The discharged vapor picks up which of the following two more additional sources of sensible heat as the suction vapor is processed through the compressor?
Motor heat and the heat of compression
What is "clearance volume" and how can it affect compressor efficiency?
The area of high pressure gas between the top of the piston and the bottom of the valve plate.
What is "volumetric efficiency"?
How well a compressor is operating compared with its design capacity
If an R404A commercial freezer has design conditions of -20� evaporator and +125� condensing, what is the compression ratio?
10.5 to 1
In the example above what is the compression ratio if the customer lowers the box temperature so that the evaporator is operating at -30� with the same head pressure?
14.1 to 1
Assume the above freezer evaporator temperature remains at -20�, but the condenser is designed for a 25� condenser split and the condensing temperature drops to 120�. What is the compression ratio?
10.5 to 1
Which situation will damage the above freezer compressor quicker, lowering the suction pressure by 5 psig or raising the head pressure by 20 psig, and why?
Lowering the suction pressure because it raises the compression ratio more than raising the head pressure
In the above question what kind of damage will occur to the compressor?
Overheating
Does lowering the suction pressure increase, or decrease, motor cooling in a suction cooled compressor?
Decreases motor cooling. At lower pressures the refrigerant gas has less density. Therefore, low pressure vapor is less effective at transferring heat.
What repairs can be made on the job to a semi-hermetic compressor?
Crankcase oil change and valve plate replacement
What is the difference between an air-cooled compressor and a suction-cooled compressor?
Air cooled is cooled by the condenser fan, suction cooled uses the cooler refrigerant vapor to cool it
What are the causes of too much oil in a compressor?
There were multiple compressor replacements, or another technician added oil
What are the causes of too little oil in a compressor?
Low refrigerant charge or low load, piping problems preventing oil return, frozen or starving evaporator, compressor short cycling
What would a tear down show if there was lack of oil?
All rods or bearings worn or scored
What causes flooding and slugging?
Too much refrigerant returning to the compressor
With the compressor running, which symptom is the most positive indicator of flooding or slugging at the compressor?
Compressor noise
If the suction valves are broken, what is the cause?
Slugging
What would cause scoring of the cylinder walls and there was blow-by?
Flooding
What is the difference between flooding and slugging?
Flooding is a little liquid refrigerant reaching the compressor; slugging is a lot of liquid refrigerant reaching the compressor all at once.
What is the primary cause of a flooded start?
Refrigerant migration
How can flooded starts be prevented?
Install a crankcase heater or pump-down solenoid
On a teardown, what would indicate a flooded start?
Erratic wear pattern on the crankshaft
What causes high motor heat on an air-cooled compressor?
Inadequate airflow from the condenser fan
What causes high motor heat on a suction-cooled compressor?
Low-mass flow of suction vapor to cool the compressor motor
What is compressor capacity control?
Controlling the amount of work a compressor can perform
Which of the following would be considered short cycling?
Six cycles per hour
Which of the following is considered compressor unloading?
...Preventing suction vapor from being compressed
What is usually used to start the unloading process?
A drop in suction pressure
As the compression ratio of a compressor is increased...
The compressor pumps a smaller volume of refrigerant, which is a reduction in the mass flow rate. 86/1
What is the maximum discharge line temperature for Copeland compressors?
225 degrees Fahrenheit
Two-stage compound systems are used in what application?
Extra-low temperature (minus 30 degrees to minus 80 degrees)
What is the "inter-stage" on a two-stage system?
Where the discharge of one system is cooled by the suction of the other 88/4
Why are scrolls considered more efficient than reciprocating compressors?
They have better volumetric efficiency
Who does Variable Frequency Drive vary compressor capacity?
It modulates the frequency of the electrical power and there by the speed of the compressor.