Unit 21 Evaporators & Refrigeration system HVAC Juniors 2019
in most cases, the refrigerant leaving the evaporator must be _______% vapor
100%
As a rule of the temperature of the evaporator should be about ____ degrees fahrenheit colder than the box temperature
15
refrigerant entering the evaporator is approximately _______% liquid
75
A typical superheat for an evaporator operating under normal conditions would be...
8 to 12 degrees Fahrenheit.
A typical evaporator operating temperature in an Air conditioning system is ____
A: 40 degrees F
If an R-12 refrigeration system is operating with a 25.4 psig suction pressure what is the evaporator superheat if the evaporator outlet temperature is 34 F?
A: 9 degrees Fahrenheit.
If a high temeperature evaporator is forming ice, which of the following could be a possible problem?
A: Evaporator fan not running.
A forced draft medium temperature evaporator would ________
A: be defrosted by off-cycle defrost
If an evaporator has a pressure drop of 10 psig and the low side pressure of is 15 psig, What is the pressure of the inlet of the evaporator?
B: 25 psig.
A direct expansion-type evaporator is also called a
B: Dry-type evaporator
Excess evaporator super heat ______
B: Will reduce the cooling capacity of the system.
What is the purpose of a multi-circuit evaporator?
By giving a refrigerant more paths to take in the evaporator.
If the application recquires a coild design to operate below 32 degrees there must be provision for
C: Defrost
The heat required for off-cycle defrost obtained from
C: The space temperature when the compressor is off.
The capacity of an evaporator is affected by __________
D all the above
If a low temperature evaporator is forming excess ice, which of the following could possibly be a problem?
D: All the above
A forced draft high temperature evaporator would ______
D: Both a&b Have its fins very close together & not need to be defrosted
A large capacity evaporator would probably have _____
D: Multiple refrigerant circuits.
If a low temperature evaporator is forming ice. which of the following could be a possible problem?
D: No problem, Ice formation normal
T or F: The device used to feed refrigerant to multiple circuits in an evaporator is called a flow control.
False
Define superheat
Heat added to vapor above boiling point.
the two primary functions of an evaporator are to remove______ and _______ from the refrigerated space
Heat and Moisture
A refrigeration system designed for the storage of flowers and candy is considered a ______ type.
High Temperature
A defrost system that pipes the compressor discharge vapor directly to the evaporator is called a _______ type
Hot gas defrost
An evaporator made up of two or more pieces of sheet metal dimpled to create tubes is called a _____ type
Plate
A low temperature evaporator always operates below freezing and the box is always below freezing as well. How is a defrost accomplished.
Source of heat is needed.
Why are fins of a low temp evaporator spaced wide apart than those in a high temperature evaporator?
To allow formation of frost without blocking the air flow.
Why is it neccersary to have superheat in the refrigerant vapor leaving the evaporater
To reduce the possibility of liquid refrigerant entering the compressor.
T or F: A stamped plate evaporator does not use a fan to move air across it.
True
T or F: Fewer fins per inch on an evaporator allow more ice to build between defrost cycles
True
T or F: The refrigerant in the evaporator must be colder than the air passing over it for heat to transfer to the evaporator.
True
T or F: When the velocity of the air moving over an evaporator is slow, the "Air Film" acts as an insulator and slows the rate of heat exchange.
True
During the defrost cycle of a forced draft low temperature evaporator, evaporator fan would?
Turn off
Define Saturated Vapor
Vapor that has been cooled to condensing temperature but has not given up any latent heat
Define Saturated Liquid
liquid that can contain all the heat it can take without boiling.
Define Sub-cooling
liquid whose temp below the normal boiling point
