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Handbook of air conditioning and refrigeration / Shan K

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System Operating Point<br />

FIGURE 20.4 System operating point.<br />

A system operating point indicates the operating condition <strong>and</strong> characteristics <strong>of</strong> a fan-duct system.<br />

Because the operating point <strong>of</strong> the fan must lie on the fan performance curve <strong>and</strong> the operating<br />

point <strong>of</strong> the duct system must lie on the system curve, the system operating point <strong>of</strong> a fan-duct system<br />

must be the intersection point P <strong>of</strong> the fan performance curve <strong>and</strong> the system curve, as shown<br />

in Fig. 20.4. At the operating point <strong>of</strong> the fan-duct system, its volume flow rate is V˙ P, <strong>and</strong> its total<br />

pressure loss is �pP. If the flow resistance <strong>of</strong> the duct system remains the same while fan performance changes, <strong>and</strong><br />

its characteristics are represented by a new fan performance curve, the operating point <strong>of</strong> the f<strong>and</strong>uct<br />

system moves to point Q in Fig. 20.4. On the other h<strong>and</strong>, if the fan performance curve remains<br />

the same while the flow resistance <strong>of</strong> the duct system is varied <strong>and</strong> represented by a new system<br />

curve, the operating point <strong>of</strong> the fan-duct system moves from P to S.<br />

The fan in a constant-volume <strong>air</strong> system should be selected with a V˙ <strong>and</strong> �pt so that the system<br />

operating point P is near the point <strong>of</strong> maximum fan total efficiency � t. The actual operating point <strong>of</strong><br />

a constant-volume system is usually within a range <strong>of</strong> � 10 percent <strong>of</strong> the wide-open volume from<br />

the point <strong>of</strong> maximum � t.<br />

Buckingham � Method <strong>and</strong> Fan Laws<br />

AIR SYSTEMS: BASICS AND CONSTANT-VOLUME SYSTEMS 20.15<br />

The relationship between various interdependent variables that represent fan characteristics for dynamically<br />

similar systems can be determined by using the Buckingham � method. For two systems

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