Chapter 5: Compressors
In which of the following compressor construction arrangements are the motor and compressor enclosed within a permanently sealed housing? (A) Hermetic (B) Open type (C) Semi-hermetic (D) Direct coupled
(A) Hermetic A hermetic or sealed compressor is one in which both compressor and motor are confined in a single outer welded steel shell. Hermetic compressors are ideal for small refrigeration systems.
In which compressor design is suction gas compressed between rotating gears (helical lobes), after having entered the compressor through a sliding vane type capacity control valve? (A) Screw compressor (B) Rotating vane compressor (C) Stationary single blade compressor (D) Scroll compressor
(A) Screw compressor A Screw compressor is a mid-size compressor, expensive to manufacture. It has helical lobes, as the rotors turn the space between them is continually reduced. Eventually the pressure of the refrigerant will rise to the required condensing pressure. This compressor is designed to maintain operating pressures above atmospheric pressure.
In a two stage centrifugal compressor with an economizer, in which stage is the refrigerant pressure greatest? (A) 1st stage (B) 2nd Stage (C) Each stage holds equal volumes (D) Varies with operating conditions
(B) 2nd Stage Remember in each stage of a multi-stage compressor the volume of refrigerant remains the same as the pressure increases. Pressure increases from one stage to another example: on a 3-stage machine the pressure of the refrigerant increases 3 times before it is discharged to the condenser.
Modern compressors use what type of material at their shaft seals? (A) Lead (B) Copper and packing (C) Low friction plastics (D) Packing glands
(B) Copper and packing Packing seals have been the most widely used sealing solution for many years. Packing seals use rope-like materials that wrap around the shaft of a compressor, filling in dead space and minimizing fluid loss.
The pressure developed by a centrifugal compressor is determined by: (A) Economizer controls (B) Impeller tip speed (C) Position of vane control (D) Refrigerant type
(B) Impeller tip speed The pressure developed by a centrifugal compressor is determined by the impeller tip speed. Tip speed is determined by diameter of the impeller and RPM's. The tip speed is simply the distance that any selected point on the peripheral of the impeller travels in a set time.
Which compressor construction arrangement is characterized by one end of the compressor's shaft extending beyond the compressor housing? (A) Hermetic (B) Open type (C) Serviceable single blade compressor (D) Centrifugal
(B) Open type In an open compressor, the motor, and the compressor housing are mounted separately. Because the open compressor lacks a seal around it, there is risk of refrigerant leakage. The advantages are that the compressor components are easily accessible for maintenance and the costs of a shell can be avoided.
Which of the following best describes how a "positive displacement" compressor increases the pressure of a refrigerant? (A) Increased velocity pressure (B) Volume reduction (C) Increasing the refrigerant's specific gravity (D) Removal of latent heat
(B) Volume reduction Positive displacement compressors increase the refrigerant pressure by reducing the refrigerant volume. Remember, as volume decreases, pressure & temperature increases.
Which of the following compressors uses non-positive (variable) displacement to increase the pressure of a refrigerant? (A) Scroll (B) Screw (C) Centrifugal (D) Rotary
(C) Centrifugal The centrifugal compressor uses variable displacement to compress vapor. The centrifugal compressor uses centrifugal force to sling the vapor refrigerant outward to increase the refrigerant pressure and the temperature and then sends it to either condenser coils or a condenser barrel where the heat is removed from the refrigerant.
In a two stage centrifugal compressor with an economizer, which stage holds the largest volume of vapor? (A) 1st stage (B) 2nd Stage (C) Each stage holds equal volumes (D) Varies with operating conditions
(C) Each stage holds equal volumes Volume as define by space is the quantity of three-dimensional space enclosed by a closed surface. For example, the space that a substance or 3D shape occupies or contains. As such, the physical dimensions of the impeller casing in a centrifugal compressor does not change from one stage to the next, therefore in a multistage compressor, each stage holds equal volumes
Which of the following best describes the operational characteristics of centrifugal refrigeration machine? (A) High volume, high pressure (B) Low volume, low pressure (C) High volume, low pressure (D) Low volume, high pressure
(C) High volume, low pressure Centrifugal compressors use variable displacement to compress vapor. Like a centrifugal pump, the restricting of the compressor discharge will reduce the quantity of vapor moved by the compressor and the corresponding horsepower. They are large capacity systems, use almost exclusively when a single compressor is to produce over 100 tons. These are low pressure systems.
