gas is
directed to the suction line of a refrigerant circuit, through the evaporator and into the
liquid line bypassing the expansion device
Rupture disk – a valve or rupture member designed to relieve excessive pressure by
mechanical failure of the disk

S
Service Valve - a valve used by service technicians to check pressures and charge
refrigerating units
Short cycling - the excessive frequency of starting and stopping in an operating system
Soft soldering – a hot joining operation in which the melting temperature of the filler
metal is lower than 850ºF (450ºC)
Solenoid Valve – a valve that is closed by gravity, pressure, or spring action and
opened by the magnetic action of an electrically energized coil, or vice versa
Steam – water in the vapor state
Subcooling – in refrigeration, the removal of heat from liquid to a point lower than the
saturation temperature corresponding to its pressure
Suction line - the tube or pipe that carries the refrigerant vapor from the evaporator to
the compressor inlet
Suction line - double riser- an arrangement of two vertical suction lines that assures oil
is entrained, even at minimum load
Superheat - extra heat in a vapor when at a temperature higher than the saturation
temperature corresponding to its pressure
Suction temperature - the temperature of the refrigerant vapor drawn into the
compressor inlet

T
Thermal conductance (C factor) – the time rate of heat flow through unit area of a
body induced by a unit temperature difference between the body surfaces. Units are
Btu/h·ft²·ºF (W/(m²·K)
Thermodynamics - the science of the relation of heat to other forms of energy
Thermodynamic properties - basic qualities used to define the condition of a
substance such as temperature, pressure, volume, enthalpy, entropy etc
Ton (of refrigeration) – a time rate of cooling equal to 12,000 Btu/h (approximately
3517 W)
Triple point – the physical state temperature at which solid, liquid and gaseous phases
exist in equilibrium
Two phase flow – simultaneous flow of two phases of a fluid, usually gas-liquid flows

U
Unitary refrigeration system – a complete factory assembled and tested refrigeration
system comprising one or more assemblies that may be shipped as one unit or
separately, but are designed to be used together

V
Vacuum – state in which the gas pressure is lower than atmospheric pressure
Vacuum test – a test to check the gas tightness of an uncharged refrigeration system by
drawing a vacuum upon it
Vapor- a gas, particularly one that is near equilibrium with its liquid phase and tht does
not follow the gas laws. The term is usually used instead of gas to refer to a refrigerant ,
or , in general, to any gas below the critical temperature
Velocity (mass velocity) – the ratio of the mass flow rate of a fluid in a pipe to the cross
sectional area of the pipe
Viscosity – the property of a fluid to resist flow pr change of shape
Viscosity index – an empirical number evaluated by comparing the viscosity of a
substance with that of a standard substance
Volumetric Efficiency – the ratio of volume induced, at suction conditions, by a
compressor in a given time to the swept volume as measured of the same time

W
Wax Content – the wax contained in lubricating oil at a specified temperature
Wet compression – a system of refrigeration in which some liquid refrigerant is mixed
with vapor entering the compressor to cause the discharge vapors from the compressor
to be saturated rather than superheated.