Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 7 | 399

much money will be saved as a result of reducing the pressure of the

compressed air.

Solution It is observed that the compressor of a facility compresses the air

to much higher pressures than needed. The cost savings associated with

pressure reduction of the compressor are to be determined.

Assumptions 1 Air is an ideal gas. 2 Compression process is isentropic, and

thus nk1.4.

Analysis The fraction of energy saved as a result of reducing the pressure

setting of the compressor is

That is, reducing the pressure setting will reduce the energy consumed by the

compressor by about 6 percent. Then,

Therefore, reducing the pressure setting by 100 kPa will result in annual

savings of $720 in this case.

Cost savings 1 Current cost 2 freduction 1 $12,000>yr 21 0.06 2 $720/yr

 1 

1 885.6>85.6 21 1.4^1 2>1.4 1

1 985.6>85.6 21 1.4^1 2>1.4 1

0.060

freduction 1 

1 P2,reduced>P 121 n^1 2>n 1

1 P 2 >P 121 n^1 2>n 1

There are also other ways to reduce the cost of compressed air in industrial

facilities. An obvious way is turning the compressor offduring nonproduc-

tion periods such as lunch hours, nights, and even weekends. A considerable

amount of power can be wasted during this stand-by mode. This is especially

the case for screw-type compressors since they consume up to 85 percent of

their rated power in this mode. The reciprocating compressors are not

immune from this deficiency, however, since they also must cycle on and off

because of the air leaks present in the compressed-air lines. The system can

be shut down manually during nonproduction periods to save energy, but

installing a timer (with manual override) is preferred to do this automatically

since it is human nature to put things off when the benefits are not obvious

or immediate.

The compressed air is sometimes cooled considerably below its dew

point in refrigerated dryersin order to condense and remove a large fraction

of the water vapor in the air as well as other noncondensable gases such as

oil vapors. The temperature of air rises considerably as it is compressed,

sometimes exceeding 250°C at compressor exit when compressed adiabati-

cally to just 700 kPa. Therefore, it is desirable to cool air after compression

in order to minimize the amount of power consumed by the refrigeration sys-

tem, just as it is desirable to let the hot food in a pan cool to the ambient

temperature before putting it into the refrigerator. The cooling can be done

by either ambient air or water, and the heat picked up by the cooling medium

can be used for space heating, feedwater heating, or process-related heating.

Compressors are commonly cooled directly by air or by circulating a liq-

uid such as oil or water through them in order to minimize the power con-

sumption. The heat picked up by the oil or water is usually rejected to the

Air

Cooling liquid

from compressor

Liquid-to-air

heat exchanger

Damper

(summer mode)

Damper

(winter mode)

Heated

air

Outside

Inside

facility

FIGURE 7–80

Waste heat from a compressor can be

used to heat a building in winter.

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