1406
Modern Refrigeration and Air Conditioning
diac. A solid-state device that allows current to flow in both
directions. For current to flow through a diac, the applied volt-
age must reach or exceed the diac’s breakover voltage. (11)
diagonal pliers. A type of pliers used in electrical work with
angled jaws designed to make nearly flush cuts. (3)
dielectric. An insulating material, such as paper, mica,
or ceramic, used in a capacitor to separate conducting
surfaces. (9)
dielectric strength. A measure of the amount of electrical
current that a refrigeration lubricant can withstand without
breaking down. Lubricants should have a minimum value
of 25,000 volts. (6)
differential. The number of units (the difference) between a
control system’s cut-out value and cut-in value. (13)
differential adjustment. An adjustment built into a tem-
perature or pressure control that increases or decreases the
difference between the cut-in and cut-out values. (13)
diffuser. A mechanical device designed to evenly distribute
the flow of air. (23)
digital charging scale. An electronic scale on which a
refrigerant cylinder is placed to measure the amount of
refrigerant entering or leaving the cylinder by tracking the
cylinder’s change in weight. (7)
diode. A solid-state device composed of a P-type material
and an N-type material. Electrons will flow through a diode
in only one direction—from the cathode to the anode. A
check valve for electrons. (11)
direct current (dc). Electric current in which electrons flow
in only one direction. Direct current is the type of current
produced by batteries. (9)
direct digital control (DDC). The use of a computer or
microprocessor that accepts both analog and digital input
signals from all parts of an HVAC or automated building
system. This information is processed per the user’s preset
instructions and sends out control signals to condition cer-
tain areas of the building. (40)
direct expansion evaporator. An evaporator into which
refrigerant is fed at the same rate that it evaporates. (26)
direct radiant heat. An electric heating system in which
the heating elements are mounted in a fixture that focuses
the heat energy emitted by the elements on the objects to be
warmed. (37)
direct return hydronic system. A type of two-pipe hydronic
system with branches of varying lengths. The terminal unit
that is closest to the boiler has the shortest length of sup-
ply and return piping, and the terminal unit that is farthest
away from the boiler has the longest length of supply and
return piping. (33)
direct-acting reversing valve. A type of reversing valve
that uses the direct action of the solenoid plunger to
move the piston and sliding section of tubing inside the
valve. (34)
direct-drive compressor. An open-drive compressor that is
driven by a motor connected directly to the compressor by a
coupling between their shafts. (15)
direct-expansion (DX) heat pump. A type of ground-source
heat pump that circulates refrigerant through tubing buried
underground. Also called direct-exchange heat pump. (34)
direct-fired. A term describing commercial absorption sys-
tems that are directly heated by a burner or other heating
element. (27)
direct-spark ignition (DSI) system. A type of gas furnace
ignition system that uses an electric spark to ignite the gas
burners. (35)
direct-venting system. A two-pipe venting system for a fur-
nace. One pipe is dedicated to venting flue gases, and the
other pipe is dedicated to bringing in fresh air for combus-
tion. (35)
dirty sock syndrome. A foul odor emanating from room
registers that is caused by the growth of mold and bacteria
on the indoor coil of a heat pump system. (38)
discharge line pressure switch. A pressure switch installed
in the compressor discharge line between the compressor
and condenser that turns off the compressor when the dis-
charge pressure gets too high. (44)
discharge service valve (DSV). A high-side service valve
connected directly to a compressor at its outlet, considered a
compressor service valve. (7)
dispensing freezer. A commercial refrigeration system that
cools or fast-freezes and dispenses liquid mixes into soft
serve or batch ice cream, shakes, or frozen beverages. (42)
dissolved air. Air trapped between molecules of circulat-
ing water in a hydronic system. Air scoops or air separators
can be installed to reduce the amount of dissolved air in a
system. (33)
distillation quality. The ability of a fuel to vaporize. Used
to describe fuel oil in oil-fired heating systems. (36)
distributed system. In commercial refrigeration, a unit
containing control devices and multiple compressors that
circulate refrigerant through remote condensers and the
evaporators in nearby conditioned spaces. (42)
distributor. A device that splits the flow of refrigerant into
several paths. (16)
DOP HEPAP Method. An efficiency test used mainly with
high-efficiency air filters. (22)
doping. The addition of impurities to a pure semiconductor
to change its conduction traits. (11)
double flare. A flare made with a double thickness of tub-
ing metal, recommended only for larger size tubing with a
diameter greater than 5/16″. (4)
double-cut file. A type of file that has teeth cut in two direc-
tions, used to remove metal quickly. (3)
double-effect absorption system. An absorption system
that uses a secondary, smaller, low-temperature genera-
tor to recover some of the waste heat from the primary
generator. (27)
downflow furnace. A furnace design in which return air
is taken in from the top and forced downward around the
heat exchanger. It is commonly used in houses where the