Rent Smart: Air compressors
| May 28, 2009
Renting the right compressor for your jobsite is a matter of understanding your needs.
All compressors basically do the same thing: build air pressure and use that pressure for applications such as powering pneumatic tools, sandblasting and painting.
“The first question a rental manager will ask is how you are using the compressor,” says Marc James, product manager for Ingersoll Rand. “He may ask about your air requirements, so it is good to know those. If you are getting a tow-behind, you want to make sure you have a vehicle large enough to tow it. The mid-range and large units can be pretty heavy.”
Before you go to your rental dealer, however, you need to think about the power source that will run the compressor. A common mistake renters make is getting a compressor that has too much or too little power for what they need to do. Another mistake is not considering how you will keep the compressor powered on the jobsite.
Compressors can be powered by electricity, gas or diesel. Some diesel compressors come with electrical outlets that can provide power simultaneously to electric tools. Unless you are working inside, you’ll likely rent a gas or diesel compressor.
One of the compressor’s most important specs is the cubic foot per minute. Your compressor’s cfm refers to air volume and having the proper amount is essential to the completion of your application. The higher the cfm on the compressor, the more work it is able to do.
A small compressor’s cfm ranges from 65 to 600, and a mid-range compressor’s cfm is 600 to 925. Anything over 925 is usually considered a large unit.
A 60-pound breaker for will require a compressor that generates about 75 cfm of air, while a 90-pound breaker will require at least a 90 cfm compressor. Other applications may vary. Sandblasting, for example, requires 100 to 1,000 cfm. Rental rates vary depending on the cfm output and size of the compressor.
“The smaller units with ranges of 185 to 375 cfm rent daily or weekly,” says Steve Mowbray, vice president of rental services for Altorfer Rents, a Caterpillar dealer. “The larger units with cfm of 750, 1,200 or 1,600 typically go out on a monthly basis.”
According to Mowbray, the most popular units rented are the single- axle, 185-cfm range compressors powered by a diesel engine. Smaller compressors can be used for applications such as air nailers that can be carried or mounted on small wheels and pulled around by hand. These units are usually electric or gas powered. The larger units are normally on single- or two-axle trailers that can be towed by a truck or loaded on a trailer for transportation.
Compressors can be divided into two camps: positive displacement and centrifugal. The type most often rented is the positive displacement compressor. Centrifugal compressors are usually stationary and are impractical as a rental. Positive placement compressors such as rotary screw and reciprocating work by filling an air chamber with air and then reducing the chamber’s volume. Here is a brief summary of two most common positive displacement compressors available for rent:
Rotary Screw Compressor: The positive displacement rotary screw compressor creates air pressure through a simple concept. Air enters a sealed chamber where it is trapped between two contra-rotating rotors. As the rotors intermesh, they reduce the volume of trapped air and deliver it compressed to the proper pressure level. The compressor operates at cooler temperatures than the reciprocating compressor and can run continuously for 24 hours if necessary, making it ideal for use in industrial applications. A disadvantage of the rotary screw compressor is that it doesn’t always operate well in dirty environments.
Reciprocating Compressor: Like an automobile engine, the reciprocating air compressor uses pistons driven by a crankshaft to deliver small quantities of air at high pressure. Compression is achieved by a change in volume caused by the piston as it moves to the end of the cylinder. The compression may be lubricated by oil or in some cases require little or no lubrication in the cylinder. Reciprocating compressors can also come in stages, meaning that the air is compressed in the first stage and the air is cooled before entering the second stage. Reciprocating compressors operate at higher temperatures than rotary screw compressors so it is important that the parts are maintained constantly.
Compressors use oil to keep parts moving smoothly, although some rotary screw and reciprocating compressors are oil-free with moving parts made from Teflon. Oil-free compressors tend to run at higher temperatures and require vigilant maintenance, so it is more likely you will rent a machine with oil.
Maintenance and safety
Any major maintenance problems should be left to the rental dealer, but some basic maintenance may be necessary. For short-term rentals in the range of a few days to a few weeks, maintenance will probably consist of checking for air and oil leaks. Longer rentals – those that will last for months – mean you will have to change the oil and air filters during the rental period.
Other maintenance procedures include draining fluid traps on a regular basis and making sure that the motor belts are not loose and are in good condition.
Because you are dealing with compressed air, safety is always a concern. Any holes or ruptures in the compressor body or the hoses that connect to it can literally have explosive consequences, so proper eye protection and other precautions such as hard hats, hearing protection and gloves are a must.
There are also other pieces of equipment that can act as safety devices.
“If the hose ever comes loose, it can be deadly,” James says. “When you are using compressed air you want to use what is called a whip check cable, which can be attached to the end of the hose and the compressor itself. A swing check valve mounts on the compressor side of the hose and if there should be a tear or a burst in the hose, the swing check valve will shut off the flow of air from the compressor and prevent that whip.”
Also, when you are ready to do the daily maintenance make sure you turn the power off.
Transporting the compressor is just as important as knowing the unit’s power source and capacity.
Some of the smaller compressors can be towed, so ask the rental dealer which models are easy to transport if that’s a concern. According to James, diesel-powered rotary screw tow-behinds are the most commonly rented compressor.
“Tow-behinds last longer, have a lot more fuel storage,” James says. “You get more capacity with them. When you are towing a compressor, you have to make sure it complies with the local towing standards. Check the lights, turn signals and if you are putting it on a trailer, use tie downs and chock blocks. You have to use common sense when you are towing them or parking them. Compressors are designed to be towed at only 55 miles per hour.”