DUAL DUCT SYSTEMS

[mechanicaldude]

A project I'm working on is being served by a 40,000 cfm dual duct air handling system. This is my first time actually working on such a system. The Cold deck is sized as a 40" round duct and the hot deck is about a 36" round. Can someone tell me how you typically size the cold and hot decks for such a system. Do both ducts need to be sized for 100% of the supply, or 50% hot deck and cold deck ...?

If I have a dual duct box that supplies air to a space that requires 3000 cfm how much of that is from the cold deck and how much is from the hot deck.

Thanks for your help.

 

[GMcD]

It depends on the zone being served and whether it requires both heating and cooling (perimeter zone, seasonal) or cooling only, and the hot deck is used for reheat trim. How did you calculate the 3,000cfm? Cooling load or heating load?

 

[mechanicaldude]

Well, the 3000 cfm is a the air flow for the existing space according to the design documents I was givin. I'm trying to reuse these Dual Duct boxes where ever I can in the new space renovation, but I want to make sure I don't exceed the total airflow that the ducts and the system can handle. So I'm trying to get a general idea of how these things are sized. What about the main ducts too? what percentage of the total AHU cfm is for the hot deck and cold deck ?

 

[GreaterSDAC]

this system should be a VAV system....so there will be a diversity in your main duct sizing........As for the 3000 cfm box, if it serves an exterior zone....rule of thumb here in California is using a 65% diverstiy of the 3000 cold deck supply cfm to find the hot deck cfm....so if you have 3000 supply cfm your hot deck side would be 1950 cfm....for interior zones...your diversity would be 30% to find the hot deck cfm. Does that make sense??

 

[imok2]

You need to say if it's a vav system of a constant volume. A constant volume is sized at a 50% of cold deck and 50 % hot deck based on the cooling and heating loads. The chances of all the mixing boxes all calling for cooling or all heat are slim. So if the mixing box is maintaining the proper set point temperature in a room (theoretically) the mixing box dampers are both at a 50% open and suppling the proper amount of cfm at the proper temperature. With a vav dual duct system you need to consider a diversity factor which is around 70%

 

[CptHk]

cd,

We are just finishing a design of a renovation that includes a dual duct constant volume system. In our case we are modifying to a VAV system and utilizing the existing distribution and making all necessary modifications.

It sounds like you should dig a little deeper to find out the original intent of the design and what is it you are trying to achieve now with the redesign/renovation. Maybe VAV is possible, maybe even modifying some ductwork and adding an extra fan to use the return air to be the hot deck supply when available.

We could not do the latter due to space constraints.

 

[mechanicaldude]

Its mostly an interior remodel so making changes to the AHU isn't really feasible. I'm guessing that they are constant volume mixing boxes because there is only one fan on the supply. The dual duct boxes they list on the equipment schedule are Agitair. Anyone ever hear of them. The equipment schedules aren't too clear or thourough so it's hard to find out much about the boxes they used.