Compressors

Compressor is the most critical component of the VCRS system because

1- it is the major cost equipment 2- it has a great influence on -energy consumption -Reliability -Serviceability of the system

Types of compressors

compressors are categorized by: The seal type -completely sealed or Hermetic -semi-sealed or semi hermetic -open typeThe operation type -Positive displacement compressor -Rotodynamic compressor

Hermetic Compressor

hermetic compressors will have direct connection to the motor, and sealed in a welded casing. This type of construction allows no maintenance except in factories with proper repair tools and skill

Reciprocating

Semi Hermetic compressor

semi-hermetic compressors will have direct connection to the motor. However, the sealing is not a complete one. There will be a provision to dismantle the casing for parts replacement, and maintenance

Open type compressor

open type, has the compressor and driver separated from each other. The compressor and the driver will be separate entities, connected by a coupling. Sealing of refrigerant gas from atmosphere is achieved by means of labyrinth and mechanical seals. Yes, this compressor construction is suitable for high capacity cooling

Compressor Types

Compressors can be classified into1.Positive displacement type -Reciprocating -Rotary -Screw -Scroll2. Roto-dynamic type -Centrifugal -Axial

1. Rotary- Small appliances2. Screw- Large Chillers3. Scroll- Central Air Conditioning; Newer

Chillers4. Centrifugal- Large Chillers5. Reciprocating- Refrigeration and A/C

Systems

Reciprocating compressor

Reciprocating Compressor

The reciprocating compressors comprise of the piston and the cylinder arrangement similar to the automotive engine\

Apart from the piston and the cylinder arrangement, the reciprocating compressor also comprises of the crankshaft, connecting rod and other small connecting elements.

The crankshaft is connected to the electric motor directly by coupling or by belt and driven by the pulley arrangement.

The rotary motion of the crankshaft is converted into the reciprocating motion of the piston inside the cylinder via the connecting rod

Rolling Piston Compressor

The Rolling piston or fixed vane type compressors are used in small refrigeration systems such as domestic refrigerators or air conditioners.

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 

This eccentricity of the shaft with respect to the roller creates suction and compression of the refrigerant . A single vane or blade is positioned in the non-rotating cylindrical block. The rotating motion of the roller causes a reciprocating motion of the single vane. 

Rolling Piston rotary Compressor

Multiple vane rotary compressor

In multiple vane type compressors, the axis of shaft concurs with the centre of the roller, however, it is eccentric with respect to the centre of the cylinder.

The rotor consists of various spaces with sliding vanes. Amid the running of the compressor, the sliding vanes, which are regularly made of non-metallic materials, are held against the chamber because of centrifugal forces.

Multiple vane compressor

Scroll Compressor

The scroll compressor comprises of two interleaved scrolls of which one is fixed and the other orbits eccentrically without rotating. During its motion small gaps are created between the scrolls where the refrigerant gets compressed.

Scroll Compressor

Scroll Compressor

screw compressors: 

 The rotary screw compressors can be either twin-screw type or single-screw type. 

Abundant lubrication and subsequent cooling of the oil using a special heat exchanger guarantees higher compression ratios than reciprocating compressors, meaning wide use in both refrigeration and air-conditioning (large chillers operating on R410A).

Screw Compressor

As it is a positive displacement machine, high pressure refrigerant as in reciprocating compressors such as R22 and ammonia are used in it.

As it is a high speed rotary machine, a large volume, as in centrifugal compressors can be handled by it.

It is therefore found extremely suitable for large capacity low temperature application such as in food refrigeration and also in large capacity central air conditioning plant with R-134a and R22

Single screw type

single screw compressors consist of a solitary helical screw and two planet wheels or gate rotors.

Twin screw type

The twin-screw type compressor consists of two mating helically grooved rotors, one male and the other female. Generally the male rotor drives the female rotor. 

Centrifugal compressors

The fluid is drawn in by the impeller near its axis and due to centrifugal force is pushed to the edge of the compressor casing. The fluid leaves the impeller with significant kinetic energy, which is then converted into pressure energy in the diffuser.

