Flow meters-Flow nozzle




DESCRIPTION

The Flow-Dyne "Critical Flow Nozzles" are converging-diverging nozzles designed for the accurate measurement and control of all gaseous flow rates within the range of 0.05 pounds per minute. The internal configuration of the Flow Meter consists of a Bell Mouth converging inlet section, a minimum area throat followed by a conical diverging diffuser section as shown below.

   

Operating on the principle of critical flow, only the inlet pressure and temperature measurements are needed to determine the flow rate. The flow rate varies linearly with the upstream pressure and is not affected by downstream pressure fluctuation or changes by virtue of sonic velocity existing at the throat. The diffuser provides efficient recovery allowing critical flow to be maintained at inlet to exit pressure ratios (Pl/P3) as low as 1.2. There are no moving parts to affect reliability.

APPLICATION:

1.     

Flow standards for calibration of other meters with all gases (air N2, 02, He, H2, etc.) in the range of 0.05 pounds per minute

2.     

Working meters in Development, Qualification and Inspection tests.

3.     

Flow Capacity and pressure drop tests on valves, ducts, electronic package cooling, heat exchangers.

4.     

Wind tunnel calibration.

5.     

Flow Limiting venturis for control of gaseous flows.

6.     

Test stands and OEM products.

7.     

Measurement of airplane cabin leakage flows.


PRINCIPLE OF OPERATION:

As the gas flows through the converging section of the nozzle the inlet pressure is converted to velocity, reaching a maximum value at the throat. The diverging section then slows the gas down by reconverting the velocity back to approximately its original pressure. When the inlet pressure P1 is in the range of 100% to 120% of the downstream pressure P3 the converging diverging nozzle if equipped with a pressure tap at the throat P2 is readily recognized as a venturi flow meter. This type of meter covering all flow rates from 0.05 lb./min. is described in Flow-Dyne's Bulletin 201. Ideally all inlet pressures greater than 120% of the downstream pressure will produce a flow condition in the nozzle which is known in various fields as critical flow, sonic flow, choked flow, and limited flow-thus Flow-Dyne's Critical Flow Nozzle.

Critical flow is characterized by the gas velocity in the nozzle throat being precisely equal to the speed of sound. Under these conditions, a fixed pressure ratio exists between the inlet P1 and throat P2 for all inlet pressures, thus eliminating the need for a throat pressure tap. By virtue of the sonic velocity at the throat, downstream pressure changes cannot affect the upstream pressure and the flow rate becomes dependent upon the upstream pressure and temperature only.

The flow equation for critical flow is:

 


Where:

W = Weight flow rate
P
1 = Nozzle inlet pressure (absolute)
T
0l = Nozzle inlet temperature (absolute)
K = Calibration coefficient supplied with each nozzle includes velocity of approach and discharge coefficient resulting from calibration.

DESIGN FEATURES AND SPECIFICATIONS:

Design Standard:

The internal configuration of these nozzles is designed to optimize the theoretical prediction of discharge coefficient. External design is unlimited and can be tailored to fit any tubing, piping or bulkhead mounting system. Although there are several Standard designs available, such as the AN union pictured above, the purchaser may specify any type of fitting and delivery date will not normally be affected.

Material:

Standard Nozzles are manufactured from type 303 stainless steel for durability and corrosion resistance.

DATA REDUCTION:

To compute the flow rate through the nozzle from the measured data of upstream pressure and temperature, and the Flow Coefficient curve supplied with each nozzle, proceed as follows:

1.     

Enter the Flow Coefficient curve (sample below) at your operating pressure and extract a flow coefficient, K.

2.     

Then, using your measured data and the proper K, you may compute the flowrate through the nozzle with the following equation:

  

Where:

W = Flow Rate in lb./min.
P
1 = Nozzle, upstream absolute static pressure in psia
T
01, = Nozzle upstream total temperature in degrees Rankine (460 + deg F)
K = Flow coefficient obtained from the curve supplied with each nozzle.
(This curve is valid over a wide range of temperatures and only under extreme conditions is it necessary to supply more than one temperature line on the flow curve).

 


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Wed, 05/20/2009 - 11:27

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