Glossary
1405
counterflow. A design in which two fluids flow in opposite
direction of each other. This design is often used in cooling
towers, evaporative condensers, and water-cooled condens-
ers. (45)
counterflow 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
ductwork runs below the level of the furnace. Also called
downflow furnace. (32)
CPVC (chlorinated polyvinyl chloride). A type of light
beige plastic pipe that may be used for hot and cold water
lines and drains. (4)
cracked open. A service valve position where the valve stem
has been turned just enough to lift the valve off the back seat.
Cracked-open valves prevent sudden pressure increases from
damaging gauges, and they allow refrigerant to flow through
the system while still providing service port access. (7)
cracking. The slight opening of a valve, which causes the
valve needle or plunger to leave its seat, allowing only a
slow flow of refrigerant. (3)
crankcase heater. An electric device on or in a compressor that
supplies heat to the crankcase in order to evaporate any refrig-
erant that may be trapped in the oil or prevent refrigerant
inside the compressor from condensing into liquid. (15, 44)
crankcase pressure regulator (CPR). A valve with an adjust-
able pressure setting that prevents crankcase pressure from
exceeding a preset safe value. A CPR protects the compres-
sor from higher than normal pressure vapor from the suction
line. Also called crankcase pressure-regulating valve, suction
regulator valve, compressor low-side pressure control valve, and
reverse metering, evaporator control valve. (44, 45)
crawl space. A type of foundation that leaves just enough
space between the floor and the ground to allow access. (30)
critical pressure. Minimum pressure required for a vapor
to liquefy when it is at its critical temperature. (1)
critical temperature. The highest temperature at which a sub-
stance may be liquefied. A vapor above its critical temperature
cannot be liquefied regardless of the pressure applied to it. (1)
crossflow. A cooling tower design in which air flows across
the wet deck from side to side, while cooling water drops
downward from above. (45)
cryogenic food freezing. A process of fast-freezing using liq-
uid nitrogen or carbon dioxide to turn perishable fresh food
into long-lasting frozen food. (42)
crystalline solar cells. A solar cell made from a thin slice of
silicon known as a wafer. (39)
cumulative run-time defrost system. An efficient type of
timed defrost system that determines the defrost intervals
based on the amount of time the compressor runs. (18)
current. The flow of electrons through a conductor. (9)
current electricity. Electricity flowing through a conductor. (9)
current relay. A relay that is activated by current running
directly through it. Current relays are used to close and
open a motor’s start winding circuit to provide the motor
with more starting torque. (13)
current-limiting fuse. A fuse that opens a circuit when the
current exceeds its limit and the fuse heats up. Current-
limiting fuses use temperature-sensitive resistors that close
when they cool back down. (13)
customer relations. The interaction of a technician with a
customer. Customer relations determine a customer’s evalu-
ation of a technician by the technician’s job performance,
appearance, and attitude. (5)
cut-in. The temperature or pressure at which a device
begins operation. (13)
cut-out. The temperature or pressure at which a device
ceases operation. (13)
cylinder head. A metal plate positioned on top of the
valve plate and bolted to the cylinder block. The cylinder
head holds the valve plate in position and seals the cylin-
der. (15)
cylinder unloading. The disabling of a cylinder’s pumping
ability. Cylinder unloading can be used to reduce the load dur-
ing startup or to decrease system capacity in order to reduce
refrigeration effect and lower power consumption. (26)
Dalton’s law. A law stating that the pressure of a mixture
of gases is equal to the sum of the pressures of each gas. (1)
deaeration. The release of gases dissolved in a liquid. In a
hydronic system, deaeration can reduce corrosion caused by
gases dissolved in the water. (33)
deep vacuum. An evacuation method that uses a vacuum
pump to pull a vacuum of 250 microns or less on a refrigera-
tion system in order to dehydrate the system. (8)
defrost timer. An electric control device that starts and
stops an HVACR system’s defrost cycle. (18, 46)
degree days. A measure used to indicate the heating or cool-
ing needed for a given region. (21)
dehumidifier. A device that decreases relative humidity by
condensing moisture out of the air. It uses a cooling coil over
which air is blown to condense moisture out of the air as it
comes in contact with the cold surface. (28)
dehydration. The drying of food. (17)
demand defrost. A defrost control system that uses sensors to
measure heat pump system variables—such as the tempera-
tures of the discharge line, liquid line, indoor coil, and outdoor
coil—in order to keep defrost cycle times to a minimum. (34)
demineralized water. Water that has been treated to remove
minerals. (28)
density. A substance’s mass per unit of volume, usually
measured in lb/ft3 or kg/m3. (1)
desiccant. In an HVACR system’s refrigerant drier, a sub-
stance that collects and holds moisture. In a dehumidifica-
tion system, a hygroscopic substance that is used to adsorb
moisture from the air. (28, 46)
device pressure losses (DPL). Pressure losses in the duct
system caused by devices other than ducts. (23)
dew point. The temperature below which water vapor in the
air will start to condense, or at which the air has 100% rela-
tive humidity. (21)