VAPOR COMPRESSION REFRIGERATION EQUIPMENTS

VAPOR COMPRESSION

REFRIGERATION EQUIPMENTS 

Equipments of vapor compression

systems include compressors,

condensers, evaporators & expansion

valves. These also include piping and various

controls.

COMPRESSORS

(i)  Reciprocating compressors: used with

(a) lower boiling temperature (much below 0⁰C) refrigerants

(ii)  low specific volume at – 15⁰C,

(c) (p_cond – –p_evap) > 3.5 atm (  R-22, NH_3, etc.).

Reciprocating compressors are of three types.

(a) Hermetically sealed units

  Compressor and motor has same housing. It is 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. These use belt/gear drive. These have different RPM. Used in large capacity refrigeration plants

(ii) Rotary compressors

  Used for

(a) medium boiling point ( between +2 and -2⁰C) refrigerants,

(b) medium specific volume at -15⁰C

(c) (p_cond – –p_evap) > 1.5 atm and <3.5 atm  (isobutene)

(iii) Centrifugal compressors

 Used with

(a) higher boiling point ( much > 0⁰C) refrigerants

(b) large specific volume at -15⁰C

(c) (p_cond – –p_evap) < 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 small, small diameter shaft used 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

 Shell and Tube Condenser

The refrigerant passes through the shell (Cylinder) and the water passes through the tubes. There are number of straight tubes.  There are no bends. Descaling is easy. Cooling tower cools hot water in condenser for reuse,

THREE TYPES OF COOLING ARRANGEMENT IN

CONDENSERS

 AIR COOLED

(i) Finned tube type

Natural Air cooled  used in refrigerators

 

Forced air cooled used in water coolers, window air conditioners and deep freezers.

WATER COOLED

 Shell and tube type

Uses water in tubes and refrigerant in shell. Medium capacity plants in libraries, cinema halls use these water cooled condensers..

EVAPORATIVE COOLED 

are simultaneously Air & 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. Atmospheric air cools hot water from the condenser. Condenser reuses this cooled water. Cooling in the condenser becomes economical. There is an accumulator to send only vapors to the compressor.  Remaining liquid goes to evaporator inlet. Large capacity refrigeration plants use cooling towers.

EXPANSION VALVES

 Five types

(i) Capillary tube

(ii) Automatic expansion valve (AEV)

Fig. Automatic Expansion Valve

(iii) Thermostatic expansion valve(TEV)

(iv) Low Side float valve(LSFV)

(v) 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)

 Used with constant cooling requirements.

Not in common use as it has disadvantages.

(i) supply of more refrigerant on decrease of cooling requirement

(ii) reduced supply of refrigerant on increase of cooling requirements.

Thermostatic Expansion Valve (T E V)

 Maintains a constant degree of super heat at the outlet of the evaporator. Increases the flow of refrigerant on increase of cooling requirements. Decreases the flow on decreasing cooling requirements. There is a feeler bulb. It contains a different refrigerant. This feeler bulb feels the degree of super heat at the outlet of the evaporator.  Increase of degree of super heat increases the flow rate. Decrease of 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. The level goes down and the float permits more quantity of refrigerant. Decreased cooling requirement causes less downward movement of float. 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.

TYPES OF EVAPORATORS

Dry expansion type and flooded type

 The refrigerant becomes fully dry saturated at the evaporator exit. Refrigerators, window air conditioners, water coolers, deep freezers use dry expansion type evaporators.

Fig. Dry and Flooded Evaporators

Flooded evaporators

When both liquid and vapor exist at the evaporator outlet.  Accumulator is a separator. It sends vapors to the compressor. The liquid left goes to evaporator. Large capacity refrigeration plants use these evaporators.

Oil separator

Oil separator is a cylindrical vessel. It is after the compressor. This separator has baffles in it. It separates the lubricating oil from the refrigerant. If the oil does not separate, it flows to condenser and evaporator. This decreases the rate of heat transfer and the cooling effect. Send Oil collected to the compressor sump. Oil separator allows effectively oil free refrigerant to the condenser.

Muffler

A cylinder after the compressor reduces the velocity of refrigerant and noise in the unit.

Liquid suction heat exchanger (LSHE)

LSHE sub-cools liquid refrigerant going to the throttle valve. Side by side, it  super heats the vapors coming from the evaporator. It increases the refrigerating effect and the COP.

Drier cum Filter

 Drier cum filter fits in the liquid line. A sieve in it filters the  impurities. Silica Gel acting as a drier removes any moisture present in the refrigerant.

Suction accumulator

Used in flooded evaporators. It separates vapor refrigerant going to the compressor. It sends back liquid refrigerant to the evaporator.

Sight glass

Sight glass  is a glass window. Fit it after the condenser. It helps to see the state of refrigerant and the type of flow.

Moisture indicator

Moisture  indicator tells the moisture present in the air. Necessary steps are necessary to insure the correct amount of moisture in the conditioned air.

Solenoid valve

Fit this valve in liquid line. This operates with electricity. It fully closes in the absence of power. It is fully open with power availability.

Check valve

A valve which allows flow only in one direction.

Evaporator back pressure regulating valve (BPRV)

Hand operated valve which controls rate of refrigerant flow and pressure in evaporator. In this, there are two outlets. Through one pipe refrigerant goes to the compressor. Through the other pipe, refrigerant goes to the outlet of evaporator for the pressure adjustments.

 Relief valve

 Releases excess pressure in the receiver. It is a safety valve like a one in a pressure cooker.

Fusible plug

 Excessive heat fuses the fusible plug. It is there in a pressure cooker or in a boiler.

Pipes/Tubes in Refrigeration

Copper  and copper alloys are used in heating, air conditioning & refrigeration. These have inherent resistance to corrosion & ease of installation. There are two principal classes of copper tubes.  Freon refrigerants use Copper tubes of K, L, and M types. Copper tubing is annealed & is without threading. Join small size copper tubes by FLARE or compression fittings.

 PIPE MATERIAL WITH AMMONIA

Use mild steel pipe with ammonia refrigerant. Ammonia is highly corrosive to copper.

 

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