The evaporator in a refrigeration system must be colder Than the refrigerated space in order to absorb heat.
True
The rule of thumb for low temperature refrigeration is 5 degrees superheat.
True
Capillary tube systems are normally used for storage of already refrigerated products.
True
Fewer fins per inch on an evaporator allows more ice to build between defrost cycles.
True
Multiple circuit evaporator uses an externally equalized expansion valve.
True
Refrigerants that boil off at different rates are called zeotropic blends.
True
R-22 is considered a low pressure refrigerant.
False
High superheat means your TXV is flooding the coil.
False
The fastest defrost method is hot gas defrost.
True
Hot gas and electric defrost use the same time clock.
True
If you are a refrigeration technician, a warm coat, hat and gloves should be in your truck even in the middle of the summer.
True
The voltage that opens the point of a potential real is called the drop-out voltage.
True
The start capacitor is wired in series with the start relay.
True
Walk-in box temperature box is not needed when calculating the TD of an evaporator coil.
False
The contacts in a current type starting relay are normally closed.
False
A motor can burn-out if the supply voltage is either too high or too low.
True
A run capacitor helps a compressor run efficiently by slightly energizing the start winding.
True
The suction and discharge valves in a reciprocating compressor open and close depending on the difference in pressure across them.
True
When setting a fan cycle switch, you should set the cut-out to a pressure equivalent to 90 degrees condensing temperature.
True
Water pressure does not need to be taken into consideration when installing an ice machine.
False
What happens when a room temperature soda is poured into a glass of ice?
The ice melts because it is absorbing heat from the soda.
What position are the points in on a potential relay with no voltage applied?
Normally Closed
High temperature refrigeration is that produced by _________ .
An air-conditioning system
Good ventilation is important when working around refrigeration equipment because modern refrigerants ______________ .
Are heavier than air and may displace the oxygen around you
A thermostatic expansion valve controls refrigerant flow to the evaporator based on ____________ .
Rate of liquid vaporization, evaporator suction line superheat, and external equalizer pressure.
If the power element of a TXV loses its charge, the _______________ .
refrigerant flow to the coil will stop
Why is the temperature of walk-ins usually lower than the temperature of reach-ins?
Walk-ins are designed for long term storage and low temperatures.
Which refrigerant is used most in recently installed reach-in refrigerators?
R134a
Which refrigerant is used most in recently installed walk-in freezers and reach-in freezers?
R404a
What is considered the evaporator temperature?
The temperature of the refrigerant inside the evaporator tubing.
How do you calculate evaporator temperature from the suction pressure?
Use a P/T chart
How is evaporator TD calculated for a walk-in cooler?
Subtract Saturated Suction Temperature (SST) from the box temperature
During hot pull-down what is the evaporator experiencing?
Starving because the refrigerant is boiling off quickly
If airflow across an evaporator is decreased, what effect does it have on evaporator temperatures and suction pressure?
Evaporator temperature and suction pressure decrease.
What is the basic sequence of operation at the beginning of a freezer defrost?
Evaporator fans shut off, compressor shuts off, and heaters come on.
If a walk-in is operating at a box temperature of 34 degrees F, what type of defrost should it have?
Plan to defrost: a defrost clock only.
What is an effective way to check a Defrost Termination Fan Delay control?
Freeze the control in another freezer then measure the continuity of the contacts with an ohmmeter.
What is the primary function of the condenser?
Reject heat from the refrigerated space.
What are the 3 phases of the condenser?
De-superheat condense, and sub-cool.
After the vapor has been condensed into a liquid, what are two causes for the liquid to flash back into a vapor before it reaches the metering device?
A raise in temperature or a fall in pressure.
If the condenser split of a unit is 30 degrees F, what should the condensing temperature be with the ambient air temperature of 70 degrees entering the condenser?
100 degrees
The term "low ambient" usually describes ambient temperatures below how many degrees?
60 degrees F
What would be the number one cause of a coil iced up in a walk-in freezer?
Time clock
How does condenser flooding maintain head pressure in low ambient conditions?
The refrigerant takes up space in the condenser tubing, forcing head pressure up.
What repairs can be made on the job for a semi-hermetic compressor?
Crankcase oil and valve plate replacement.
What causes flooding and slugging?
Too much refrigerant returning to the compressor.
If the suction valves are broken, what is the cause?
Flooding and slugging
If a walk-in is operating at a box temperature of 36 degrees and above, what type of defrost should it have?
