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Air compressors:

Oct. 18, 2006
are you getting the entire story?

By Ken Stofen,
Ecology Package, Inc.,
Racine, Wis.

Figure 1. CAGI-approved system is shown at top (before). Bottom view (after) shows the Ecology Package system, which controls the temperature of the compressed air as it flows through the system.

Recently, I received a request for quotation on an 150-hp air compressor. I asked the purchasing agent for permission to survey his company's compressed air system. Naturally, he asked why — and I told him that I wanted to know if there was any other reason that they wanted a 150-hp air compressor, other than the usual request for more compressed air volume.

I wanted to talk to the end users. — the people who actually used compressed air every day on the job. From them, I found out things the buyer didn't tell me and that their engineer didn't even know.

The real story
The compressed air demand during the day ranged from 200 cfm to just over the maximum capacity of their existing air compressors — which caused the air tools on the production line to slow down. Looking at the problem from the top-down, all that mattered to the company executives was to "get more air now!" In an emergency situation like this, no one thinks clearly, so mistakes are made.

The plant engineer computed the volume of compressed air he would need if all of the compressed air operated equipment was operating at maximum capacity at the same time; then he added a 10% safety factor and came up with 650 cfm at 100 psig. That volume would require a 150-hp air compressor.

The engineer told the buyer to get quotations from three or four air compressor distributors for an 150-hp air compressor. Instead, I suggested three 50-hp air compressors. The true cost of owning an air compressor is not the initial price but the operating cost after the air compressor is installed.

Consider the facts: this company uses between 200 and 650 cfm. Simply put, 200 cfm can easily be supplied by a 50-hp compressor. Why start and operate a 150-hp compressor when you can do the job with 1 /3 of that horsepower?

Now think about maintenance when the plant is shut down and the maintenance crew needs compressed air. Even a 50-hp compressor is too big. As the demand grows in the plant for compressed air, the second 50-hp unit would start, and if still more volume is required, the third unit would start. When the demand for compressed air dropped, one by one, the 50-hp air compressors would unload for ten minutes to cool off. If there still was no demand for more compressed air, they would shut down.

An HVAC engineer and a mechanical contractor modified the three 50-hp fluid-flooded screw type air compressors to my specifications, so all three would supply oil-free instrument quality compressed air.

The customer traded in both of their old air compressors and installed the three modified 50-hp units in their place. Now he is prepared for most problems that might occur in the future. If the demand for compressed air falls to a steady 200 cfm because of a recession or a change in the products they manufacture, he can operate a 50-hp compressor for a lot less money than operating a 150-hp compressor.

If the factory expands and the demand increases for compressed air, he can install another 50-hp modified air compressor at any location in the plant that is convenient and connect that new modified air compressor to the closest air line pipe.

With three or four identical air compressors, the plant would have an instant back-up unit for an air compressor that may shut down.

Long life?
Experts tell us that the average water-cooled oil cooler lasts for one to two years. The reason for that short life span is because oil coolers are purchased by air compressor manufacturers on a bid basis. This means that the lowest price for a heat exchanger that will meet minimum standards will get the order. Then, the customer is stuck with a $1000 bill (plus labor) to replace the heat exchanger and the bearings in the air end of the compressor. If he's lucky, he may not have ruined the air end, which would cost several thousand dollars more to replace.

Synthetic oil is fine, but we found that a synthetic fluid that contains no petroleum oil is much better, because synthetic fluid reduces friction dramatically. Simply by changing from the authorized private label petroleum oil to a synthetic fluid, we were able to reduce the demand for electric power used by the same air compressor under identical conditions by 14%!

Driving off the moisture
With our Ecology Package system, we use the heat of compressing air to "drive off the moisture" and a spin-on 10 µm OEM filter to pre-filter the synthetic fluid down to 10-µm particle size. Then we use a bypass filter to filter down to less than 1 µm. The wear particles that do the most damage to moving parts are in the 5 to 25 µm range, so we analyze the synthetic fluid every six months and fill the new filters with new fluid before we install them. But we won't change the fluid until a fluid analysis indicates it should be changed.

Our oldest installation has gone 3 years and 11 months without a fluid change. That is a significant savings because the fluid costs $20/gal and the capacity of the system is 16 gal. The OEM and bypass filters cost less than $100. These are thousands of dollars of legitimate savings that are hidden from view when you agree to have an authorized air compressor distributor, "change your oil and dispose of the old oil every four months."

The Compressed Air & Gas Institute (CAGI) has done a marvelous job for its members to bring order out of chaos, but the members are free to manufacture and sell any product they want to sell, even if it doesn't meet CAGI Standards. For many years, the big manufacturers' developments were changed a little here and there but, for the most part, they were copied by other members. This follow-the-leader mindset by all manufacturers has done little to improve air compressor efficiency and longevity. The differences we have found so far are shown in the box below.

Change fluid only if an analysis indicates it should be changed. To get consistent results from a prop-erly designed compressed air system, Figure 1, you must control the temperature of the compressed air as it passes through the system.

How they do it

Use motors with 85-87% power factor
See problems from the top-down
Receivers are wet inside
Waste rejected heat when compressing air
150 hp lead compressor
150 hp back up compressor
Use air compressors without an oil pump
Recommend oil change every four months

How we do it

All motors have 95% power factor
See same problems from bottom up
Receivers are dry inside
Recycle rejected heat when compressing air
50 hp lead compressor
50 hp + 50 hp back up compressors
All air compressors have an oil pump
Analyze the fluid and change filters

Ken Stofen can be reached via email at [email protected]

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