# Motors, Horsepower, RPM and Litres per minute

How do I get a rough estimate of how many litres per minute a motor can push?

• Volume delivered per revolution * rpm Commented Apr 28, 2018 at 5:43
• As much as you want. Commented Apr 28, 2018 at 15:31

Formulating a relation will depend on the type of the pump. But if you find out the volume delivered in one revolution and multiply it by rpm , you can get volume output per minute.

• Volume swept and volume delivered are not always equal - compressibilty may need to be accounted for... Commented Apr 28, 2018 at 6:40
• Sorry for the mistake. Commented Apr 28, 2018 at 9:16

First order approximation: Do it as a work problem.

Power = energy/time

so Energy = power * time.

Energy = Work = force * distance = pressure * volume. (Do the dimensional analysis...)

So power * time = pressure * volume

rearranging again

power/pressure = volume/time

The rest is picking a compatible set of units so you don't end up with something weird like board-feet per fortnight.

So given a 5 horse motor, and a 100 foot head, how much water can I move?

1 hp = 746 watts 5 hp = 3730 watts

100 foot head = 43 psi.

If you leave it in psi you will end up with a bizarre unit. I think it would be kgm/lbft*s which gives me a headache trying to visualize.

Easier to conver 43 psi to about 280 kPa

So 3730 W/280 kPa =.013 cubic meters per second or about 13 liters per second.

Add 5% losses to the pump. 10% pump loss. 10% hydraulic losses and you get about 10 liters per second.