Refrigerant Compressors

Abhay Deep Singh 2007ME10466

Kishor Kumar 2007ME10498

Danish Sharma 2007ME10483

MEL 344

Refrigeration and

Air Conditioning

Classification of Compressors

Compressors

Positive Displacement

Reciprocating

Piston

Diaphragm

Rotary

One shaft

Sliding vane

Liquid ring

Scroll

Two shaft

Roots

Twin screw

Stepped rotor

Tooth

Three shaft

Aerodynamic

Centrifugal

Axial

Ejector

Positive Displacement

• Positive admission and delivery ,no flow reversal, (valves , etc. used)

• Intermittent operation

• Necessary discharge pressure can be developed by the application of force.

• ‘Surge’ does not take place here.

• Reciprocating and Rotary compressors come under this category Bicycle pump

Aerodynamic compressors

• Also called dynamic or non positive displacement compressor

• Continuous operation

• Pressure developed depends on the kinetic energy of the inlet fluid

• Flow reverses when pressure developed is less than discharge pressure

• Surge may occur.

• Axial , centrifugal are some of the examples.

Reciprocating compressors

•W = 1∫2 pdV +

2∫3 pdV + 3∫4 pdV +4∫1 pdV = Area 1-2-3-4

• V1 – V4 = Vp

• V1 – V3 = Vs

I. Piston II. Diaphragm

Piston Compressor Types

• Single acting

• Double acting

Piston Compressor types

• Trunk piston compressors

• Crosshead piston

compressors

Thermodynamics of Compression

• w = f(T1,p2/p1,n)

• isothermal work (n=1) :

P1v1 ln(p2/p1)

• Isentropic work (n=γ) :

(γ/(γ-1))p1v1[(p2/p1)(γ-1)/γ – 1]

• Polytropic work : (n=n)

• Isothermal Efficiency : ηt = Isothermal work / actual work

• Adiabatic Efficiency : ηa = Isentropic work / actual work

Volumetric Efficiency

• ηv = Vs/Vp = 1+c-c(p2/p1) 1/n

• P2/pmin = (1/c + 1)γ

Effect of valve pressure drops:

ηv = (1+c)(ps/p1)1/n - c(pd/p1)1/n

Leakage loss

Overall efficiency

ηv = (1+c)(ps/p1)1/n - c(pd/p1)1/n

+ 0.015r

Performance characteristics

Capacity Control

I. Speed variation

• discharge pressure

independent of speed hence power consumption reduces at reduced capacity

• Efficient for reciprocating compressors.

0 20 40 60 80 100

capacity %

Po

we

r %

0

20

4

0

6

0 8

0

1

00

Contd..

II. Valve Lifting

• Method of unloading the cylinders

• depressors or fingers are used

Single acting:

single cylinder 100% and 0%

Double cylinder 100% , 50%and 0%

Double acting:

Single cylinder 100% ,50% and 0%

And so on

contd…

Valve Lifting• Suction valve opened during compression stroke

with depressors or fingers .

• No work is involved except the friction work during idling

• Finger type unloading there is a series of fingers housed in an assembly

that are actuated by a push rod from an outside actuator

• Fingers are lowered that depress the valve sealing components and hold the valve in open position

• It directly acts on the valve sealing components so it might damage them

• Plug and Port type are also used.

Contd…

III. Clearance Pockets• Volumetric efficiency depends upon clearance

• higher the clearance – lower the volumetric efficiency and hence lower the mass flow rate

• We can have stepped clearance pockets for variable capacity control

Contd..

III. Clearance pockets

• expansion and compression processes

have efficiency less

than 100% which results in

off -load power to be a

substantial value. ideal

0 20 40 60 80 100

capacity %

po

we

r %

0 2

0

40

60

80

10

0

Contd.

IV. Compressor by-passing• When evap pressure falls below a certain value, bypass

valve opened , hot discharged refrigerant flowing back into suction

• the capacity of the compressor can be regulated quite closely

• little or no reduction in compressor power consumption at reduced refrigeration capacities

• excessive superheating of the suction gas resulting in overheating of the compressors.

• Only for small compressors• Overheating of the compressor can be reduced by sending

the hot bypass gas to the evaporator inlet - two advantages

Contd..

Contd…

V. Cyclic on-off operation• A gas receiver is at the end of discharge line

• When the receiver reaches pre-set level of pressure the compressor power supply is cut off.

