There is a considerable difference between the "fourth utility" and the traditional utilities of gas, water, and electricity: a business has complete control over the production and quality of its compressed air. Depending on your needs, you may want to take steps to improve the overall quality of your air, especially if your compressor is going to power manufacturing automation equipment or other expensive machinery.

Because of the temperature changes involved its creation, compressed air can suffer from condensation that introduces moisture into the air delivery. In sensitive equipment such as CNC routers, any water is likely to cause problems and may void your warranties. To combat this, compressors can be equipped with driers that remove all traces of moisture.

Air from a compressor is also usually warmer than ambient air. If that's not appropriate for your task, you can get coolers to lower the air temperature.

Compressed air can also accumulate small particles of oil, dust, and other debris. Relatively inexpensive filters can remove these particles and protect your equipment.

A small investment in conditioning the air that comes from your compressor can protect a much larger investment: cool, dry, clean air can extend the life of your manufacturing equipment.

Oil-free air
Because the oil required to lubricate compressor motors can sometimes get into the air supply, basic air-powered tools are generally tolerant of some oil in the compressed air. However, for applications such as working with food or semiconductors, as little as one part per billion of oil is unacceptable. In these cases, you'll need oil-free (or oilless) air.

While driers, coolers, and filters are add-ons that can work with any air compressor, oil-free compressors are a separate class entirely. Oil-free air compressors have compression chambers that are completely sealed off from the oil-lubricated parts of the machine, ensuring a steady flow of completely oil-free air. They're considerably more expensive, so you should only choose one in the most sensitive situations.

Heat and noise
You should also consider the placement of your compressor. Compressors generate considerable amounts of heat: in addition to the heat of the motor, the physics of compression are such that increasing pressure on a gas while reducing its volume also increases temperature. Compressors can also be very noisy - more than 100 decibels, in some cases. These two factors lead many businesses to set up a "compressor room" where the heat can be vented and the noise contained, while air is distributed to where's it needed through long hoses or piping.

If you don't have the luxury of space, you may want to get an enclosure for the compressor to reduce noise levels. Alternatively, you can make sure to choose a quieter type of compressor. Rotary compressors are much quieter than reciprocating models: new rotary machines are not much louder than 65 db, the same volume as typical conversation.

Power
Compressors are usually powered by electricity. The smallest models run on standard 110v, single-phase power, but most commercial and industrial models need three-phase power. Often, you'll be able to specify the voltage (208v, 230v, 460v) the compressor should run on, so make sure you know what you have available.

If you're going to use your compressor outdoors, or move it around quite a bit, you'll want to choose a compressor with a gasoline or diesel engine, instead. Internal combustion engines will greatly increase the noise and heat your compressor generates, but they are usually cheaper to operate than electric compressors.

Mobility
If your compressor is going to be mobile, you'll also want to pay more attention to the weight, wheels, and handles on the compressor. Larger engine-driven compressors for construction are often self-contained trailers, making it easy to move them from job site to job site. If you won't be moving it around but do have concerns about space, look for a compressor that's vertically oriented. It will leave you with more valuable floor space.