Vacuum piping questions 1

[mecheHOG]

Hello all,

This is my first post so please bear with me. I am trying to calculate how much vacuum I will have in a series of branching pipe system. My first question is can I use the same formulas and calculations from "branching pipe flows" for pumps or do I need equations from ductwork systems with FTP and FSP?

Next, my vacuum section looks like this:

/-- /-- /--
V---|-------|-------|
\-- \-- \--

I do not know the specs on the vacuum yet but I intend on finding it. The desired suction on each branch is 8 inches of WC. I know the lengths but the pipe size is need to be designed. Sorry if I left anything out, let me know. I would appreciate any insight or help.

Thanks,
mecheHOG

 

[BigInch]

Use absolute pressures. There may be better flow equations of one kind or another that may be more appropriate for your system, depending on what gas is flowing in those pipes. A number of different equations could be used, for example, for pressure drops for low pressure steam flow, or another for very low pressure air flow, or light or heavy gases. You might have another one or two, depending on if its relatively hot or cold, low or high velocity, etc. There are too many possibilities to mention right now, but you could look in the McGraw Hill "Piping Handbook" for some alternate equations which might apply to your specific conditions.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[a234f56]

Dear mecheHOG,

You'll find the attachment too a interesting read !!

 

[zdas04]

That is a useful link for laying piping out. It doesn't speak to calculating the pressure distribution in a piping network. Since the OP mentioned "ducts" I'm going to take a leap and assume that he's talking about air flow in an HVAC system.

Pumping equations are not going to help much, you need to go to gas-flow arithmetic. All of the predominant equations (e.g., Isothermal Gas, AGA Fully Turbulent, Panhandle A, and Weymouth) assume incompressible flow which requires that downstream pressure be a reasonable proportion of upstream pressure (most people use 90%). If your suction is 8 inH2O at sea level (about .3 psia) then if upstream pressure is more than about 9.1 inH2O you have violated that assumption.

There is a compressible-flow equation in the GPSA field data book called the Spitzglass Equation (equation 17-29 in V12) that purports to be appropriate for vacuum flows, but I've never had occasion to compare its results to field measurements. I would probably start there, but it has the limitation of assuming a constant temperature of 60[°]F.

Beyond that you might want to look at HVAC references.

David

 

[mecheHOG]

The vacuum system is being designed but I hit a wall with the pipe diameter. The system is actually vacuuming soap dust (made of mainly animal fat). I got production to admit that they wanted 8"WC of just air at the end of the vacuum hose. I have an EG&G rotron with 7.5hp that I could use with no charge but I cannot find any info about the fan system because the company went out of business years ago. Now, the hose has ID of 1 1/2", what size pipe and what size header with a fan capable of pulling x to get the 8"WC at the hose end? What equations and what assumptions can be made?