Use is limited to high cooling capacities and low compression ratios (usually large chillers running on R134A)

Evaporators

The evaporator is an equipment of the refrigeration system in which the heat is removed from the air, water or any other body to be cooled by the evaporating refrigerant

Classification of the Evaporators Based on the Construction

1- Finned Evaporators 2-Plate Type of Evaporators 3-Shell and Tube types of Evaporators a) dry expansion type b) flooded type

Finned Evaporators

the bare tube evaporators fitted with the fins, called as finned evaporators

The bare tube evaporators are made up of copper tubing or steel pipes. The copper tubing is used for small evaporators where the refrigerant other than ammonia is used, while the steel pipes are used with the large evaporators where ammonia is used as the refrigerant.

The fins on the external surface of the bare tube evaporators increases the contact surface of the of the metallic tubing with the fluid and increase the heat transfer rate

Finned Evaporator

The finned evaporators are most commonly used in the air conditioners of almost all types like window, split, packaged and the central air conditioning systems.

In these systems the finned evaporator is popularly known as the cooling coil. The hot room air flows over the finned evaporator or the cooling coil, gets chilled and enters the room to produce the cooling effect.

In window a/c the finned evaporators is located behind the beautifully looking grill. In the wall mounted split unit it located behind the front grill of the indoor unit.

Finned evaporator

Plate Type of Evaporators

In the plate type of evaporators the coil usually made up of copper or aluminum is embedded in the plate so as so to form a flat looking surface

Externally the plate type of evaporator looks like a single plate, but inside it there are several turns of the metal tubing through which the refrigerant flows.

The advantage of the plate type of evaporators is that they are more rigid as the external plate provides lots of safety.

The external plate also helps increasing the heat transfer from the metal tubing to the substance to be chilled

Plate Type Evaporator

Further, the plate type of evaporators are easy to clean and can be manufactured cheaply.

The plate type heat exchangers can be easily formed into various shapes as per the requirement.

Thus in the household refrigerators and the deep freezers, where they are used most commonly, they can be converted into the box shape to form the closed enclosure, where various food can be kept in the frozen state

Plate type evaporator

Shell and Tube Type

The shell and tube types of evaporators are used in the large refrigeration and central air conditioning systems.

The evaporators in these systems are commonly known as the chillers.

The chillers comprise of large number of the tubes that are inserted inside the drum or the shell.

Depending on the direction of the flow of the refrigerant in the shell and tube type of chillers,

they are classified into two types: a) dry expansion type b) flooded type of chillers

Dry expansion chiller

In dry expansion chillers the refrigerant flows along the tube side and the fluid to be chilled flows along the shell side.

The flow of the refrigerant to these chillers is controlled by the expansion valve

Direct expansion chiller

Flooded Chiller

In case of the flooded type of evaporators the refrigerant flows along the shell side and fluid to be chilled flows along the tube.

In these chillers the level of the refrigerant is kept constant by the float valve that acts as the expansion valve also.

Condenser

In condensers the refrigerant gives up the heat that is has absorbed in the evaporator.

There are three main types of condensers:

1.air cooled condensers, 2.water cooled condensers 3.evaporative condensers.

Air cooled condensers

Air cooled condensers are used in small units like household refrigerators, deep freezers, water coolers, window air-conditioners, split air-conditioners, small packaged air-conditioners etc

These are used in plants where the cooling load is small and the total quantity of the refrigerant in the refrigeration cycle is small.

Air cooled condensers are also called coil condensers as they are usually made of copper or aluminum coil. Air cooled condensers occupy a comparatively larger space than water cooled condensers.

Air Cooled Condenser

Air cooled condensers are of two types: natural convection and forced convection.

In the natural convection type, the air flows over it in natural a way.

In the forced air type, a fan operated by a motor blows air over the condenser coil.

Forced air cooled

Natural Air Cooled

Water cooled condensers

Water cooled condensers are used for large refrigerating plants, big packaged air-conditioners, central air-conditioning plants, etc.

These are used in plants where cooling loads are excessively high and a large quantity of refrigerant flows through the condenser.

There are three types of water cooled condensers: tube-in-tube or double pipe type, shell and coil type and shell and tube type.

Shell and tube water condenser

Shell and coil water condenser

Double Pipe HX

Evaporative condensers

Evaporative condensers are usually used in ice plants.

In these condensers the hot refrigerant flows through the coils.

Water is sprayed over these coils. At the same time the fan draws air from the bottom side of the condenser and discharges it from the top side of the condenser.

The spray water that comes in contact with the condenser coil gets evaporated in the air and it absorbs the heat from the condenser, cools the refrigerant and condenses it.

