Pressure Head loss w/out a given diameter

[iamjustanewbie]

Hi

How can i get the Diameter of a duct or pipe if I only have Flow Rate?

And where did the 0.1inch/100ft of friction head loss comes from? Is it from experience? I can't find it in ASHRAE Handbook.

Thanks in advance.

 

[willard3]

A. You can't


B. Accepted in the Engineering community.

 

[dynamo78]

Kuya, If this is your first time to size duct system then you have to establish the air duct usage, like; ventilation, conditioned supply air or return. then is it industrial? commercial? or residential? after establishing those, look for their corresponding velocities available in ASHRAE and other HVAC resources. Hope that helps in a way.
Other parameters maybe added by other members.

 

[urgross]

I typically try to use 0.08 IWC/100', but that is a matter of considering energy vs. first cost.

If you have determined to use either 0.1 IWC or 0.08 IWC and know the flow, then use the SMACNA HVAC System Duct Design Manual, Duct Friction Loss Chart, to get the equivalent round duct size. If not using round, then go to the rectangular conversion table.

 

[iamjustanewbie]

Thanks for all your reply ^_^

Please correct me if I'm wrong because I am just a newbie in HVAC design.

@willard3: Sir, you are right that I can't get the diameter if I only have flow rate that is why I am looking for the basis of 0.1 inch WC/100ft (supply) & 0.08 inch WC/100ft (return). The example in ASHRAE Handbook Fundamentals 2009, the diameter is given & that is why they were able to get the friction head loss through friction chart or Darcy Weisbach equation maybe. I was able to read some blogs & they say that the most commonly used, (maybe rule of thumb?), is 0.1 inch WC/100ft. So, I am thinking if they got it from their own experience.

@dynamo78: Sir, from your own point of view, which is better in designing Ductworks, Velocity Method or Equal Friction Method?

@urgross: Sir, I was able to read a blog & it says that 0.08 inch/100ft is for the Return Duct & the 0.1 inch/100ft is for the Supply. Is it correct?


Thanks in advance.

 

[urgross]

It's whatever you decide, or your customer requests. None of it is "correct" they are all options.

You might want to look at static regain, as long as you are beginning in HVAC.

 

[dynamo78]

Kuya, Velocity reduction method is ok if you are quite familiar with HVAC ductwork systems. you can used that for relatively small units up to maybe 10 TR, and in open space where air balancing is not that critical.You need a little bit of judgement and experience in there Bro. the best and easier for me is equal friction method. it takes care of balancing the system.
HTH Kuya.but I maybe older than you.LOL

 

[iamjustanewbie]

@urgross: Thank You so much. I will review static regain method.

@dynamo78: Are you also using the 0.1 inch/ 100ft Rule of Thumb for Equal Friction Method?

 

[dynamo78]

Yes, I use .1"/100 ft. its the usual recommended value.

 

[iamjustanewbie]

Alright. Thanks so much for your time answering my questions dynamo78