REF UNIT 25 Flashcards
(25.2) What does an EPR do?
keeps refrigerant pressure in evaporator from going below a predetermined point
(25.2) What is the opening pressure and what is the closing pressure in an EPR?
the evaporator pressure is the opening force, and the spring pressure is the closing force. the evaporator outlet pressure cancels itself out.
(25.2) what are EPR valves AKA?
ORI (OPEN on RISE of INLET) or holdback valve
(25.2) on a multiple evaporator system, which evaporator does not need an EPR
the one with the lowest SST
(25.2) how do EEPR valves work?
they use a stepper motor that responds to microprocessor that responds to box temp
(25.3) what do CPR (crankcase pressure regulating) valves do?
limits pressure to compressor in suction line. Keeps compressor from overloading.
(25.3) What type of valve is a CPR?
CRO (CLOSE on RISE of OUTLET)
(25.3) When should the CPR be adjusted?
during hot pulldown
(23.3) T or F: Oil separators have a float inside that let’s oil out when it rises.
true
(25.3) what is the purpose of the crankcase heater?
protects the compressor from refrigerant migration. it heats during off cycle.
(25.3) oil pumps are usually used in what capacity compressors?
over 3 HP
(25.3) what are the two types of oil pumps?
gear type or eccentric type
(25.3) How do eccentric oil pumps work?
it squeezes the oil though small holes
(25.4) what are the 4 most common methods of keeping head pressures up in low ambient conditions?
fan cycling, fan speed control, shutters, condenser flooding
(25.4) what does condenser splitting do?
both condensers used in summer, one used in winter
(25.4) why is a high-pressure control more necessary on water-cooled compressors than air-cooled condensers?
because an interruption of water is more likely than an interruption of air.
(25.4) what is low-ambient fan control?
stops condenser fan in response to low head pressure.
(25.4) how does an oil pressure control work?
it works on a pressure differential, ensuring there is positive oil pressure in the crankcase, usually about 20psi
(25.4() T or F: the oil pressure control uses a time delay so that the compressor has time to build oil pressure
true
(25.4) what is used so that the oil pressure control doesn’t open if an internal motor protection device opens?
a current-sensing relay
(25.4) what are pilot-operated solenoid valves?
solenoids that mostly use difference in pressure so that it isn’t a massive load to operate
(25.4) how does an automatic pump down system work?
closes a liquid line solenoid valve when de-energized
(25.7) what is the first on the evaporator called?
hoar frost
T or F: the colder the evaporator, the more fins it needs
false. it’s the opposite.
(25.7) what is off-cycle defrost?
using the air in the box to defrost the coil in off-cycle
(25.7) what is random defrost?
defrost that is done when the system has the capacity to perform a defrost cycle.
(25.7) T or F: defrost on low-temp evaporators are always planned.
true
(25.7) why is it important for the evaporator fan to be off during defrost?
so that the fan doesn’t blow the dripping moisture all up in the place.
(25.7) what is the fail-safe?
when the timer on the time lock runs a certain amount of time during defrost if the coil isn’t sensed to be above freezing so that electric heaters don’t cause damage.
(25.3) why should oil separators be kept warm?
so that liquid refrigerant doesn’t condense and then re-enter the crankcase on start up.
