Rotary and liquid rim compressor

ROTARY COMPRESSOR AND LIQUID RIM

COMPRESSOR

By Shayak Das -015

& Sumon Roy -014

B.Power Engg.

Jadavpur University

3

How Rotary Compressor Comes In Market!

The rotary compressor concept was invented before the 1st W. W., due to lack of advanced technology, it was only

about 20 years ago that the first rotaries appeared on the market. It has since almost completely replaced the

reciprocating type. All new air-conditioning units that are manufactured around the world in 2kW to 7.5kW

cooling capacity feature rotaries, so it is only a matter of time till “reciprocating” will reside in our memories only.

HITACHI was the first leading manufacturer that moved rotaries from the design board to mass production.

Rotary’s success is contributed to several factors: they are smaller and lighter, they are more efficient, more

resistant to liquid refrigerant presence and they tolerate higher working temperature. This has not been achieved

merely by the way the vapour gets compressed, but rather that the whole concept of a hermetic compressor has

been re-designed.

How Rotary Compressor is different from Reciprocating Compressor?

There is basic differences between rotary and reciprocating compressor.

1)A reciprocating air compressor works much like the engine in your car. An electric motor turns the crankshaft which

causes the piston to rise & fall within the cylinder, compressing the air.Rotary compressors use two precisely machined

screws to continuously compress the air. Since the screws never change direction, there is less noise, heat & vibration than

with a piston pump.

2)Another difference is that oil is used to seal the gaps between the rotary’s screws during compression while the

reciprocating pumps have piston rings that will eventually wear out.

3)A reciprocating (piston) compressor will be ideal for household or small purposes. A reciprocating compressor is not

designed to run continuously. Whereas Rotary compressors built to run continuously, producing a strong yet consistent

airflow.

So a rotary compressor is best for commercial use and when we need a constant demand for air, but it will not work

efficiently if it's cycled on and off too much.

In reciprocating type, returning gas enters the compressor housing and on its way to cylinder(s) it cools down the

casing, electric windings and oil. The vapour then gets sucked via suction reed(s) by a downward moving piston.

The compression begins with an upstroke and ends when the piston reaches TDC and the compressed gas gets

discharged into manifold and out to condenser.

Weakness of this design is its vulnerability for liquid refrigerant related damages. Especially floodback: during an

on-cycle when the amount of superheat is insufficient the droplets of non-evaporated refrigerant pour in. At first

they evaporate and by doing so, the compressor’s temperature descends. The lower the temperature, the more

droplets escape evaporation and soon they mix with oil, wearing all bearing surfaces. Many of these droplets get

sucked into cylinders. Here they position themselves on top of the upward moving pistons and smash the valve

reeds at the speed of a Boeing. The impact creates disastrous pressure in all directions and inflicts multiple

damages. Dented suction valve reeds damaged piston crowns and conrods pins are the usual casualties. During the

off-cycle, on the other hand, the refrigerant migrates to the crankcase and creates havoc on start-up (Liquid

Washout, Slugging). Unfortunately, refrigerant migration is bad news for all compressors.

In rotary compressor: the returning gas first enters a suction line liquid accumulator that is fixed to the

compressor’s casing and from there it gets sucked directly into the compression chamber. The suction line liquid

accumulator’s primary function is to compensate during the reversed cycle for the volumetric difference between

the condensing and evaporating coils. However, it successfully doubles, as a liquid inhibitor. In

the chamber, an eccentric shaft rotates a roller in a cylinder. Extremely high tolerances - .01 mm made it

possible to do away with sealers. As the roller (also called an impeller or a rolling piston) revolves, the blade traps

quantity of vapour and compresses it into an ever-decreasing space, building up pressure and temperature. Finally

the vapour is discharged through the exhaust port into the compressor housing. Here the hot gas “cools down” the

electric windings and oil. These components are designed to work happily in 100°C to 120°C. The practical

implication of the design is twofold: revolving way of compression cause that the compressing mechanism is more

