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normally drawn from inside the building or the shelter. However, in many

locations the air temperature in the building is higher than the outside air

temperature, because of space heaters in the winter and the heat given up by

a large number of mechanical and electrical equipment as well as the fur-

naces year round. The temperature rise in the shelter is also due to the heat

dissipation from the compressor and its motor. The outside air is generally

coolerand thus denserthan the air in the compressor room even on hot

summer days. Therefore, it is advisable to install an intake ductto the com-

pressor inlet so that the air is supplied directly from the outside of the build-

ing instead of the inside, as shown in Fig. 7–78. This will reduce the energy

consumption of the compressor since it takes less energy to compress a

specified amount of cool air than the same amount of warm air. Compress-

ing the warm air in a building in winter also wastes the energy used to heat

the air.

4 Reducing the Air Pressure Setting

Another source of energy waste in compressed-air systems is compressing

the air to a higher pressure than required by the air-driven equipment since it

takes more energy to compress air to a higher pressure. In such cases consid-

erable energy savings can be realized by determining the minimum required

pressure and then reducing the air pressure control setting on the compressor

accordingly. This can be done on both screw-type and reciprocating com-

pressors by simply adjusting the pressure setting to match the needs.

The amount of energy it takes to compress a unit mass of air is determined

from Eq. 7–89. We note from that relation that the higher the pressure P 2 at

the compressor exit, the larger the work required for compression. Reducing

the exit pressure of the compressor to P2,reducedwill reduce the power input

requirements of the compressor by a factor of

(7–96)

A power reduction (or savings) factor of freduction
0.08, for example, indi-

cates that the power consumption of the compressor is reduced by 8 percent

as a result of reducing the pressure setting.

Some applications require slightly compressed air. In such cases, the need

can be met by a blower instead of a compressor. Considerable energy can be

saved in this manner since a blower requires a small fraction of the power

needed by a compressor for a specified mass flow rate.

freduction

wcomp,currentwcomp,reduced

wcomp,current

 1 

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

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

398 | Thermodynamics

Wall

Compressor

Outside

air

Air intake duct

Air filter

FIGURE 7–78

The power consumption of a

compressor can be reduced by taking

in air from the outside.

Air inlet

85.6 kPa

800 kPa

900 kPa

Air

Compressor

Motor

Compressed

air

FIGURE 7–79

Schematic for Example 7–23.

EXAMPLE 7–23 Reducing the Pressure Setting to Reduce Cost

The compressed-air requirements of a plant located at 1400-m elevation is

being met by a 75-hp compressor that takes in air at the local atmospheric

pressure of 85.6 kPa and the average temperature of 15°C and compresses it

to 900 kPa gauge (Fig. 7–79). The plant is currently paying $12,000 a year

in electricity costs to run the compressor. An investigation of the compressed-

air system and the equipment using the compressed air reveals that

compressing the air to 800 kPa is sufficient for this plant. Determine how

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