refrigerants and refrigeration systems
list the different types of cooling mechanisms
1. Evaporative 2. thermoelectric 3. absorption 4. mechanical compression
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.
288,000 btu/24-hr
a ton of refrigerant is equal to
reciprocating compressors
have a piston and cylinder
r-502
refrigerant with the lowest boiling temperatures @ atmospheric pressure
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.
list the four major components of the compression refrigeration cycle in order
1. compressor 2. condenser 3.metering device 4. evaporator
name the three common refrigerant lines found on a compresion system
1. discharge line 2. suction line 3 liquid line
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"
evaporator
The component in a refrigeration system that absorbs heat into the system and evaporates the liquid refrigerant.
refrigerant
a chemical that evaporates at a low temperature and removes heat as it changes from a liquid to a gas; used in cooling systems
what is the difference between a liquid 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.
true
an expansion valve is the pressure reducing device that controls the flow of liquid refrigerant into the evaporator
evaporator and condenser
at what location or locations in a refrig. system does the saturation pressure correspond to the saturation temperature
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
Explain why boiling is considered a cooling process
boiling requires heat. when a liquid boils, it extracts heat from whatever its touching
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
discharge line muffler
"high side" dampens pulsations and noise created by the compressor
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
liquid line filter-drier
"high side" removes moisture and impurities from the liquid refrigerant. helps protect the system expecially the metering devise
oil separator
"high side" removes the oil entrained in the discharge gas and returns it to the compressor crankcase.
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.
evaporator pressure regulator
"low side" controls the pressure in the evaporator. it will not allow the evaporator pressure to drop below the EPR setpoint
oil level controls
"low side" used on systems with multiple compressors which are piped into common dishcare and suction headers
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.
when a refrigerant enters the compressor, it is a LOW PRESSURE LOW TEMPERATURE SUPER HEATED VAPOR and when it leaves the compressor it is a HIGH PRESSURE HIGH TEMPERATURE SUPER HEATED VAPOR
...
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.
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
list the three types 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 types 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.
substitute for R-12
R34A
temperature/ pressure relationship
This refers to the temperature/pressure relationship of a liquid and vapor in a closed container. If the temp increases, the pressure will increase. If the temp is lowered the pressure will decrease.
saturation temperature
depends on the pressure of the liquid/vapor mixture. the pressure is called the saturation pressure. the higher the pressure , the higher the saturation temperature of the liquid and the vapor mixture. the lower the pressure, the lower the saturation temperature.
where is evaporative cooling the most effective
dry climates, like the south western united states
are heavier than air and may displace the oxygen around you
good ventilation is important when working around refrigeration equipment because modern refrigerants
ice melts in iceboxes because
heat flows through the insulated walls heat enters when the door is opened heat enters when warm food is placed in the icebox answer is ALL OF THE ABOVE
an air-conditioning system
high temperature refrigeration is that produced by
what is the purpose of the metering device ?
it reduces the boiling point of the refrigerant far enough below the temperature of the load to get the required heat transfer rate .
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
true
only liquid refrigerant should enter the metering device
when the refrigerant leaves the metering device , it is not 100% liquid . why?
part of the liquid is vaporized to cool the refrigerant down to the boiling point of the refrigerant in the evaporator .
compressor
pumps the refrigerant throughout the system.. As a basic component of an air-conditioning system, a compressor draws vaporized refrigerant from the evaporator, compresses it, and discharges it to a condenser.
condenser
rejects the heat from the refrigerant
the ice melts b/c it is absorbing heat from the soda
room temperature soda is poured into a glass of ice
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.
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 liquid line
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
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 .
enthalpy
the amount of heat a refrigerant contains with a -40 degree F as the 0 heat content reference point is called _____
true
the boiling point of a refrigerant should be low enough @ atmospheric pressure to keep the system pressure above 0 psig when operating @ low temperature
what components are responsible for maintaining the high and low pressures in an absorption refrigeration system
the compressor and the metering device
true
the discharge gas from the compressor is a high pressure ,high temp., superheated vapor.
true
the evaporator can be thought of as a "heat sponge"
true
the evaporator in a refrigeration system must be colder than the refrigerated space in order to absorb heat
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 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
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
compressor and expansion device
the pressure in a refrigeration system is changed in the
refrigeration
the process of removing heat from a place where it is not wanted to a place where it makes little or no difference
what is the purpose for subcooling the liquid refrigerant before it leaves the condenser?
the purpose is to reduce the amount of flash gas and increase the capacity of the system . it helps the evaporator operate more efficiently .
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
condensor and evaporator
the refrigerant in a refrigeration system changes state in the
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
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
suction line
the refrigerant vapor picks up additional heat as it flows from the evaporator to the compressor . the heat is picked up from the _____
screw
the________ is another positive displacement compressor and is used for larger a/c and refrigeration applications.
centrifugal
the_________ is referred to as a kinetic displacement compressor.
which type of metering device provides the most efficient operation
thermostatic expansion valve
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
desuper heats , condenses, and subcools the refrigerant
what 3 effects does the condenser have on the refrigerant vapor entering the coil?
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
the law requires technicians to recover refrigerant from all refrigeration systems so that they
will not be released into the atmosphere may be used again may be reclaimed for future use ALL OF THE ABOVE
