1. ## Duct Calculation (ACGIH)

Dear Friends

I am designing ducting system for a dust collector (Bag Filter) according ACGIH in this regard i have questions.

Question:

1- Static Pressure Ratio between two path is : 2.24 and other one is 2.88 i want to know can i balance them with blast gate.

2- in ACGIH example i see that acceleration factor in Hood is 1 ? why ? i didn't any reason for it.

3- what's the maximum ratio for static pressure that we can balance with blast gate?

thanks

2. ## Duct calculation

The branch with the higher static pressure is the one which governs the selection of the system exhaust fan.

Don't worry about static pressure ratio. Just allow for a blast gate in every branch. The final adjustment is made at commissioning and system "balancing" to ensure that each branch carries its designed flow.

Sometimes the S.P. in the governing branch is much higher than elsewhere. Here, experience dictates that a booster fan in that branch will allow the exhaust fan to be sized for the lower branch S.P. This is an economical solution when the governing branch carries a small proportion of the total system flow.

There is no hard and fast rule for maximum S.P ratio that I know of. You must calculate the fan power for the alternatives to find the best solution.

3. ## thanks

Originally Posted by Michael Reid
The branch with the higher static pressure is the one which governs the selection of the system exhaust fan.

Don't worry about static pressure ratio. Just allow for a blast gate in every branch. The final adjustment is made at commissioning and system "balancing" to ensure that each branch carries its designed flow.

Sometimes the S.P. in the governing branch is much higher than elsewhere. Here, experience dictates that a booster fan in that branch will allow the exhaust fan to be sized for the lower branch S.P. This is an economical solution when the governing branch carries a small proportion of the total system flow.

There is no hard and fast rule for maximum S.P ratio that I know of. You must calculate the fan power for the alternatives to find the best solution.
thank you for your response, but i think maybe we will have problem with blast gate for control this S.P ratio.
do you have any standard about set blast gate?
thanks

4. ## Duct calculation

In your calculations, make a small allowance for the S.P loss across the blast gate, say 100 Pa. Setting of blast gates (balancing) is a trial and error execise at commissioning.

The gate in the governing run will be nearly full open. Those closer to the dust collector need to be partly closed to adjust the flows. Setting is an iterative process of measurement and adjustment. You cannot design the final gate position before commissioning.

The attached file is a record of a typical procedure.

Michael Reid.