Evaporative Condenser

keeping the evaporative condenser clean and free of scale is very difficult and requires lots of maintenance

Compressor Condenser

Metering deviceEvaporator

Direction of Refrigerant Flow

Throttling or Expansion Devices

In the throttling valve the pressure of the refrigerant reduces suddenly and excessively. With this the temperature of the refrigerant also reduces drastically.

This low pressure and low temperature liquid refrigerant then enters the evaporator and absorbs heat from the substance or the space to be cooled.

Meters the correct amount of refrigerant to the evaporator

Installed in the liquid line at the inlet of the evaporator

Throttling devices

Common devices:

1.fixed bore (capillary tube)2.thermostatic expansion valve, 3.Automatic expansion valve,

Less common devices:

1. High-side float, 2. low-side float

Capillary tubeCapillary tube is one of the most commonly used

throttling devices in the refrigeration and the air conditioning systems.

The capillary tube is a copper tube of very small internal diameter.

It is of very long length and it is coiled to several turns so that it would occupy less space.

The internal diameter of the capillary tube used for the refrigeration and air conditioning applications varies from 0.5 to 2.28 mm (0.020 to 0.09 inches).

Capillary tube

Capillary tube

Capillary tubing is used for small refrigerating and air-conditioning systems like household refrigerators, water coolers, deep freezers, window air-conditioners, split air-conditioners, small packaged air-conditioners etc.

The capillary tube is a very simple device that can be manufactured easily and it is not very costly.

Provides a constant flow (or feed) of refrigerant

How Capillary Tube Works?

When the refrigerant leaves the condenser and enters the capillary tube its pressure drops down suddenly due to very small diameter of the capillary.

The decrease in pressure of the refrigerant through the capillary depends on the diameter of the capillary and the length of the capillary.

Smaller is the diameter and more is the length of the capillary more is the drop in pressure of the refrigerant as it passes through it.

TXV/TEV

TEV

Thermostatic expansion valve or TEV is one of the most commonly used throttling devices in the refrigerator and air conditioning systems.

The thermostatic expansion valve is the automatic valve that maintains proper flow of the refrigerant in the evaporator as per the load inside the evaporator.

If the load inside the evaporator is higher it allows the increase in flow of the refrigerant and when the load reduces it allows the reduction in the flow of the refrigerant.

This leads to highly efficient working of the compressor and the whole refrigeration and the air conditioning plant

TEV

the thermostatic expansion valve is used in the systems of higher capacities.

It is commonly used in the industrial refrigeration plants, high capacity split air conditioners, packaged air conditioners, central air conditioners and many other systems

The thermostatic expansion valve has a constriction or orifice due to which the pressure of the refrigerant passing through it drops down suddenly to the level of the evaporator pressure.

Due this the temperature of the refrigerant also drops down suddenly and it produces cooling effect inside the evaporator.

R-134a

Evaporator pressure 18.4 psig

26.1 psig

30°F

Spring pressure = ?

26.1 psig = Ps + 18.4 psig

Ps = 7.7 psig

TXV RESPONSES TO LOAD CHANGES

When load increases

Refrigerant boils faster and the suction line temperature increases

Valve opens to feed more refrigerant to the evaporator

When load decreases

Refrigerant takes longer to boil Valve closes to feed less refrigerant to the evaporator

TXV WITH EXTERNAL EQUALIZER

Used if an evaporator has more than a 2.5 psig drop from inlet to outlet

The evaporator pressure is sensed at the outlet of the coil instead of the inlet

Used to prevent the coil from starving

Connected to the evaporator outlet after the thermal bulb

Used to compensate for pressure drop in the

evaporator

Saturated refrigerant to the evaporator

Liquid refrigerant to the expansion valve

External equalizer line connected to the outlet of the

evaporator coil

Evaporator pressure pushing up on the diaphragm

Diaphragm

Solid brass divider

TEV

Maintains a constant evaporator superheat

Automatic Expansion Valve

Also known as an AEV or a constant pressure valve.

It is not seen as much as the TXV.

The AEV responds to a load change exactly opposite from the TXV.

As the load increases the AEV will start to starve the evaporator, thus maintaining pressure and boiling point.

As the load drops the AEV will begin to open and allow the pressure to remain constant as well as boiling point.

Diaphragm

Needle and Seat

Spring

Liquid refrigerant from condenser or receiver

Saturated refrigerant to the evaporator

Evaporator pressure

Spring pressure

The force that operates an AEV is the evaporator pressure. This is the upward force on the bottom of the diaphragm that tends to close the valve

Systems with AEV’s and most systems with TXV’s should have a receiver to ensure a proper refrigerant flow to the valve.

