15783 components vc_rs

Various Components of Vapor Compression Refrigeration

Systems

Minesh Vohra (15783) Asst. Professor (SME-M3-MD08)

Detailed Study of various Components:

Compressors Condensers Evaporators Expansion Devices


Compressor’s Classifications1. Based on Working Principle:

Positive Displacement TypeReciprocating typeRotary type with sliding vanes (rolling piston type or multiple vane type)Rotary screw type (single screw or twin-screw type)Orbital/Scroll compressors

Roto-DynamicsRadial flow type, orAxial flow type

2. Based on arrangement of compressor motor or external drive:Open type Hermetic (or sealed) typeSemi-hermetic (or semi-sealed) type


Positive Displacement TypeThe flow of refrigerant to the compressor is not steady, the positive displacement type compressor is a pulsating flow device. Disadvantages:

Prone to high wear Vibrations High noise level.

Roto-Dynamics TypeIn roto-dynamic compressors, the pressure rise of refrigerant is achieved by imparting kinetic energy to a steadily flowing stream of refrigerant by a rotating mechanical element and then converting into pressure as the refrigerant flows through a diverging passage.


Positive Displacement Type


• Rotary Compressors Used in small refrigeration systems (upto 2 kW

capacity) Compression is achieved by reducing the volume of

the refrigerant. In this type of compressors, the rotating shaft of the

roller has its axis of rotation that matches with the centerline of the cylinder, however, it is eccentric with respect to the roller

The rotating motion of the roller causes a reciprocating motion of the single vane.


• Rotary Screw(twin) Compressors:

1. Scroll compressors are orbital motion.

2. Suction and compression is obtained by using two mating, spiral shaped, scroll members, one fixed and the other orbiting.

3. The compression process involves ORBiTING SCROLL and then as pressure increases the discharge takes place axially

• Rotary Screw(twin) Compressors:

Two mating helically grooved rotors, one male and the other female.

The male rotor has lobes, while the female rotor has flutes or gullies.

The flow is mainly in the axial direction. Suction and compression take place as the rotors unmesh

and mesh.


Roto-Dynamic COMPRESSORS Centrifugal compressors; also known as turbo-compressors. Pressure rise takes place due to the continuous conversion of angular

momentum imparted to the refrigerant vapour by a high-speed impeller into static pressure.

Centrifugal compressors are steady-flow devices subjected to less vibration and noise.

As the refrigerant flows through the blade passages towards the tip of the impeller, it gains momentum and its static pressure also increases.

From the tip of the impeller, the refrigerant flows into a stationary diffuser (4).

In the diffuser, the refrigerant is decelerated and as a result the dynamic pressure drop is converted into static pressure rise, thus increasing the static pressure further.

The vapours from the diffuser enters the volute casing (5) where further conversion of velocity into static pressure takes place due to the divergent shape of the volute.

Finally, the pressurized refrigerant leaves the compressor from the volute casing (6)


Condenser: Unit used to reject heat after sub-heating and also during sub-cooling at constant pressure

Classifications:Air Cooled CondenserWater Cooled CondenserEvaporative Condensers


Air Cooled Condensers….Air is used as fluid for cooling purpose i.e. refrigerant rejects heat to air flowing over a condenser.Further Classifications:

1. Natural Convection Type: The finned type condensers are mounted either below

the refrigerator at an angle or on the backside of the refrigerator.

In case, it is mounted below, then the warm air rises up and to assist it an air envelope is formed by providing a jacket on backside of the refrigerator. The fin spacing is kept large to minimize the effect of fouling by dust and to allow air to flow freely with little resistance.

A thin wires are welded to the serpentine tube coil. The wires act like fins for increased heat transfer area. Figure 1. shows the schematic of a wire-and-tube type condenser commonly used in domestic refrigerators. Hence these condensers are used for small capacity refrigeration systems like household refrigerators and freezers.


Air Cooled Condensers….2. Forced Convection Type:

In forced convection type condensers, the circulation of air over the condenser surface is maintained by using a fan or a blower.

These condensers are normally uses fins on air-side for good heat transfer. The fins can be either plate type or annular type.

Forced convection type condensers are commonly used in window, air conditioners, water coolers and packaged air conditioning plants.


