pressure drop across 2 duct silencers

[airductservices]

I have just completed a university project. Air balancing contractor is coming back with excessive resistance on REF1.
REF1 is a booster behind the mixing box on 30ton air handler. REF1 is rated for 11,000 cfm. On each side of REF1 I have and Industrial accoustics 5ES silencer 42x36-60 with a face velocity of 1000fpm and a static pressure drop of .24

The ductwork is the same size as the silencers so I cant see how this is correct or why they are not a little bigger to allow less static given the fact that there is 1 each side of the fan

AIr balance contactor states that of the 11,000 we are only getting about half that at the air hadler. Can anyone do a quick calculation on this to see if a mistake was made on the plans? system design is .75 TSP @ 11,000cfm current reading is 5,340cfm .73 TSP

 

[willard3]

There is not enough information given to hazard a guess.

What is:

The fan size
Fan hp
Fan Type
The ductwork layout

Fan laws will be your friend.

 

[ChrisConley]

As a quick comment (really not enough information as willard points out), I'd say that the pressure drop of your silencers likely assumes ideal inlet conditions. If you don't have a long length of duct up and downstream, the actual pressure drop of the silencer will be much higher.

 

[DrRTU]

Actually, I think your duct is oversized to fit the sound traps. The face on an 5ES is 1048 FPM, the selection seems correct. The DP is correct at 0.24" You stated fan selection for the system TSP is .75", this seems wrong. The traps alone are taking .48 at full flow. You still have the duct, inlet/outlet, AHU and grilles to account for. It sounds like and incorrect fan selection and an odd selection of duct size for 11,000 CFM. Is this VAV or CV? I guess by the name, this is a return / exhaust fan? We would need a lot more information to get the root cause of low flow. As pointed out by others, inlet and discharge conditions will greatly affect DP. These traps have flow arrows in both directions.

 

[MintJulep]

I am assuming that the .24 DP on the silencers is at the rated flow.

If you are getting only half the flow, the DP will be much less.

So the question to ask is "what is eating the fan's pressure?"

Of course, it could be something stupid-simple. Like the fan turning backwards.

 

[mgmasc]

any air balancer worth his salt would have given you (at a minimum) a complete static pressure profile which would include the delta P across each component in the duct system. To walk away and only furnish air flow and fan pressure is pathetic.
A good balancer hangs in there and helps trouble shoot the problem.

 

[airductservices]

Ok here it goes. Fan size is 5hp belt driven 32" diameter 10 blade . I have 2- 26"x26" ducts coming into a 36x42-36 duct. at the end of that duct is the first silencer 36x42-60. then a 2 ft square to round into the fan 36x42-32". out of the fan an identical square to round to another identical silencer. then duct kicks down from that silencer down to 20x72 duct into the mixing box on the air handler. on the output side of the air handler I also have another silencer of the same size. my output ductwork is 20x72 transitions to 36x42 to the silencer then back down to the 24x48 as per the plans. The original plans called for a 24x48 duct on the return also. but given the space, I could not transition up to 36x42 within the space I had so I made the duct the smae size as the silncer there.

THis is a VAV system. with VF drives on REF1 and AHU1 driven by automated logic.

 

[airductservices]

And yes DR.RTU this is a return exhaust fan. I have 2 24x24 intake ducts on the top of airhandler going 70 feet to the exterior of the building and a 20x72 exhaust duct that transitons to 24x48 then out to the exterior of the building seventy feet also.

Tomorrow I am going to take a scan of this section on the print and post it.

 

[willard3]

Look at system effects from AMCA manuals.

If this stuff is too close to the fan outlet, the fan will not reach full capacity because the pressure drop is way higher than you think it is. First devices, including nasty fittings, should be 8-10 feet from the fan discharge on a straight duct to let flow develop.

A fan curve will also help with this problem; do you have one? Is this a FC, BI, Airfoil, Radial blade?