1400
Modern Refrigeration and Air Conditioning
automatic defrost system. Defrost system in which a timer
or control mechanism operates heater elements during a
refrigeration system’s Off cycle. (18)
automatic expansion valve (AXV). A metering device oper-
ated by low-side pressure that throttles the liquid refrigerant
in the liquid line down to a constant pressure. Also called a
pressure-controlled expansion valve. (16)
auxiliary heat. A supplementary heat source in a heat pump
system that is turned on when the heat pump cannot extract
enough heat from the outside air or when the heat pump is
in defrost mode. Auxiliary heat is most often supplied by
electric heating elements. (34)
available static pressure (ASP). The amount of friction or
resistance used to design the longest duct run. (23)
axial flow fan. A propeller fan in which air leaves the fan
along the direction the axle is pointing. (23)
azeotropes. Refrigerant blends with fixed boiling and con-
densing points. An azeotropic blend responds to pressure
and temperature changes like a single refrigerant. Also
called azeotropic blends and azeotropic refrigerants. (6)
back seated. A service valve position used for normal system
operation. A back-seated service valve closes off the service port
so no procedures or pressure readings can be performed. (7)
backflow preventer. A hydronic system check valve installed
in the makeup water line that prevents water from flowing
back into the water main. (33)
BACnet. A common protocol or set of rules for software
and hardware communication that allows controllers from
one company to work with those from another company.
BACnet is short for building automation control network. (40)
bacteria. Simple, single cell microorganisms responsible for
the transfer of many diseases. (22)
baffles. Surfaces in and along air ducts that direct airflow
through an evaporator and throughout a conditioned space
cabinet. (46)
balance point. The point at which a heat pump’s total heat
output equals the heat loss of the conditioned space. If the
ratio of heat pump output to heat loss is less than the balance
point, auxiliary heat is required. (34)
balanced pressure steam trap. A steam trap filled with a
mixture of water and mineral spirits. The mixture vaporizes
and condenses just below the temperature of steam to trig-
ger the steam trap’s valve. (33)
balancing. Sizing the ducts and adjusting the dampers to be
sure that each room of a forced-air HVAC system receives
the correct amount of air. (24)
balancing valve. A hydronic system valve that adjusts the
water flow to each terminal unit or zone in a system so
that heat is evenly distributed throughout the system. Also
called globe valve. (33)
bar. A unit of pressure equal to 14.5 psia, which is approxi-
mately equal to one atmosphere. (1)
below-atmospheric-pressure element. A type of control device
that opens an electrical circuit on temperature rise. (13)
belt-driven compressor. An open-drive compressor that is
connected to its drive motor by a belt and pulley system. (15)
bending spring. A tool used to make bending tubing easier and
to reduce the danger of flattening the tubing while bending it. (4)
biflow thermostatic expansion valve. A type of thermo-
static expansion valve (TXV) that allows refrigerant to flow
in both directions in a heat pump system. With a built-in
passage regulated by a check valve, a biflow TXV meters
refrigerant flowing in one direction and allows refrigerant
to flow unrestricted in the opposite direction. (34)
bimetal coil. A thermostatic control made of a coil of two
different metals that expands and contracts with changes in
temperature, causing a mercury switch mounted on one end
to tilt and open or close a circuit. (13)
bimetal device. A device made of two different met-
als bonded together and formed into a particular shape.
Temperature changes cause the metals to contract or expand,
which makes the device bend in the desired direction. (13)
bimetal disc. A thermostatic control that consists of a con-
cave disc composed of two different metals. The disc is
dished in one direction when it is cooled and snaps into a
dished position in the opposite direction as it warms. (13)
bimetal steam trap. A steam trap that contains two dis-
similar metal strips bonded together. Temperature changes
cause the bimetal strip to bend in one direction or the other
to trigger the steam trap’s valve. (33)
bimetal strip. A thermostatic control with electrical contacts
at either end. As the strip reacts to different heat conditions, it
opens and closes the contact points of an electrical circuit. (13)
bioaerosols. Airborne microorganisms such as viruses, bac-
teria, fungi, protozoa, mold, rust, dust mites, mildew, and
yeasts. (22)
blank-off plates. Sheet metal plates that support the evapo-
rator coil and block off areas of the plenum so that all the air
passing through the plenum is directed through the evapo-
rator coil. (31)
blast chiller. A commercial refrigeration system that cools
a large amount of cooked hot food rapidly and uniformly
without freezing it. (42)
blast freezer. A commercial refrigeration system that cools
and freezes a large amount of cooked hot food rapidly and
uniformly. Also called shock freezer. (42)
bleed resistor. A resistor that slowly discharges a capacitor
when power to the capacitor’s circuit is turned off. (14)
blowback. The ignition of a large amount of vaporized fuel
oil that blows soot backward out of the combustion cham-
ber. (38)
blowdown. Cooling tower water that is intentionally
drained for the purpose of reducing certain mineral concen-
trations in the water. (45)
blower. A motor-driven fan used to circulate air. (32)
blown fuse. An electrical fuse in which the conductive ele-
ment inside the fuse has melted to open the circuit and pre-
vent excess current flow. (13)