How to determine duct sizes from calculated pressure drops? 2

[oaskar]

I'm trying to redesign a pneumatic transport system, so I know that the optimum velocity to move my product is 5000 fpm and i know our production rates, my question is how do i know if the current duct sizes are suitable for the new air volume that i assumed.

 

[ChasBean1]

Your title question is different from the question you wrote. For pressure drop based on 5,000 fpm duct velocity, try a ductulator:

http://www.me.ua.edu/me407/f04/E-Ductulator.xls

If you know the duct dimensions, you know cfm, and pressure drop can be obtained from the ductulator.

e.g.,
12" round would be 3,927 cfm with 2.17 in. w.c. pressure drop per 100 linear feet;
10" x 10" would be 3,472 cfm with 2.43 in. w.c. drop per 100 feet;
24" x 24" would be 20,000 cfm with 0.85 in. w.c. drop per 100 feet.

What are the current duct sizes?

 

[oaskar]

Well it's a whole system so there's different sizes but let's take 26" round Sch.40 Steel pipe 40' long with 5000fpm velocity. Does this calculator only apply to Galvanized Steel pipes?

Thank you

 

[willard3]

The only thing that will change is the relative roughness (E/d) and it's not that different for sched 40 pipe than it is for galv. sheet metal ductwork.

In fact, the pipe is a little smoother.....

I'll guess your flow is turbulent.

 

[ChasBean1]

It looks like this calculator has a drop down menu of different roughnesses for different ducts. Unfortunately, I didn't see a way to manually type one in.

The flow through your 26" x 26" duct would be 23,470 cfm, but pressure drop will vary based on the duct: galvy or spiral would be .31 in. w.c. in the 40 foot run; PVC or aluminum .30; 'smooth liner' .42; fiber liner .57; flex duct .49.

They don't seem to vary tremendously, probably because it's a pretty big duct and the bulk of the stream would be unhindered by friction. Maybe use the fiber liner model as a conservative approach (assume a pressure drop of 0.6 in. w.c.).

 

[oaskar]

I'm trying to convey 125lbs/min of dry product pneumatically to a cyclone separator. The optimum velocity is 5000 fpm in a 10"d*100'L pipe. In trying to size a fan with the capacity to do that, I know i can use darcy's on the air part of the flow but what about the product?
thanks

 

[ChasBean1]

Interesting dilemma. It works out to 2,727 cfm of air with a 2.7 in. w.c. pressure drop in the duct. Three notes:

1. Particle size matters.

2. You're asking 205 lb/min of air (density 0.075 lb/ft3) to move 125 lb/min of product (density in maybe the 40-80 lb/ft3 range?). Is that practical or do you need a bigger duct moving more air? I anticipate the product would fill up and dam the duct in short order.

3. We get 2.7" pressure loss in the duct from friction, considering air density alone. What about the pressure loss associated with forcing the air and product into the cyclone? Does the cyclone manufacturer have a spec for inlet pressure required to be effective?

 

[quark]

I presume you are talking of dilute phase conveying. Like suggested earlier, your design depends upon the denstity of product and solids ratio. If you are dealing with plastic pellets (PP or HDPE) scoring(to prevent angel hairs) of the pipes increase the pressure drop.

You will find an excellent article on pneumatic conveying system design from the link below.

http://www.cheresources.com/pnuconvey.shtml