Table 5-10 shows that "compressed Air, cubic feet per minute," can be used to
establish the effect of velocity on pressure drop in several common pipe
sizes. (The actual velocity can be determined, if desired, by dividing the
compressed flow by each pipe area in square feet as given in Table 5-11).
Thus, if the required flow of compressed gas (as calculated by Equation (3))
is approximated in Table 5-10, an immediate indication of the best pipe size
can be obtained by an examination of the relative pressure-drop values. With
some idea of the allowable pressure drop based on inlet or outlet pressure
conditions, it is usually possible to select one pipe size for further
examination.
CAUTION:
The tabulated pressure-drop values must be increased
for an increase in pipe schedule number as described
in the table.
When a tentative pipe size has been selected, the total equivalent pipe
length is determined as previously described, and the total pressure drop is
calculated. This value must be multiplied by the ratio of the gas density at
the estimated effective pressure to the density of air at 100 psia as used in
Table 5-10 (0.596 pounds/feet3) because, as shown in Equation (1), the
pressure drop is proportional to the gas density.
(4) Approximation for Water, Low Flow. Let the maximum required
flow in the system shown in Figure 5-3 equal 20 gallons of water per minute.
An examination of Table 5-9 shows a range in velocity from 12.03 feet/second
for 3/4-inch pipe to 0.868 feet/second for 3-inch pipe. If this is for
general service to the hyperbaric chamber, Table 5-6 shows a reasonable
velocity range of from 4 to 10 feet per second. A 3/4-inch pipe with a water
flow velocity of 12.03 feet per second and a pressure drop per 100 feet of
37.8 psi seems to be on the high side, while a 1-1/4-inch pipe with a
velocity of 4.29 feet per second and a pressure drop per 100 feet at 2.8 psi
is definitely on the low side. Thus, a selection of 1-inch pipe with a
velocity of 7.43 feet per second and a pressure drop of 10.9 psi per 100 feet
appears reasonable.
The total length of pipe as shown in Figure 5-3 is 120 feet. Using figure
5-2, it is determined that the equivalent 1-inch pipe lengths for the three
fittings are: (1) for the globe valve - 25 feet, (2) for the standard elbow -
approximately 3 feet, and (3) for the angle valve - 15 feet. Thus, the
pressure drop for the equivalent pipe length of 163 feet is 10.9 psi times
1.63 or approximately 18 psi. This does not seem excessive in terms of
buying an appropriate pump or using a building water supply with a supply
pressure of approximately 75 psi. On the other hand, use of 3/4-inch pipe
would have resulted in a pressure drop of approximately 37.8 psi times 1.63,
or about 62 psi. This would normally be excessive.
NOTE:
With a short length of pipe and only one valve, use of a
3/4-inch pipe would provide a pressure drop within acceptable
limits.
(5) Approximation for Water, High Flow. Let the maximum required
flow in the system shown in Figure 5-3 equal 600 gallons of water per minute.
An examination of Table 5-9 shows a range in velocity from 15.12 feet/second
for 4-inch pipe to 2.44 feet/second for 10-inch pipe. The velocity for the
4-inch pipe seems too high according to Table 5-6, but the pressure drop is
only
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