friction loss per 150' of duct 3

[emalsyd]

i'm trying to size ductwork based on a total run of 150'. is there a formula to determine duct sizing or some way to convert the duct sizing on a ductulator to handle the friction loss for 150' vs. the standard 100'?

 

[AbbyNormal]

Usually those ductilators are based on 100 feet of straight duct.

Scale the pressure down by a ratio of 100/150. 0.1" per 100 ft is the same as 0.067 per 150 ft as an example.

Again that is for straight duct, some use equivalent lengths for fittings and add it to the straight lengths for an equivalent length.

I prefer using the coefficients that make fitting losses proportional to the velocity head myself.

 

[katmar]

I suspect that it would be the inverse of what AbbyNormal has said. If the pressure drop is 0.1" over 100 ft, then I would expect it to be 0.15" over 150 ft.

If your actual length is 150 ft and you want the pressure drop to be 0.1", but your ductulator gives the pressure drop for only 100 ft, then calculate the size duct to give you 0.067" over the 100 ft and that should give you 0.1" over the full length of 150 ft.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[AbbyNormal]

Correct I am dyslexic, use 0.067 per 100 to get 0.1 per 150. Good call

 

[emalsyd]

thanks for the help.

 

[quark]

I would prefer 0.1" per 100 ft. In case of return ducting, you can use 0.08" per 100 ft.

The approximate duct size for 1000 cfm and 0.1"/100' will be about 12"x13" and 12"x16" if you use 0.1"/150'. You have to go for 75 sq.ft of extra ducting in this case. However, 0.05" doesn't make much difference with respect to a fan static pressure and subsequent power consumption.

 

[HVAC68]

The initial size of the duct should be based on velocity. Subsequent sizing of the duct downstream can be done based on the same friction - as per equal friction method. Setting 0.1" per 100' cannot be generalised as the velocity varies depending on the air quantity handled.

For 1000 cfm - 0.1" per 100' friction corresponds to 950 fpm.

For 10000 cfm - 0.1" per 100' friction corresponds to 1600 fpm

For 50000 cfm - 0.1" per 100' friction corresponds to 2300 fpm

The best way would be to fix the initial velocity based on the application - I use 1250 to 1500 fpm for comfort air-conditioning and 2000 fpm for ventilation.


HVAC68

 

[willard3]

I use static regain and have a basic program I wrote for calculations. Final pressure calculation is total pressure. Total pressure used in specifications makes it much easier to get the Air Handling Unit that you want.

I find that fitting cost frequently exceeds the cost of keeping the duct the same size.

Balancing is easier with static regain as static is pretty much constant along the duct length.

Fan energy is less with static regain and fan energy is a large fraction of HVAC operating costs

This also decreases velocity downstream of the fan causing less noise in occupied spaces.

 

[lilliput1]

It is not economical to design low velocity system (less than 2000 fpm) using static regain. My recommendation is similar to Quark. Note also that equipment in ductwork, elbows, transition causes more pressure drop than the straight run.
lilliput1