Short answer: size the fan on the heat your numbers leave behind, and for this machine that is about 75,000 BTU/h — roughly 1,750 CFM of outside air to hold 80 F [27 C] inside with 40 F [5 C] coming in, so install about 2,500 CFM of delivered flow. A 300 HP shaft load on a 95% efficient motor brings 804,000 BTU/h of electrical energy across the enclosure wall, and 731,000 BTU/h of it leaves again in the 4,100 CFM process stream at 205 F [96 C]. Only the difference heats the room. The popular shortcut — counting 33% of input power as room heat — points to 376,000 BTU/h and an 8,700 CFM fan, five times more machine than this house needs. Here is the boundary math that lands on the right fan.
The boundary decides what stays inside
Draw the box around the motor and blower, then list everything that crosses the skin. Electrical power enters through the motor leads. Process air enters through its own duct at 40 F and leaves through the discharge pipe at 205 F, carrying most of that electrical energy with it. Cooling air enters cold and leaves through the louvers warm, and the walls shed a little more on a cold day. Your fan load is the input minus what the piped stream carries away — nothing else.
The split surprises people the first time. A TEFC motor sheds its own losses off the frame into the room: 300 HP at 95% efficiency loses 300 x (1/0.95 - 1), about 16 HP, or 40,000 BTU/h of warm air at the motor. The blower's inefficiency takes the other road: it heats the gas the machine is already moving, which is why the discharge pipe reads 205 F. That heat rides out to the process and never loads your louvers.
The discharge temperature is your lie detector. If the room truly had to shed 376,000 BTU/h, the process stream could carry only 428,000, and 4,100 CFM would leave at 137 F [58 C] instead of 205 F. A hot pipe leaving the building is the receipt for the energy that stayed with the gas. For a sealed box full of drives and PLCs, the enclosure heat calculation guide walks the standard version of this sum; a blower house differs in one way — its biggest heat customer leaves through a pipe.
Six numbers before you start
Six numbers fix the fan, and five of them are on the datasheet: shaft power at the operating point (300 HP here, 75% of the 400 HP nameplate), motor efficiency (95%), process flow and the condition it was measured at (4,100 CFM at the inlet, 40 F), discharge temperature (205 F [96 C]) and outside design temperature (40 F [5 C]). The sixth is yours: the room band you will hold, 77–86 F [25–30 C] in this case.
Two of the six deserve a second look. The 400 HP on the nameplate is a rating; the operating point is the load, and an ammeter reading settles the difference: 300 HP at the shaft is comfortable. Treating 300 HP as useful air power against 67% combined efficiency implies a 448 HP draw, which pushes the frame past its rating. The flow figure matters at its measurement condition, because mass flow is what the physics conserves: 4,100 CFM at the inlet is about 308 lb/min [140 kg/min], and the same pounds squeeze into roughly 2,550 CFM at the 15 psig [1 bar g] discharge.
The calculation, step by step
- Electrical input. Shaft power over motor efficiency: 300 / 0.95 = 316 HP, which is 316 x 2,545, about 804,000 BTU/h [236 kW] crossing the wall on the motor leads.
- What the process stream takes out. With the 1.08 factor for standard air (0.075 lb/ft3 [1.2 kg/m3], Cp 0.24 BTU/lb-F), the pipe carries 1.08 x 4,100 x (205 - 40), about 731,000 BTU/h, out of the box. For damp intake air, do this term psychrometrically instead — calculating leaving wet bulb from an enthalpy change covers that method.
- What stays. 804,000 - 731,000 = 73,000 BTU/h; call it 75,000 for design. The density basis moves this: at the true 40 F inlet density of 0.079 lb/ft3, the stream carries about 770,000 and only 34,000 stays, so hold the band as margin you will spend in summer.
- The extras. A bare 12 in [300 mm] discharge pipe with 30 ft [9 m] of run inside the house adds about 20,000 BTU/h by convection and radiation, so insulate it or route it out short and straight. Outdoors, add the sun before you fix the fan; the solar heat load walkthrough shows that sum.
- Airflow. CFM = Q / (1.08 x allowed rise). Holding 80 F inside on 40 F air: 75,000 / (1.08 x 40) = 1,736, so 1,750 CFM [3,000 m3/h]. The factor is for standard air; colder intake is denser and carries a few percent more per cubic foot, so the number leans safe.
- Installed margin. Filters load, louvers resist and the density band is real, so buy 40–50% over the sheet number: 1,750 x 1.4, about 2,500 CFM [4,250 m3/h] of delivered flow. Two smaller fans give you redundancy and keep the noise down in what is already a noise enclosure.
| Hold inside | Rise over intake | Cooling airflow |
|---|---|---|
| 77 F [25 C] | 37 F [21 C] | 1,900 CFM [3,200 m3/h] |
| 80 F [27 C] | 40 F [22 C] | 1,750 CFM [3,000 m3/h] |
| 86 F [30 C] | 46 F [26 C] | 1,500 CFM [2,500 m3/h] |
One credit works in your favor. A painted steel house 20 x 12 x 10 ft [6.1 x 3.7 x 3.0 m] sheds about 43,000 BTU/h through 1,120 ft2 [104 m2] of wall at 80 F inside and 40 F outside (k = 5.5 W/m2K), which is more than half the load on the winter design day. Bank it as margin rather than downsizing the fan; summer takes it back.
