Patent ID: 7543596

Claim:
Fluid flow control apparatus, comprising: a proportional fluid control valve having a fluid inlet and a fluid outlet; a pneumatic proportional control valve in communication with said proportional fluid control valve for modulating said proportional fluid control valve; means for measuring pressure drop; a controller in communication with said pressure drop measuring means and with said pneumatic proportional control valve for controlling the flow of fluid through said proportional fluid control valve in response to said measured pressure drop; said apparatus configured to interchangeably contain one of a plurality of frictional flow elements having a frictional flow element fluid inlet in fluid communication with said fluid outlet of said proportional fluid control valve and having a frictional flow element fluid outlet spaced from said frictional flow element fluid inlet, each of said frictional flow elements comprising a helical coil having a unique length or diameter relative to any other of said plurality of frictional flow elements and creating a pressure drop between said frictional flow element fluid inlet and frictional flow element fluid outlet that is measured by said means for measuring pressure drop.