Which of the following best describes the state of the refrigerant at the inlet and outlet of the compressor? (A) Low pressure liquid in, low pressure liquid out (B) Low pressure vapor in, high pressure liquid out (C) Low pressure vapor in, high pressure vapor out (D) High pressure vapor in, high pressure liquid out
(C) Low pressure vapor in, high pressure vapor out Refrigerant enters the compressor though the suction valve as a low-pressure vapor (LPV) and exits through the discharge valve as a high-pressure vapor (HPV).
A double acting compressor has which of the following operational characteristics? (A) Produces discharge at one end of the cylinder (B) Produces discharge at both ends of the cylinder (C) Produces suction and discharge at both ends of the cylinder (D) Produces suction on alternate strokes
(C) Produces suction and discharge at both ends ofthe cylinder Remember the Horizontal double acting "HDA" compressor has a suction and discharge valves at both ends of the cylinder Therefore, refrigerant gas is compressed on two sides of the piston.
For a given operating condition and motor horsepower, which of the following compressors will have the smallest displacement? (A) Centrifugal (B) Screw (C) Rotary (D) Scroll
(C) Rotary Rotary hermetic compressors use the rotating action of a roller inside a cylinder to compress the refrigerant. Rotaries, by design, include less parts than other types of compression technology and provide an alternative, efficient option for various applications.
If a refrigeration system is said to have (3) stages. This implies: (A) Three steps of capacity are obtainable (B) The system can cool, heat, or dehumidify (C) The pressure of the refrigerant is increased (3) times before it is discharged to the condenser (D) The unit was factory shipped in (3) parts
(C) The pressure of the refrigerant is increased (3)times before it is discharged to the condenser On a 3 stage centrifugal machine, the pressure of the refrigerant is increase 3 times before it is discharged to the condenser.
What type of compressor is most likely to be used in a system which is operating in a vacuum? (A) Screw (B) Scroll (C) Rotary (D) Centrifugal
(D) Centrifugal Centrifugal compressors are use in large capacity systems, uses almost exclusively when a single compressor is to produce over 100 tons. Capable of maintaining vacuum pressures, below atmospheric pressure.
Which of the following is closest to the crankcase pressure of an operating reciprocating compressor? (A) Condensing pressure (B) Atmospheric pressure (C) Mid-point between evaporator and condenser (D) Evaporator pressure
(D) Evaporator pressure Remember; the evaporator, which is located on the low side of the refrigeration system is an open-type heat exchanger, refrigerant inside the evaporator, freely flows from the evaporator, through the suction valve and into the compressor, thus, having a uniformed pressure up to the crankcase of the compressor.
Which of the following states one of the functions of a compressor? (A) Compress refrigerant so that it will all fit into the condenser (B) Compress liquid refrigerant so that it will vaporize (C) Circulate the refrigerant through the condenser tubes (D) Provide the differential pressure needed to circulate the refrigerant through the cycle
(D) Provide the differential pressure needed to circulate the refrigerant through the cycle Compressors: Maintain the required suction pressure on the evaporator, move the refrigerant through the system, increase the pressure of the refrigerant up to the required condensing pressure as well as separate the low and high side of the system.
A two cylinder single acting vertical compressor has a suction pressure of 45 PSIG. The purpose of the shaft seal is to keep: (A) Air from entering (B) Air and moisture from entering (C) Oil from escaping (D) Refrigerant and oil from escaping
(D) Refrigerant and oil from escaping Open compressors shaft seals are used to prevent any refrigerant or oil to leak out of the compressor. As the compressor is at a much greater pressure than the external atmospheric pressures. Remember, high pressure systems operated above atmospheric pressure therefore a seal prevents refrigerant from coming out of the system at the shaft seal of the compressor.
What advantage do semi-hermetic compressors have over hermetic compressors? (A) Light weight design (B) Less expensive (C) Field serviceable (D) Consume less energy per ton of cooling
C) Field serviceable On a semi-hermetic compressor, the motor and compressor are housed together, but they can be accessed as needed. This enables maintenance checks and repair or replacement of parts as they degrade rather than a total failure of the system.
What type of compressor valves need the most clearance space when used on vertical compressors? (A) Ribbon (B) Ring Plate (C) Poppet (D) Feather
C) Poppet A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into a compressor. It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usually a disk shape on the end of a shaft known as a valve stem. Due to the valve stem, this type of valve needs more clearance space.
