3.Compressed Air System

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Screw compressor



Centrifugal Compressor



Compressor efficiency

¼½

»¬«

¹ º

¸©ª

¨v 1-P

P 0.612/PQ1-K NK / NK

1-K

s

ds

N = No. of stagesK = Ratio of specific heats (1.35 for air)

Ps = suction pressure in kg/cm2

Pd = Discharge pressure in kg/cm2

Q = Actual air flow (m3/min.)

Actual kW = � 3 V I v PF as measured

Efficiency of compressor and motor combination =kWActual

kWlTheoretica100 v

Theoretical kW =



Energy Efficiency practices incompressed air systems



Effect of Intake Air temperatureon Power Consumption

Inlet

Temperature (0C)

Relative Air

Delivery (%)

Power Saved

(%)

10.0 102.0 + 1.4

15.5 100.0 Nil

21.1 98.1 - 1.3

26.6 96.3 - 2.5

32.2 94.1 - 4.0

37.7 92.8 - 5.0

43.3 91.2 - 5.8

Every 40C rise in inlet air temperature results in a higher energy consumption by 1 % to

achieve equivalent output. Hence, cool air intake leads to a more efficient compression.



Air Inlet Filter on Power

Consumption

Pressure Drop

Across air filter

(mmWC)

Increase in Power

Consumption (%)

0 0

200 1.6

400 3.2

600 4.7800 7.0

For every 25 mbar pressure lost at the inlet due to choked filters, the

compressor performance is reduced by about 2 percent.



ElevationPerce tage elative

Vol metric Efficie cy

om are ith ea Levelltit e

Meters

Barometric

Press re

Mbart 4 bar t 7 bar

Sea level 1013 100.0 100.0500 945 98.7 97.7

1000 894 97.0 95.2

1500 840 95.5 92.7

2000 789 93.9 90.0

2500 737 92.1 87.0

It is evident that compressors located at higher altitudes consume more power

to achieve a particular delivery pressure than those at sea lvel, as the

compression ratio is higher.



Efficacy of Inter and AfterCoolers

DetailsImperfect

Cooling

Perfect

Cooling

C illed Water

Cooling

1 Stage inlet temperature 0C 21.1 21.1 21.1

2 Stage inlet temperature 0C 26.6 21.1 15.5Capacity (m3/min) 15.5 15.6 15.7

Shaft Power (kW) 76.3 75.3 74.2

Specific energy consumption

kW (m3/min)

4.9 4.8 4.7

Percent Change + 2.1 - - 2.1

It can e seen from t e ta le t at an increase of 5.50C in t e inlet to t e second stage

results in a 2 % increase in t e specific energy consumption. Use of cold water

reduces power consumption.



Cooling WaterRequirement

Com ressor y e

Mi im m q a tity of

Cooli g ater req ire

for 2.85 m3

/mi . F at 7bar (l m)

Single-stage 3.8

T o-stage 7.6

Single-stage ith a ter-cooler 15.1

T o-stage ith a ter-cooler 18.9



Power Reduction throughPressure Reduction

Pressure

ReductionPower Reduction (%)

rom( ar)

To ( ar)

Single-

stageWater-

cooled

Two-stageWater-

cooled

Two-stage Air-

cooled

6.8 6.1 4 4 2.6

6.8 5.5 9 11 6.5

A reduction in the delivery pressure of a compressor would

reduce the power consumption.



Expected Specific Power Consumption ofReciprocating Compressors (based on motor

input)

Press re bar No. of tages ecific Po er

k /170 CMH

1 1 6.29

2 1 9.64

3 1 13.04

4 2 14.57

7 2 18.34

8 2 19.16

10 2 21.74

15 2 26.22



Energy Wastage due toSmaller Pipe Diameter

Pi e

Nomi al

Bore (mm)

Press re ro (bar)

er 100 meters

Eq ivale t o er

losses (k )

40 1.80 9.5

50 0.65 3.4

65 0.22 1.2

80 0.04 0.2

100 0.02 0.1

Typical acceptable pressure drop in industrial practice is 0.3 bar in

mains header at the farthest point and 0.5 bar in distribution system



Discharge of Air throughOrifice

Gauge

Pressure

ar

0.5 mm 1 mm 2 mm 3 mm 5 mm 10 mm 12.5 mm

0.5 0.06 0.22 0.92 2.1 5.7 22.8 35.5

1.0 0.08 0.33 1.33 3.0 8.4 33.6 52.5

2.5 0.14 0.58 2.33 5.5 14.6 58.6 91.4

5.0 0.25 0.97 3.92 8.8 24.4 97.5 152.0

7.0 0.33 1.31 5.19 11.6 32.5 129.0 202.0



Cost of Air Leakage

Orifice Size

mm

KW

Wasted

* nergy Waste

(Rs/Year)

0.8 0.2 8000

1.6 0.8 32000

3.1 3.0 120000

6.4 12.0 480000

* based on Rs. 5 / kWh ; 8000 operating hours; air at 7.0 bar



Heat Recovery

A s noted earlier, compressing air generates heat. In fact,industrial-sized air compressors generate a substantial amount of heat that can be recovered and put to useful work. More than80% of the electrical energy going to a compressor becomesheat. Much of this heat can be recovered and used for

producing hot water or hot air.

