I have a question regarding writing a shell balance for a cylindrical system with transport in one direction (in any area of transport phenomena). When we set up the conservation equation(say steady state), we multiply the flux and the area at the surfaces of our control volume and plug them into the eqn. Afterwards I believe that we divide the resulting equation by the volume of the control volume before taking the limit as DelX,DelZ or DelR approaches 0. In cartesian coordinates we divide this by delXdelYdelZ, but why in cylindrical CV do we divide by 2.pi.DelR.L instead of 2.pi.R.DelR.L (which is the volume of our CV)?