https://www.mesubjects.net/wp-admin/post.php?post=4410&action=edit Open and Hermetically sealed compressor units
https://www.mesubjects.net/wp-admin/post.php?post=5067&action=edit Refrigeration equipment-2
https://www.mesubjects.net/wp-admin/post.php?post=4198&action=edit MCQ Vapor Compression Refrigeration
https://www.mesubjects.net/wp-admin/post.php?post=4194&action=edit MCQ non-conventional refrigeration
Reciprocating, rotary and centrifugal compressors are used for refrigerants having lower, medium and high boiling points. Air, water and evaporative condensers are used in domestic, commercial and industrial plants. Cooling tower cools the hot water from the condenser for its reuse.
COMPRESSORS:(i) Reciprocating compressors: used with
(a) lower boiling temperature (much below 00C) refrigerants
(ii) low specific volume at – 150C,
(c) (pcond – –pevap) > 3.5 atm ( R-22, NH3, etc.).
Reciprocating compressors are of three types.
(a) Hermetically sealed units: Has both compressor and motor in one housing, used in refrigerators, water coolers, window air conditioners, display cabinets
(b) Semi-open units: There is a separate housing for compressor and motor, directly coupled having same RPM, used in units of 100 TR capacity refrigeration plants
© Open units: It has a separate housing for motor and compressor, connected by belt/gear drive, different RPM, used in large capacity refrigeration plants
(ii) Rotary compressors: This is used for
(a) medium boiling point ( between +2 and -20C) refrigerants,
(b) medium specific volume at -150C
(c) (pcond – –pevap) > 1.5 atm and <3.5 atm (isobutene)
(iii) Centrifugal compressors: It is used with
(a) higher boiling point ( much > 00C) refrigerants
(b) large specific volume at -150C
(c) (pcond – –pevap) < 1.5 atm such as R-113
L/D RATIO FOR VARIOUS RECIPROCATING COMPRESSORS
Type of unit L/D
Vacuum pumps 0.5
High speed air compressors 0.5
Reciprocating Freon Compressors 0.8
Ammonia Reciprocating Compressors 1.0
High pressure compressors 4.0
( To keep piston force per unit inertia small, diameter is kept small for high pressure compressors)
DIFFERENT TYPES OF CONDENSERS:
(a)Finned tube type
(b) Double tube type
(c) Coil shell type
(d) Shell and tube type
Q. Give the brief description of Shell and Tube Condenser.
The refrigerant passes through the shell (Cylinder) and the water passes through the tubes. There are number of straight tubes and hence there are no bends to reduce the pressure drops. It is also easy to clean the tubes when the scale is formed in the tubes. Hot water coming out of the condenser is cooled in the cooling tower for reuse,
THREE TYPES OF COOLING ARRANGEMENT IN CONDENSERS
AIR COOLED: Finned tube type: natural Air cooled in refrigerators, forced air cooled in water coolers, window air conditioners and deep freezers.
WATER COOLED: Shell and tube type: water in tubes and refrigerant in shell. These are used in medium capacity plants in libraries, cinema halls.
EVAPORATIVE COOLED (Simultaneously Air as well as water cooled ): bare tube type: used in large cooling plants in cold storage’s, ice plants, wine industries, milk plants and refineries.
Cooling Tower: It is a heat exchanger where hot water from condenser outlet is cooled by atmospheric air. This cooled water is reused in the condenser. Thus, it makes the cooling in the condenser economical. There is an accumulator to send only vapors to the compressor and remaining liquid to the evaporator inlet. These are used in large capacity refrigeration plants.
EXPANSION VALVES: Five types: Capillary tube, Automatic expansion valve (AEV), Thermostatic expansion valve(TEV), Low Side float valve(LSFV) and high side float valve(HSFV).
Capillary tube: Used in small capacity plants like refrigerators, window air conditioners, water coolers, display cabinets, and deep freezers.
Automatic expansion valve(A E V): It is used with constant cooling requirements: not very common as it has disadvantages like supply of more refrigerant on decrease of cooling requirement and reduced supply of refrigerant on increase of cooling requirements.
Thermostatic Expansion Valve (T E V): It maintains a constant degree of super heat at the outlet of the evaporator under all conditions of cooling load. It increases the flow of refrigerant on increase of cooling requirements. It decreases the flow on decreasing cooling requirements. There is a feeler bulb containing a different refrigerant than the one circulating in the refrigerating machine. This feeler bulb feels the degree of super heat at the outlet of the evaporator. In case of increase of degree of super heat it increases the flow rate. When degree of super heat decreases the flow rate of refrigerant.
Low Side Float valve (L S F V): A float valve is located in the evaporator itself. If cooling requirement increase causing increase in evaporation. Due to this, the level goes down and the float permits more quantity of refrigerant. When cooling requirement decreases, there is less rate of evaporation. Hence, float goes down by lesser height and refrigerant flow decreases.
High Side Float Valve (H S F V): Float valve is located in the receiver itself. More cooling requirements increase the pressure in the evaporator. This increased pressure lifts the float and increases the supply of refrigerant in the evaporator and vice versa. On less cooling requirement, pressure decreases and there is lesser flow of refrigerant.