Typical uses for recovered heat include supplemental spaceheating, industrial process heating, water heating, makeup airheating, and boiler makeup water preheating. Recoverable heat from a compressed air system is not, however, normally hot

enough to be used to produce steam directly. A s much as 80-93% of the electrical energy used by anindustrial air compressor is converted into heat. In many cases,a properly designed heat recovery unit can recover anywherefrom 50-90% of this available thermal energy and put it touseful work heating air or water



Heat Recovery with Air-Cooled Rotary Screw

Compressors Air-cool ed packaged rotary scr ew compr essors ar e very amenabl e to heat r ecovery for spac e heating or other hot air us es. Ambi ent atmospheric air is heat ed by passing it across the syst em's aft ercool er and lubricant cool er, wher e it extracts heat from both the compr ess ed air and the lubricant that is us ed to lubricat e and cool the compr essor.

Sinc e packaged compr essors ar e typically enclos ed in cabin ets and

alr eady includ e heat exchangers and fans, the only syst em modifications n eed ed ar e the addition of ducting and another fan to handl e the duct loading and to eliminat e any back pr essur e on thecompr essor cooling fan. T hes e heat r ecovery syst ems can bemodulat ed with a simpl e thermostatically-controll ed hinged vent. When heating is not r equir ed -- such as in the summer months -- thehot air can be duct ed outsid e the building. T he vent can also bethermostatically r egulat ed to prov id e a constant t emperatur e for a heat ed ar ea.

Hot air can be us ed for spac e heating, industrial drying, pr eheating aspirat ed air for oil burn ers, or any other application r equiring warm air. As a rul e of thumb, approximat ely 50,000 B tu/hour of en ergy is av ailabl e for each 100 cfm of capacity (at full-load). Air t emperatur es of 30 to 40oF above the cooling air inl et t emperatur e can be

obtain ed. Recovery effici enci es of 80-9 0% ar e common



Steps n s mp e s op- oormethod for leak

quantificationShut off compressed air operated equipments (or conduct test when no equipment is using compressed air).

Run the compressor to charge the system to set pressure of operation

Note the sub-sequent time taken for on load and off load cycles of the compressors. For accuracy, take ON & OFF timesfor 8 10 cycles continuously. Then calculate total ON Time(T) and Total OFF time (t).

The system leakage is calculated asSystem leakage (cmm) = Q v T / (T + t)

Q = A ctual free air being supplied during trial, in cubicmeters per minute

T = Time on load in minutes

t = Time unload in minutes



Leak test: exampleCompressor capacity (CMM) = 35Cut in pressure kg/SQCMG = 6.8

Cut out pressure kg/SQCMG = 7.5

On load kW drawn = 188 kW

Unload kW drawn = 54 kW

Average On-load time = 1.5 minutes Average Unload time = 10.5 minutes

Comment on leakage quantity and avoidable loss of power due toair leakages.

a) Leakage quantity (CMM) =

= 4.375 CMM

b) Leakage per day = 6300 CM/day

c) Specific power for compressed air generation=

= 0.0895 kwh/m3

d) Power lost due to leakages/day = 563.85 kWh

355.105.1

1.5v

CMH6035

k h188

v



Capacity Assessment inShop-floor

Isolate the compressor along with its individual receiver being taken for test

from main compressed air system by tightly closing the isolation valve orblanking it, thus closing the receiver outlet.

Open water drain valve and drain out water fully and empty the receiver and thepipe line. Make sure that water trap line is tightly closed once again to start the test.

Start the compressor and activate the stop watch.

Note the time taken to attain the normal operational pressure P 2 (in the

receiver) from initial pressure P 1.Calculate the capacity as per the formulae given below :

Actual ree air disc arge Min./ NMT

V

P

PP Q 3

0

12v

!

Wher e

P2 = Final pr essur e af er f illing ( g/cm2

a)

P1 = Initial pr essur e ( g/cm2a) af ter bleeding

P0 = Atmospher ic Pr essur e ( g/cm2

a)

V = Stor age volume in m3

which includes r eceiver af ter cooler and deliver piping

T = Time take to build up pr essur e to P2 in minutes

3.Compressed Air System

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