Dual Duct VAV vs. Single Duct VAV w/Reheat
Dual Duct VAV vs. Single Duct VAV w/Reheat
(OP)
Could someone please explain the benefits of one system over the other?
Thanks
Thanks
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Dual Duct VAV vs. Single Duct VAV w/Reheat
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RE: Dual Duct VAV vs. Single Duct VAV w/Reheat
Dual duct systems are designed by static regain calculations and use high velocity air transport when compared to a "VAV" system which may be designed with equal friction methods which I tend to do.
I recently designed a replacement AHU for a dual duct system serving a hospital laboratory. The AHU submittals are on the way as I type ..... The unit is designed for additional capacity in the event they need it and incorporates an enthalpy wheel heat recovery. It weighs 45,000 lbs. These units are custom AHU's not a typical Trane climate changer or packaged rooftop unit. We replaced 2 of the 5 dual duct AHU's in that hospital in the last 5 years. Those existing units were built in place and used a concrete pad as the base of the air handler and tongue/groove walls.
The ASHRAE handbook does not give much info on dual duct systems. They went by the wayside since the energy crisis of the 70's although there is a slew of them still operating in the northeast USA. They were the choice in those days.
I have never designed a dual duct system and the preferred system of a hospital is constant volume reheat.
I'd have to check the energy codes if a DD system is even allowed these days.
What type building are you contemplating?
With vav/rh, you need an airflow station measuring and logic to adjust the OSA as the supply cfm modulates. You also need a method of fan modualation just like a DD AHU.
Typical min reheat position is 50% of cooling however beware that if a space needs 100 cfm and you try heating with 50 cfm it just won't work. You need to tailor the min heating setpoint.
RE: Dual Duct VAV vs. Single Duct VAV w/Reheat
Dual Fan, Dual Duct Goes to School.
Warden, David
ASHRAE Journal; v46 n5 , p18-20,22,24-27 ; May 2004
Describes this type of system, citing a Canadian installation that enables the school to use fifty percent less heat than the next most efficient school in the system. Advice for applying the system in other climates and building types is offered.
RE: Dual Duct VAV vs. Single Duct VAV w/Reheat
RE: Dual Duct VAV vs. Single Duct VAV w/Reheat
RE: Dual Duct VAV vs. Single Duct VAV w/Reheat
VAV adds some energy savings, but by sacrificing desired flowrates and ventilation.
Constant Volume is plagued with the massive control and capacity requirements of sub-cool and re-heat - a certain, but hugely wasteful process.
Multi-zone units suffer from numbers of zones that can be handled, and the dedicated ductwork per zone. A point of diminishing returns is met when numbers of multi-zone units proliferate, and separate duct runs fill the space.
Dual-Duct can handle these circumstances at an advantage. A school with different size rooms, and differing internal loads could be a likely candidate. Ventilation needs are high and constant. I have also seen this in many animal facilities - where environmental requirements for diverse species vary from room to room, yet ventilation flow rates remain high.
Digital controls are easy and cheap, and can correct for the deficiencies of the other systems to a large degree. So, admittedly, dual-duct systems are not very common anymore. Yet, there are inherent advantages with some applications.
RE: Dual Duct VAV vs. Single Duct VAV w/Reheat
RE: Dual Duct VAV vs. Single Duct VAV w/Reheat
benefit. Ventilation air is delivered not only in the
primary airstream from the central system but also
in the local secondary recirculation air from local
air sources, such as the return plenum. As the example
will show, secondary recirculation greatly
improves system ventilation effi ciency, which
reduces outdoor air intake fl ow compared to singlepath
ventilation systems.
For more on this go here: http://