This is an excellent question because the wrong air compressor can cost you big bucks and cause untold headaches. Each type of compressor (reciprocating, rotary screw, and rotary vane are the primary types) has its own “pros and cons”, but it is not difficult to determine the most appropriate type for your application once you know a little about each one.
Reciprocating, or piston, air compressors are the most familiar. An industrial quality machine will last a long time with routine maintenance. Reciprocating compressors are also very efficient because they only run when there is a demand for compressed air. When there is no demand, they turn off, saving wear and tear on the machine and, most importantly, saving on your power bill. In fact, most of these machines must turn off regularly to cool. An ideal on/off ratio is 60/40. Reciprocating air compressors are available in a wide range of sizes from fractional horsepower up through hundreds of horsepower. A reliable rule of thumb is that each horse power will produce four cubic feet per minute (4 cfm) of compressed air at 120-150 pounds per square inch (psi) pressure.
Rotary screw air compressors run continuously, producing a steady flow of compressed air. They are suited for applications that require a constant supply of compressed air that would never give a reciprocating machine rest. However, if the air demand decreases, a rotary screw machine will produce compressed air that is not needed. This inefficiency will show up in your power bill. Sometimes a rotary screw machine is used to provide a base amount of compressed air for which there is a constant demand and a reciprocating machine is used to “trim” the system, operating only when demand exceeds the capacity of the rotary screw. This arrangement can be very efficient. Also, newer technology variable speed motors can reduce the power consumption of rotary screw machines when air demand diminishes. Rotary screw compressors are available in the range of five horsepower up. Each horsepower will produce about 4-5 cfm of compressed air depending on the specific design.
Rotary vane air compressors are suitable for the same types of applications as rotary screws, but they have some advantages. They operate with fewer moving parts, have fewer potential oil leak points, and utilize a superior hydraulic-controlled inlet system. Some air compressor service companies shy away from working on rotary vane machines because they are not very familiar with them. Air Compressor Service has more expertise and experience with rotary vane machines than any other compressor service company in Washington State. Rotary vane machines are generally available from two horsepower through 300 horsepower. With proper preventive maintenance, these machines can serve well for 100,000 hours or more.
The best way to size your air compressor is to measure your actual compressed air use for at least one week of typical operations and allow for reasonable estimates of expected changes. Air Compressor Service can perform this service for a reasonable cost (free for our existing customers). Industrial air compressors are too expensive to make a mistake by buying a machine too small for your application, or too big, which is the more common mistake. One might logically assume that if a 15-hp compressor is good for your application, then a 30-hp would be better, but that is not the case, at least not for rotary machines. Although a rotary machine (unless it is a variable speed unit) will run at a constant speed even with fluctuating air demand, if the demand is too low for a sufficient time, the machine will shut off. Then when demand increases it will turn back on. Frequent on/off cycling causes the machine to “burp” oil each time it shuts down which can cause a variety of problems.
There are some telltale symptoms of a seriously oversized rotary air compressor:
• The compressor is an oily mess,
• You have to buy extra oil between services,
• You are having problems with your condensate drains,
• You are frequently replacing downstream filters, or
• You are experiencing corrosion in your compressed air system.
If you suspect your rotary air compressor is oversized, ask Air Compressor Service to inspect your compressed air system and determine if changes are indicated.
Obviously, your business does not involve using your compressed air to spray paints, or you would know of problems a little water can cause. But water’s rap sheet is long and varied and is not limited to crimes against paint jobs. Water in your compressed air will flush away lubricants in your air tools and air powered machines, increasing wear and causing premature breakdowns. Also, it can mix with soluble and corrosive solder fluxes in your compressed air piping system and corrode machines and tools down stream. Water is also corrosive when mixed with compressor oil mist. Fortunately, water is easy to outsmart. Appropriately sized refrigerated or desiccant air dryers effectively remove all the moisture from your compressed air. Add particulate and coalescing filters to remove particulate and oil contamination, and your compressed air will be clean, dry and ready to effectively perform the work you prescribe for it. However, as with your compressor, the appropriate size and type of dryer is critical. You should request that a professional air compressor service company, such as Air Compressor Service, look at your operation and recommend the appropriate equipment.