Utility Load Math

How to Calculate Compressed Air, Steam, and Refrigeration Loads in a Plant

The four load calculations every utility engineer runs: compressed air demand, refrigeration tonnage, steam demand, and boiler load, worked end to end with real units.

Compressed air demand starts with summing point-of-use consumption, not compressor nameplate. Add each tool or process in scfm, apply a duty cycle, then a use factor. A grinder rated 30 scfm running 40 percent of the shift draws 30 x 0.40 = 12 scfm average. Sum every load, add 10 to 20 percent for leakage on a maintained system, and multiply by a diversity factor of 0.7 to 0.85 since not all loads peak together. Ten stations averaging 12 scfm give 120 scfm, times 1.15 for leaks and 0.8 diversity, lands near 110 scfm system demand. The Compressed Air Demand calculator sums stations and applies these factors directly.

Convert that demand to compressor horsepower to size the machine. A rule that holds well is 4 to 5 scfm per horsepower at 100 psig for a lubricated rotary screw, so 110 scfm needs roughly 110 / 4.5 = 24.4 hp, meaning a 25 hp unit at that pressure. Every 2 psi of extra discharge pressure adds about 1 percent to power, so running 110 psig instead of 100 costs 5 percent more energy for no work. Size storage receiver volume at 1 to 4 gallons per scfm; 110 scfm suggests a 240 to 440 gallon tank to ride through demand spikes without short cycling.

Refrigeration tonnage measures heat removal, where one ton equals 12,000 Btu/hr, the heat to melt one ton of ice in 24 hours. For a process cooling load, Q = m x Cp x deltaT. Cooling 40 gpm of water from 55 F to 45 F: mass flow is 40 gpm x 8.34 lb/gal x 60 = 20,016 lb/hr, times Cp of 1.0 Btu/lb-F, times 10 F drop, equals 200,160 Btu/hr, or 200,160 / 12,000 = 16.7 tons. The Refrigeration Tonnage calculator runs this from flow and temperature drop, and the Chiller Load calculator extends it to full building and process loads with diversity.

Chiller electrical draw follows from tonnage and efficiency, expressed as kW per ton. A modern water-cooled centrifugal runs 0.5 to 0.65 kW/ton at full load; an older air-cooled unit sits at 1.0 to 1.4 kW/ton. Our 16.7 ton load on a 0.6 kW/ton chiller pulls 16.7 x 0.6 = 10 kW. Efficiency also reads as COP, where COP = 3.517 / (kW/ton), so 0.6 kW/ton is a COP of 5.86. Part-load matters more than peak: IPLV weights performance at 25, 50, 75, and 100 percent load, and most plants run below 60 percent capacity the majority of hours.

Steam demand sums process heat, then converts to pounds per hour of steam. Q = m x Cp x deltaT for sensible loads; divide the total Btu/hr by the latent heat of the steam to get lb/hr. Heating 500 lb/hr of product by 120 F with Cp 0.6 needs 500 x 0.6 x 120 = 36,000 Btu/hr. At 100 psig, saturated steam carries about 881 Btu/lb of latent heat, so 36,000 / 881 = 41 lb/hr of steam. Add distribution and warm-up losses of 10 to 25 percent. The Steam Demand calculator totals process, tracing, and heat-up loads into a single lb/hr figure.

Boiler load is total steam demand converted to boiler horsepower and fuel input. One boiler horsepower equals 34.5 lb/hr of steam from and at 212 F, or 33,475 Btu/hr. A plant needing 6,900 lb/hr of steam requires 6,900 / 34.5 = 200 boiler hp. Fuel input divides output by combustion efficiency; at 82 percent, input is 6,900 lb/hr x roughly 1,000 Btu/lb net / 0.82, near 8.4 million Btu/hr, or about 8,400 cubic feet of natural gas per hour at 1,000 Btu/ft3. The Boiler Load calculator handles the horsepower and fuel conversions across pressures and efficiencies.

Steam trap loss quantifies wasted steam through a failed-open trap, using orifice flow. The Napier equation approximates discharge as W = 24.24 x P_abs x D^2 for saturated steam, where W is lb/hr, P_abs is absolute pressure in psia, and D is orifice diameter in inches. A 1/8 inch trap orifice failed open at 100 psig (114.7 psia) blows 24.24 x 114.7 x 0.125^2 = 43 lb/hr continuously. Multiply by 8,760 hours and the failed trap wastes 377,000 lb of steam a year. The Steam Trap Loss calculator estimates this per trap so you can rank a survey by leak rate.

Pump energy for cooling water and condensate closes the utility picture. Hydraulic horsepower = (gpm x head in feet x specific gravity) / 3,960. Moving 300 gpm at 80 feet of head for water: 300 x 80 / 3,960 = 6.06 hydraulic hp. Divide by pump efficiency of 0.70 and motor efficiency of 0.92: 6.06 / (0.70 x 0.92) = 9.4 input hp, or 7.0 kW. The Utility Pump Energy Cost calculator turns this into annual cost from run hours and rate, and the Compressed Air Leak Loss calculator applies the same waste logic to air escaping through undersized orifices.

Published 2026-07-01.