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Handbook of air conditioning and refrigeration / Shan K

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21.50 CHAPTER TWENTY-ONE<br />

The drawbacks <strong>of</strong> a parallel fan-powered box include the need for a careful balance <strong>of</strong> flow <strong>and</strong><br />

total pressure <strong>of</strong> two <strong>air</strong>streams, the difficulty in selection <strong>of</strong> the size <strong>of</strong> the box, <strong>and</strong> its more complicated<br />

controls.<br />

The size <strong>and</strong> capacity <strong>of</strong> a fan-powered box is determined by the capacity <strong>of</strong> the fan, the VAV<br />

box, <strong>and</strong> the heating coil. The heating coil in a fan-powered box can be a hot water coil or an electric<br />

duct heater located downstream from the fan-powered box. Fan-powered boxes are usually<br />

built in sizes corresponding to the associated VAV box primary <strong>air</strong> volume flow rates from 300 to<br />

3200 cfm (150 to 1600 L/s) <strong>and</strong> fan supply volume flow rates <strong>of</strong> 250 to 4000 cfm (125 to 2000<br />

L/s). Most fan-powered boxes have a fan total pressure between 0.5 <strong>and</strong> 0.8 in. WC (125 <strong>and</strong> 200<br />

Pa) <strong>and</strong> an external pressure loss <strong>of</strong> about 0.25 in. WC (63 Pa) to <strong>of</strong>fset the pressure drop <strong>of</strong> downstream<br />

flexible ducts <strong>and</strong> diffusers. The pressure drop <strong>of</strong> the VAV box <strong>and</strong> the corresponding pressure<br />

drop <strong>of</strong> flexible ducts <strong>and</strong> diffusers due to cold primary <strong>air</strong> should be <strong>of</strong>fset by the supply fan<br />

in the primary <strong>air</strong> AHU or PU.<br />

The speed <strong>of</strong> the centrifugal fan in the fan-powered box can be adjusted to high, medium, or low<br />

or even variable-speed drive. Both radiated <strong>and</strong> duct-borne sound paths should be checked to determine<br />

whether they can meet the space <strong>air</strong> required NC criteria. Because the fan total pressure is<br />

higher in a fan-powered box than in a fan-coil unit, a temperature rise due to fan power heat gain <strong>of</strong><br />

0.75°F (0.4°C) should be considered in psychrometric analysis.<br />

The VAV box a in fan-powered box can be either pressure-dependent or pressure-independent.<br />

Pressure-independent fan-powered boxes are widely used.<br />

Fan Characteristics in Parallel Fan-Powered Boxes<br />

In a fan-powered VAV system using a parallel fan-powered box, two fan-duct systems are connected<br />

in parallel: the cold primary <strong>air</strong>stream discharged from the supply fan F1 in the AHU or<br />

PU <strong>and</strong> the recirculating plenum <strong>air</strong>stream extracted by fan F2, as shown in Fig. 21.15. At the<br />

combining point A, the pressures <strong>of</strong> these two <strong>air</strong>streams must be equal, <strong>and</strong> the volume flow rate<br />

is the sum <strong>of</strong> the volume flow rates <strong>of</strong> the cold primary <strong>air</strong> V˙ sn,c <strong>and</strong> the recirculated plenum <strong>air</strong><br />

V˙ sn,r.<br />

For a control zone in the perimeter zone using a parallel fan-powered box at its zone peak supply<br />

volume flow rate with conventional <strong>air</strong> distribution V˙ sn,d (a supply <strong>air</strong> temperature <strong>of</strong> 55°F, or<br />

12.8°C), in cfm (L/s), the distribution is as follows:<br />

Zone supply volume flow rate <strong>of</strong> control zones<br />

at summer design conditions<br />

Zone supply volume flow <strong>of</strong> recirculating<br />

plenum <strong>air</strong> from parallel fan-powered box 0<br />

According to Yu et al. (1993), the zone supply volume flow rates <strong>of</strong> control zones in the interior<br />

zone using a parallel fan-powered box with cold <strong>air</strong> distribution at summer design conditions are as<br />

follows:<br />

Supply volume flow rate <strong>of</strong> 44°F (6.7°C) cold<br />

primary <strong>air</strong>, cfm (L/s)<br />

Supply volume flow rate from fan-powered<br />

box (recirculating plenum <strong>air</strong>), cfm (L/s)<br />

Zone supply volume flow rate for control zones in the perimeter zone at winter heating design conditions<br />

must meet the following requirements:<br />

Cold primary <strong>air</strong> for ventilation<br />

Supply temperature differential T s � T r<br />

1.0V˙ sn,d<br />

0.6V˙ sn,d<br />

0.4V˙ sn,d<br />

0.3V˙ sn,d<br />

� 15°F (8.3°C)

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