Unit 13, COMPRESSURES
What is the compression ratio of a compressor with a suction pressure of 110 psig and a discharge pressure of 360 psig?
(360+15) / (110+15) = 375/125 = 3 to 1
List the ways to change the refrigeration capacity to match the system load.
1. Cycling multiple compressors on and off . 2. Cylinder unloading. 3. Hot-gas bypass. 4. Speed control a. Two speed. b. Variable-frequency drives (VFD)(Pg.170)
What are the three different screw compressor types?
1. SINGLE- SCREW with gate rotors. 2. TWIN-SCREW 3. TRIPLE-SCREW
What are some common applications for rotary compressors?
1. Small refrigerated appliances 2. Window air conditioners. 3. Package terminal air conditioners. 4. Ductless split systems.
What controls the internal unloading solenoid coil of the two-step scroll.?
24V AC CONTROL MODULE. This control module receivesa 24V AC signal from the thermostat's second stage. Within the module, the 24 V AC is rectified to DC, which operates the internal unloading solenoid coil. (Pg. 162)
How does a rotary-vane compressor work?
A large off-center rotating shaft carries multiple blades in slots, and they have a reciprocating action as they move in and out, following the cylinder walls.
How does a two-step scroll compressor achieve capacity reduction?
A rotating internal plate opens and closes two drilled ports in the scroll. When these two ports are open, refrigerant exits the scroll in the first compression chamber. (Pg. 161)
Above what temperature do most refrigerant oils begin to break down chemically?
At a temperature of 350°F and above, most refrigerant oils begin to break down chemically. (Pg.173)
What type of a system are most centrifugal compressors used on?
Centrifugal compressors are widely used in single-stage, multi-stage, open, and semihermetic configurations to chill water for building air conditioning in capacities from 100 tons to 8000 tons and higher. (Pg. 165)
How can oil be delivered to internal moving compressor parts?
Delivery of lubricant can be by simple splash or a separate oil pump. A common splash system uses a slinger that slings the oil around the inside of the compressor. (Pg. 165)
At what two capacities can a digital scroll compressor operate?
Digital scroll compressors get their name from the fact that they only operate under two conditions: either at 100% capacity or 0% capacity. (Pg. 162)
Why is it incorrect to close the suction line service valve to test a compressor?
Do not use this procedure! It can result in deep vacuum, which can allow internal arcing to occur in the electrical windings of the compressor.
What is cylinder gas reexpansion, and why is it a problem?
Gas trapped in the compressor clearance space is expanded at the beginning of the suction stoke. This causes inefficiency because new suction gas cannot enter the cylinder until the trapped gas is at a pressure below the suction pressure. (Pg.158)
What is a hermetic compressor?
HCs have their mechanical components and electric motor enclosed in a single welded shell.
What gives the digital scroll the appearance of varying it's capacity?
It is to the ability to switch very quickly between full-on capacity to full-off capacity that allows digital scrolls to produce an infinite range of capacities. (Pg. 162)
What problem can be caused if liquid refrigerant enters the compressor?
Liquid does not compress. If the liquid cannot leave quickly, the mechanical force is quickly converted into thousands of pounds of hydraulic pressure, and break compressor parts like valves, valve plates, rods, and pistons. (Pg. 172)
How can a technician determine if an operating compressor is experiencing floodback?
Liquid refrigerant floodback occurs while the system is operating. A system operating with a suction line superheat of 5° or less is probably experiencing flooding. (Pg. 171)
What is the purpose of a crankcase heater on a compressor?
Oil and refrigerant liquid are miscible, so it is possible to have refrigerant migrate during long off cycles to a compressor in a cold location. A crankcase heater can be added to the outside of hermetic compressors to raise the temperature of the oil to prevent this migration from occurring. (Pg. 168).
How are the small gaps on the screw compressor sealed?
Oil flooding is used for sealing as well as lubrication and cooling. (Pg. 164)
List two advantages of open-drive compressors.
