Velocity Pressure Head
Dynamic pressure or velocity head.
The dynamic pressure in a fluid flow can be expressed as
p d = 1/2 ρ f v 2 (1)
where
p d = dynamic pressure in fluid (Pa, ps f (lb/ft 2 ))
ρ f = density of fluid (kg/m 3 , slugs /ft 3 )
v = fluid velocity (m/s, ft/s)
- 1 Pa = 10 -6 N/mm 2 = 10 -5 bar = 0.1020 kp/m 2 = 1.02x10 -4 m H 2 O = 9.869x10 -6 atm = 1.45x10 -4 psi (lbf/in 2 )
- 1 psf (lb f /ft 2 ) = 47.88 N/m 2 (Pa) = 0.006944 lb f /in 2 (psi)
Head loss as water column can be calculated by dividing the dynamic pressure with the specific weight of water like
h w = p d / γ w
= ( 1/2 ρ f v 2 ) / ( ρ w g)
= ρ f v 2 / (2 ρ w g) (2)
where
h w = head water (m Water, ft Water)
γ w = ρ w g = specific weight water (9807 N/m 3 , 62.4 l b f /ft 3 )
where
ρ w = density of water (1000 kg/m 3 , 1.940 slugs/ft 3 )
g = acceleration of gravity ( 9.81 m/s 2 , 32.174 ft/s 2 )
Water Flow
If the flowing fluid is water - then ρ f = ρ w , and (2) can be modified to
h w = v 2 / 2 g (3)
Or simplified to
Metric Units
h w (m H2O) = 0.051 v 2 (3b)
h w (mm H2O) = 51 v 2 (3c)
where
v = velocity (m/s)
Imperial Units
h w (ft H2O) = 0.0155 v fps 2 (3d)
h w (in H2O) = 0.186 v fps 2 (3e)
where
v fps = velocity (ft/s)
or
h w (ft H2O) = 0.0155 (v fpm / 60) 2 (3f)
h w (in H2O) = 0.186 (v fpm / 60) 2 (3g)
where
v fpm = velocity (ft/min)
Water Flow - Head to Velocity Calculator
This calculator can be used to calculate water velocity in a pipe or water stream from a measured head of water column. The dynamic pressure as head is typically measured with a pitot tube .
Water Flow - Velocity and Dynamic Head Chart
Water Flow - Velocity Heads
Water Flow Velocity - v - (ft/sec) | Velocity Head - h w - (ft H2O) |
---|---|
0.5 | 0.004 |
1.0 | 0.016 |
1.5 | 0.035 |
2.0 | 0.062 |
2.5 | 0.097 |
3.0 | 0.140 |
3.5 | 0.190 |
4.0 | 0.248 |
4.5 | 0.314 |
5.0 | 0.389 |
5.5 | 0.470 |
6.0 | 0.560 |
6.5 | 0.657 |
7.0 | 0.762 |
7.5 | 0.875 |
8.0 | 0.995 |
8.5 | 1.123 |
9.0 | 1.259 |
9.5 | 1.403 |
10.0 | 1.555 |
11.0 | 1.881 |
12.0 | 2.239 |
13.0 | 2.627 |
14.0 | 3.047 |
15.0 | 3.498 |
16.0 | 3.980 |
17.0 | 4.493 |
18.0 | 5.037 |
19.0 | 5.613 |
20.0 | 6.219 |
21.0 | 6.856 |
22.0 | 7.525 |
- 1 ft (foot) = 0.3048 m = 12 inches = 0.3333 yd
Air Flow
Metric Units
If the flowing fluid is air with ρ f = 1.205 kg/m 3 (20 o C) and the reference fluid is water with ρ w = 1000 kg/m 3 - (2) can be simplified to
h w (m H2O) = ((1.205 kg/m 3 ) v 2 ) / (2 ( 1000 kg/m 3 ) 9.81 m/s 2 )
= 6.1 10 -5 v 2 (4)
where
v = air velocity (m/s)
or
h w (mm H2O) = 6.1 10 -2 v 2 (4b)
Note! - these simplified equations must be relaculated for other air temperatures and densities .
Imperial Units
h w (ft H2O) = (v / C) 2 / 12 (4c)
h w (in H2O) = (v / C) 2 (4c)
where
v = velocity (ft/min)
C = constant (4005 for temperature 70 o F)
The constant C and temperature:
Air Temperature ( o F) |
C |
35 | 3872 |
40 | 3891 |
45 | 3910 |
50 | 3932 |
55 | 3952 |
60 | 3970 |
65 | 3987 |
70 | 4005 |
75 | 4025 |
80 | 4045 |
85 | 4064 |
90 | 4081 |
95 | 4101 |
100 | 4117 |
150 | 4298 |
Air Flow - Head to Velocity Calculator
This calculator can be used to calculate air velocity in a duct or air stream from a measured head of water column. The dynamic pressure as head is typically measured with a pitot tube .
Velocity and Dynamic Head Chart
Charts are based on air density 1.205 kg/m 3 and water density 1000 kg/m 3 .
Air Flow - Velocity Heads
Table based on ρ f = 1.205 kg/m 3 (C = 4005, temperature 70 o F) and ρ w = 1000 kg/m 3 ( 1.940 slugs/ft 3 ) .
Air Flow Velocity - v - (ft/sec) | Velocity Head - hw - (ft H2O) |
---|---|
5 | 0.00047 |
10 | 0.0019 |
15 | 0.0042 |
20 | 0.0075 |
25 | 0.012 |
30 | 0.017 |
35 | 0.023 |
40 | 0.030 |
45 | 0.038 |
50 | 0.047 |
55 | 0.057 |
60 | 0.067 |
65 | 0.079 |
70 | 0.092 |
75 | 0.11 |
80 | 0.12 |
85 | 0.14 |
90 | 0.15 |
95 | 0.17 |
100 | 0.19 |
Related Topics
-
Fluid Mechanics
The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
Related Documents
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Dynamic Pressure
Dynamic pressure is the kinetic energy per unit volume of a fluid in movement. -
Pipe and Duct Systems - Total Head Loss
Major and minor loss in pipes, tubes and duct systems. -
Pipe Line Velocities vs. Fluid
Typical fluid flow velocities for common liquids, gases and vapors. -
Pitot Tubes
Pitot tubes can be used to measure fluid flow velocities by measuring the difference between static and dynamic pressure in the flow. -
Static Pressure vs. Head
Static pressure vs. pressure head in fluids. -
Velocity-Area Flowmetering
Flow rate or discharge in an open conduit, channel or river can be calculated with the velocity-area principle.