Duct sizing

[WilliamTz]

Hello

I am upgrading an old 1980's Office that uses circular duct throughout for HVAC. I want to retain the old circular duct downstream of the air handler as much as possible up as far as new VAV boxes I intend installing..
I usually use 1pa/m when sizing ductwork and that is how I intend sizing the duct down stream of the new VAV boxes.
However can I go up to 1.5pa/m for the main circular duct serving these VAV boxes with a max velocity of approximately 7.5m/s?...duct are mostly 300-400mm dia. off the air handlers main supply header.

Regards
W

 

[HVAC-Novice]

In a VAV system there is diversity. So the main trunk flow will be less than the sum of all zone peak flows. How much less is hard to determine and depends on the nature of loads. Office with east-west exposure and people going from offices into meeting rooms etc, I'd say diversity could be 70% or even less. but this is hard to estimate. If a VAV serves a 24/7 server closet, obviously this will be 100% most the time - at least you should assume for design. But an East facing office won't have much load in the afternoon....

 

[Snickster]

Recommended velocities in ducts are given in HVAC hanbooks such as ASHRAE, etc. The "Handbook of Air Conditioning Design" by Carrier Corporation gives the following recommended velocities for offices:

Main Supply Ducts:

General Offices = 1500 FPM based on noise, 2000 FPM based on friction pressure drop
Private Offices = 1200 FPM based on noise, 2000 FPM based on friction pressure drop

Branch Supply Ducts:

General Offices = 1600 FPM based on friction pressure drop
Private Offices = 1600 FPM based on friction pressure drop

But I believe that for the frictional pressure drop consideration the governing criteria is intial versus operating cost of the system. In other words if you have smaller duct to provide lower duct cost but higher friction then larger size blower will be required that uses more energy over the lifetime. Therefore as far as limit to friction pressure drop is concerned it is up to you if you want to pay higher cost for blower and operating cost over lifetime and allow for higher pressure drop in system. In case of the noise criteria you cannot exceed limits set or you will have too noisy environment for office workers.

If you are designing HVAC systems you should get some good reference books which go over design requirements for HVAC systems. The one I mentioned above can be found at the following link which I have used whenever I needed to design HVAC systems:

https://www.amazon.com/sp?ie=UTF8&s...illed=0&asin=007010090X&ref_=olp_merch_name_1