Horsepower to BTU/h - Generator Room Heat Rejection

Sizing a Plant Room Around the Heat a Generator Engine Throws Off

A standby set is specified in electrical kW, but the plant room has to cope with the engine behind it. Ventilation openings, radiator discharge ducts and the room's temperature rise are all worked out in BTU per hour, so the first move is to put the engine's horsepower rating into the same unit as every heat-rejection line on the manufacturer's data sheet.

Conversion factor: 1 hp = 2,544.433579 BTU/h. The 150 hp diesel behind a 100 kW set therefore delivers 381,665 BTU/h of shaft work — and since that is only about 36 % of the fuel it burns, roughly 680,000 BTU/h more has to leave through the radiator, the exhaust and the room air.

The Four Places That Heat Goes

Radiator discharge

Jacket water and charge-air cooling take roughly a quarter of the fuel energy, near 1,840 BTU/h per engine hp. It leaves in the radiator air stream and must be ducted straight outdoors, never recirculated.

Exhaust gas out of the building

Around 30 % of the fuel — about 2,120 BTU/h per hp — goes up the stack. It never loads the room air, but any uninsulated run of pipe inside the building hands part of it back.

Radiation off the engine

Block, manifolds, turbo and lagging shed on the order of 4 % of the fuel, near 283 BTU/h per hp, directly into the room. This is the number that drives ventilation air, not the radiator figure.

Alternator losses on top

The generator end is 92–96 % efficient, and everything it loses becomes room heat: a 100 kW set adds roughly 26,000 BTU/h that appears nowhere in the engine's own heat balance.

From the Engine Rating to Louvre and Duct Sizes

Work the numbers in the order the installation drawing needs them: shaft power first, then the heat streams, then the air that has to carry them away.

Enter the engine's rated output

Put the shaft rating from the engine sheet — 150, 510, 1,460 hp — into the hp field. The BTU/h figure appears immediately; a comma is accepted as the decimal mark and spaces typed inside the number are ignored.

Scale it into the heat streams

Shaft power is about 36 % of the fuel, so the other branches follow from the same base: multiply engine hp by roughly 1,840 for jacket water, 2,120 for exhaust and 283 for radiated heat to get each stream in BTU/h.

Turn room heat into airflow

Divide the heat staying in the room by 1.08 times the temperature rise you will accept in °F. Radiated 42,000 BTU/h plus 26,000 from the alternator, held to a 15 °F rise, needs about 4,200 CFM through the louvres — before the radiator's own air.

Swap to read the data sheet back

Manufacturer tables publish heat rejection in BTU/h. Press the swap button (↔) to run BTU/h → hp and see what engine size a published figure implies — handy when a ventilation table arrives without the engine model on it.

Do not add the radiator load to the room: a set-mounted radiator pushes its air out through a duct to a discharge louvre. If that duct leaks, or the discharge recirculates back round to the intake, the same heat re-enters the room and the engine overheats at a fraction of its rated ambient.

Heat Rejection by Generator Set Size

Typical figures for diesel sets at full load. The shaft column is the exact hp → BTU/h conversion; the rest are the heat streams that follow from it at a nominal 36 % brake thermal efficiency. Check them against the specific engine's published data before ordering louvres.

Set rating	Engine	Shaft power (BTU/h)	Jacket water (BTU/h)	Exhaust (BTU/h)	Radiated by engine (BTU/h)
20 kW	30 hp	76,333	55,000	64,000	8,500
60 kW	90 hp	228,999	165,000	191,000	25,000
100 kW	150 hp	381,665	276,000	318,000	42,000
200 kW	290 hp	737,886	533,000	615,000	82,000
350 kW	510 hp	1,297,661	937,000	1,082,000	144,000
500 kW	730 hp	1,857,437	1,342,000	1,548,000	207,000
1,000 kW	1,460 hp	3,714,873	2,683,000	3,097,000	413,000

Read down the last column and the ventilation problem is obvious: a 1,000 kW set sheds more heat into the room by radiation alone than a 60 kW set produces as electricity. Read across a row and the cooling problem appears too — jacket water is a load comparable to the useful output, and it has to move through a fixed louvre area at whatever face velocity the acoustic attenuators leave you.

How This Helps on the Installation Drawing

Walk a whole set schedule in one pass

Both fields stay editable and update as you type, so a page of engine ratings can be converted one after another without resetting anything between entries.

Read a heat-rejection line backwards

The swap button turns a published BTU/h figure into horsepower, useful when you want to know what engine a supplier's ventilation table was actually built around.

Electrical and mechanical units side by side

The searchable dropdowns hold kW and MW as well as hp, PS, BTU/min and TR, so the set's electrical rating and the engine's mechanical one can be compared without leaving the page.

Figures formatted for a schedule

Thousands are spaced for legibility at the millions-of-BTU end, and the copy button hands over the plain number, ready to drop into a load sheet cell.

Engine Room Ventilation Questions

How much of a set's heat actually stays inside the plant room?

Far less than the total rejected. The exhaust leaves through the stack and the radiator air is ducted out, so what remains is the engine's radiated share — roughly 4 % of the fuel — plus the alternator's losses, plus whatever an unlagged exhaust run and silencer give back. For a 100 kW set that is on the order of 68,000 to 90,000 BTU/h, against more than a million BTU/h passing through the room in total.

What airflow do the intake louvres need for a given engine size?

Two demands add up. Cooling the room takes BTU/h divided by 1.08 times the allowed rise in °F — about 4,200 CFM to hold 15 °F on that 100 kW set. The radiator then draws far more: shifting 276,000 BTU/h of jacket heat across a 30 °F air rise is roughly 8,500 CFM, which is why louvre free area, not the room's own heat, usually sets the opening size. Combustion air, around 2.5 CFM per hp, is small by comparison but still has to be there.

Do alternator losses have to be counted separately?

Yes. Engine data sheets stop at the flywheel, so the generator end's copper, iron and windage losses appear in none of the engine's heat-rejection lines. Take the electrical output, divide by the alternator efficiency, and the difference is heat: 100 kW out at 93 % means about 7.5 kW lost, which is 25,600 BTU/h dumped straight into the room air. Bigger machines are more efficient in percentage terms but shed more in absolute terms.

How warm is the room allowed to get before the set derates?

Ratings are declared at standard reference conditions near 25 °C, and radiators are quoted with an ambient capability — commonly 40 to 50 °C at the core intake. Above that, output falls by a few percent for every 10 °C. Designing for a 5 to 8 °C (10–15 °F) rise over the outdoor design temperature keeps the intake inside that declared capability on the hottest day, which is exactly the day the set is most likely to be called on.

When does the job need a remote radiator instead of the set-mounted one?

When the discharge path costs more resistance than the engine-driven fan can push. Those fans typically tolerate only about 0.5 in w.g. of external static pressure, which a long duct run, a tight bend and an acoustic attenuator use up quickly. A basement room with no external wall, a heavily attenuated enclosure, or a set sitting far from any louvre are the usual cases — the price is an electrically driven remote cooler, its own supply, and a pumped circuit to maintain.

Horsepower to BTU per Hour