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43 terms

refrigerants and refrigeration systems

STUDY
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list the different types of cooling mechanisms
1. Evaporative
2. thermoelectric
3. absorption
4. mechanical compression
Briefly explain the fundamental principles which make the compression refrigeration cycle work
1. "Cold" inst a substance or an energy form, its the lack of heat.
2. heat travels from hot to cold.
3. liquids absorb large amounts of heat when boiled off to a gas.
3. gases give off large amounts of heat when condensed into a liquid.
4. when somthing is boiling or evaporating the temperature stays the same.
5. the temperature at which a liquid boils is controlled by its pressure.
Explain why boiling is considered a cooling process
boiling requires heat. when a liquid boils, it extracts heat from whatever its touching
list the four major components of the compression refrigeration cycle in order
1. compressor
2. condenser
3.metering device
4. evaporator
why do both the compression cycle and absorption cycle have a high pressure side and a low pressure side
the low-pressure side allows refrigerant to boil at a temperature below the temperature of the space being cooled, absorbing heat. the pressure present in the high side allows the refrigerant to condense at a temperature higher than the outdoor temperature, releasing heat
what components are responsible for maintaining the high and low pressures in an absorption refrigeration system
the compressor and the metering device
what components are responsible for maintaining the the high and low pressures in an absorption refrigeration system
the generator, the absorber, and the solution pump
explain how an evaporative cooling system works
water flows over media and air is pulled across the wetted media. some of the water evaporates, cooling off the air
where is evaporative cooling the most effective
dry climates, like the south western united states
explain how a thermoelectric refrigeration system works
when DC current is passed through a thermocouple masde of two dissimilar metals, one junction absorbs heat and the other releases heat. this is called the pelletier effect
where in the compression refrigeration cycle is the rrefrigerant superheated
the refrigerant is superheated from the end of the evaporator, through the compressor, up to the beginning of the condenser
why is superheating desirable
superheatingthe refrigerant leaving the evaporator insures that no liquid enters the compressor, if the liquid does enter the compressor, it can be damaged because liquid is noncompressible .
where in the compression refrigeration cycle is the refrigerant subcooled
the refrigerant is subcooled from the outlet of the condensor up to the metering device
why is subcooling desirable
subcooling ensures that no flash gas occurs in the luquid line on the way to the metering device because flash gas in the liquid line reduces system capacity.
where in the compression refrigeration cycle is the refrigerant saturated
the refrigeant is saturated in all the evaporator except for the very end. its also saturated in most of the condenser except for the very beginnging and very end
explain the heat balance in a compression cycle refrigeration system
the heat picked up in the evaporator plus the heat compression equals the heat rejected in the condenser
what is the difference between a liquied receiver and suction accumulator
a liquid receiver is used after the condenser on the high side. its designed to allow only liquid to leave. an accumulator is located on the low side just before the compressor. its designed to allow only vapor out.
list some of the optional components that can be found on the high side of the refrigeration system
discharge line muffler, oil separator, liquid receiver, liquid line filter-drier, sight glass, luquid line solenoid, and suction to liquid heat exchanger
list some of the optional components that can be found on the low side of a refrigeration system
suction to luquid heat exchanger, evaporator pressure regulator, suction line filter, suction accumulator, crankcase pressure regulator, and oil level controle
list the trypes of compressors and briefly explain how they operate
1. Reciprocating- uses pistons in a cylinder. gas is compressed by squeezeing it between the piston and a valve plate at the top of the cylinder
2. Rotary- uses a roller that rotates in an orbital motion within a cylinder. gas is compressed by squeezing it between the roller and a spring-loaded valve.
3. Screw- uses two rotating screws shaped like augers. gas is compressed by squeezing it between the two screws.
4. Centrifugal- has a very high-speed centrifugal impeller that spins within a housing. the gas enters the impeller in the center and is thrown outwards by centrigugal force, compressing the gas.
list the three trypes of air cooled condensers and briefly explain the difference between them
1. Natural draft- doesnt use a fan, works by natural convection only
2. Induced draft- uses a fan to suck air across the coil
3. Forced draft- uses a fan to blow air across the coil
list the different types of water cooled condensers and give a description of each type
1. double pipe
2. open vertical shell and tube
3. horizontal shell and tube
4. shell and coil
what are the two large classes of metering devises
1. fixed restriction
2. thermostatic expansion valves "txv"
which type of metering device provides the most efficient operation
thermostatic expansion valve
compare coil design between older, less efficient air dcondetioning systems and newer high SEER air conditioning systems
new high-efficiency coils are physically larger than older lower- efficiency coils. the newer coils use less temperature and pressure difference than the older systems
name the three common refrigerant lines found on a compresion system
1. discharge line
2. suction line
3 liquid line
which line is the larger of the two lines going to the compressor
suction line
which two lines connect the inside and outside units of the split system
suction line and luquid line
why is the refrigeration cycle called a cycle
because its a circle. the refrigerant circulates from one side of the system to the other and back again
describe the refrigeration cycle
1. Compressor suction 118 psig 50 degrees low temperature, low pressure, super heated gas.
2. Compressor discharge 365 psig 160 degrees high temperature, high heat, super heated gas.
3. beginning of the condenser 365 psig 160 degrees high pressure, high heat, super heated gas
4. center of the condensor 365 psig 110 degrees saturated 50/50 saturated mix
5. end of condensor 365 psig 100 degrees subcooled liquid
6. metering device inlet 365 psig 100 high pressure warm subcooled liquid
7. metering device out 118 psig 40 degrees low temperature, low pressure, 75/25 satruated mix
8. evaporator in 118 psig 40 degrees 75/25 saturated mix
9. evaporator center 118 psig 40 degrees 50/50 saturated mix
10. nearing the end of the evaporator 118 psig 40 degrees 25/75 saturated mix
11. leaving the evaporator 118 psig 50 degrees 100% low pressure, low temperature, super heated gas
discharge line muffler
"high side" dampens pulsations and noise created by the compressor
oil separator
"high side" removes the oil entrained in the discharge gas and returns it to the compressor crankcase.
liquid line filter-drier
"high side" removes moisture and impurities from the liquid refrigerant. helps protect the system expecially the metering devise
sight glass
"high side" allows the technician to visually see the dcondition of the refrigerant in the line. many sight glasses have a moisture indicator which changes color when exposed to moisture.
liquid line solenoid
"high side" an electrically operated valve used to control the flow of liquid refrigerant. with a normally closed solenoid valve, the refrigerant will only flow through when the solenoid is energized. no flow when not energized.
suction to liquid heat exchanger
"high side" exchanges heat between the warm liquid line and the cool suction line. this increases both the suction gas superheat and the luquid line subcooling
evaporator pressure regulator
"low side" controls the pressure in the evaporator. it will not allow the evaporator pressure to drop below the EPR setpoint
EPR valves
"low side" used on systems with multiple evaporators operating at a different pressures and temperatures. the evaporators share a common suction line but are able to maintain separate evapoator pressure because of the evaporator pressure regulators
suction line filters
"low side" used to protect the compressor from contaminants. typicaslly suction line filters are used when a replacement compressors are installed. the filter is removed after the system has operated long enough for the filter to do its job.
suction line accumulator
"low side" are used to ensure that no liquid enter the compressor. commonly used on heat pumps
Crankcase pressure regulators
"low side" are used to keeep the suction pressure to the compressor from rising above the setpoint of the CPR valve
CPR valve
"low side" comonly used on commercial freezers. they keep the compressor from bing overloaded when the evaporator tremperatur is above its normal operating temperature.
oil level controles
"low side" used on systems with multiple compressors which are piped into common dishcare and suction headers