Mechanical Science - Refrigeration Machines
Objective 7: Explain the operation of the following metering devices: (a) Automatic expansion valve (b) Thermostatic expansion valve (c) Thermal-electric (solid-state) expansion valve (d) Capillary tube
(a) Automatic expansion valve
Objective 4: State the function of the following components: (a) compressor (b) metering device (c) condenser (d) receiver (e) evaporator (f) accumulator
(a) compressor Functions of the Compressor: - Receives or removes the refrigerant vapor from the evaporator, so that desired pressure, and temp can be maintained - Increases the pressure of the refrigerant vapor through the process of compression and simultaneously increases the temp of the vapor so that it will transfer its heat to the condenser cooling medium There are 5 primary types of compressors: - Reciprocating, rotary, screw, centrifugal, scroll Reciprocating compressors make up the largest % of compressors used for refrigeration systems. A compressor is basically a pump designed for gases and vapors, instead of liquids. The compressor raises the pressure of the gas (vapor) and it provides the motive force to drive the fluid through the system.
Objective 10: Describe causes for the following refrigeration system conditions: (a) high head pressure (b) low suction pressure (c) high suction pressure
(a) high head pressure High head pressure is caused by either restriction in the compressor discharge line or by poor heat transfer in the condenser. These conditions can be caused by: - Insufficient condensing medium (air, water, etc.) - High condensing medium temperature - Dirty or partially blocked condenser - Air or other non-condensable gases in system
Objective 1: Define the following terms as they apply to refrigeration systems: (a) latent heat (b) sensible heat (c) subcooling (d) superheat
(a) latent heat Latent heat is heat added to a liquid to bring abt a change in phase, w/ no change in temp. Heat added to a liquid @ its boiling temp will vaporize the liquid, w/o changing its temp.
Objective 5: Explain the operation of the following compressor designs: (a) reciprocating (b) centrifugal (c) rotary (d) screw (e) scroll
(a) reciprocating
Objective 7: Explain the operation of the following metering devices: (a) Automatic expansion valve (b) Thermostatic expansion valve (c) Thermal-electric (solid-state) expansion valve (d) Capillary tube
(b) Thermostatic expansion valve A thermostatic expansion valve (TXV) controls the refrigerant flow in the evaporator by the temp of refrigerant @ the evaporator outlet. There are 2 basic types of thermostatic expansion valves (TXVs), sensing bulb and thermal-electric.
Objective 5: Explain the operation of the following compressor designs: (a) reciprocating (b) centrifugal (c) rotary (d) screw (e) scroll
(b) centrifugal Centrifugal compressors are used successfully in large refrigerating systems. In this type of compressor, vapor, as it is moved rapidly in a circular path, moves outward. This action is called centrifugal force. The vapor is fed into a housing near the center of the compressor. A disk w/ radial blades (impelllers) spins rapidly in this housing forcing vapor against the outer diameter. This uses the same principles of operation as a centrifugal pump. The pressure gained is so small that several of these compressor wheels or impellers are put in series to create greater pressure difference and to pump a sufficient volume of vapor. This type of compressor looks like a steam turbine or an axial-flow air compressor for a gas turbine engine. The centrifugal compressor has the advantage of simplicity. There are no valves, or pistons, and cylinders. The only wearing parts are the main bearings. Pumping efficiency increases w/ speed, so centrifugal compressors operate at high speeds. The rotor or impeller in a centrifugal compressor is keyed to the compressor shaft. The impeller is made of cast iron, steel, or aluminum, and is specially designed to move the vapors w/o going above gas velocity limits and w/o having vapor trapping pockets.
Objective 10: Describe causes for the following refrigeration system conditions: (a) high head pressure (b) low suction pressure (c) high suction pressure
(b) low suction pressure Either a restriction in the suction line or poor heat transfer in the evaporator causes low suction pressure. These conditions can be caused by: - Insufficient air or heat load on evaporator coil - Poor distribution of air over evaporator coil - Restricted refrigerant flow - Faulty metering device - Low discharge pressure - Undercharge of refrigerant
Objective 4: State the function of the following components: (a) compressor (b) metering device (c) condenser (d) receiver (e) evaporator (f) accumulator
(b) metering device
Objective 1: Define the following terms as they apply to refrigeration systems: (a) latent heat (b) sensible heat (c) subcooling (d) superheat
(b) sensible heat Sensible heat is heat added to a substance, which results in a temp change to that substance, and no change in phase. Heat added to water vapor (steam) will increase the temp of the steam.
Objective 1: Define the following terms as they apply to refrigeration systems: (a) latent heat (b) sensible heat (c) subcooling (d) superheat
(c) Subcooling Subcooling is the temp of a liquid, below saturation (condensing) temp.
Objective 7: Explain the operation of the following metering devices: (a) Automatic expansion valve (b) Thermostatic expansion valve (c) Thermal-electric (solid-state) expansion valve (d) Capillary tube
(c) Thermal-electric (solid-state) expansion valve
Objective 4: State the function of the following components: (a) compressor (b) metering device (c) condenser (d) receiver (e) evaporator (f) accumulator
(c) condenser
Objective 10: Describe causes for the following refrigeration system conditions: (a) high head pressure (b) low suction pressure (c) high suction pressure
(c) high suction pressure Either heavy load conditions or improper metering of the refrigerant to the evaporator causes high suction pressure. These conditions can be caused by: - Heavy load conditions - Low superheat adjustment - Improper expansion valve adjustment - Poor installation of sensing bulb - Inefficient compressor - High head pressure on capillary tube systems - High head pressure
Objective 5: Explain the operation of the following compressor designs: (a) reciprocating (b) centrifugal (c) rotary (d) screw (e) scroll
(c) rotary There are 2 basic types of rotary compressors. 1 has blades that rotate w/ the shaft and the other has a stationary blade. The low pressure refrigerant vapor from the suction line is drawn into the opening and fills the space behind the blade as it revolves. As the blades revolve, trapped vapor in the space ahead of the blade is compressed until it can be pushed into the exhaust line to the condenser.
