Tricky Duct Static Loss Calculation

[BronYrAur]

I have been challenged to correct a horrible ductwork installation. I have a host of problems causing me to lose static, but the biggest is described below. I will do my best to explain. Anyone know how to calculate the friction loss in the following ductwork:

3' section of 206" x 21" vertical plenum that drops out of a 54,000 CFM AHU.

A transition from 206" x 21" to 40" x 40". This transition takes place over about 6' of vertical distance. It is a very irregular fitting. If it were piping, I would describe it as an eccentric reducer. It's not quite straight on one side, but the vast majority of the reduction takes place from one end of the long 206' run. Hope that makes sense.

The 40" x 40" duct is only about 3' long. The air dead-heads into the bottom of a 40" x 40" x 40" tee with no turning veins. Coming out the two sides of the tee, the duct expands up to 54" x 20" on one side and 84" x 42" on the other.

I am trying to calculate how much static I am losing on this crazy layout. My AHU is designed for 54,000 CFM at 3" ESP.

 

[AbbyNormal]

Can't measure static before and after?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[BronYrAur]

I knew someone would come up with that smart-#%& answer. Just kidding! Yes I can and will. Just thought I would see what it looks like on paper.

 

[AbbyNormal]

it was not a smart ass answer, I took a stab at it at another thread. Can't remember the type of blower, maybe scan a aketch

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

then to account for the difference between what you calculate vs the big drop you measure, call it 'system effect'

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[BronYrAur]

Thanks, I was just kidding with the smart-ass remark!

 

[lilliput1]

I estimate 0.87" wg pressure drop to the 54"x20" branch assuming it handles 11,000 CFM and 0.32" wg pressure drop to the 84" x 42" branch assuming it hanles 43,000 CFM. I included fire dampers out of the airstream at each duct. You should verify each with actual measurment since theoretical callculations assume straight air flow in & out each fitting.

 

[Auth]

In fact this can not be calculated precisely, because each loss parts are too much close, it is hard to calculate interaction loss between these parts.

I calculated the max flow speed which is about 25m/s, the main losses are at the parts of tee and expansion, use a approximate part to estimate them.

If the loss is important, a CFD software can give a more precise result.

System design/optimization for Airplane Environment Control System
and Evaporating refrigerating system

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