Sizing the Orifice

The flow versus pressure curve shown in Fig. 4 can be used to size limiting orifices properly for different gases, gas mixtures, and required flow rates. To determine flow rates for gases otherthat nitrogen, use Fig. 4 to determine the flow rate at the desired pressure through the 0.006-in. orifice. Next, use the correction formula below to find the flow for a gas of different molecular weight. 

For gas mixtures the effective molecular weight is determined as follows  

Continuing the example of GeH4 above, with a minimum acceptable flow rate of 3 L/min, the orifice would need to be increased to 0.008-in. diameter (the flow rate scales as the area of the orifice). 

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Apart from the size of the orifice, the nature of the orifice is of considerable importance. Thus the use of an orifice and a tube of the same diameter yields different bubble sizes. The orifice plate is found to yield a higher bubble volume than the corresponding tube. 

At 150 psi the flow rate through the 0.006-in. orifice is 2.8 L/min nitrogen. This must be taken into consideration when sizing the orifice for process flow. 

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