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Building Design and Construction Handbook - Merritt - Ventech!

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13.84 SECTION THIRTEEN<br />

Dual-duct VAV control units are specifically designed for dual-duct systems<br />

that require VAV for perimeter areas. These units feature pressure-compensated<br />

shut-off operation with pneumatic, electric, or system-powered controls, with a<br />

variable-volume cold deck <strong>and</strong> constant-volume or variable-volume hot deck. They<br />

are available with multiple outlets <strong>and</strong> factory-calibrated mixing points. Dual-duct<br />

control units maximize energy savings with complete control flexibility when fans<br />

modulate both heating <strong>and</strong> cooling air flows. Heating <strong>and</strong> cooling air can overlap<br />

to maintain minimum air flows with superior room-temperature control.<br />

13.31.4 Dual-Duct VAV Control Diffusers<br />

These assemblies consist of the dual-duct VAV control unit <strong>and</strong> linear-slot diffusers<br />

(Fig. 13.39). They are used for VAV dual-duct perimeters <strong>and</strong> special-use areas.<br />

They provide all the features <strong>and</strong> benefits described previously.<br />

FIGURE 13.39 VAV terminal with linear-slot<br />

diffuser.<br />

13.31.5 Noise Considerations<br />

Linear-slot diffusers are available<br />

for all VAV systems. These diffusers<br />

maintain the desired co<strong>and</strong>a effect down<br />

to �5% of air flow. They are available<br />

with one- or two-way throws, with up<br />

to four slots at 1000 cfm per diffuser.<br />

They may operate with aspirating electric,<br />

pneumatic or system-powered, unitmounted<br />

thermostats. Factory-installed<br />

fire dampers are also available. Benefits<br />

include elimination of dumping at reduced<br />

flows, ease of relocation, <strong>and</strong><br />

high capacity at low first cost, as well<br />

as reduced control costs with unitmounted<br />

thermostats.<br />

Rooftop packaged units used for VAV air conditioning may transmit high levels of<br />

noise to occupied spaces. Such noise is generally caused by compressor, condenser<br />

fans, or supply-duct exhaust fans. Computerized duct-design programs are available<br />

that indicate the sound generation that can be expected at any point in the duct<br />

distribution system. Examination of the predicted levels gives designers the opportunity<br />

to make design alterations to avoid adverse acoustical conditions.<br />

Location of rooftop units is also important. They should not be installed over<br />

offices, conference rooms, or other critical areas. Units preferably should be located<br />

over storage rooms, corridors, or utility areas, where higher sound levels would be<br />

acceptable.<br />

(H. E. Bovay, Jr., ‘‘H<strong>and</strong>book of Mechanical <strong>and</strong> Electrical Systems for <strong>Building</strong>s,’’<br />

N. R. Grimm <strong>and</strong> R. C. Rosaler, ‘‘H<strong>and</strong>book of HVAC <strong>Design</strong>,’’ McGraw-<br />

Hill Publishing Company, New York; B. Stein et al., ‘‘Mechanical <strong>and</strong> Electrical<br />

Equipment for <strong>Building</strong>s,’’ John Wiley & Sons, Inc., New York.)

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