What does DICPA stand for?

Diameter of the imaginary circle of primary air

In machines like coal-fired boilers, “primary air” is the first air that helps carry and burn the fuel. It goes into the furnace through a pipe or a nozzle. Now imagine the end of that pipe. Engineers think of a circle there to understand how the air flows. This is not a real circle—it is imaginary, which means it is just for calculation. The diameter is the straight-line distance from one side of the circle to the other, passing through the center. So, this phrase means the width of that imaginary circle where the primary air flows.