Cfm

MaintenanceCircleTeam

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April 22, 2007

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NEWSLETTER FOR MAINTENANCE COMMUNITY Word for the day: CFM

CFM – short form for Cubic Feet per Minute - is the commonly used measure of volumetric flow from an air compressor, a fan etc. In industries, it is common to refer to this term with reference to an air compressor and hence we will discuss about this. Let us consider a simple single cylinder reciprocating compressor to understand how CFM is generated. For understanding purpose the driving electric motor, with 600 RPM is connected to the compressor. L = Stroke length of the piston = 2 feet D = Diameter of the piston = 1 feet N = Revolutions per minute of motor = 600 RPM Since we are immediately interested in calculating CFM, we have all the dimensions in feet. Later, we will look at conversion of feet into meter and vice versa. Area of the Piston, A = πD2 / 4 = π (1 x 1) / 4 = 0.7854 Square feet Therefore, theoretical volume of the cylinder, V = L X A = 2 x 0.7854 = 1.5708 Cubic Feet For every revolution of the piston it “displaces” a volume of V cubic feet. For N revolutions per minute, volume displaced, Q = V X N = 600 x 1.5708 = 942.48 Cubic feet per minute. Since valve clearance and other mechanical variations affect the output, the actual output may be little less than the theoretical output and can reduce over a period of time, if no adequate maintenance care is given. Since 1 meter = 3.33 feet (1 feet = 0.3 meter), to calculate Q in Cubic meter per minute, Q = 942.48 x 0.3 x 0.3 x 0.3 = 25.447 Cubic meter per minute. Hence 1 cubic meter per minute equals 37.037 cubic feet per minute. We can alter the CFM by adjusting L, D or N within the limitations of the design attributes. If a two-stage compressor is considered, the volume will theoretically double. A Cubic feet per minute is specified under standard conditions, since density of air changes with temperature and other parameters affect the output. For air compressors, it is customary to specify CFM at rated pressure, temperature and leaving the outlet free into atmosphere (this is also referred as FAD or Free Air Delivery) which will be stamped on name plate of the compressor. Now let us arrive at how to calculate the CFM requirement for an application. Let us consider a simple cylinder with diameter (also called Bore) of 1 feet and stroke length (distance of travel) of 3 feet. This cylinder will be pushing an application of 1000 Kilograms (Kg) weight and expected to complete the stroke in 3 seconds. L = Stroke length of the piston = 3 feet D = Diameter of the piston = 1 feet T = time of travel = 3 seconds Since we know the travel time, let us calculate the piston velocity, V = L / T feet per second = 3 / 3 = 1 feet per second Area of the Piston, A =πD2 / 4 = π (1 x 1) / 4 = 0.7854 Square feet. Since we know the velocity, now we can calculate the volume displaced every second Q = A x V = 0.7854 x 1 = 0.7854 Cubic feet per second = 0.7854 x 60 = 47.124 Cubic feet per minute (CFM). Therefore, this piston requires around 48 CFM of air to complete the task in 3 seconds. As the application speed changes, CFM requirement also changes. Now, for understanding purpose, let us If you like to improvise this article or contribute or comment please mail us at: [email protected]

This document contains information, for reference only. We assume no responsibility for its implication.

MaintenanceCircleTeam

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April 22, 2007

calculate the pressure required to complete the task. The pressure should not only push 1000 kilograms of application weight (W), but also the dead weight of the piston-rod assembly along with certain friction (F), which for understanding purpose let us assume as 100 kilograms. Therefore the total weight to be pushed is 1000 + 100 = 1100 kilograms. Area of the piston, A = 0.7854 Square feet = 730 Square Centimeters (1 feet = 12 inches = 12 x 2.54 = 30.48 centimeters). Pressure (P) = Force (W + F) / Area (A) = 1100 / 730 = 1.5 Kg per Square Centimeter. Therefore, to complete the task satisfactorily, we need 48 CFM of air at a minimum pressure of 1.5 Kg/Sq.cm. from the source. So, we have to choose a compressor which can meet (exceed) these requirements. If more than one application is desired, the CFM requirements of all should be added and then suitable compressor should be selected. Compared to reciprocating type, calculating the CFM of a screw compressor is complex and hence we usually take the value from chart or manufacturer’s specification sheet. Even though CFM is very critical in compressed air application, it finds equal importance in selection of air handling units, centralized air conditioning and handling units, industrial fans, burners and many more. Now let us look at a simple field method of measuring CFM from a compressor in a typical shop floor. This also helps in finding out the efficiency of the compressor unit. Of course, there are better and highly sophisticated methods available to carry this task more precisely. Let us have a compressor capable of delivering 200 CFM at 7 Kg/Sq.cm. connected to an “intermediate” airreceiver of 40 cubic feet capacity before being connected to any downstream unit. Two valves, V1 & V2 can isolate compressor and receiver respectively. And let us assume the volume of pipe length between V1, V2 and receiver as 10 cubic feet (it is important to calculate the volume of interconnecting pipes in such a set up to avoid errors in calculation). A standard pressure gauge – 0 to 10 Kg/Sq.cm. range – is fitted to the receiver. Close the valve V2 and start the compressor. From basics of physics, we know that for every 1 Kg/Sq.cm. increase in pressure, we are filling 40 + 10 = 50 cubic feet of compressed air. To keep errors minimum, let the pressure rise in receiver till 4 Kg/Sq.cm. Now start a stop watch and measure the time required till pressure rises to 6 Kg/Sq.cm. (6 is only an indicative reference. It can be adjusted as needed. The purpose is to create 2 Kg/Sq.cm. difference). Perform following calculations: Initial Pressure, P1 = 4 Kg/Sq.cm. Final Pressure, P2 = 6 Kg/Sq.cm. ∆P = P2-P1 = 6-4 = 2 Kg/Sq.cm. = 100 Cubic feet. Stop watch reading: 45 seconds = (45/60) = 0.75 minutes = T Therefore, CFM measured = 100 / T = 100 / 0.75 = 133 Cubic feet per minute!! The compressor is approximately delivering 133 CFM against its 200 CFM standard value and hence its efficiency is only 66.5% (133/200). So, we can perform a backward analysis and troubleshoot the problem to increase its CFM. Similarly, we can approximately find out the CFM requirement of a plant during its peak hours. When the plant is running sufficiently loaded, allow the receiver to be filled with air at 6 Kg/Sq.cm(6 is only an indicative reference. It can be adjusted as needed. The purpose is to create 2 Kg/Sq.cm. difference). Now close the valve V1 and start the stop-watch simultaneously. Record the time taken for pressure to drop from 6 to 4 Kg/Sq.cm. Let us say it is 30 (0.5 minutes) seconds. Therefore, the plant needs (100/0.5) = 200 CFM of air during peak hours. The selected compressor should be able to match (exceed) the present and to some extent “future” demand of the plant. As a redundancy practice, it is normal to install combination of compressors at a specified “utility” location, whose total CFM should match the requirement. CFM calculation and selection is very vital for successful running of a plant since any wrong approach will lead to many factors some of which include: (a) Oversized compressors, consuming too much power (b) Unwanted pressure drops at consumer point resulting in machine stoppages and downtime (c) High initial capital investment

If you like to improvise this article or contribute or comment please mail us at: [email protected]

This document contains information, for reference only. We assume no responsibility for its implication.