Eclipse Engineering Guide

SIMPLIFIED SELECTION OF AIR, GAS AND MIXTURE PIPING SIZE
Air, gas and mixture piping systems should be sized to
deliver flow at a uniform pressure distribution and without
excessive pressure losses in transit.
Two factors cause air pressure loss and consequent pressure
variations:
1) Friction in piping and bends, and
2) Velocity pressure losses due to changes in direction.
In combustion work, piping runs are usually short (under
50 ft.), but often have many bends. By assuming that all
velocity pressure is lost or dissipated at each change of direction
and by using a pipe size to give a very low velocity pressure,
other losses can be disregarded. In general, a velocity
pressure of 0.3 to 0.5″ w.c. satisfies this need. This is equivalent
to air velocities of about 2200 to 2800 ft/minute. For
other gases, this velocity is inversely proportional to their
gravities; consequently, higher velocities can be tolerated
with natural gas, but propane and butane piping should be
sized for lower velocities than air.
The accuracy of orifice meters is also sensitive to pipe
velocity, so every effort should be made to keep velocity pressure
below 0.3″ w.c. in metering runs.
Pv, "wc
Nat. Pro-
Velocity
Bu- Ft/Min
Pipe Size
2-1/2"
1-1/2"
Gas Air panetane x1000
10
1/4" 3/8" 1/2" 3/4" 1" 1-1/4" 2" 3"
3.0 5.0
4.0
9
8
2.0
1.5
1.0
3.0
2.0
1.5
5.0
4.0
3.0
2.0
5.0
4.0
3.0
7
6
5
1.0 1.5
2.0 4
1.0
1.5
1.0
3
2.5
0.5
0.4
0.3
0.2
0.15
0.1
0.5
0.4
0.3
0.2
0.15
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.1 0.15 0.2
0.05
0.1 0.15
1
Shaded Areas
100
Indicate Recommended
Velocity Pressure
Range
2
1.5
2 3 4 6 8 1000
If pipe sizing charts or tables aren’t available, you can
quickly estimate the maximum air flow capacity of a pipe
with these simple equations:
Maximum cfh air = (Nominal pipe size) 2 x 1000
The result will correspond to a velocity pressure of about 0.5″
w.c., the maximum recommended for low pressure air systems.
Optimum cfh air = (Nominal pipe size) 2 x 750
QUICK METHOD FOR SIZING AIR PIPING
15
The graph below shows the relationship between velocity,
velocity pressure and flow for various pipe sizes handling air,
natural gas, propane, and butane. Because the specific gravity
of most air-gas mixtures is close to that of air, mixture piping
can be sized the same as air piping. The error will be
insignificant.
Example: A burner requires 10,000 cfh air at a static pressure
of 13″ w.c. The blower supplying this burner develops 15″
w.c. static pressure. Piping between the two will run 15 feet,
including four 90° bends. What size piping is required?
Solution: Total pressure available for piping losses is
15″ w.c. - 13 ″ w.c. = 2″ w.c.
This allows a velocity pressure loss of:
2 ÷ 4 = 0.5″ w.c. for each of the four elbows.
Under the “Air” column on the left-hand side of the Pv
graph, locate 0.5″ w.c. velocity pressure. This is equivalent to
about 2800 ft/minute air velocity. Locate the intersection of
the 2800 ft/minute line and the 10,000 cfh line, then drop
down to the first curve below this point, in this case, 4″ pipe.
This is the pipe size that should be used.
12" 14" 16"
2 3 4 6 8 10,000 2 3 4 6 8 100,000 2 3 4 6 8
Flow, cfh
4"
This will produce a flow rate equivalent to about 0.3″ w.c.
velocity pressure.
Example: What is the maximum air flow rate for 21 ⁄ 2" pipe?
(21 ⁄ 2) 2 = 6.25
6.25 x 1000 = 6,250 cfh air.
6"
8"
10"
18"
10
9
8
7
6
5
4
3
2.5
2
1.5
1