I am new to this forum. I am an applications engineer. I design, build and program industrial automated control systems. I am currently working on a project for a lime calcining kiln. Our project involves an upgrade of the coal-fired kiln's fuel delivery system. This involves a coal grinding mill with a large fan that forces air through the coal mill and carries the coal particulates in suspension into the kiln via an air stream. We have replaced the fixed speed fan with a variable speed fan. Our objective is to maintain an air mass to fuel mass ratio of 1.8 to 1 in order to maximize fuel efficiency. Our fuel mass is maintained at a relatively constant rate by a closed loop control on a weighfeeder belt feeding coal to the mill through an airlock valve that maintains a seal to atmosphere so that the mill is part of a closed circuit regarding the air flow. We have installed a hot wire air mass sensor in the return air line from the kiln to the coal mill. It is not feasible to measure the air stream in the outgoing pipe between the coal mill and the kiln because of the suspended coal particulates in that air stream, although this is the region of the air circuit for which we are actually interested in the quantity of the air mass. There is a significant temperature differential between the incoming and outgoing air stream and the incoming pipe is about twice the diameter of the outgoing pipe. The incoming air is approximately 535 degrees Fahrenheit while the outgoing air from the mill is approximately 180 degrees Fahrenheit. Our instrument compensates for this temperature differential and calculates the actual air mass in pounds per hour rather than reporting the volume of air flow in CFM. I know that Ohm’s law dictates that electrical current in a closed circuit must be the same at all points in the circuit. Does this same rule apply to air mass in a closed circuit? In other words, are we to be assured that the air mass entering the mill is equal to the air mass exiting the mill at a lower temperature? This would imply that the volume of air in the outgoing portion of the circuit must be less than volume of air entering the mill. We are experiencing a lower reading of air mass than expected. I am concerned that this is being caused by turbulence in the pipe in which our instrument is installed. We were not able to install the sensor in as long a run of straight pipe as would have been ideal because of the required geometry of the piping between the kiln and the coal mill. I have maintained that the mass must be equal at all points in our air circuit, but other engineers involved in the project disagree with my assertion. The situation is complicated by the fact that the circuit is actually open within the firing end of the kiln and the air stream exiting the coal mill isn’t pure air, but an air/fuel mixture. Am I right or wrong about air mass entering the mill = air mass exiting the mill?