16.01.2013 Views

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

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

15.6 CHAPTER FIFTEEN<br />

1. Total pressure-volume flow �pt-V˙ 2. Static pressure-volume flow �ps-V˙ 3. Velocity pressure-volume flow pv-V˙ The point where the �pt-V˙ or �ps-V˙ curve intersects with ordinate is called the shut<strong>of</strong>f point. At this<br />

point, the block tight or completely shut-<strong>of</strong>f volume flow rate V˙ block � 0.<br />

Moreover, static pressure<br />

is equal to total pressure at this point.<br />

The volume flow rate at the point <strong>of</strong> intersection either between the �pt-V˙ <strong>and</strong> pv-V˙ curves or<br />

between the �ps-V˙ curve <strong>and</strong> the x axis is called free delivery, or the wide-open volume flow rate.<br />

At the point <strong>of</strong> free delivery, �ps � 0, �pt � pv, <strong>and</strong> fan static efficiency �s � 0.<br />

In Fig. 15.2, the other curves shown are the fan total efficiency curve, the fan static efficiency<br />

curve, <strong>and</strong> the fan power input P-V˙ curve. The efficiency curves show that at the shut<strong>of</strong>f<br />

point, both �t <strong>and</strong> �s are zero, <strong>and</strong> at free delivery �s is again zero. It is important that the fan be<br />

operated near maximum efficiency as much as possible. Because <strong>of</strong> friction <strong>and</strong> dynamic losses, fan<br />

power input at the shut<strong>of</strong>f point is not equal to zero.<br />

� s-V˙<br />

Influence <strong>of</strong> Elevation <strong>and</strong> Temperature<br />

FIGURE 15.2 Fan performance curves.<br />

Both elevation <strong>and</strong> temperature have an influence on <strong>air</strong> density � a, in lb/ft 3 (kg/m 3 ), so they affect<br />

the fan total pressure. The fan total pressure �p tf, in in. WC (Pa), required at high elevations <strong>and</strong><br />

temperatures with corrections can be calculated as<br />

�p tf � K TK el �p t,s<br />

� t-V˙<br />

(15.11)

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

Saved successfully!

Ooh no, something went wrong!