Different Types of Rotary Compressors

1)Rolling piston or fixed vane type compressors: These type of compressors are

used in small refrigeration systems (upto 2 kW capacity) such as domestic refrigerators or air conditioners. These

compressors belong to the class of positive displacement type as 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 center line 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

As shown in Figure

this type of compressor does not require a suction valve but requires a discharge valve. The sealing between the

high and low pressure sides has to be provided:

1.Along the line of contact between roller and cylinder block

2.Along the line of contact between vane and roller, and

between the roller and end-pates

3.The leakage is controlled through hydrodynamic sealing and matching between the mating components. The

effectiveness of the sealing depends on the clearance, compressor speed, surface finish and oil viscosity. Close

tolerances and good surface finishing is required to minimize internal leakage.

Unlike in reciprocating compressors, the small clearance volume filled with high-pressure refrigerant does not

expand, but simply mixes with the suction refrigerant in the suction space. As a result, the volumetric

efficiency does not reduce drastically with increasing pressure ratio, indicating small re-expansion losses. The

compressor runs smoothly and is relatively quiet as the refrigerant flow is continuous.

2)Multiple vane type compressor - In multiple vane type compressor, the axis of rotation

coincides with the center of the roller (O), however, it is eccentric with respect to the center of the cylinder

(O’). The rotor consists of a number of slots with sliding vanes. During the running of the compressor, the sliding

vanes, which are normally made of non-metallic materials, are held against the cylinder due to centrifugal

forces. The number of compression strokes produced in one revolution of the rotor is equal to the number of

sliding vanes, thus a 4-vane compressor produces 4 compression strokes in one rotation.

In these compressors, sealing is required between the vanes and cylinder, between the vanes and the slots on

the rotor and between the rotor and the end plate. However, since pressure difference across each slot is only a

fraction of the total pressure difference, the sealing is not as critical as in fixed vane type compressor.

This type of compressor does not require suction or discharge valves, however, check valves are used on discharge side to prevent reverse rotation during off-time due to pressure difference. Since there are no discharge valves, the compressed refrigerant is opened to the discharge port when it has been compressed through a fixed volume ratio, depending upon the geometry. This implies that these compressors have a fixed built-in volume ratio. The built-in volume ratio is defined as “the ratio of a cell as it is closed off from the suction port to its volume before it opens to the discharge port”. Since no centrifugal force is present when the compressor is off, the multiple vanes will not be pressed against the cylinder walls during the off-period.

high pressure refrigerant from the discharge side can flow back into the side and pressure equalization between high and low pressure sides take place. This is beneficial from the compressor motor point-of-view as it reduces the required starting torque.

Cylinder block

Sliding vanes

O’

O

Suction Discharge

3)Rotary screw compressors:-

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

a) Twin-screw compressor:- 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. 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. When one lobe-gully combination begins to unmesh the opposite lobe-gully combination begins to mesh.

If the built-in pressure at the end of compression is less than the condensing pressure, high pressure refrigerant from discharge manifold flows back into the interlobe space when the discharge port is uncovered. This is called as undercompression. On the other hand, if the built-in pressure at the end of compression is higher than the condensing pressure, then the compressed refrigerant rushes out in an unrestrained expansion as soon as the port is uncovered (over-compression). Both under-compression and over-compression are undesirable as they lead to loss in efficiency.

Lubrication and sealing between the rotors is obtained by injecting lubricating oil between the rotors. The oil also helps incooling the compressor, as a result very high pressure ratios (upto 20:1) are possible without overheating the compressor.

b)Single-screw compressors:- As the name implies, single screw compressors consist of a single helical screw and

two planet wheels or gate rotors. The helical screw is housed in a cylindrical casing with suction port at one end and

discharge port at the other end. Suction and compression are obtained as the screw and gate rotors unmesh and mesh. The

high and low pressure regions in the cylinder casing are separated by the gate rotors. The single screw is normally driven

by an electric motor. An oil separator, oil cooler and pump are required to circulate the lubricating oil.

