2 replies to this topic

[rbel038]

Hi,

 

I am performing a calculation to size the flow rate through a small packed column to atmosphere (scrubber). The upstream flow rate is a maximum 500 Pascal (discharge pressure of purge fan) , according to the laws for choked flow of ideal gas, critical pressure ratio of air is around 0.528. If i do the quick calculation based on my upstream pressure I get 260 Pa as my downstream critical pressure.

 

Sure for such low pressure air cannot reach sonic velocity???

 

Appreciate your knowledge

[Bobby Strain]

You must use absolute pressure.

 

Bobby

[serra]

Bobby is correct, consider absolute pressure,

as far as I know there are no limits due to pressure for sonic velocity

(assuming you are estimating correctly the value),

the main problem with very low pressures in piping / vessels is that gas density

becomes low and  this can reduce maximum mass fllux (limited by sonic speed)

below the required values (at that point you have to increase pressure).