Unit 21 Quiz Flashcards
Relates to the velocity of the medium passing over the exchange surface. Film of air or liquid next to the tube in the heat exchanger.
Film factor
Reduces pressure drop inside the evaporator.
Multiple circuit coil
Medium temperature refrigeration where evaporator coil operates below freezing but the air in the cooler is above freezing. Coil is defrosted by air inside the cooler while the compressor is off.
Off cycle defrost
To remove moisture.
Dehumidification
Dry type evaporator where all the refrigerant is boiled off before leaving the evaporator.
Direct expansion evaporator.
Determines the coil operating temperature.
Boiling temperature
A refrigeration system operated with the liquid refrigerant level very close to the outlet of the evaporator coil for improved heat exchange.
Flooded evaporator
Vapor at the end of the evaporator.
Saturated vapor
When a metering device is not feeding enough refrigerant to the coil resulting in higher superheat and rise in temperature.
Starved coil
Two pieces of metal stamped with the impression of a pipe passage through it.
Stamped evaporator
Can be accomplished with heat from the system or using electric heater.
Defrost
Absorbs heat into the system from whatever medium is to be cooled.
Evaporator
The difference in temperature between the boiling refrigerant temperature and the evaporator outlet temperature.
Superheat
28 F to 40 F household refrigerator.
Medium temperature refrigeration
What are three functions of the evaporator?
Absorbs heat, converts liquid refrigerant into vapor, superheats
What are three methods used to defrost an evaporator?
Hot gas defrost, cool gas defrost, electrical defrost.
The best heat exchange takes place between _____.
Liquid and liquid
Calculate the superheat for a system using R-22 when the temperature at the exit of the evaporator is 2 F and the suction pressure is 15.1 psig.
14
If the evaporator temperature is below 32 F, there must be some provision for _______.
Defrost
During pump down, the temperature in the evaporator will _____.
Decrease as pressure decreases
If hot gas is used to defrost the evaporator, what stops the condensed refrigerant in the evaporator from going back to the compressor?
Suction line accumulator
What improved the flow of air passing over the coil to increase efficiency of the heat exchange?
Electric fan/blower
Refrigerant entering the evaporator is approximately _____ vapor.
25%
A saturated vapor condition exists when ________.
All of the liquid has boiled to vapor.
An evaporator made up of two pieces of sheet metal that have an impression of refrigerant piping is called a _____ evaporator.
Stamped
The _____ of the liquid refrigerant determines the coil operating temperature.
Boiling point
What prevents liquid refrigerant from going back to the compressor in a system designed to use a flooded evaporator?
Suction line accumulator
Name three materials that evaporators can be made of.
Copper, steel, aluminum
True or false: Superheat can be measured when the evaporator is completely blocked with ice.
False
True or false: Before checking superheat, sub cooling of the condenser must be known.
False
True or false: Optimum range for superheat is 8 to 14 degrees of superheat.
False
True or false: a dry-type evaporator is also known as a DX (direct expansion) coil.
True
True or false: During a hot gas defrost, liquid refrigerant may leave the evaporator.
True
True or false: Cool gas defrost uses liquid from the top of the receiver.
False
True or false: A receiver properly sized for a system will be able to hold the entire refrigerant charge.
True
True or false: A refrigeration system designed for the storage of flowers or candy is considered a medium temperature type.
False
True or false: The finned-tube evaporator is the most used design of evaporator than any other type of heat exchanger between air and refrigerant.
True
True or false: A shell and tube type evaporator reduces the incoming water temperature by 20 degrees F.
False
The greatest amount of heat taken into the system occurs when the liquid refrigerant is changing to vapor in the evaporator.
True
A water fountain uses what type of evaporator to chill the water.
Drum type evaporator
How does a multi-circuit evaporator maintain the same superheat in each circuit?
By having the same amount of refrigerant & pressure circulating throughout all circuits.
When liquid refrigerant in the evaporator is boiled away and exits the evaporator, the refrigerant should be 100% _____.
Saturated vapor
What two things will happen if the evaporator fan is not turned off during defrost?
- Warm air from defrost will raise produce temperatures.
- Defrost time will take longer.
Calculate superheat for a system that uses R-22 when the suction pressure is 64.2 psig. and the temperature at the exit of the evaporator is 52 degrees F.
52-37= 15F
A system that uses R-407C requires a superheat of 14F. What should the suction pressure be if the temperature at the exit of the evaporator is -2F?
-2-14= -16
Extra Credit:
1. How many microns is equal to 1 inch?
25,400 microns
Extra Credit:
2. When performing a vacuum decay test and the micron gauge begins to rise by 10 microns per second and stops at 800 microns after 10 minutes, can the system be charged with refrigerant?
NO
Extra Credit:
3. In an alternating current circuit that has a capacitor, the current ____ the voltage.
Leads
(I C E)
Extra Credit:
4. A contactor in the outdoor condensing unit has a resistance reading of 10ohms. The low voltage transformer supplies 25V to operate the contactor. How many amps will the contactor draw when the contactor coil is energized?
Extra Credit:
5. Starting weight of the recovery cylinder is 25 lbs 7 oz. What should the total weight of the cylinder be if 8 lb 15 oz of refrigerant is added? Give your answer in pounds and ounces.
