Handbook of air conditioning and refrigeration / Shan K
16.01.2013 • Views
Share Embed Flag

Handbook of air conditioning and refrigeration / Shan K

Handbook of air conditioning and refrigeration / Shan K

Handbook of air conditioning and refrigeration / Shan K

SHOW MORE
SHOW LESS
ePAPER READ
DOWNLOAD ePAPER
gmpua.com

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

START NOW

21.48 CHAPTER TWENTY-ONE<br />

Fan-Powered VAV Boxes<br />

A fan-powered VAV box, or simply a fan-powered box, consists <strong>of</strong> a small forward-curved centrifugal<br />

fan, a VAV box, a heating coil (hot water or electric heater), a low-efficiency disposable filter,<br />

dampers, an outer casing, <strong>and</strong> corresponding controls, as shown in Fig. 21.14. Fan-powered VAV<br />

boxes can be classified as series <strong>and</strong> parallel boxes according to their construction <strong>and</strong> operating<br />

characteristics.<br />

Series Fan-Powered Box. In a series fan-powered box, a small centrifugal fan is connected in series<br />

with the cold primary <strong>air</strong>stream flowing through the VAV box, as shown in Fig. 21.14a. The<br />

volume flow <strong>of</strong> the cold primary <strong>air</strong> is varied at part-load operation, but after this <strong>air</strong> mixes with the<br />

induced plenum recirculating <strong>air</strong>, the fan delivers a continuously operated, nearly constant-volume<br />

flow rate <strong>of</strong> the mixture <strong>of</strong> cold primary <strong>air</strong> <strong>and</strong> plenum recirculating <strong>air</strong>. The mixture is then supplied<br />

to the control zones, either heated at the heating coil or without heating depending upon the<br />

zone load.<br />

Because the <strong>air</strong>flow from the series fan-powered box is constant, it is easier to select ceiling diffusers<br />

to achieve good space <strong>air</strong> diffusion. In a series fan-powered box, cold primary <strong>air</strong> is thoroughly<br />

mixed with plenum recirculating <strong>air</strong>. The main disadvantage <strong>of</strong> a series fan-powered box is<br />

its larger fan, which consumes more energy than a parallel fan-powered box.<br />

Parallel Fan-Powered Box. In a parallel fan-powered box, the induced recirculating plenum<br />

<strong>air</strong>stream discharged from the centrifugal fan is connected in parallel with the cold primary<br />

<strong>air</strong>stream discharged from the VAV box in the fan-powered box. These two <strong>air</strong>streams are mixed either<br />

before or after the heating coil <strong>and</strong> then are supplied to the control zones.<br />

A parallel fan-powered box with a heating coil located upstream from the fan is called a drawthrough<br />

parallel fan-powered box, as shown in Fig. 21.14b. When the heating coil is located downstream<br />

from the fan, the box is called a blow-through parallel fan-powered box, shown in<br />

Fig. 21.14c. The <strong>air</strong> temperature entering the heating coil <strong>of</strong> a draw-through unit is the temperature<br />

<strong>of</strong> recirculating plenum <strong>air</strong>, which is higher than that <strong>of</strong> the mixture <strong>of</strong> cold primary <strong>air</strong> <strong>and</strong> recirculating<br />

plenum <strong>air</strong> in a blow-through box.<br />

A parallel fan-powered box operates only when there is a need to mix the warm plenum <strong>air</strong> with<br />

the cold primary <strong>air</strong> at summer cooling mode part-load operation, when the volume flow <strong>of</strong> the cold<br />

primary <strong>air</strong> is at its minimum setting or other preset value, or when the mixture <strong>of</strong> cold primary <strong>air</strong><br />

<strong>and</strong> warm plenum <strong>air</strong> is heated at the heating coil to <strong>of</strong>fset the zone winter heating load. When a<br />

parallel fan-powered box is used to mix 40 to 50°F (4.4 to 10.0°C) low-temperature primary <strong>air</strong><br />

with warm plenum <strong>air</strong> in a cold <strong>air</strong> distribution system, it is also operated continuously during summer<br />

cooling mode.<br />

In a parallel fan-powered box when the cold primary <strong>air</strong>stream is reduced to its minimum setting<br />

during cooling mode part-load operation, the volume flow rate <strong>of</strong> the extracted plenum recirculating<br />

<strong>air</strong> plus the cold primary <strong>air</strong> is usually equal to 0.80 to 1.0 <strong>of</strong> the zone summer peak supply volume<br />

flow rate . The fan in a parallel fan-powered box extracts only recirculating plenum <strong>air</strong>, <strong>and</strong><br />

V˙ sn,d<br />

therefore it has a smaller volume flow rate <strong>and</strong> a lower fan total pressure than that in a series fanpowered<br />

box. Because <strong>of</strong> its smaller fan size, it consumes less fan power in cooling mode<br />

operation.<br />

Current parallel fan-powered box designs also provide a good mixture <strong>of</strong> cold primary <strong>air</strong> <strong>and</strong><br />

extracted plenum <strong>air</strong>. Some <strong>of</strong> the recent models use a variable-speed small parallel fan. It energizes<br />

as soon as the supply volume flow <strong>of</strong> the cold primary <strong>air</strong> drops at cooling mode part-load operation.<br />

The extracted recirculating plenum <strong>air</strong> gradually increases as the volume flow <strong>of</strong> cold primary<br />

<strong>air</strong> decreases up to a maximum limit between 0.4 <strong>and</strong> 0.5 . Under this condition, the zone<br />

supply volume flow rate is kept nearly constant. Parallel fan-powered boxes are <strong>of</strong>ten used in cold<br />

<strong>air</strong> distribution because <strong>of</strong> their lower initial cost <strong>and</strong> lower fan energy consumption.<br />

In a parallel fan-powered box, a backdraft damper should be installed upstream from the fan.<br />

This damper is normally closed to prevent the backward flow <strong>of</strong> cold primary <strong>air</strong> in case the fan is<br />

deenergized. When the fan is turned on, the backdraft damper is fully opened.<br />

V˙ sn,d<br />

V˙ sn,d

logo

products

Resources

Company

Terms of service
Privacy policy
Cookie policy
Cookie settings
Imprint
Change language
Made with love in Switzerland
© 2024 Yumpu.com all rights reserved

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!