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The following concept has been
published in a pocket reference booklet for many years with minor
changes uncorrected.
AND I QUOTE:
“Figures in this chart show theoretical SCFM air flow
through sharp edge orifices
In practice only about 2/3rds
of this flow is obtained. ` ` ` ` ` ` ` Use about half of the flow
shown because a 4-way valve is not a sharp edge orifice and will
usually pass only half as much as a sharp edge orifice.”
END QUOTE!
This particular pocket reference
is handy. I use it often but avoid their sharp edge orifice concept.
Repeated for all these years the myth may
have become fact to many including at least one well-read authority.
The concept presented in error
is basically that a sharp edge orifice passes 1/3rd more flow than
other orifices. This is the exact opposite of the facts. It is
the sharp edge orifice that will only pass 2/3rds of full flow.
To build a sharp edge orifice secure a thin
plate approximately 1/16th thick to block flow through a fitting,
tube, pipe or similar passageway and drill a hole of the desired
diameter through the thin plate (before or after). Be sure the drill
is sharp and the hole perpendicular to the thin material. Be careful
not to chamfer or round the edges of this orifice.
The sharp edge orifice has been
widely used in flow measurement, laboratory work and analysis
because it is easily made and consistent when properly constructed
and kept clean. The flow figures obtained with the sharp edge
orifice may be divided by 2/3rds to find the full (100%) flow.
Making a sharp edge orifice is a slam-dunk when compared to a
well-rounded orifice, Delaval orifice, venturi or some other flow
path device. Possibly because the sharp edge orifice was widely used
the data book authors assumed it had higher flow or flow close to
100% rather than actually lower but consistent flow. For
example if you use this formula; Q(scfm) = 14.5 x Dia” x Dia” x
[P1(psig) + 14.7] (essentially the accepted Moss formula with the
constants crunched) This full (100%) flow in SCFM will be the same
as most orifice charts including the one published in the errant
reference booklet. Then to get the flow expected if the orifice were
a qualified sharp edge orifice just multiply by 2/3ds (67%).
Also as stated in error, it is
improbable and impractical to attempt to compare the flow through a
valve to a sharp edge orifice rather than a normal full flow
orifice. I suggest you use the Cv rating for the Valve in question
to determine flow.
The flow through any device may
be measured with flow meters. To equate the empirical or measured
flow to an orifice for the sake of comparison use the modified Moss
formula above and solve for the equivalent orifice diameter.
Orifice Diameter” = Square Root
of [Q (scfm) / (14.5 x (P1+14.7))]
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This is a theoretical full flow
(100%) orifice. The misconstrued sharp edge orifice is not part of
this comparison.
What causes the sharp edge
orifice to consistently restrict air flow to only 2/3, 67%, of full
flow? Enter the Vena Contracta.
The stream lines of flowing air
passing through a sharp edge orifice constrict the flow path to a
smaller diameter than the orifice itself to bypass turbulence
created by the abrupt (sharp) edge at the inlet to the orifice. This
smaller diameter is a dynamically induced restriction that
constricts the flow path area. As the flowing air exits the sharp
edge orifice the flow restriction continues to converge to a
position just past the outlet of the orifice called the Vena
Contracta. The area of the Vena Contracta is a close
approximation to 2/3rds of the area of the sharp edge orifice.
Texts on Physics are explicit
and consistent on the Vena Contracta caused by a sharp edge orifice.
It is time to debunk the leap of logic that turned this concept
upside down and lead many astray.
There are many styles, types and
shapes of orifices and flow passages with 67% flow from a sharp edge
to 97% through a well-rounded orifice. The Moss orifice flow formula
and most others are based upon a passage of a given diameter or area
times the standard density of air and the velocity of sound at sea
level. This would yield 100% flow in theory. Any reduction of flow
based on orifice shape, style, construction etc. may be applied
after the fact but should not become an implied standard by any
means.
The Sharp Edge Orifice error
described above is one of several erroneous concepts and old wives
tales that have been widely published and repeated by others who
develop their own status as respected authorities and continue to
mislead.
May I suggest that as you read
or listen to any and all concepts you keep your BS-amometer turned
on. Please be a critical thinker when reading material in ALL ABOUT
AIR. If you have doubts or questions it would be a pleasure to give
the rhyme and reason for claims we make. |