• It saves power but it is generally used in small scale applications as continuous on-off leads to motor heating

• In large applications , it is limited to the number of starts per hour – typically 3 to 4.

Compressor temperature and pressure limitations

Temperature

• Discharge temperature should be low so that suction volume is larger for maximum compressor performance.

• The compressor should be covered by cooling jacket to avoid temperature distortions (especially gases with high γ)

• Excessive temperature can cause piston seizure

• Synthetic lubricants can be used

• Typical maximum temperature is 150⁰ C (302⁰F)

Pressure

These can be used for a wide range of discharge pressures varying from 1 bar to 1000 bars.

Other parameters

Piston Speed

The materials of construction and the type of lubricants determine the reliable maximum speed.

Typical piston speed limitations :

maximum speed (m/s)

Lubricated cylinder in vertical , ‘V’ or ‘W’ configuration 4.3

Lubricated cylinder in horizontal configuration 3.8

Non-lubricated cylinders in vertical , ‘V’ or ‘W’ configuration 3.5

Non –lubricated cylinder in horizontal configuration 3.0

Compressor Size

Generally they come under the range 3000-5000 m³/hr

Some of them also available at 20000 m³/hr

Scroll

Compressors

Scroll Compressors

• Scroll compressors are orbital motion, positive-displacement machines that compress with two inter fitting , spiral-shaped scroll members (ASHRAE 2004).

• Structure – fixed scroll and orbiting scroll

• Two types : compliant design & Non-compliant design

Compression Process

• Built-in pressure ratio = (Built-in volume ratio) (cp/cv)

Efficiencies

• Compared with equivalent up-to-date reciprocating designs of similar sizes.

1 2 3 4 5 6pressure ratio

Vo

lum

etri

c e

ffic

ien

cy

%0

2

0

40

6

0

8

0

10

0

Capacity control

I. speed variation

Good method – wide range of speeds

Standard frequency inverters are available foe small power inputs which will provide a wide range of speeds from a standard AC motor

In compliant design there may be minimum speeds below which effective sealing may be lost.

No difficulty with non-compliant design

Contd…

II. Variable-Displacement

• Porting holes in the fixed scroll member. The control mechanism disconnects or connects compression chambers to the suction side by respectively closing or opening the porting holes.

• When all porting holes are closed, the compressor runs at full capacity; opening all porting holes to the suction side yields the smallest capacity..

Summary

• Used in wide variety of refrigeration and HVAC applications

• Manufactured in variety of sizes up to 25 tonnes

Advantages

• High efficiency no clearance volume as compared to reciprocating compressors

• Few moving parts , low vibration

• Low noise levels

Disadvantages

• Difficult to disassemble in field for maintenance

Centrifugal Compressors

Centrifugal compressors

• They are called “dynamic” machines because they continuously exchange angular momentum between a rotating mechanical element and a steadily flowing fluid.

• Because their flows are continuous, turbomachines have greater volumetric capacities, size for size, than do positive-displacement devices

• Ability to produce a high pressure ratio(upto 8:1) and mass flow rates of upto 15m3/sec

• Can be single or multi stage( for process refrigeration)

Centrifugal compressor

Main components:

i. Inlet casing

ii. Impeller

iii. Diffuser

iv. Volute casing

Work done in compression =

(outlet ang. Momentum-inlet ang. Momentum)*ω

Reaction of an impeller

• The ratio of pressure-producing work to total work is known as the impeller reaction. Some of work increasing the velocity of flow.

• Ψ varies from about 0.4 to about 0.7.

Isentropic analysis

W = (h2o– h10

) = (h4 – h1) + (c42 – c1

2)/2

• ήs varies from about 0.62 to about 0.83, depending on the application. The same compressor produces different adiabatic results with different refrigerants and with the same refrigerant at different suction conditions. A compressor’s overall adiabatic efficiency does not completely indicate its individual stage performance.

• This efficiency cannot be used in case we have multistaging.

Compressor characteristics

Suppose we are operating at

point D. We decrese the mass

flow rate , pressure decreses at

compressor outlet .In case the

pressure in discharge tube

downstream doesn’t fall quickly

enough. The flow tends to

reverse due to adverse pressure

gradient. The pressure ratio

keeps on decreasing. No

stability in region where slope of

this curve is positive.

Surging

Surging

• Part-load range is limited (on the left side of the performance map) by a surge envelope.

• In surging refrigerant alternately flows backward and forward through the compressor, accompanied by increased noise, vibration, and heat.