Random defrost: no clock or heaters, just off cycle defrost with thermostat.
What is the primary cause of a flooded start?
Refrigerant migration
How can flooded starts be prevented?
Install a crankcase heater or a pump-down solenoid.
Why are scrolls considered more efficient than reciprocating compressors?
They have better volumetric efficiency.
Why is superheat more important in a TEV system?
It allows the evaporator to operate at its maximum efficiency & it provides a margin of safety, preventing compressor damage due to flooding or lack of cooling.
Why are externally equalized expansion valves needed on some evaporators?
Because of high pressure drop in the distributor and / or evaporator.
How do you know a TEV is hunting?
Superheat fluctuates
What 3 things would you check if a TEV is hunting?
Valve sizing, liquid to the valve, and if opening the valve stops the hunting.
If a replacement tube has a larger ID than the original, should it be longer or shorter than the original tube?
Longer
What is the primary function of a thermostatic expansion valve?
Maintain the proper evaporator superheat.
The typical evaporating temperature in an air conditioning system is _____________ ?
40 degrees indoor coil
The condenser in a refrigeration system does what?
It must reject the heat absorbed in the evaporator to the outside of the refrigerated space
A thermostatic expansion valve controls refrigerant flow through the evaporator based on _______________ .
Bulb pressure, evaporator pressure, & spring pressure
What are the 3 common methods of head pressure control during low ambient conditions?
Dampers, fan cycle controls, & condenser flooding
If the head pressure is high and the flow rate is high, what is the condition of the water cooled condenser?
Too much mineral build-up in the condenser
What are the causes of too little oil in the compressor?
Low refrigerant charge or low load, piping problems preventing oil return, & frozen or staving evaporator.
What is the maximum discharge line temperature for Copeland compressors?
225 degrees F
What are the 3 primary forces on a TEV?
Bulb pressure, Evaporator pressure, & spring pressure
What type of evaporator will need an external equalized valve?
A single circuit evaporator without an equalizer tube & large evaporators with substantial pressure drop
What happens to the capacity of a TEV as the head pressure decreases?
The capacity decreases because of less pressure drop across the valve
What are the 3 criteria for a TEV to work correctly?
Valve sized correctly, bulb installed correctly, & a complete column of liquid to the valve
The TD of a properly operating walk-in cooler should be ________ ?
10 degrees F
What terminal on a time clock is energized to take the clock out of defrost when the evaporator is warm enough?
terminal X
A low pressure control has a "high event" and a "differential setting. The technician wants the system to "cut-in" at 28 PSIG and cut-out at 2 PSIG, what would the proper setting be?
High event: 28 PSIG, differential: 26 PSIG
Abnormally high subcooling is a sign of ________ ?
too much refrigerant in the condenser
A R-22 medium temperature walk-in cooler at 95 degree ambient and 30 degree split should run a head pressure of about ________ ?
278 PSIG
What component is used to prevent refrigerant from flashing?
Heat exchanger
Positive displacement compressors use crankcase heat to do _________ ?
prevent refrigerant migration to the compressor
What is considered the "normal" box temperature of a walk-in refrigerator?
35 to 37 degrees F
What is considered the "normal" box temperature of a reach-in refrigerator?
38 to 40 degrees F
What is considered the "normal" box temperature of a walk-in freezer?
-10 degrees F
What is considered the "normal" box temperature of a reach-in freezer?
0 degrees F
What is "hot pull-down"?
When the evaporator is subject to higher temperatures and loads than normal operation
Why is it necessary for the fin spacing in a freezer evaporator to be wide?
Frost build-up will not occur as fast if the fin spacing is wide
Why does the suction vapor have to be increased in temperature before it can be condensed?
So it can be higher than the ambient temperature
When does condensing start?
When the discharge gas is above condensing temperature
Fan cycling controls can respond to what 2 conditions?
Ambient air temperature and head pressure
What problems can occur if the condenser fan stays off too long?
Air-cooled compressor motor overheating and erratic TEV operation result
What problem occurs when the condenser fan motor short cycles?
The fan motor can be damaged
What 4 things must a technician know before diagnosing problems in a HPR valve?
Ambient temperature, head pressure, liquid line temperature, & valve rating
What does tube-in-tube mean when referring to water cooled condensers?
The water pipe is surrounded by the refrigerant tubing
What is the approximate condensing temperature of a water-cooled condenser?
90 degrees F
What is volumetric efficiency?
How well a compressor is operating compared to its design capacity