Application Note

Application Note

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

If you knew how

much it cost to run

that equipment

—could you make better operational and

maintenance decisions?

Comparing 200-horsepower air

compressors

Consider this manufacturing plant example. The

facility owned two 200-horsepower air compres-

sors used for supplying plant air. Since compressor

number one was rated at slightly more capacity

in cubic feet per minute (cfm) than compressor

number two, the decision had been made many

months before to run compressor number one as

the main compressor, and use compressor number

two as the “trim” compressor. Thus, the trim com-

pressor would run only when compressor number

one was not able to maintain system pressure.

This sounds logical, especially since they are both

the same horsepower—there should be little or no

difference in electrical operating costs.

Power loggers were installed on each unit for

several days to determine the actual electrical

energy operating cost. Each unit was run by itself

to make sure its operation was not influenced by

the other unit. Compressor number two was found

to be significantly more energy efficient, and was

also quite capable of supplying plant air needs.

Data logger software automatically graphs the energy usage for a

200-horsepower air compressor. This quick overview shows the

compressor is averaging just over 50 kW of energy consumption during

a three day period of operation. Knowing the electrical rate charge

of $/kWh, it would be easy to estimate the cost of operating this

compressor during this period.

Decisions have to be made—to save energy, to save dol-

lars. Having hard data on which to base those decisions

removes the “guess factor” and ultimately has a positive

effect on the bottom line. Can recording data, analyz-

ing results, and then making intelligent decisions really

have that large of a dollar effect, though? The answer is

“Absolutely yes!”

PAGE 1
If you knew how much it cost to run that equipment —could you make better operational and maintenance decisions? Application Note Decisions have to be made—to save energy, to save dollars. Having hard data on which to base those decisions removes the “guess factor” and ultimately has a positive effect on the bottom line.
PAGE 2
Compressor number two was a newer, more energy-efficient design than its counterpart. The facility was being charged $0.07 per kilowatt hour (kWh). The run times and kilowatt energy usage were extracted from the logged data and the actual operating costs determined. Such calculations are relatively simple and can easily be built into a spreadsheet for plant-wide usage. The bottom line: Operating compressor number two as the primary plant air compressor resulted in an annual estimated savings of $29,510.
PAGE 3
For example, a 460-volt, three-phase, standard efficiency 100 HP motor operating under full load for 8,760 hours per year will probably cost just over $48,000 per year to operate at an electric rate of $0.10 per kWh. But, what if the motor is not operating under full load at all times? Then, the total operating cost can drop significantly. The only way to know for sure is to measure and record data. Then, analyze data and determine the true dollar value that the motor is contributing to that utility bill.