One advantage of an open-drive compressor is that the motor and compressor are separate units so that the motor and compressor can be repaired or replaced without having to buy a complete system. Another advantage is that the motor's heat is absorbed in the refrigerant, so it does not have to be rejected in the condenser like most other compressor types.
Explain the difference between a positive-displacement compressor and a nonpositive-displacement compressor.
Positive-displacement compressors move the gas by taking it into a place and then replacing that space with something physical, like a piston. Non positive-displacement compressors do not fill the gas space with something else. They operate like giant fans or turbines turning at very high speeds.
List the major types of compressors by mechanical design.
Reciprocating Rotary Scroll Screw Centrifugal
What is the only thing all compressors must pump.?
Refrigerant vapor.
How does a scroll compressor work?
Scroll compressors compress gas between two close-fitted spiral scroll members. One is fixed and the other moves (but does not rotate) in an orbital path.
What is the lowest percentage of full capacity at which a digital scroll can be operated, and why?
The 10% minimum capacity is designed to ensure that the compressor gets proper lubrication. (Pg. 162)
Why is the oil separator important to screw compressors?
The compressor could quickly run out of oil without the oil separator returning oil to the compressor. (Pg. 164)
What is the function of the compressor within the refrigeration cycle?
The compressor is a mechanical device for pumping refrigerant vapor from the low-pressure side in the evaporator to the high pressure side in the condenser. The compressor's job is to raise the pressure and temperature of the refrigerant so that it can be condensed.
What factors affect compressor capacity?
The evaporator temperature. The condenser temperature. Refrigerant properties. Compressor RPM
Scaling
The formation of lime and other deposits on the water side surfaces of heat exchangers.
What is the purpose of compressor capacity control?
The idea of capacity control is to make the compressor capacity match the load . (Pg. 170)
How does a centrifugal compressor work?
The impeller acts like a fan. Refrigerant enters in the middle of a centrifugal impeller and is thrown outward by centrifugal force. (Pg. 165)
Why is more horsepower required to produce a ton of cooling at an evaporator saturation temperature of 0°F than at 40°F ?
The lower temperature evaporator increases the pressure difference that the compressor must maintain.
Why are hermetic compressors designed for specific applications?
The match of the motor and compressor is dependent upon the factors affecting compressor capacity.
How can a technician tell if a failed compressor has been operating at excessive discharge temperatures?
The maximum discharge line temperature should be between 180°F and 220°F. Varnished oil deposits on the head and valve plate indicate that this system has been operating at consistently high temperatures. (Pg. 172)
Why should hermetic compressors not be operated in a vacuum?
The motor varnishes used in the motor winding lose some of their insulating ability under deep vacuums, and motors can arc out very quickly, even before they have time to overheat. (Pg. 154).
What rotates in a scroll compressor?
The only rotating part in a scroll compressor is the motor shaft.
Condenser
The part of the refrigeration system that receives the high pressure superheated vapor from the compressor and extracts the heat in vapor reducing it to a high pressure saturated liquid.
WHAT is the clearance volume of a reciprocating compressor?
The space between the top of the piston and the cylinder head.
What opens and closes the valves in a reciprocating compressor?
The valves are self-operating: they open and close due to the pressure difference.
Why are the intake ports on low and medium-temperature compressors larger in size than the intake ports on air-conditioning compresses.
They have to be larger to allow enough low density, low pressure refrigerant vapor into the compression cylinder.
What is an advantage of the semihermetic compressor design.
They provide the advantagesof semihermetic compressors but are still field or factory replaceable.
How does a rolling-piston rotary compressors work?
They use an offset rolling piston that seals against a blade that remains stationary and moves in and out of it's slot to seal against the rolling piston.
Explain what compliance means for scroll compressors.
This allows the scroll flanks to maintain close contact, and yet the orbiting scroll can separate and unload to pass a small slug of liquid. .
What is the purpose of the shaft seals on an open compressor?
To prevent refrigerant from leaking around the crankshaft.