# Convert horsepower to m^3/sec at atmospheric pressure of air

Discussion in 'Hydrodynamics and Aerodynamics' started by Zha, Nov 13, 2018.

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### ZhaJunior Member

Hey,
Does anyone know how much 1hp equals to in cubic feet per minute or cubic meters per second at the pressure of the atmosphere near the ground (needed to find airflow for a fan)? Internet gives 1hp=15.59 cfm and also 1hp=0.69 cfm which one is right? Or I am missing something?
Thank you in advance!

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### Mr EfficiencySenior Member

Not sure what your enquiry is, but 15.59 cubic feet per minute going through a fan powered by 1 hp seems very low. That is a cubic foot in 4 seconds !

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### DCockeySenior Member

The power required depends on the size of the fan and total pressure increase across the fan as well as the volumetric flow rate. What is the reason for the question? For a design? For a school assignment? Or something else?

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### JamesG123Senior Member

Sounds like a skewl assignment.
Also;
"pressure of the atmosphere near the ground" does not mean anything. "Near the ground" at sea level and the peak of Mt. Everest are totally different. But I guess you mean sea level since that is the default in engineering quizes.

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### baeckmoHydrodynamics

If it is an axial (=propeller) fan, operating in a free volume, just "stirring" the air, there is a strict relation between shaft power and propeller disc area; nothing else! This can be treated by traditional propeller theory. In this, there is but one unknown factor, which is the pumping efficiency, aka "fluid-mechanical efficiency" of the machinery, be it a single rotor, or a combination of rotor and stator. This factor is generally established from experiment; but a reasonable starting point for a decent blade shape lies in the range of 75 to 85 %.

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### HeimfriedSenior Member

If you multiply the flow rate in m³/s by the increase of the total pressure of the air [Delta p (total) = p (total after fan) - p (total befor fan) ] in Pascal (= N/m²) you will get a (physical quantity) power in Watt. This theoretical power divided by the efficiency factor of the ventilator gives the needed power.

To obtain the Delta p (total) you need to know some things about the resistance in the path of the hauled air. This path is normally extended from intake from the free atmospheric air to the outlet into free atmospheric air. Often you will find the resistance of single obstacles in the air path (as grille, duct, bend tube, etc.) given in pressure loss in Pascal (all depending on the flow rate). So mostly you will be able to sum this losses up to get a reasonably value of Delta p.

Link with link:
Engine Room ventilation - Air out issue https://www.boatdesign.net/threads/engine-room-ventilation-air-out-issue.60351/#post-830685

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### DragonpointJunior Member

Does anyone know how much 1hp equals to in cubic feet per minute or cubic meters per second at the pressure of the atmosphere near the ground (needed to find airflow for a fan)? Internet gives 1hp=15.59 cfm and also 1hp=0.69 cfm which one is right? Or I am missing something?
You can actually have a One HP Fan evacuating an Engine Room and no air moves at all.
The Room has no air inlet , and so the fan runs and runs , but doesn't move any air , the inside exists in a sort of vacuum. Eventually what happens , the
electric fan seizes , because cooling air is not present .

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### DragonpointJunior Member

Does anyone know how much 1hp equals to in cubic feet per minute or cubic meters per second at the pressure of the atmosphere near the ground (needed to find airflow for a fan)? Internet gives 1hp=15.59 cfm and also 1hp=0.69 cfm which one is right? Or I am missing something?
As an Aero engineer , what you want to find out is , can the fan evacuate a room which contains 15.59 Cubic Feet of Ambiant Air ?
The second number definitely erroneous , it means nothing .
This equation is an Important one if you know an exhaust port , or exhaust pipe , is delivering 15.59 Cubic Feet of Exhaust per Minute .
You need fresh air , especially inside a marine hull , because it will fill up with engine exhaust otherwise.
Let's say the Engine Room measures 5 feet x 5 feet , square ( Cubed ) .
5 x 5 is 25 Cubic Feet .

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### gonzoSenior Member

To get any circulation there has to be an inlet and outlet.
Dragonpoint: The exhaust in a marine installation always goes outside the engineroom.

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