Chapter hvac engineering Fundamentals: Part 1 Introduction

238
Chapter Eight
Figure 8.19
Orifice plate flow measurement.
where
Q
⫽
flow rate
A
⫽
cross-sectional area of conduit
H
⫽
static-pressure change
C
⫽
constant relating to orifice and conduit areas and fluid
density
All values must, of course, be in consistent units.
The orifice plate is simple and relatively inexpensive, and therefore
it is widely used. It creates a dynamic loss because of the abrupt con-
traction and expansion. Its accuracy falls off rapidly as flow decreases
below about 20 percent of design. (The turndown ratio is, therefore,
about 5:1.)
The dynamic losses of an orifice meter can be eliminated or de-
creased by using a
venturi
(Fig. 8.20), in which the area changes are
gradual. The flow equation for the venturi is
Q
⫽
C
兹
H
(8.6)
where
C
is a constant determined by the manufacturer and related to
size and physical characteristics.
A
turbine flow meter
is a small propeller mounted in the fluid stream
and connected by a gear train to a measuring / totalizing mechanism.
Design Procedures: Part 6
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.

---