Rotameter operation 1

[garybawden]

Hello,
I am new to this forum and i was wondering if anyone can explain the fluid mechanics behind the operation of a rotameter. I am interested in the principle of operation and the science behind it. Can anyone help?

Many thanks

Garybawden

 

[TD2K]

Very briefly, a rotameter consists of a float suspended in the upwards flowing fluid. The float sits in a tapered body. As the fluid flows around the float, a pressure drop occurs. The float moves up or downwards in the fluid until the force on the float created by the pressure drop equals the weight of the float less the bouyancy effect of the fluid. When the float moves up or down in the tapered tube, the annular area for the fluid to flow through changes, thus changing the dP on the float (in reality, the dP is 'fixed' because of the force balance and the dP is in turn 'fixed' by changing the orifice area for flow eg. where the float sits in the tube). A force mass balance shows this best.

Selecting a float is therefore directly affected by the flow rate you want to measure and the fluid's properties. Using a old rotameter in a new service means checking the original conditions, frequently the information with the rotameter will tell you how to compensate for the new conditions.

If you can find a copy of Perry Handbook of Chemical Engineering, it has a good picture and explanation of this (likely far better than I've done). Also, try searching on

http://www.google.com

for rotameters and see what comes up.