duct sizing

[new2hvac]

I suspect this is a pretty simple question but can't seem to get a straight answer. Exactly how do you go about sizing a duct? I've looked at the graphs in the ASHRAE handbooks and it appears that the duct size is inversely proportional to the friction loss per 100 ft of duct.

It does not seem intuitive that the less friction loss you have, the larger the duct would have to be.

Can someone please point out what I'm missing? Specifically, I'm trying to size ducts for a 10-ton unit (~4000 cfm).

 

[DouginMB]

The friction drops as the duct gets larger because the velocity of air moving through the duct decreases. This assumes a constant air volume flow rate (cfm).

I size duct based on a value of 0.1"/100ft for supply and 0.08"/100ft. for return. Based on 4000 cfm, this comes to 21"x21" (or 24" diam. for round). I normally round up to the nearest even dimensions, so I would select a 22"x22" rectangular. The return gives you the same dimensions in rectangular or 25" round.

 

[new2hvac]

That makes sense. Thanks!

Are the values of .1"/100ft SA and .08"/100ft RA rules of thumb? When would you deviate from this?

 

[mzahran]

As you have been told from the other follow up about the friction loss decreases if the duct size gets bigger because of the velocity, but I have a small comment.
There are two ways to size a duct:
1- by using the speed of air method and that is by starting at the end outlet of the duct and choosing the appropriate speed for the outlet and going up to the machine by increasing the speed for example for a residential application the outlet air speed should be around 600 ft/min, of course these values depend on the kind of application.
2- By using the constant pressure drop method and that is done by specifying a value for the pressure drop (friction losses) for the duct and sizing by it. For an example the value of design for a residential application is 0.03 to 0.05 in/100ft and all these values are found in the ASHREA standard.
Personally I prefer to use the constant pressure drop method.


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[DouginMB]

The .1"/100' and .08"/100' are rules-of-thumb that I was taught my first day on the job for sizing low velocity duct. As mzahran points out, this is the constant friction method and works well for most instances. I usually don't deviate from it if I can help it. If I do, it's because there might be space restrictions on the duct size or I'm purposely running higher velocity duct. The velocity method he mentions could be used, but i don't use it except when noise is a critical consideration (like with exposed duct or duct running over a church sanctuary). Most of the duct I design will run in an attic or ceiling plenum where noise is almost never a problem.

The rules of thumb I gave you will keep you out of trouble for "normal" duct systems. For a 10 ton system that would be a 0.5" to 1.5" external static pressure range. Since you're new to HVAC design, it would behoove you to manually calculate the duct static pressure so you get an idea of what is a normal duct system and what's not normal. After doing this enough, you will be able to determine if you need to deviate your duct sizes from the "rules of thumb".

There's also the Static Regain method for sizing duct, but I have never used it or tried to. Look it up in the ASHRAE handbook if you're interested.

 

[mdonner]

I want to refer to DouginMB's last remark: static regain is usually worth using in high velocity duct systems, such as induction units and industrial ventilation.