Buy delivered airflow, not free-air CFM
A fan's free-air rating is a bench number. Add the louvers, guards and a wall sleeve and the system sits near 0.2–0.4 in. w.g. [50–100 Pa] of static, where a typical fan moves 20–30% less than its sticker. Pick from the fan curve at 0.3 in. w.g., and start from the guard airflow loss and grille pressure drop numbers when you add layers of hardware; the fan-cooled enclosure airflow note shows the whole delivered-flow method.
Louvers set the intake velocity, so keep it under about 500 fpm [2.5 m/s] on free area. Your 2,500 CFM needs 5 ft2 [0.5 m2] of free opening; two 36 in [914 mm] square storm louvers at roughly 50% free area give 9 ft2 and run near 280 fpm. Outdoors, hood the intake, fit a filter rack and bird screen, and weigh what louvers do to a weather rating — a ventilated house holds only what the louver assembly holds, which is the whole argument of enclosure vent versus sealed box. For a sealed Type 4X / IP66 house — rated by the manufacturer, not UL listed — the fans disappear in favor of a closed loop, covered in the decision below.
Checks that prove the fans hold the line
Run the machine at full load for a few hours, then read three temperatures. Across the exhaust louvers the rise should land near 75,000 / (1.08 x 1,750), about 40 F; a 55 F rise means about 1,270 CFM is moving, so clean or enlarge the intake path before you blame the fan. At the wall sleeve the discharge pipe should still read about 205 F [96 C]; a cooler pipe is giving up heat to the room along its run. At the motor, the air entering the frame should sit at or below your 86 F [30 C] target.
Lock out and test for absence of voltage before opening the house, and work the fan wiring de-energized like anything else in there.
Where this calculation goes wrong
Six mistakes account for nearly every oversized or undersized blower-house fan, and each one shows its number:
| Mistake | What it costs you | The fix |
|---|---|---|
| All input power treated as room heat | 804,000 BTU/h, an 18,600 CFM fan — ten times too big | Subtract the process stream first |
| 33% of input counted as room heat | 376,000 BTU/h, an 8,700 CFM fan — five times too big | Blower inefficiency leaves with the gas; frame and casing losses stay |
| Nameplate 400 HP used as the load | Residual near 340,000 BTU/h, four times the fan | The operating BHP is the load; an ammeter settles it |
| Discharge CFM multiplied by atmospheric density | Mass flow off by about half | Convert at the inlet: 4,100 CFM x 0.075 = 308 lb/min |
| 15 psig read as 15 psia | Pressure basis off by 2x | 15 psig is 29.7 psia absolute |
| Free-air CFM treated as delivered flow | 20–30% short at 0.3 in. w.g. | Read the fan curve at installed static |
Ventilated or sealed: the decision
Ventilate this house. The 75,000 BTU/h load is 6.3 tons of cooling, and 2,500 CFM through two louvers holds 80 F on a 40 F day with margin to spare. Re-run step 5 at your summer design temperature before you order: with outside air only 10 F below the target, the same load wants 75,000 / (1.08 x 10), about 6,900 CFM, four times the winter figure. Once outside air reaches the target, no fan closes the gap, and that day belongs to closed-loop cooling.
If the spec seals the house — washdown, or fumes that cannot reach the electronics — size the closed-loop gear on the same 75,000 BTU/h, a 6–7 ton class air-to-air heat exchanger or air conditioner. The fan-versus-cooler comparison walks that choice, and closed-loop cooling where fumes are in the air covers the dirty-air case. Had the shortcut been right, 376,000 BTU/h is 31 tons and no practical sealed cooler covers it, which is the strongest reason to close the boundary before you buy.
The enclosure order follows the airflow. 304 stainless suits an indoor or sheltered house and 316L where salt or chlorides reach it, with fan cutouts, louver openings and pipe sleeves called out on the drawing; custom enclosures are built to that drawing and delivered in 10 business days from your approval, and the enclosure selector pairs the size with the environment.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
The datasheet gives a heat-rejected-to-room figure. Can I use it directly?
Yes. A packaged machine's published heat rejection has already done the boundary math, so run CFM = Q / (1.08 x allowed rise) on it. A machine quoting 60,000 BTU/h to the room needs 60,000 / (1.08 x 40), about 1,390 CFM at a 40 F rise. Add your own extras on top: sun, hot piping and any other equipment in the house.
How much of the load is the TEFC motor itself?
Most of it. At 300 HP shaft and 95% efficiency the motor sheds 16 HP, about 40,000 BTU/h, off its frame into the room, which is 930 CFM of your 1,750 at a 40 F rise. Blower casing and bearing losses add a few percent of shaft power, and the rest of the machine's waste heat rides out in the process air.
What happens to the fan size in summer?
It climbs fast as the allowed rise shrinks, because flow is inversely proportional to the temperature difference. Against 95 F [35 C] outside air and an 86 F target there is no rise left and ventilation is done; at a 15 F rise the same 75,000 BTU/h wants about 4,600 CFM. Fix the summer design temperature first, then choose between more fan and a closed loop.
How do I know whether 4,100 CFM is inlet, standard or discharge flow?
Ask for the basis, then convert everything to mass flow. 4,100 CFM at 40 F and atmospheric pressure is about 308 lb/min [140 kg/min]. The same mass at the 15 psig [1 bar g], 205 F discharge occupies about 2,550 CFM, because cubic feet follow pressure and temperature while pounds do not. Run the whole balance on pounds per minute and the ambiguity disappears.
