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8/6/2019 3.Compressed Air System
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Screw compressor
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Centrifugal Compressor
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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 =
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Energy Efficiency practices incompressed air systems
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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.
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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.
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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.
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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.
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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