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Application
Engineering Basics
Air
Density and Horsepower
The Air Density
varies in inverse proportion to the absolute temperature. Example
- A blower is to handle 200°F air at 3 PSI pressure. What pressure
(standard air) blower is required?
Let:
P = Pressure hot air (3 PSI)
P
= Pressure standard air
AT
= Absolute temperature hot air (200+460=660°F)
AT
= Absolute temperature standard air (68+460=528°F)
A blower is capable
of delivering 3 PSI pressure with standard air. What pressure will
it develop handling 200°F inlet air?
Pressure varies
in direct proportion to the density.
Example - A 3 lb. (standard air) blower is to be used to
handle gas having a specific gravity of 0.5. What pressure does
the blower create when handling the gas?
Let:
Pa = Air pressure (3 lb.)
Pg = Gas pressure
SG = Specific gravity of gas (0.5)
Pg = Pa x SG
= 3 x .5 = 1.5 lb.
If we are required
to handle a gas having a specific gravity of 0.5 at 1.5 lb. pressure,
we can determine the standard air pressure blower as follows:
The following
table gives the barometric pressure of various altitudes:
Absolute Pressure At Altitudes Above Sea Level (Based on U.S. Standard
Atmosphere)
Altitude
Feet |
Pressure |
Altitude
Feet |
Pressure |
| In. Hg. |
PSIA |
In. Hg. |
PSIA |
0
500
600
700
800
900
1,000
1,500
2,000
2,500
3,000
3,500
4,000
|
29.92
29.38
29.28
29.18
29.07
28.97
28.86
28.33
27.82
27.31
26.61
26.32
25.84 |
14.70
14.43
14.38
14.33
14.28
14.23
14.18
13.90
13.67
13.41
13.19
12.92
12.70 |
4,500
5,000
5,500
6,000
6,500
7,000
7,500
8,000
8,500
9,000
9,500
10,000 |
25.36
24.89
24.43
23.98
23.53
23.09
22.65
22.22
21.80
21.38
20.98
20.58 |
12.45
12.23
12.00
11.77
11.56
11.34
11.12
10.90
10.70
10.50
10.90
10.10 |
Horsepower
The horsepower
changes as the cube of the speed ratio.
Example - A blower is operating at a speed of 3500 RPM and
requiring 50 horsepower. If the speed is reduced to 3000 RPM, what
is the new required horsepower?
HP
= Original Horsepower (50)
HP
= New Horsepower
RPM
= Original Speed (3500 RPM)
RPM
= New Speed (3000 RPM)
The above is
known as the 1-2-3 rule of blowers.
Horsepower
vs. Specific Gravity & Ratio of density.
The horsepower varies in direct proportion to the specific gravity
(ratio of density of gas to density of air).
Example - A standard air blower requires a 10 HP motor. What
horsepower is required when this blower is to handle a gas whose
specific gravity is 0.5?
HP = 10 x 0.5
= 5 horsepower
It is possible
that several of the above modifications may be required on one installation.
Therefore, it may be necessary to use various combinations of these
formulae.
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