1.3 Fluid Dynamics
7
P T = P s + P v
(1.9)
P s
P v
P T
Fig. 1.5 Types of fluid pressure in forced conduction, static pressure (P s ), dynamic pressure (P v )
and total pressure (P T )
1.3.2 Pressure Units
The pascal (Pa) is the International System (SI) pressure unit. It is the pressure exerted
by a force of 1 N on a 1 m
2 surface normal to it. Other units exist. Their conversion
factors are listed in Table 1.1.
From the definition of pressure as force (F) per unit area (A):
P =
F
A
(1.10)
Multiplying up and down by the displacement (d), it becomes:
P =
F d
A d
This relationship means that pressure can be expressed in units of energy or work
(W ) per units volume (V ) (Eq. 1.11):
Table 1.1 Pressure units most frequently used in engineering
bar
Pa
atm
kgf × cm −2 mmHg
mmwc
1 bar
1
1 × 10 5
0.987
1.02
7.5 × 10 2
1.02 × 10 4
1 Pa
1 × 10 −5
1
9.87 × 10 −6
1.02 × 10 −5 7.5 × 10 −3
0.102
1 atm
1.013
1.013 × 10 5 1
1.033
7.6 × 10 2
1.033×10 4
1 kgf × cm −2 0.981
9.807 × 10 4 0.968
1
7.356 × 10 2
10 4
1 mmHg
1.333 × 10 −3 1.333 × 10 2 1.316 × 10 −3 1.36 × 10 −3 1
13.59
1 mmwc
9.807 × 10 −5 9.807
9.677 × 10 −5 10 −4
7.354 × 10 −2 1