4)Scroll compressors:- Scroll compressors are orbital motion, positive displacement type compressors, in

which suction and compression is obtained by using two mating, spiral shaped, scroll members, one fixed and the other

orbiting.The compression process involves three orbits of the orbiting scroll. In the first orbit, the scrolls ingest and trap

two pockets of suction gas. During the second orbit, the two pockets of gas are compressed to an intermediate pressure. In

the final orbit, the two pockets reach discharge pressure and are simultaneously opened to the discharge port. This

simultaneous process of suction, intermediate compression, and discharge leads to the smooth continuous compression

process of the scroll compressor. One part that is not shown in this diagram but is essential to the operation of the scroll is

the anti-rotation coupling. This device maintains a fixed angular relation of 180 degrees between the fixed and orbiting

scrolls. This fixed angular relation, coupled with the movement of the orbiting scroll, is the basis for the formation of gas

compression pockets.

Liquid Ring Compressor

Liquid Ring Compressors are a special type of Rotary compressor. Two versions of Liquid Ring Compressor are available:-

1. Single stage &

2. Two stage.

The single stage compressors reach a discharge pressure of 5.5 bar the two stage units go up to 14 bar. Conical gas distributors

permit large inlet gas passage openings, minimising capacity losses. To balance out the radial forces on the compressor shaft

the machines are built in double acting construction. The rotor is arranged overhung requiring only one shaft seal. The

bearings are located in an externally accessible bearing housing and are grease lubricated. The rotor turns inside the casing

without metal-to-metal contact minimising wear.

.

Working Principle In Liquid Ring Compressor, the rotor is positioned centrally in an oval-shaped body. By the

rotation,which proceeds without metal to metal contact, a ring of liquid is formed which moves with the rotor and follows

the shape of the body. At the two points of the nearest proximity of the rotational axis and body, this completely fills the

chambers of the rotor and as rotation proceeds, it follows the contour of the body and recedes again, leaving spaces to be

filled by the incoming gas. These spaces are connected via the cone porting to the inlet of the compressor. As a result of

the suction action thus created, gas is pulled into the compressor. As the rotation progresses, the liquid is forced back into

the chambers, compressing the gas. This gas is forced out of the discharge port and then leaves the compressor via the

outlet flange. The compressor is fed continuously with liquid which serves to seal the clearances between inlet and

discharge port and remove the heat of compression. This liquid leaves the compressor together with the gas to be

compressed and is separated from the gas in a discharge separator.

Conclusion This is the whole about Rotary Compressor and Liquid Rim Compressor. In Heat Power Lab,we did

the experiment on reciprocating compressor and measures the performance means brake thermal efficiency,volumetric

efficiency etc. Here we discuss about the rotary types compressor and by doing that we knew where are the main

difference between them. We also discuss about the special type Liquid ring compressor. Rotary compressors are now-a-

days widely used in industries as well as commercially for its advantages.

Acknowledgement We are very much thankful to Our Professor Ad sir to choose this type excellent

topics for us. It will definitely help us in our practical field. Not only that We are also very much thankful to

Professor Dr. Amitava Dutta, Our lab teacher Mithun Das, Pritam Kumar Das and other lab assistant and also

our other batch mates for helping us to complete our presentation. This will be remain as a life time benefit for us.

References1.R&AC Lecture 2014 Rotary Compressor.pdf

2. SB_Serie112 liquid ring compressor and its principle.

3.Nash_Compressor manual,a gardner Denver product, www.gdnash.com.

4.Compressor world ,www.powershow.com.

5.www.directindustry.com.

6.en.m.Wikipedia.org// liquid ring compressor .

7. en.m.Wikipedia.org// Rotary Compressor.

8.Engineering Thermodynamics – P.K.Nag.

Thank You