Chapter hvac engineering Fundamentals: Part 1 Introduction

5.3.1
Pressures
In the design of duct systems, three pressures are specified: the total
pressure
P
t
, static pressure
P
s
, and velocity pressure
P
v
. The total pres-
sure is the sum of the velocity and static pressures:
P
⫽
P
⫹
P
(5.8)
t
s
v
In HVAC work, these pressures are usually measured and expressed
in terms of the height of a water column supported by the pressure.
Static pressure is the pressure which exists in the duct independent
of the velocity. Static pressure is considered to be uniform in all di-
rections. The velocity pressure is that due to the inertial or kinetic
energy of the flowing fluid (air in the HVAC system) and is measured
in the direction of flow. For standard air conditions (density of 0.075
lb / ft
3
), the equation for velocity pressure is
Design Procedures: Part 3
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