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Compressed Air Systems – click the questions below to go the answers
Q. My air compressor seems to turn on and off frequently. Is this an indication of an upcoming failure or some other matter? The Pro Says: The relationship of the amount of compressed air you need to the amount you produce is important. There are a few factors to consider. If your compressor is oversized for the air demand and the type of control on the machine is (load/no load) then compressor will CYCLE. Cycling is when the compressor responds to an air demand and then either shuts off or unloads. If the volume delivered is much greater than the need, the compressor is continually starting and stopping (CYCLING). Most compressors with (load/no load) control are equipped with a timer that can be adjusted to allow the compressor to idle rather than turn off, reducing the high demand on frequently starting and stopping the motor. Another important point to consider is the size and location of your air receiver tank, A general rule of thumb for (load/no load) controls is 5-gallon tank capacity for every horsepower, i.e. 50 HP For a 250-gallon tank. Compressors equipped with modulation controls reduce the cycling effect by responding to the actual need through pressure identification. As the air demand increases, the compressor automatically provides more air volume and reduces volume as the demand declines. There are several types and styles of modulating controls, each compressor manufacturer offers different designs to accomplish the same result. The cost of operation can be a direct result of proper controls to meet your specific application. Lastly, you may simply have too large of an air compressor to meet your needs.
Q. I'm adding about a quart of oil a week to my 100 HP rotary screw compressor, is that about average? The Pro Says: That depends on the type, age and condition of your compressor and if you’re operating 8, 16 or 24 hours per day. Assuming your machine is 10 years old or less and you’re running 24 hours per day, that sounds about right. Most manufacturers state their oil carry-over at 2-3(PPM) parts per million.
Q. What is the most economical way to remove moisture (water) from my air system? The Pro Says: Water removal is not as much of an economical issue, but rather an application issue. A refrigerated air dryer is designed to reduce the dew point down to + 38º F. This dew point feels dry to the touch and is acceptable for most industrial applications. A desiccant dryer will typically reduce the dew point down to a -40ºF level. It can however, be adjusted anywhere from 0º F to – 100º F if required. A point to note is that desiccant dryers require purge air for operation. Depending on the design, the losses are as high as 15 to 17% of the incoming air. (Example), 500cfm incoming air saturated at 100º F and 100 psig will deliver 420cfm outlet air at (-) minus 40º F.
Q. What causes pressure drops? The Pro Says: The simple answer is any obstruction or restriction in the air line. However, the size of the air pipe, the distance the air travels, the number of bends in the line and the type of accessory components used (filters, dryers, etc.), all contribute to line losses. Frequently when plants expand they simply add more pipe to the end of the line, as well as sometimes adding a compressor. When you deliver out of the compressor at a higher pressure to compensate for line losses you are spending premium dollars. A properly designed air system with the correct compressor location and adequate pipe sizing can save as much as 20% in energy costs.
Q. I know air leaks are costly, can you give me a general rule as to how costly? The Pro Says: A good general rule is: Based on 24 hours per day / 7 days per week operation a compressed air system operation at 100 psig with a cost of power at $0.05 per kilowatt-hour with (one) 1/4" size opening will waste $8,382.00 annually. A 1/4" opening at 100psig will lose approximately 100cfm or 25 horsepower of air.
Q. What is the difference between cfm (cubic feet per minute) and acfm (actual cubic feet per minute)? The Pro Says: cfm free air is air delivered to a certain point at a certain condition, converted back to ambient conditions. acfm is the flow rate of air at a certain point at a certain condition at that point.
Q. How can we determine the cost of air at our plant? The Pro Says: A recent survey by the U.S. Department of Energy indicated that for a typical industrial facility, approximately 10% of the electricity consumed is used to generate compressed air. For some facilities, compressed air generation may account for 30% or more of the electricity consumed. Compressed air is an on-site generated utility. Very often, the cost of generation is not known; however, some companies use a value of 18-30 cents per 1,000 cubic feet of air. Compressed air is one of the most expensive sources of energy in a plant. The overall efficiency of a typical compressed air system can be as low as 10-15%. For example, to operate a 1 HP air motor at 100 pounds per square inch gauge (psig), approximately 7-8 HP of electrical power is supplied to the air compressor. To calculate the cost of compressed air in your facility, use the formula shown below:
Where: Example
Compressed Air System Control Strategies Matching Supply with Demand Individual Compressor Controls Multiple Compressor Controls Pressure/Flow Controllers
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