Water Cooled Condenser

Depending upon the construction it is further Classified as:

1.Double pipe or tube-in-tube type2.Shell-and-coil type3.Shell-and-tube type


Water Cooled Condenser…

1. Double Type Tube-In-Tube type:Double pipe condensers are normally used up to 10 TR

capacity.The cold water flows through the inner tube.Refrigerant flows through the annulus in counter flow. Headers are used at both the ends to make the length of

the condenser small and reduce pressure drop.The refrigerant in the annulus rejects a part of its heat to

the surroundings by free convection and radiation. The heat transfer coefficient is usually low because of

poor liquid refrigerant drainage if the tubes are long.



2. Shell-and-coil type::These condensers are used in systems up to 50

TR capacity. The water flows through multiple coils, which

may have fins to increase the heat transfer coefficient. The refrigerant flows through the shell.

In smaller capacity condensers, refrigerant flows through coils while water flows through the shell.

Figure 4 shows a shell-and-coil type condenser. When water flows through the coils, cleaning is done by circulating suitable chemicals through the coils.



3. Shell-and-tube type::•Used in systems from 2 TR upto thousands of TR capacity.•The refrigerant flows through the shell while water flows through the tubes in single to four passes. •The condensed refrigerant collects at the bottom of the shell.•The liquid refrigerant is drained from the bottom to the receiver. •A vent connecting the receiver to the condenser for smooth drainage of liquid refrigerant. •The most common type is horizontal shell type. •Vertical shell-and-tube type condensers are usually used with ammonia in large capacity systems so that cleaning of the tubes is possible from top while the plant is running.


Evaporative Condenser• Both air and water are used to extract heat from

the condensing refrigerant. • Evaporative condensers combine the features of a

cooling tower and water-cooled condenser in a single unit.

• In these condensers the water is sprayed from top part on a bank of tubes carrying the refrigerant and air is induced upwards. There is a thin water film around the condenser tubes from which evaporative cooling takes place.

• The heat transfer coefficient for evaporative cooling is very large and can be operated at low condensing temperatures (about 11 to 13 K above the wet bulb temperature of air)..

• The role of air is primarily to increase the rate of evaporation of water.

• The required air flow rates are in the range of 350 to 500 m /h per TR of refrigeration capacity


Expansion Devices• Constant pressure or Automatic Expansion Valve (AEV) • Thermostatic Expansion Valve (TEV) • Capillary Tubes


Expansion DevicesConstant pressure or Automatic Expansion Valve (AEV):

•The adjustment spring is usually set such that during off-cycle, the needle is pushed against the orifice. Hence,

•During the off-cycle, the refrigerant will vaporize in the evaporator as a result the evaporator pressure rises during the off-cycle .

•During on- refrigerant is not fed to evaporator while the compressor removes the refrigerant from the evaporator and hence, the evaporator pressure Pe starts decreasing at a very fast rate since valve is closed


Expansion DevicesThermostat Expansion:•A thermostatic expansion valve maintains a constant degree of superheat at the exit of evaporator


Capillary tube• It is a fixed restriction type device.• 0.5m to 5m length and 0.5mm to 2.25mm diameter.• Pressure drop due to friction and length of tube• Advantages are simplicity, low cost, absence of moving parts.

EVAPORATOR:Flooded Type Evaporator• The amount of liquid refrigerant

circulated through the evaporator is considerably in excess of that which can be vaporised.• Float valve is used for maintaining

the constant liquid level.• Accumulator is used to prevent

liquid carry over to compressor.• Give high rate of heat transfer • Used in large installations, chemical

and food processing industries.

Dry or direct expansion type evaporator• Amount of liquid refrigerant fed into the evaporator is limited to

that which can be completely vaporised by the time it reaches the end of evaporator.• Mostly used in producing refrigeration and having capacity

below 150 tonnes of refrigeration. Vapours

Liquid- Vapours

Liquid

Bare tube evaporator• Usually constructed of either steel pipe or copper tubing.• Copper tubing is used in small evaporators using freon as refrigerant

whereas steel pipe is employed with large evaporators using ammonia as refrigerant.• Used in household refrigerators.• Principle same as dry expansion type evaporator.

Plate surface Evaporator• Made in different designs• Tubing is attached to the plates whereas in other cases two sheets

may be pressed together to provide a path for refrigerant flow between them.• Used in domestic refrigerators or frozen food industry.

Finned Tube evaporator• Finned coils are bare tube coils upon which metal plates or fins have

been installed.• Fins increase the surface area and heat transfer rate• Best suited for window type AC and other air-cooling application

where temperature is maintained above 1˚C.• Adding a fan to the finned evaporator increases the performance.

15783 components vc_rs

Engineering

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