Fluid Power
10-98
 Technical Feature  

 

Air cooling is cheaper by design

Designers have found that taking to the air can be cheaper than taking to the water. Tom Shelley reports

An air cooled, oil free compressor has been made cheaper than water cooled versions through careful attention to design.

It boasts less noise than machines of similar size, while there is no need to worry about freezing or the functioning of water pumps or the effects of corrosion.

Oil free compressors of this type are intended for any application where the compressed air comes into contact with the customer's products, or where the customer's equipment is sensitive to presence of oil. Obvious applications include the pharmaceutical, food and beverage, hospitals, chemical processes, and electronic industries.

Most of the approaches applied to designing this compressor -- the latest in the 'Dryclon' range of industrial compressors from CompAir BroomWade -- are appropriate to designing any kind of air cooled machine system.

The company already makes a series of water cooled models, but there are many advantages to be obtained from turning to air cooling, especially in freezing climates. The machine shown to Eureka is a 90kW prototype, the first of a full range of machines.

One of the problems with designing an air cooled machine is that is requires movement of much more air than a water cooled system. Product manager Andy Pulley says that if water cooled, his machine would need to move 113 m3/min of air but the air cooled version needs to move 538 m3/min.

The compression jacket is oil cooled; the oil is then air cooled. In addition, there has to be an air cooled intercooler, acting on the partially compressed air between the two compression stages, and a compressed air aftercooler. The three coolers are each 125mm deep aluminium finned units, stacked one above the other.

Less noise from the fans
One problem in a system with big fans is noise. This arises from the blade passing frequency, and from the compression chamber, which in a water cooled machined is partly silenced by the water jacket. One of the goals of the design team was to reduce noise to below 80dBA and preferably 75dBA, which is quieter than the older generation of compressors on construction sites and much quieter than some of the other equipment. For comparison, conversation in a quiet room tends to be around 35dBA, while a jet engine can reach 110dBA or more.

Part of the solution is to use two smaller fans instead of one big one. These fans will also be buried in the heart of the machine. Noise getting out of the cooling air intake will be partly suppressed by using a louvered panel. The exhaust cooling air is ducted along the top of the equipment, and turns through 90 degrees before going out through the roof. A considerable amount of noise attenuation can be achieved by turning air through a right angle in this way and lining the insides of the corners with noise absorbing foam. If this proves insufficient, there will also be foam lined baffles along the inlet and exit ducts.

Nowadays, Pulley says the makers of noise suppressing foam have devised materials which damp out noise in particular frequency bands. Catalogues include sound damping frequency spectra and there is no a great deal more understanding of the problems and applying science to solving noise problems than there was a few years ago.

The best material of all to absorb noise remains mineral wool held in place by perforated sheet steel. This solution is still available to the noise conscious designer if all else proves insufficient.

One of the goals of the design is to segregate the cooling installation from the compression section. In this way, there is no risk of the incoming ambient air picking up heat from the compression section. All parts of the machine are accessible through two access doors. The layout of the prototype machine looks neat tidy and efficient.

The capital cost of the basic compressor is 5 to 10 per cent greater than that of a water cooled machine because of the large coolers. The overall total capital cost of the installation is lower, because there are no water pumps, radiators or water pipe work. The only down side is that the installation is larger, because of the big air ducts.

The general trend in compressor sales is to produce machines which produce higher quality compressed air. Statistics show that sales of oil free compressors are growing faster than oil injected compressors. This is partly because the food and beverage, pharmaceutical, health care and electronics industries are growing markets, and partly because of health and environmental considerations. Oil in condensate water is very difficult to separate. Most countries will not allow oil containing condensate to be disposed of untreated.

Air cooling is long established, as any Volkswagen Beetle owner or former owner will testify. It avoids quite a few problems associated with freezing cooling water and the need for anti-freeze, not to mention leaks caused by corrosion in radiators and perished and split rubber hoses.

 

Design Pointers
An air cooled, oil free compressor has a lower capital cost than its water cooled equivalent

Air cooling removes the need for anti-freeze in freezing climates, pumps, radiators and water pipework

Noise problems have been overcome by careful design

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