• Prolonged operation under these conditions can damage the compressor.

• Surging can be distinguished from other kinds of noise and vibration by the fact that its flow reversals alternately unload and load the driver. Motor current varies markedly during surging.

Characteristic curve

ηp = ∆hois/∆ho ∆ho = h40

– h10= (h4 + c4

2/2) - h10

∆hois= h40is

– h10= (h4is + c4is

2/2) - h10

Pressure ratio developed in a centrifugal stage

• Overall, polytropic work and efficiency are more consistent from one application to another because they represent an average stage aerodynamic performance.

System balance and capacity control

• Normally refrigerant compressors are designed to take care of the most severe operating conditions cooling load is high and heat sink temperature.

• However, when the operating conditions are not so severe, i.e., when the cooling load is low and/or the heat sink temperature is low, then the compressor becomes oversized.

• If no control action is taken, then the compressor adjusts itself by operating at lower evaporator temperature, which may affect the refrigerated space

temperature.

• Operating at low evaporator temperature may lead to other problems such as

1. Low air humidity,

2. frosting of evaporator coils

3. freezing of the external fluid

Besides affecting the temperature of

refrigerated space

• In order to maintain almost the same evaporator outlet conditions and condensor conditions with decreasing load , we need to control the mass flow rate of refrigerant, or the volume flow rate and decrease the work input also

• Capacity control is done by 2 ways-

1. Variable Speed drive(VSD)

2.Variable inlet whirl vanes

System characterstics

Hot gas bypass

• E, F,G,H are non attainable points beyond surge envelope.

• ADD hot gas from the compressor discharge to the evaporator, or compressor suction piping. When hot-gas bypass is used, no further power reduction occurs as load decreases. The compressor is artificially loaded to stay out of the surge envelope. The increased volume caused by hot-gas recirculation performs no useful refrigeration.

Variable inlet whirl vanes

• Prerotation vanes modulate capacity by altering the direction of the fluid flow entering the impeller relative to the impeller blade leading edge.

.

Compressor characterstics

1.The theoretical curve of a compressor can bederived from the velocity triangles.

2. A spin-free inflow is to be present.

3.Furthermore we take the assumption ofequal c m (which is characterizing for the flowrate) at entry and exit.

Assuming ϐ1= 0 Assuming cr1=cr2

W= ∆angular momentum * ω

W= (u2 * (u2 - cr1cotϐ2)) - (u1

2) (1)

V=A1*cr1 (2)

W= (u22 - u2 *V/A1/cotϐ2)) - (u1

2) (3)

W=a - b*V

• Above expression can be written as

• W= c * r(n-1)/n - d (4)

• Comparing (3) and (4) we get

• A - B*V = r(n-1)/n

The straight line , which is derived by this, is only correct for an infinite number of blades,

which means it is a theoretical straight line. If you look upon a limited number of blades the

lift of delivery is reduced.

• If you look upon a limited number of blades the lift of delivery is reduced about Δyz.

• The friction losses Δyr cause a further sinking of the characteristic curve, which becomes the larger, the higher the flow rate is.

• The collision losses, which result from an unfavourableincident flow of the profile nose, are considered by Δyst. The collision losses become the larger the further away by the design point the machine is operated.

• Finally the clearance losses cause a further shift of the characteristic around ΔV, which causes an additional waste of the efficiency.

Rotary compressors

• These are used primarily for lower refrigeration capacities ( e.g 1 ton).

• Small and compact in size

• They are of two types

1. Rolling pisten/fixed vane

2. Rotary vane types

• Consists of a roller mounted on the eccentric of a shaft with a single vane or blade suitably positioned in the non rotating cylindrical housing, generally called the cylinder block.

• Produces large torsional vibrations.

• Rotary compressors have a high volumetric efficiency

Rotary vane compressor

• Rotary-vane compressors have a low mass-to-displacement ratio, which, in combination with compact size, makes them suitable for transport application.

• Capacity of order of 2 to 40 kW

• The eight discrete volumes are referred to as cells. A single shaft rotation produces eight distinct compression strokes. Although conventional valves are not required for this compressor, suction and discharge check valves are recommended to prevent reverse rotation and oil logging during shutdown

• Built in compression ratio.

Rotary Screw Compressors

Rotary screw compressors

• Use a rotary type positive displacement mechanism.

• One rotating or two counter rotating intermeshed helical screw elements housed within a specially shaped chamber.

