What are the criteria for making an informed and lasting decision when selecting an air compressor?
With all the different options available on modern air compressors, the proper selection can be a guessing game without the right set of guidelines. There are nearly as many different air compressor and option combinations as there are types of applications. Making a mistake in the selection process can cost you efficiency, extra service and even production.
Before buying any pieces of equipment, there are always some key considerations to keep in mind to help guide the selection. For air compressors, these ideas can be simply stated:
- How much pressure is required?
- How much air flow is required?
- What is the air quality required?
- What is the operating duty cycle?
- What is the first cost?
- What is the total cost of ownership?
- What special requirements dose this application demand?
To answer the first three questions, simply determine: What will the compressed air be used for? The most common uses for compressed air are simply described as shop, plant, or air.
In the shop capacity, the air compressor is being used to supply an energy source for tools like impact wrenches, nail guns, drills and grinders and blow guns. There are hundreds of stationary and portable tools that use compressed air for their power source. Each device has manufacturer’s specifications for the appropriate amount of pressure (psig, bar or kPa) and air flow (cfm, l/sec or m3/min). The performance of the device is critically dependent on these factors, and these must be kept in mind when selecting an air compressor.
For example, the chart below shows some common hand tools and the required air supply conditions to make them perform as the manufacturer states:
If the compressed air is to be used a power source for certain precision equipment, the selection becomes even more critical. For instance, a pneumatic drill can operate with a certain maximum level of contamination that would not be acceptable for a precision dentist’s drill. Even though both drills must have the incoming air filtered to remove as much dust and water before entering the tool, a dentist’s drill should not receive any contamination at all, including the oil that is commonly used in operating an air compressor. Special types of compressors are available that use no lubricating fluid in the compression chamber – either oil free or oil less.
To further clarify the compressor selection, it is important to consider the duty cycle of the equipment. Certain types of compressors are preferred for continuous duty based on their robust design and ability to run at constant temperature, regardless of demand.
Air-cooled reciprocating (piston type) compressors are generally ideal for intermittent duty usage that requires them to operate at most 60-70% of the time. If the compressor is not allowed to unload, or rest, at least 30% of its operating cycle, there is a possibility of heat damaging the head and valves.
Certain reciprocating compressors include forced lubrication, heavy duty valves and efficient inter-cooling which allow the machines to run at much higher duty cycles and still provide years of service. Rotary screw and centrifugal compressors are designed for continuous duty. Because they have internal cooling, this type of compressor can operate 24 hours a day, 365 days a year as required. This type of design, however, comes at a premium.
The ‘first cost’ of any equipment is always a deciding factor when selecting one model over another. First cost is affected by many factors, like design, pressure and flow, lubricated or oil-free (oil-less) and any special features that may be required. Generally speaking, rotary screw and centrifugal compressors tend to be more expensive than reciprocating compressors because they are designed for more severe duty and longer maintenance intervals. The second part of the cost equation is overall cost of ownership.
In general, the first cost of an air compressor represents about 10% of the cost to own and operate the compressor in the first five years. The other components are installation and piping, maintenance and power. The largest contributor to cost of ownership is power, ranging from the same cost per year as the initial investment all the way to three or four times the initial cost per year. In selecting a relatively low price up front, a customer could be paying almost twice as much for power versus a slightly higher initial cost for a more efficient compressor. To help in this part of the decision, all manufacturers of industrial equipment provide data sheets that clearly indicate the power and capacity of their standard air compressors. These data sheets can be obtained from each manufacturer, or from their local distributor.
A final factor, but not the only remaining factor by any means, involves the amount of customization required for the compressor to meet the needs of the application. This can include special cooling equipment for hot or cold environments, sound attenuation devices (enclosures or active noise cancellation), control devices to integrate into existing equipment, non-standard voltage…the list is nearly endless. If a standard piece of equipment nearly meets the requirements, it is a great advantage to make a compromise in the name of economy and long term service.
To make this information more meaningful, consider the following table:
|Pressure Range**||Capacity Range**||Air Quality||Duty Cycle||Initial cost||5 Year Cost||Options Available|
*Reciprocating compressors are considered to be single acting, air cooled
**Normal range is for non-specialized equipment, available from catalog
+Certain manufacturers, like Quincy Compressor, offer extreme duty air cooled reciprocating compressors with 100% duty cycle
The selection process should take these factors into consideration. While the above information is not meant to make a final selection, it serves to give the reader an idea of the questions to ask when dealing with the issue of equipment selection. The main purpose is to make sure the impact of a proper selection, based on real requirements like air flow, pressure and ultimate lifecycle cost, are considered rather than simple power rating and first cost. Making a decision to buy an air compressor is a decision that lasts a long time – the power to make the best, informed decision is based on asking the right questions.
To find out more about the best model for your needs, contact a local Quincy dealer near you!