Objective 7: Explain the operation of the following metering devices: (a) Automatic expansion valve (b) Thermostatic expansion valve (c) Thermal-electric (solid-state) expansion valve (d) Capillary tube
(d) Capillary tube The capillary tube is a popular type of refrigerant control, especially in smaller units. This control is simply a length of tubing w/ a small inside diameter. It acts as a constant throttle on the refrigerant. It is made of seamless tubing and has a small and accurate inside diameter. It is usually equipped w/ a fine filter or filter-drier at its inlet, to remove any moisture or dirt from the refrigerant. The amt of refrigerant in the system must be carefully calibrated, since all of the liquid refrigerant will move into the low side during the off cycle as the pressures balance or equalize. Too much refrigerant will cause the unit to frost back on the low side. The capillary tube must be used w/ a thermostatic motor control. The capillary tube refrigerant control has no moving parts. Therefore, it has 2 advantages. 1st, there are no parts to wear or stick. 2nd, the pressures balance in the system when the unit stops. This condition places a minimum starting load on the motor.
Objective 1: Define the following terms as they apply to refrigeration systems: (a) latent heat (b) sensible heat (c) subcooling (d) superheat
(d) Superheat Superheat is the temperature of a vapor, above saturation (boiling) temp.
Objective 4: State the function of the following components: (a) compressor (b) metering device (c) condenser (d) receiver (e) evaporator (f) accumulator
(d) receiver Receivers are installed to collect the high pressure, low temp liquid refrigerant as it leaves the condenser. In some models, the lower section of the condenser is used as the receiver. A receiver serves as a storage for refrigerant, maintains a liquid seal on the liquid line, and vents any air or non-condensable gases back to the condenser. Receivers are usually designed to be large enough to hold the complete charge of refrigerant required to operate the unit. Receivers are equipped w/ isolation valves on the inlet & outlet lines to permit the service person to pump the unit down when work is to be performed on another component in the system. Some receivers are also equipped w/ a small vent line from the uppermost section to the top of the condenser shell. Thus, any air or non-condensable gases can escape back to the condenser where they can be purged from the system. During normal operations, the receiver is abt 1/3 full of liquid refrigerant. Sight glasses may be installed to show liquid level. On some models, the level is detected by an electronic, capacitance-type probe mounted in the receiver and indicates a % of fullness on an externally mounted meter.
Objective 5: Explain the operation of the following compressor designs: (a) reciprocating (b) centrifugal (c) rotary (d) screw (e) scroll
(d) screw The screw-type compressor uses a pair of special helical rotors. The rotors trap and compress vapor as they revolve inside an accurately machined compressor cylinder. These compressors are available in either external drive or hermetic construction. They are used in large systems of 20 tons & up. The 2 rotors are not the same shape. 1 is male, and the other is female. The male rotor is driven by the motor. It has 4 lobes. The female rotor meshes w/ it and is driven by the male rotor. It has 6 interlobe spaces. The cylinder encloses both rotors. When the lobes from each screw mesh, the vapor that was in the spaces between each lobe is squeezed out of that space and exits through the discharge valve. Since there are 4 lobes on the male rotor, and 6 on the female rotor, the male rotor will revolve more rapidly than the female rotor. The rotors are helixes so the pumping action will be a continuous action, rather than pulsating as w/ a reciprocating compressor. In the absence of this reciprocating motion, there is very little vibration during operation. Many screw-type compressors operate w/ oil injection. This seals the clearance btwn the rotors and btwn the rotors and the cylinder. It also helps cool the compressor. The efficiency of these compressors is quite good. They may be used w/ most of the common refrigerants.
Objective 4: State the function of the following components: (a) compressor (b) metering device (c) condenser (d) receiver (e) evaporator (f) accumulator
(e) evaporator
Objective 5: Explain the operation of the following compressor designs: (a) reciprocating (b) centrifugal (c) rotary (d) screw (e) scroll
(e) scroll The scroll compressor uses 2 offset spiral disks to compress the refrigerant vapor. The upper scroll is stationary, and the lower scroll is the driven scroll. The intake of the vapor is at the outer edge of the scroll, and the discharge occurs @ the center of the stationary scroll. The driven orbiting scroll is rotated around the fixed scroll in an orbiting motion. During this movement, the suction vapor is trapped btwn the 2 scrolls. As the driven scroll rotates, it compresses the refrigerant vapor through the discharge port. The scroll compressor has fewer moving parts than reciprocating compressors and has less torque variation. This results in very smooth and quiet operation.
Objective 4: State the function of the following components: (a) compressor (b) metering device (c) condenser (d) receiver (e) evaporator (f) accumulator
(f) accumulator
Objective 6: Explain the factors that affect compressor performance.
2 useful measures of compressor performance are: (1) capacity (related to compressor displacement) (2) performance factor
Objective 8: Describe failure mechanisms and symptoms associated w/ a refrigeration system.
In small refrigeration units, the major problems that occur are: - insufficient cooling - improper refrigerant charge Other troubles may occur in the electrical circuity, but they will have to be determined on case-by-case basis. 3 main conditions in units that are operating, but not cooling satisfactorily are: - high head pressure - low suction pressure - high suction pressure