• As the mechanism rotates, the meshing and rotation of the two helical rotors produces a series of volume-reducing cavities. Gas is drawn in through an inlet port in the casing, captured in a cavity, compressed as the cavity reduces in volume, and then discharged through another port in the casing

Rotary screw compressors

• The effectiveness of this mechanism is dependent on close fitting clearances between the helical rotors and the chamber for sealing of the compression cavities.

Twin screw compressors

• The twin-screw type compressor or twin-screw blower is a positive displacement type device that operates by pulling air through a pair of meshing high-tolerance screws similar to a set of worm gears.

• The intake is located at one end of the two screws, which overlap, but not completely, leaving a small pocket open.

• As the air moves axially through the housing, space becomes smaller and the screw continues to pressurize the air charge. This air gets released via the built in pressure ratio discharge, which holds the compressed air to a specified pressure before exiting the compressor

Twin screw compressors

Twin screw compressors

A twin-screw

compressor consists

of accurately

matched rotors (one

male and one

female) that mesh

closely when rotating

within a close

tolerance common

housing. One rotor is

driven while the other

turns in a counter-

rotating motion.

Single screw compressors

• A single-screw compressor uses a single main screw rotor meshing with two gate rotors with matching teeth. The main screw is driven by the prime mover, typically an electric motor

• The screw-like grooves gather vapors from the intake port, trap them in the pockets between the grooves and compressor housing, and force them to the discharge port along the meshing point path. This action raises the trapped gas pressure to the discharge pressure.

Single screw compressors

Screw

compressor

s have the

capability

to operate

at pressure

ratios

above 20:1

single

stage

Single screw compressors

Oil-flooded screw compressors

• Oil is injected into the compression cavities to aid

sealing and provide cooling sink for the gas charge.

• The oil is separated from the discharge stream,

then cooled, filtered and recycled.

Oil-flooded screw compressors

• It is usual for some entrained compressor oil to carry over into the compressed gas stream downstream of the compressor. In some applications, this is rectified by filter vessels.[2]

• In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be removed from the compressed air system via condensate management equipment.

• Standard oil-flooded compressors are capable of achieving output pressures over 200 psig, and output volumes of over 1500 cubic feet per minute (measured at 60 °C and atmospheric pressure).

Oil-free screw compressors

• the air is compressed entirely through the action of the screws, without the assistance of an oil seal.

• They usually have lower maximum discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psig, and output volume of over 2000 cubic feet per minute (measured at 60 °C and atmospheric pressure).

Under compression

• If the internal volume ratio is too low for a given set of operating conditions, the gas pressure in the trapped volume remains below the desired discharge pressure.

• the discharge port opens The high pressure gas from the discharge line flows back into the flute space, equalizing the flute pressure that must be compressed again

Under compression

Extra work

is required

to

compress

the same

gas twice.

Over compression

• If the internal volume ratio of the compressor is too high for a given set of operating conditions the discharge gas remains trapped longer, leading to pressure rise above the discharge pressure .

• In this case, the gas is compressed to a value above the discharge pressure and when the discharge port opens, the high pressure gas in the screw flutes expands out of the compressor into the discharge line.

Over compression

This takes more

energy than that

required if the

compression had

been stopped

sooner when the

internal pressure

was equal to the

system discharge

pressure.

Capacity control

• Compressors that have only axial ports will have fixed volume ratios. The built-in volume ratio can be changed by changing the size or location of the discharge port.

• Another way to change the volume ratio is with the help of a slide valve. For these compressors, the volume ratio can be varied according to requirements while the compressor is in operation, to get better part-load efficiency.

Capacity control

• Capacity is controlled with slide valves. The design uses a slide valve that opens to vent some gas back to the suction port,

reducing both the net gas flow and power input.

Capacity control

A slide valve for

capacity control

of a screw

compressor:

(a) its position

relative to the

rotors,

(b) slide at full-

capacity position,

and

(c) slide at

reduced-capacity

position.

Capacity control

It can be seen that

for a given built-in

volume ratio, the

efficiency reaches a

peak at a particular

optimum pressure

ratio. The value of

this optimum

pressure ratio

increases with built-in

volume ratio

References

• www.google.com

• www.wikipedia.org

• Refrigeration Handbook ASHRAE

• Industrial Compressors – P.A. O’Niell

• Refrigeration and Air-Conditioning – C.P. Arora