alternative differential pressure generating device

[ChronisT]

Hi there,

I have been assigned the task to evaluate the performance of a differential pressure flow device in which the differential pressure is generated from a custom-made geometry setup which resembles a vertical wing inside a rectangular part of the air duct. This wing is centered in the air duct and it looks like a water drop. It creates two (2) rectangular openings for the air flow to pass through. Differential preesure is measured in both vertical sides of this wing against a third point of common pressure.

For information, I have uploaded a relevant schetch of the geometry. It is definetely not a venturi tube or an orifice. External dimensions of the duct are ~1.8m x 1.4m.

This custom-made geometry generates differential pressure, which is used for air flow calculation.

My 1st question: is bernulli equation still applicable for this geometry? Are there any assumptions that have to be made?
My 2nd question: does flow orientation affects the measurement?
My 3rd question: does pressure/temperature correction from operating to Normalized conditions work like in all common flow measurements?

I am a bit confused in which way to start, so thanks guys in advance for any help.

C

 

[quark]

That is Aerofoil Flow Device. 1. Yes, Bernoulli is applicable. Use the flow coefficient of the device. 2. I didn't get this question 3. Yes, if there is not much variation in temperature pressure conditions then you need to correct only once.

 

[LittleInch]

ChronisT

It sure looks like a venturi tube to me in section...

1) yes I think bernouilli would apply
2) Flow is only possible in one orientation. Do you vertical orientation versus horizontal of the device? - If this is a gas then no impact
3) Yes

Where is the pressure measurement?

why are you seemingly reinventing the wheel?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChronisT]

Thanks for the very helpfull replies guys,

I checked them only today, i was out of office,

It is not a venturi, it is an Aerofoil Flow device as quark mentioned, put in service in 1983, still in operation and it needs maintenance and verification. Unfortunately no documantation is available, regarding its flow coefficient.

Fluid medium is air @ 294oC-1.2bars.

With Flow orientation I mean the direction of the Flow through the airfoil device.

The temperature measurement is located close upstream. pressure measurement is located upstream too but far away (~100m of piping length).

Regarding pressure temperature correction, the correction factor to normalize the flow from operating value to reference value is the product of pressures ratio x temperature ratio (Pact / Pref) x (Tref / Tact). Act stands for actual flow conditions,Ref stands for reference (or “N”) flow conditions.

Now it comes the good stuff:

In Endress+Hauser manuals the correction factor (Pact / Pref) x (Tref / Tact) is not square-rooted.

But according to an instruction I received lately from Emerson technical department, (Pact / Pref) x (Tref / Tact) is square-rooted.

So, is the correction factor square-rooted or not?

C









Z is the compressibility, let us forget it for now.

So this leaves us with the product of the two fractions: (Pact / Pref) x (Tref / Tact). Is it square rooted in the computation or as it is?

 

[LittleInch]

If you search aerofoil flow measurement device a whole bunch of stuff appears.

Flow is one way - in your original diagram from top to bottom.

See to be mainly indian companies making them, e.g.

http://www.eurekaflow.com/pdf/Aerofoil.pdf

Correction factor is not square rooted, but pressure is absolute pressure and temperature is absolute temperature (Kelvin)



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

I think the OP was asking what would happen if the flow direction was intentionally reversed.

TTFN (ta ta for now)
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[ChronisT]

Thnaks guys for the information,

really helpful,

C

 

[hacksaw]

just use a pitot tube, they work fine,

 

[quark]

I can't comment on E+H not recommending square rooting of ratio of pressure temperature ratio, unless I see the manual.

Qv ∝ (ΔP/ρ)^1/2 (volumetric flowrate) and
Qm ∝ (ΔPxρ)^1/2 (mass flowrate

So, your compensation factor for density is (PactxTbase/PbasexTact) in the denominator under square root if you are measuring volumetric flowrate or in numerator under square root if you are measuring mass flowrate.

Flow direction is important for any dP device.