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

The hissing of air leakscan sometimes be heard even in high-noise manu-

facturing facilities. Pressure dropsat end-use points in the order of 40 per-

cent of the compressor-discharged pressure are not uncommon. Yet a

common response to such a problem is the installation of a larger compres-

sor instead of checking the system and finding out what the problem is. The

latter corrective action is usually taken only after the larger compressor also

fails to eliminate the problem. The energy wasted in compressed-air systems

because of poor installation and maintenance can account for up to 50 per-

cent of the energy consumed by the compressor, and about half of this

amount can be saved by simple measures.

The cost of electricity to operate a compressor for one year can exceed the

purchase price of the compressor. This is especially the case for larger com-

pressors operating two or three shifts. For example, operating a 125-hp com-

pressor powered by a 90-percent efficient electric motor at full load for 6000

hours a year at $0.085/kWh will cost $52,820 a year in electricity cost, which

greatly exceeds the purchase and installation cost of a typical unit (Fig. 7–72).

Below we describe some procedures to reduce the cost of compressed air

in industrial facilities and quantify the energy and cost savings associated

with them. Once the compressor power wasted is determined, the annual

energy(usually electricity) and cost savingscan be determined from

(7–87)

and

(7–88)

where hmotoris the efficiency of the motor driving the compressor and the

unit cost of energy is usually expressed in dollars per kilowatt hour (1 kWh 

3600 kJ).

1 Repairing Air Leaks on Compressed-Air Lines

Air leaks are the greatest single cause of energy loss in manufacturing facili-

ties associated with compressed-air systems. It takes energy to compress the

air, and thus the loss of compressed air is a loss of energy for the facility. A

compressor must work harder and longer to make up for the lost air and

must use more energy in the process. Several studies at plants have revealed

that up to 40 percent of the compressed air is lost through leaks. Eliminating

the air leaks totally is impractical, and a leakage rate of 10 percent is consid-

ered acceptable.

Air leaks, in general, occur at the joints, flange connections, elbows,

reducing bushes, sudden expansions, valve systems, filters, hoses, check valves,

relief valves, extensions, and the equipmentconnected to the compressed-air

lines (Fig. 7–73). Expansion and contraction as a result of thermal cycling

and vibration are common causes of loosening at the joints, and thus air

leaks. Therefore, it is a good practice to checkthe joints for tightness and to

tightenthem periodically. Air leaks also commonly occur at the points of end

use or where the compressed-air lines are connected to the equipment that

operates on compressed air. Because of the frequent opening and closing of

the compressed-air lines at these points, the gaskets wear out quickly, and

they need to be replaced periodically.

Cost savings 1 Energy savings 21 Unit cost of energy 2

Energy savings 1 Power saved 21 Operating hours2>hmotor

392 | Thermodynamics

Compressor: 125 hp = 93.21 kW

Operating hours: 6000 h/yr

Unit cost of electricity: $0.085/kWh

Motor efficiency: 0.90

Annual energy usage: 621,417 kWh

Annual electricity cost: $52,820/yr

FIGURE 7–72

The cost of electricity to operate a
compressor for one year can exceed
the purchase price of the compressor.

Compressed

air

Joint

Air leak

FIGURE 7–73

Air leaks commonly occur at joints
and connections.