A dual duct system provides constant airflow to the spaces it serves. The dual duct box has a MOD for each deck (hot and cold) and both modulate to maintain space temperature. With dual duct, both fans are operating and you have roughy twice the amount of ductwork. The hot deck is usually 80% the cfm of the cold deck.
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.
There was an interesting article in the ASHRAE Journal sometime back. If you can access it through the ASHRAE Website it may be worth the read:
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.
Actually this is a high school and we have been consulted to do a review of a proposed design. I myself have never done a dual duct system either. I understand how each system functions i was just wondering if there are any benefits dual duct has over single duct vav. To me it seems the dual duct system costs more upfront and there are no energy savings or other benefits (iaq, comfort, etc) to warrant that additional cost. And about the code issue, around here ashrae 90.1 is the energy code and the building as a whole will meet code if it is shown to be as efficient as a base 90.1 design building using the ECB method in 90.1.
Where I have seen dual duct systems are in places that require constant volume or instances required to maintain pressure balances. Hospitals and labs in particular. For instance with a lab exhausting a lot of air, if all the makeup air was cooled to 55F and dumped into the space it would overcool and make it a little chilly in the space. A dual duct box could repond quicker to a lab switch being flicked on for instance. Single duct with reheat would be better suited for true variable volume situation but I guess could be used to accomplish the same thing (you would just have to pipe hhw to each box). Better apps for the single duct system would be offices, etc. For labs with a lot of exhaust hoods in particular, you can use a dual duct with smaller box (over the single duct vav) if the thermal load air volume is less than what is required for makeup air volume (the box would get the makeup air through BOTH inlets). Big issue is application and space available.
Dual Duct HVAC is best used when numerous spaces with highly variable sensible loads are combined with the need for high flowrate ventilation.
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.
This is probably not helpful but in my opinion, it is hard to beat a good old fashioned VAV w/ reheat at box system in a school. I can have one room at 85 degrees in heat mode and the room next door at 65 degrees in cooling mode. It is typically cost effective and offers a tremendous level of control. Schools do not typically need constant volume flow rates with the exception of ventilation. Maybe constant volume boxes at exterior zones with heat but other than that, straight VAV is great. Ventilation is easily controlled and provided by this single duct system. Facilities guys typically like this also because it is simple and reliable. The simplier the system and controls, the better for most school districts.
Dual-path ventilation systems offer a unique
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.
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