Compressed Air Best Practices - March 2009 - (Page 26)

® | 03/09 Focus Industry ENERGY MANAGEMENT | DEMAND-SIDE SYSTEM OPTIMIZATION: POINT-OF-USE A I R C Y L I N D E R S , VA LV E S , D I S T R I B U T I O N A N D S T O R A G E The chart below is from another production machine and this time the cycles aren’t as fast as the previous ones so the pressure does come back to the header pressure after each cycle. But all point-of-use piping and fittings are undersized and not allowing the required volume of air per actuation to occur. The results are quite clear in the pressure drops of 20 psig. An example of a typical air cylinder found on a machine would be 8-inch diameter by 8-foot stroke. Let’s say it strokes in three seconds and the pressure is 80 psig. What is the actual flow into the cylinder? This one cylinder would require a flow of 363 scfm! What size piping is needed here? The 363.4 is the actual standard flow rate during the actuation. This should be the value used to size the proper valves, hoses, filters and regulator for this particular cylinder. Note that other cylinders might be actuating at the same time on a piece of equipment and their flows must be considered when sizing the branch piping to feed this machine. The manufacturer might average the flow over a minute and therefore the published flow would be lower. Average flow should not be the guideline for sizing components. If we could allow the flow that is required to enter the machine as needed, there would be minimal pressure drop. But the problem lies in the fact that when cylinders actuate, there is a very large peak flow of air required to move them through their respective stroke lengths at the desired pressure. When peak flows exceed the flow rating of valves and piping, pressure drop occurs. The fix is usually a modification of the local piping/FRL’s or added point-of-use storage to compensate for undersized components. Let’s take a look at how to calculate the flow into a cylinder as it actuates. FLOW COEFFICIENT OR CV When selecting valves for air cylinders, an important consideration is their ability to pass the required volume of air at an acceptable pressure drop. This is referred to as the flow rating. A common method of rating flow is by CV factor. The CV factor is derived from an expression which gives the number of gallons of water per minute that will pass through the valve with a 1 psi differential between the valves inlet and outlet. In many valve designs, the variation in capacity between different flow paths may vary up to 50%. One manufacturer’s ½-inch port valve may actually pass less flow then another ¼-inch port valve. The National Fluid Power Association is currently using the following formula for obtaining the capacity coefficient or Cv: FLOW RATE INTO A CYLINDER AND FLOW COEFFICIENT The flow rate into a cylinder is based upon the amount of air needed to move the piston load and to force out exhaust air from the other side of the cylinder at a specified speed. Because it will require a specific mass of air to perform the required work, the flow rate must be stated in scfm (standard cubic feet per minute) which is the only compressed air term that states airflow in mass flow rate. A standard cubic foot of air is defined as air at a barometric pressure of 29.92 inches of mercury (we will use 14.5 psia) with a temperature of 68°F and a relative humidity of 0% and a weight of 0.0750 lbs. The standard flow rate can be calculated by first determining the volume of the cylinder: V (in3) = A (area) x S (stroke) Next we calculate the standard compression ratio: Pressure at cylinder (psig) + absolute pressure (psia) Absolute pressure at site (psia) We can calculate the standard flow rate using the following formula: SCFM = Vin3 (standard compression ratio) (time to fill cylindersec )28.8 Cv = Q 22.48 T1xG ΔPx ( P2 + Pa ) Where: Cv = Capacity coefficient Q = Flow in scfm (14.5 psia, 0% RH, 68 °F) G = Specific gravity of the fluid (G = 1 for air) T1 = Absolute temperature °R (460 + degrees F) ΔP = Allowable pressure drop P2 = Final outlet pressure P1 = Inlet pressure (ΔP = P2 – P1) Pa = Atmospheric pressure in psia 26 26 www.ai b estpractices.com www.airbestpractices.com http://www.airbestpractices.com

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Compressed Air Best Practices - March 2009 Contents From the Editor Utility-Air News Compressed Air Audit of the Month Air Standards Assessment Improves Electroplater Production and Saves Energy Demand-Side System Optimization Seven Sustainability Projects for Industrial Energy Savings Personal Productivity Resources for Energy Engineers Wall Street Watch Advertiser Index Classifieds

Compressed Air Best Practices - March 2009

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