Air on Demand

February 19, 2009 06:00 PM

Farmers routinely deal with nuts and bolts on equipment that tax 1"-drive air impact wrenches. Painting and sandblasting are now common activities in farm shops. Some farmers even find it economical to have their own tire machines to mount and dismount tires.

All of the bigger more specialized air tools consume massive volumes of compressed air. Many farmers are upgrading their compressed-air systems to keep up with the increased demand. That requires taking an inventory of current air-powered shop tools, as well as a look into the future.

"Take the rated cfm [cubic feet per minute] of all the air tools that you currently might use at the same time and add them together,” says Vinson Sill, light industrial marketing manager for Quincy Compressor. "Add another 20% to 25% for additional tools or future growth and that's an idea of the minimum cfm your air compressor should produce.”

Most air tool manufacturers have charts on their Web sites that list the individual cfm ratings of their tools. Local air tool retailers can also help customers calculate the potential cfm requirements for their shops.

Choosing an air compressor. Once your maximum air requirements are calculated, the next step is to select an air compressor and storage tank that will meet those needs.
Single-stage compressors use a single piston/cylinder to compress air in one step. Two-stage compressors use a primary piston/cylinder to initially compress air and a secondary piston/cylinder to bring air pressure to the final desired level.

"Single-stage compressors work well at up to 125 psi working pressure,” Sill says. "Two-stage compressors are better at maintaining more than 125 psi. Many manufacturers offer splash- or pressure-lubricated two-stage compressors. The better lubrication systems produce better durability.”

Direct-drive compressors that run off the motor's driveshaft, rather than through the belts and sheaves, are not the best choice for heavy use in farm shops,
according to Sill.

"Direct-drive compressors are compact and lightweight, but they aren't as durable as belt-driven units,” Sill says. "The compressors run at the same speed as their electric motor, around 3,600 rpm. Our industrial-duty, belt-driven compressors run from 600 rpm to 9,050 rpm. Which one do you think will last longer? Heat due to speed is what reduces the life of an air compressor.”

Farmers shopping for air compressors may encounter rotary-screw air compressors. Rotary-screw compressors compress air with supercharger-type rotors, rather than the reciprocating-piston design of conventional air compressors.

Rotary-screw air compressors are dramatically quieter than reciprocating air compressors. They are also significantly more expensive and generally unsuited for farm use.

"Most rotary-screw compressors are designed to run all the time in a factory where there is constant air use,” explains Randy Sutton, system engineer for Ingersoll Rand. "We have a smaller rotary-screw compressor that is designed to start and stop, more like a traditional reciprocating compressor, but it's considerably more expensive and better suited for a farm shop that is busy enough to use a certain amount of air all day, every day.”

Storage tanks are generally part of the compressor package. Vertical storage tanks have a smaller footprint and take up less floor space than horizontal tanks.
"In my experience, an industrial-duty 5-hp to 7½-hp, two-stage, cast-iron compressor with a 60-gal. to 80-gal. [storage] tank is a good fit for most farm shops,” Sutton says.

Water removal. Sutton and Sill recommend a water removal device on air systems and storage tanks.

"If you're in a humid part of the country, it's surprising how much water can condense and accumulate in your system,” Sill says. "I've seen an 80-gal. air tank that we literally drained 75 gal. of water out of. The guy couldn't figure out why he had no capacity for his air tools and was getting so much water through his air lines.”

A simple drain petcock on the supply tank is the minimum water control solution, but effective only if drained regularly. Automatic tank drains can be programmed to drain tanks daily or weekly. An air-to-air aftercooler that is mounted on the compressor can reduce moisture in air systems by 50%. An actual air dryer is the best solution if dry air is absolutely necessary.

"If you're going to be doing much painting, then you've got to have a dryer,” Sill says. "Expect to pay 20% to 25% of the cost of the compressor to add a dryer. A dryer for a 5-hp compressor is around $700 to $1,200.”

Air delivery. Delivering compressed air to air-powered tools can be simple or complicated. A simple 25'- or 50'-long, 3⁄8" inside diameter (i.d.) rubber air hose coupled directly to the air supply tank is as simple as it gets. More and bigger air tools require a more sophisticated delivery system.

Many farm shops connect their air compressor to a "header” air pipe located along one wall, with quick-connect couplers T'd into the header pipe at regular intervals. Both 3⁄8" and ½" i.d. piping are common.

"If you're going to use 1" impact wrenches or other tools that need a large charge of air, then a 5⁄8" or 1" i.d. header pipe might be a good idea,” Sutton says. "You can still use the lighter 3⁄8" air hoses for normal use, but you'll want a ½" i.d. air hose on the big-impact guns to take full advantage of the higher cfm that the larger header pipe can deliver.”

Schedule 40–type black pipe, copper pipe and aluminum pipe are options for delivery piping. Black pipe is low-cost but requires cutting and threading for installation. Copper is currently a high-priced option, and joints must be "sweated” during installation. Aluminum air piping is less expensive than copper and uses compression fittings for quick and easy installation.

Sutton acknowledges that there are OSHA-approved acrylonitrile-butadiene-styrene (ABS) plastic air lines available on the market.

"I can't recommend plastic lines,” he says. "If somebody uses the wrong kind of plastic [piping] or mixes [OSHA-] approved plastic fittings with nonapproved fittings, you end up with a dangerous situation.”

The problem with using nonapproved plastic air piping, Sutton explains, is that the oils inherent to the inside surfaces of all air systems will attack nonapproved plastic and make it brittle.

There have been instances when the weakened non-OSHA-approved plastic lines used in air systems were bumped or accidentally tapped and they exploded, he adds.
The final link between air systems and air tools is the rubber air hose and its couplers. The 3⁄8" i.d. rubber hoses and connectors are adequate for most air tools up to the ½"-drive impact wrenches. Air tools with ¾"- and 1"-drive accessories benefit from the higher cfm flow of ½" i.d. and larger air hoses and connectors.

"It's possible to temporarily bump up air pressure to 125 psi or more to get more power, but it's hard on air tools,” Sutton says. "Most manufacturers recommend setting [air pressure] regulators at 90 psi. If you need more power, go with more cfm from the compressor and a larger delivery system than with more psi.”

You can e-mail Dan Anderson at

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