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Low Pressure Air Flows Thru Orifice Meter Plate Question


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#1 sfhh

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Posted 24 February 2023 - 01:15 PM

I have a question as the followings, and seek for your advice. Thanks!

 

There is a fan with discharge pressure about 700 mmH2O(G).

I am trying to design an orifice meter plate with 0~100 mmH2O range

corresponding to 0~1500 NM3/H air. It is  in  8"  sch 10 pipe.

(There is about 500 mmH2O(G) back pressure downstream the orifice) 

 

After sizing the orifice, the result is

beta ratio=0.63

permanent pressure loss= 60 mmH2O

max differential pressure is 2100 mmH2O ( in Vena contracta ).

 

My question is it means at some point downstream the orifice plate,

the pressure will become vacuum, then recover some pressure to 700-60=640 mmH2O(G)

 

 

Is there any problem for the flow thru this orifice?

i.e. Is flowing possible through this orifice, as it will create vacuum somewhere?

 

Thanks!



#2 Bobby Strain

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Posted 24 February 2023 - 01:19 PM

It is unusual to use an orifice plate in such low pressure service. I would look for a low pressure drop alternate.

 

Bobby



#3 sfhh

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Posted 24 February 2023 - 01:24 PM

Thanks. That is why i use only 100 mmH2O range.

But i am still doubting: is the flow possible ?

If not possible, then i will change to use pilot tube (annubar meter).

 

Still looking for the answer if the flow is possible ? or any problem ?



#4 Pilesar

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Posted 24 February 2023 - 07:41 PM

An instrument engineer would not choose an orifice meter for a low pressure vapor flow meter, but likely choose an annubar. How much energy would be required to recover the permanent pressure loss of an orifice?



#5 breizh

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Posted 24 February 2023 - 10:01 PM

Hi,

Consider these documents to support your query.

Breizh



#6 sfhh

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Posted 25 February 2023 - 12:40 AM

HI,

 

Thanks for all your input.

 

According to the calculation program, the max power loss is 243.1 watt.

As we will buy a new fan, so fan differential static pressure will

include this pressure loss across orifice 60 mmH2O.



#7 snickster

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Posted 25 February 2023 - 09:31 PM

In English units the upstream pressure is 15.7 psia and the downstream pressure is about 12.7 psia with flowrate of 883 SCFM.  A 3 psi drop looks reasonable and if you've done the calculation correct it is not at choked (sonic) flow conditions.  The pressure is not really vacuum per se as there is still 12.7 psia absolute downstream of the orifice place which is still a positive pressure.  The maximum flow possible is when flow reaches sonic conditions in the vena contracta which will be at about 8.29 psia if small friction loss across orifice plate is neglected.



#8 sfhh

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Posted 26 February 2023 - 09:56 PM

Hi,

 

I choose 100 mmH2O (0.142 psi) range for this orifice meter.

There is no choked flow.

But in the  vena contracta, the pressure is 700 mmH2O (G)- 2100 mmH2O= -1400 mmH2O (G)

= -1.99 psig= 12.7 psia

 

In my thought, the gas flow is OK through the orifice, although

it is vacuum in the vena contraca point.

Of course, if liquid, it is not ok to become vacuum.

But it is gas flow, so should be OK.

 

Thanks!



#9 snickster

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Posted 27 February 2023 - 04:57 PM

I don't understand.  You are trying to measure flow with orifice plate which measures the flow by measuring the delta P and converting to velocity using energy equation relationships that relate the drop in pressure to increase in velocity.  You say you selected orifice based on 100 mm delta P but the actual delta P is 2100 mm at highest flowrate?  Are you saying that you are measuring at flange taps rather than vena contracta taps?

 

There are better flow meters for low pressure blower piping and ducts per follows

 

HVAC Air Flow Meters for Duct Flow Measurement | Paragon Controls


Edited by snickster, 27 February 2023 - 04:58 PM.


#10 sfhh

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Posted 02 March 2023 - 06:33 AM

Hi

 

The sizing range for orifice plate i use is 0~100mmH2O which corresponding to 

0~1500 NM3/H (actual Operating T is 40C, inlet P= 700 mmH2O)

 

The program shows the max differential pressure drop is 2100 mmH2O which ia at Vena Contraca .

but permant pressure drop is about 60 mmH2O, as the pressure will recover after Vena contraca .

 

I plan to use flange tap.

 

Thanks!



#11 katmar

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Posted 03 March 2023 - 02:20 AM

It is not at all abnormal to have a local point with a pressure below atmospheric (even with a liquid).  The energy is not lost - it has simply been converted to kinetic energy at the vena contract and is then recovered as the flow area expands and the velocity decreases. What you are seeing is exactly what happens in an ejector, which uses a nozzle in the place of your orifice to increase the efficiency. In an ejector the existence of the localized vacuum is taken advantage of to draw in some other fluid.


Edited by katmar, 03 March 2023 - 02:21 AM.


#12 sfhh

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Posted 03 March 2023 - 09:11 AM

Hi Thanks for your input. For liquid in vacuum, i think it has to be careful if the liquid vapor pressure at the operating temp is greater than the vacuum pressure at the vena contrat point. It might flash to choke the flow. Thanks!




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