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

specifications, on the other hand, may drop from 91 percent at full load to 75

percent at quarter load. The first motor is obviously better suited for a situa-

tion in which a compressor is expected to operate at quarter load during a sig-

nificant portion of the time. The efficiency at part-load conditions can be

improved greatly by installing variable voltage controllers if it is economical

to do so. Also,oversizinga motor just to be on the safe side and to have some

excess power just in case is a bad practice since this will cause the motor to

operate almost always at part loadand thus at a lower efficiency. Besides,

oversized motors have a higher initial cost. However, oversized motors waste

little energy as long as they operate at loads above 50% of design.

Using a Smaller Motor at High Capacity

We tend to purchase larger equipmentthan needed for reasons like having a

safety margin or anticipated future expansion, and compressors are no excep-

tion. The uncertainties in plant operation are partially responsible for opting

for a larger compressor, since it is preferred to have an oversized compressor

than an undersized one. Sometimes compressors that have several times the

required capacity are purchased with the perception that the extra capacity

may be needed some day. The result is a compressor that runs intermittently

at full load, or one that runs continuously at part load.

A compressor that operates at part load also causes the motor to operate

less efficiently since the efficiency of an electric motor decreases as the point

of operation shifts down from its rated power, as explained above. The result

is a motor that consumes more electricity per unit power delivered, and

thus a more expensive operation. The operating costs can be reduced by

switching to a smaller motor that runs at rated power and thus at a higher

efficiency.

3 Using Outside Air for Compressor Intake

We have pointed out earlier that the power consumed by a compressor is

proportional to the specific volume, which is proportional to the absolute

temperatureof the gas at a given pressure. It is also clear from Eq. 7–89

that the compressor work is directly proportional to the inlet temperatureof

air. Therefore, the lower the inlet temperature of the air, the smaller the

compressor work. Then the power reduction factor, which is the fraction of

compressor power reduced as a result of taking intake air from the outside,

becomes

(7–95)

where Tinsideand Toutsideare the absolute temperatures (in K or R) of the ambi-

ent air inside and outside the facility, respectively. Thus, reducing the absolute

inlet temperature by 5 percent, for example, will reduce the compressor

power input by 5 percent. As a rule of thumb, for a specified amount of com-

pressed air, the power consumption of the compressor decreases (or, for a

fixed power input, the amount of compressed air increases) by 1 percent for

each 3°C drop in the temperature of the inlet air to the compressor.

Compressors are usually located inside the production facilities or in

adjacent shelters specifically built outside these facilities. The intake air is

freduction

Wcomp,insideWcomp,outside

Wcomp,inside



TinsideToutside

Tinside

 1 

Toutside

Tinside

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