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Handbook of air conditioning and refrigeration / Shan K

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22.6 CHAPTER TWENTY-TWO<br />

Recirculating<br />

damper<br />

Exhaust<br />

damper<br />

rt<br />

2.0<br />

pt , in WG<br />

1.0<br />

p o<br />

p ro<br />

0<br />

�1.0<br />

�2.0<br />

�3.0<br />

�4.0<br />

O<br />

Return<br />

fan<br />

Return duct<br />

�p r,s<br />

Exhaust<br />

<strong>air</strong><br />

Return<br />

<strong>air</strong><br />

�p rf<br />

rfi<br />

Mixing<br />

exhaust<br />

section<br />

ru rec<br />

rfo<br />

�p ret<br />

O<br />

Minimum outdoor <strong>air</strong> damper<br />

Mixed plenum<br />

Supply fan<br />

m cc<br />

Economizer<br />

<strong>air</strong> damper<br />

Recirculating/<br />

exhaust chamber<br />

AFD AFD<br />

Outdoor<br />

<strong>air</strong><br />

�p om<br />

P2<br />

�p sf<br />

sfi<br />

sf<br />

sfo<br />

Supply duct<br />

�p sf,var<br />

Supply <strong>air</strong><br />

�p sf<br />

�p var <strong>and</strong> fixed part �p fix, in in. WC (Pa), <strong>and</strong> it can be calculated as<br />

2 �psys ��pvar ��pfix � RvarV˙ var ��pfix<br />

� (�p r, s ��p PU ��p sd,var) ��p fix<br />

� (�p r, s ��p sf,var) ��p fix<br />

(22.1)<br />

where �pr,s � pressure loss <strong>of</strong> return <strong>air</strong> system, in. WC (Pa)<br />

�pPU � pressure drop across PU, in. WC (Pa)<br />

�psd, var � pressure loss <strong>of</strong> supply duct system before duct static pressure sensor, in. WC (Pa)<br />

�psf, var � pressure loss <strong>of</strong> variable part <strong>of</strong> supply fan, in. WC (Pa)<br />

Rvar � flow resistance representing variable part <strong>of</strong> VAV system, in. WC/(cfm) 2 (Pa� s2 /m6 )<br />

� supply volume flow rate <strong>of</strong> VAV system, cfm (m3 V˙<br />

var<br />

/s)<br />

In a VAV system with a supply <strong>and</strong> return fan combination connected in series, the supply<br />

volume flow rate flowing through the supply fan is different from the return volume flow rate<br />

flowing through the return <strong>air</strong> fan. This makes the pressure-volume operating characterirstics more<br />

complicated.<br />

If the pressure sensor is installed closer to the supply fan, the fan power savings at reduced volume<br />

flow decreases because <strong>of</strong> the increase in �pfix. The set point <strong>of</strong> the static pressure sensor may<br />

vary from 0.7 to 1.5 in. WC (125 to 375 Pa).<br />

In a VAV system serving a single floor in a multistory building, if the maximum design <strong>air</strong> velocity<br />

in the main duct vmax is 3000 fpm (15 m/s), <strong>and</strong> if the length <strong>of</strong> main duct is 150 ft (45.7 m),<br />

pt<br />

d<br />

P1<br />

VAV<br />

box<br />

�p sf,fix<br />

�p sf,fix<br />

FIGURE 22.1 System pressure diagram for a supply-return fan combination <strong>air</strong> system (connected in series) (rfi �<br />

return fan inlet; rfo � return fan outlet; sfi � supply fan inlet; sfo � supply fan outlet; �p rf � return fan total pressure;<br />

�psf � supply fan total pressure; AFD � adjustable-frequency, variable-speed drive).<br />

s<br />

Space<br />

p r<br />

p ro � 0.03 in WG<br />

p o

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