Double Duct Systems

Where temperatures must be closely controlled in a large number of rooms or zones, double-duct systems (Fig. 26-4) circulate both heated and cooled air to a control box in each zone. Double-duct systems offer superior comfort control and flexibility for simultaneous heating and cooling zones. Double-duct CAV systems use two supply ducts and a separate return duct. Both warm and cool air ducts supply large mixing boxes controlled by dampers. In summer, the cooling air stream is used alone, and in winter, the warming one is used. The mixing boxes proportion and blend warm and cold air to the desired temperature before delivering it to the zone or space.

Air quality in a double-duct system is controlled through positive and negative pressures. Spaces that generate odor, excess heat and humidity, or pollutants, such as kitchens, toilets, or retail pet stores, are supplied with

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mixing box

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Figure 26-4 Double-duct system.

less air than adjacent spaces, resulting in lower air pressure. Air will naturally flow from higher-pressure spaces into lower-pressure ones, and the lower-pressure spaces can then be exhausted directly outdoors, keeping pollutants from spreading into adjacent spaces.

In some spaces, like shopping malls, the corridors of apartment houses, and stair towers, more air is introduced into the space than is mechanically removed. These spaces are kept under positive pressure, so that air tends not to flow into them. This pressurization helps prevent unheated or uncooled outdoor air or smoke from a fire from entering these spaces. Higher air pressures also reduce discomfort from drafts and uneven temperatures from infiltration of air through the building envelope.

Double-duct CAV systems provide the best comfort under conditions where the amount of actual heating or cooling is below the maximum for which the system was designed, as in a partially occupied room. They work less well with perimeter zones in cold climates. They are usually high-velocity systems, so smaller duct sizes can reduce installation space, but they still use a great deal more space and energy than a single-duct VAV system. The mixing boxes are also expensive and require maintenance. Double-duct systems are very expensive to install, and are usually used only in larger buildings, such as hospitals, where a variety of zones with differing needs justifies a separate CAV for each zone.

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