From Working Guide to Process Equipment, Third Edition
I imagine that many readers might skip this chapter. After all, a nozzle is simply a hole in a vessel, flanged up to a pipe (see Fig. 14.1). Why a whole chapter? Well, it is not that simple. Lots of process problems occur due to improperly designed draw-off nozzles.
Figure 14.1: A draw-off nozzle.
14.1 Nozzle Exit Loss
The pressure drop of a fluid flowing through a nozzle is equal to
where ? H
= pressure loss of the fluid as it flows through the nozzle, in inches of fluid
= velocity of the fluid, as it flows through the nozzle, in feet per second
This equation assumes that before the fluid enters the nozzle, its velocity is small, compared to its velocity in the nozzle. The increase in the velocity, or the kinetic energy, of the fluid in the nozzle comes from the pressure of the fluid. This is Bernoulli s equation in action. The energy to accelerate the fluid in the draw-off nozzle comes from the potential energy of the fluid. This is Newton s second law of motion.
The coefficient used in the equation above (0.34) assumes the process fluid has a low viscosity. For most process nozzles, this is a reasonable assumption. Detailed information on draw-off nozzle coefficients has been published in Crane. 1
For me, I don t trust published coefficients. I like to prove it myself: prove it by experimentation. Often, Liz (my co-author) calls to me in the bathroom, Norm, what are you doing?
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