Duct Pressure Loss Calcs - HELP

[pkendall]

Hi!
Am currently struggling with a calculating the static pressure loss thru a dust collection fitting ... none of the books I am referencing give a clear step-by-step procedure.
The fitting is a "Tapered Hood" type approx 8"x4" (32 sq. in.) inlet with a shallow transition to a 4" (12.6 sq. in.)diameter fitting. Airflow is 350CFM.
What is the most straight forward way of going about this?
Thanks in advance.

pkendall

 

[quark]

That's too low a size for 350 cfm. Generally, I will ignore the losses in the hood and the transition and consider only the duct.

You are getting about 7.5"/100ft.

Get your hands on the ductsizer, a nice little program from Mcquay.

http://www.mcquay.com/McQuay/DesignSolutions/Software

Regards,

 

[pkendall]

Thanks quark ... forgot to mention that in the system we are tryingto design, we want a carrying velocity of 4000fpm ... which corresponds to a 4" dia. duct @ 350CFM.
Will check out the McQuay software.
Have a nice day!

pkendall

 

[CanuckMiner]

pkendall,

"Industrial Ventilation", published by ACGIH, gives hood entry loss factors for various tapers. This publication is one of the best resources when it comes to designing dust collection systems.

Cheers,
CanuckMiner

 

[pkendall]

Hi fellow Canuck (I'm in Nfld.) ... I have a 1983 copy of that very book here on my desk ... fig 6-10 shows factors for hood entry losses, but am having trouble determining the total loss for the described fitting unfortunately ...

 

[CanuckMiner]

Perhaps you should look into updating your library. ;-)

I have the 22nd edition here...1995. Not too recent either, but it does have a Hood Entry Loss factors table/figure dated 1 - 95 (presumably, January 1995).

The table illustrates entry losses (in terms of velocity pressure) for round and rectangular ducts for various included angles, and has a graph illustrating the loss for a transition from rectangular to round.

Cheers,
CanuckMiner

 

[pkendall]

Thanks again for the reply Canuck ... I finally figured it out ... closer scrutiny of fig. 6-10 helped ... also found an easier-to-follow example in the front of the '83 "Industrial Ventilation" ... anyhoo, the Static Pressure of the hood (SPh) = velocity pressure of the duct (VPd) + the entry loss of the hood (he), where he = transition loss factor (Fh) x VPd ... Fh from fig. 6-10 = approx. 0.45

SPh = VPd + (Fh x VPd) VPd = (4000fpm/4005)2 = 0.9975
SPh = 0.9975 + (0.45 x 0.9975)
SPh = 0.9975 + 0.448
SPh = 1.45" (361 Pa)

Thanks again everyone!

 

[JStephen]

ASHRAE handbooks have similar info.

 

[safetydan]

An even better book would be Burton's Industrial Ventilation Workbook. He really makes going through some of the calculations much easier.