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Handbook of air conditioning and refrigeration / Shan K

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15.32 CHAPTER FIFTEEN<br />

TABLE 15.2 Comparison between Various Types <strong>of</strong> Fans<br />

Case Study—Selection <strong>of</strong> Fans<br />

Select the appropriate type <strong>of</strong> fan for the following applications:<br />

1. The weaving room <strong>of</strong> a textile mill<br />

2. A convenience store<br />

3. A high-rise <strong>of</strong>fice building<br />

Solution<br />

1. In the weaving room <strong>of</strong> a textile mill, the sound power level <strong>of</strong> the looms is far greater than the<br />

sound power level <strong>of</strong> the fan noise transmitted through the ductwork <strong>and</strong> supply <strong>and</strong> return<br />

grilles. Also the fan total pressure required for the <strong>air</strong> system used in the weaving room is usually<br />

less than 3 in. WC (750 Pa). Therefore, a tube-axial or vane-axial fan would be suitable.<br />

2. A convenience store usually requires an <strong>air</strong> system with a volume flow rate less than 5000 cfm<br />

(2360 L/s). Usually, a small or a medium-size ro<strong>of</strong>top packaged unit is used. Forward-curved<br />

fan is usually used in this type <strong>of</strong> packaged unit.<br />

3. A high-rise <strong>of</strong>fice building may use a central hydronic VAV system or a unitary packaged VAV<br />

system. Design criteria also vary considerably according to the location, occupancy, <strong>and</strong> design<br />

criteria <strong>of</strong> the building. A more detailed <strong>and</strong> comprehensive analysis must be made to determine<br />

whether backward-curved centrifugal fans with <strong>air</strong>foil blades, forward-curved fans, vane-axial<br />

fans, or a combination <strong>of</strong> fans should be used.<br />

15.8 COIL CHARACTERISTICS<br />

Backward, <strong>air</strong>foil Forward-curved<br />

centrifugal fan centrifugal fan Vane-axial Propeller fan<br />

Fan total pressure �p tf Higher �p t Comparatively Higher �p t Low �p t<br />

lower �p t<br />

Flow rate All flow rates Larger flow rate All flow rates Larger flow rate<br />

Fan power input Nonoverloading Overloading Nonoverloading Nonoverloading<br />

Fan modulation Inlet vanes Inlet vanes Controllable pitch<br />

AC inverter AC inverter AC inverter<br />

Fan total efficiency 0.7 to 0.86 0.6 to 0.75 0.7 to 0.88 0.45 to 0.6<br />

Sound power level Lower, higher L w Medium, higher L w Medium, difference Higher, higher L w<br />

at low frequencies at low frequencies <strong>of</strong> L w values is at high frequencies<br />

small at various Hz<br />

Airflow direction 90° turn 90° turn Parallel to axle Parallel to axle<br />

Volume <strong>and</strong> weight Greater Less Greater Medium volume <strong>and</strong><br />

lower weight<br />

Initial cost Higher Medium Higher Low<br />

Applications Large HVAC&R Lower pressure, Large HVAC&R Low-pressure, highsystems<br />

small HVAC&R systems volume flow<br />

systems exhaust systems<br />

Coils are indirect-contact heat exchangers that transfer heat between <strong>air</strong> <strong>and</strong> another medium (such<br />

as water, refrigerant, steam, or brine) for the purpose <strong>of</strong> heating, cooling, dehumidifying, or doing a

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