Backpressure and Hose Length in non-exhaust application.

[Joseph West]

Hello,

First, thank you for any assistance. I am a mechanic and my Super want's me to design a back-pressure system that will simulate field use of my machines in my shop. We spray several types of fireproofing thru hose's that vary in length between 50 and 500 feet.

We use blower's that put out 15-20 psig and 300-25,000cfm.

I was told by the material manufacturer that the back pressure on a 50' hose with the optimum material in it should be between 3.5-4 PSIG but I can't seem to figure out how to relate that to a longer hose or if 3.5-4 PSIG of back pressure is (or can be considered) a lot of air pressure.

Can you have 5 PSIG with a 15,000 CFM? How does that work.

I am sorry I am not an engineer or physics specialist so much of this I just don't understand so any help is truly appreciated.

Joseph L. West

 

[256bits]

The resistance of movement of fluid in your hose dictates what pressure the blower will output. From the pump you will have pressure P and at the hose end you will have Patm = 0 pressure guage. Along the length of the hose, the pressure is a function of distance from the pump. If the hose is length L, then at the pump there is pressure P, one quarter the way down, the pressure is (3/4)L, all the way to the end where P=0, assuming you do not have many twists, turns and elbows of sorts.

Size and length of hose matters. Longer hoses have more pressure drop ( back pressure ). Smaller diameter hoses have more pressure drop.

To relate PSI and CFM of the blower, you have to look at the manufactuere's fan curve for your particular blower (and rpm). It should translate that if the blower is producing more PSI then it is outputting less CFM( ie longer or smaller hose), less PSI means more CFM (shorter or larger diameter hose).

Can you have 5 PSIG with a 15,000 CFM? How does that work.

Sure, why not. Depends on your blower selection and hose size and length.