The systems with a capillary tube will never (or rarely) have a receiver.

The receiver is a type of storage tank to hold extra refrigerant.

Float Valve

The float valve is used as the throttling device in the large refrigeration systems with flooded chiller.

There are two types of float valves:

1.low side float valves 2.high side float valves

Low Side Float Valves

The float valve maintains the constant level of the liquid in the flooded evaporator

The float of the low side float valve is placed in the evaporator, which is at low pressure.

The construction and the working of the low side refrigeration float valve are similar to the float valve used in the water tank used for maintaining the level of the liquid.

Low side Float valve

The hollow ball or float floats on the refrigerant inside the evaporator and moves up and down as per the level of the liquid.

The hollow ball is connected to the needle and valve seat via the float arm.

Thus as the ball moves up and down the float arm also moves that allows for the opening or the closing of the orifice.

High Side Float Valve

The refrigerant condensed in the condenser moves to the chamber of the high pressure float valve.

As the level of the refrigerant rises the float ball moves up and opens the float valve that allows for the passage of the refrigerant through needle valve.

The level of the refrigerant would rise in float chamber when more refrigerant is coming from the condenser that means there is more load on the plant.

Thus when there is higher load on the plant there is increase in the flow of the refrigerant through the float valve

Cascade system

each cascade uses a different refrigerant, it is possible to select a refrigerant that is best suited for that particular temperature range.

Very high or very low pressures can be avoided

Migration of lubricating oil from one compressor to the other is prevented

Cascade Systems

In a cascade system a series of refrigerants with progressively lower boiling points are used in a series of single stage units.

The condenser of lower stage system is coupled to the evaporator of the next higher stage system and so on

The component where heat of condensation of lower stage refrigerant is supplied for vaporization of next level refrigerant is called as cascade condenser

this system employs two different refrigerants operating in two individual cycles

They are thermally coupled in the cascade condenser. The refrigerants selected should have suitable pressure-

temperature characteristics. An example of refrigerant combination is the use of carbon

dioxide (NBP = -78.4 oC, Tcr= 31.06 oC) in low temperature cascade

and ammonia (NBP = -33.33oC, Tcr= 132.25 oC) in high temperature cascade

Applications of cascade systems: 1.Liquefaction of petroleum vapours 2.Liquefaction of industrial gases 3.Manufacturing of dry ice 4.Deep freezing etc.

air conditioner compressors are categorised. These are categorised by,

the seal type (e.g. completely sealed, semi-sealed and open type), and

the operation type (e.g. piston, scroll, rotary and centrifugal)

Sealing of compressors:There are three types of air conditioner

compressors construction. Thecompletely sealed, semi-sealed, and the open type.

Completely sealed compressors are also known as the hermetic type, and the semi-sealed are known as the semi-hermetic type.

hermetic compressors will have direct connection to the motor, and sealed in a welded casing. This type of construction allows no maintenance except in factories with proper repair tools and skill

semi-hermetic compressors will have direct connection to the motor. However, the sealing is not a complete one. There will be a provision to dismantle the casing for parts replacement, and maintenance

open type, has the compressor and driver separated from each other. The compressor and the driver will be separate entities,connected by a coupling. Sealing of refrigerant gas from atmosphere is achieved by means of labyrinth and mechanical seals. Yes, this compressor construction is suitable for high capacity cooling

Air conditioner compressors for residential, and for average-large sized

office buildings, will normally be of hermetic, and semi-hermetic construction.

Reciprocating and rotary-screw compressors are common in food manufacturing operations that use ammonia refrigeration. Reciprocating compressors generate pressure from pistons that compress the refrigerant within a cylinder

While a piston-type, reciprocating compressor is still popular in a variety of refrigeration systems, it isn’t the most efficient type of compressor, due to the clearance space required at between the top of the piston and the valve plate.

A rotary compressor that is often found on smaller refrigeration systems such as household refrigerators, drinking fountains operates more efficiently than a piston compressor since it performs both the suction and discharge processes simultaneously.

A rotary compressor is more efficient than a piston compressor because it can do two things at once…suction and discharge.

Scroll Compressor

The scroll compressor comprises of two interleaved scrolls of which one is fixed and the other orbits eccentrically without rotating. During its motion small gaps are created between the scrolls where the refrigerant gets compressed.