Air Compressor CFM and Duty Sizing From Tool Demand

Compressor sizing from the sum of tool nameplate ratings buys a machine two or three times too big, and sizing from the largest tool buys one that cannot keep up. A tool's rated CFM is its consumption WHILE THE TRIGGER IS DOWN, and almost nothing runs at 100% duty: an impact wrench on an assembly line might see 50%, a blow gun sees 10%. Weighting each tool by its realistic duty and summing is what produces a compressor that neither short-cycles nor starves, and the receiver is what absorbs the difference between average demand and instantaneous peaks. A shop with two impact wrenches at 5 cfm and 50% duty, a sander at 12 cfm and 70%, and a blow gun at 3 cfm and 10% averages 13.70 cfm -- against a connected load of 25 cfm, which would buy 6.3 hp for a shop that needs about 4.7. THE LEAK ALLOWANCE IS THE HONEST PART. A typical industrial system leaks 10 to 20% of its output and a neglected one leaks 30% or more, so a compressor sized with no leak budget is undersized on the day it is installed. It is carried explicitly here rather than buried, because a leak allowance you can see is a leak allowance you might fix -- and fixing it is far cheaper than the horsepower it buys. The duty-weighted demand relation is the same one the air receiver calculator uses, so the two cannot disagree about what a set of tools actually draws. This sizes on tool demand the user supplies, and every input is an estimate: duty cycles in particular are guesses until somebody logs them, and a metered week beats any table. It does not correct rated CFM to the actual working pressure, which matters because a tool rated at 90 psig draws more at 100; it does not correct for altitude, which reduces the mass a compressor delivers at the same volumetric rating; and it does not address the distribution piping, whose pressure drop can starve a tool the compressor is perfectly capable of feeding. It does not size the receiver, the dryer, or the aftercooler, select between reciprocating, rotary screw and centrifugal, or evaluate duty rating -- a machine that can make the CFM but is not rated for continuous duty will not last. The compressor and tool manufacturers' data govern.

average demand = the sum over tools of quantity x rated CFM x duty cycle; with leaks = that x (1 + leak allowance); design CFM = that x (1 + growth allowance); motor power is approximated at about 4 CFM per horsepower for a two-stage unit at 100 psig.

The duty-weighted compressed-air demand build-up as standard practice, by name. The duty-weighted sum is the SAME relation the air receiver calculator uses, so the two cannot disagree about what a set of tools draws. A tool's rated CFM is its consumption with the trigger down. The compressor and tool manufacturers' data govern.

A weighted sum on tool ratings and duty cycles the user supplies; no manufacturer table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: tool1_qty, tool1_cfm, tool1_duty, tool2_qty, tool2_cfm, tool2_duty, tool3_qty, tool3_cfm, tool3_duty, tool4_qty, tool4_cfm, tool4_duty, leak_allowance_pct, growth_allowance_pct, cfm_per_hp, connected_cfm, average_cfm, with_leaks_cfm, design_cfm, motor_hp, connected_hp

• Duty cycles are estimates until logged a metered week beats any tablecompressed air practice
• Rated CFM is not corrected to working pressure a tool rated at 90 psig draws more at 100the tool manufacturer's data
• Altitude is not corrected it reduces the mass delivered at the same volumetric ratingcompressed air practice