The Ratio Control
Architecture
By Allen Houtz and Doug Cooper
ABSTRACT: The ratio control architecture is used to maintain the flow rate of one stream in a process at a defined or specified proportion relative to that of another. A common application for ratio control is to combine or blend two feed streams to produce a mixed flow with a desired composition or physical property. Consistent with other articles in this e-book, applications of interest are processes with streams comprised of gases, liquids, powders, slurries or melts.
Ratio Control and Metered-Air Combustion Processes
By Allen Houtz and Doug Cooper
ABSTRACT: A ratio control strategy can play a fundamental role in the safe and profitable operation of fired heaters, boilers, furnaces and similar fuel burning processes. This is because the air-to-fuel ratio in the combustion zone of these processes directly impacts fuel combustion efficiency and environmental emissions. A requirement for ratio control implementation is that both the fuel feed rate and combustion air feed rate are measured and available as process variable (PV) signals.
Override (Select) Elements and Their Use in Ratio Control
By Allen Houtz and Doug Cooper
ABSTRACT: A select element receives two input signals and forwards one of them onward in the signal path. A low select passes the lowest of the two signals, while a high select passes the larger value onward. A select element can be implemented as a DCS or PLC function block, as a few lines of computer code, or as a simple hardware circuit. And while the elements above are using electrical current, they can also be designed to select between high and low voltage or digital (discrete) counts.
Ratio with Cross-Limiting Override Control of a Combustion Process
By Allen Houtz and Doug Cooper
ABSTRACT: We explored override control using select elements in a previous article and learned that environmental and energy efficiency concerns for metered-air combustion processes can be partially addressed with a single select override element. Examples illustrated how a select override can either prevent having too much fuel or too much air in the air/fuel mixture fed to the burner of a combustion process, but one override element alone is not capable of preventing both scenarios. In this article we explore the addition of a second select override element to create a cross-limiting architecture that prevents the air/fuel ratio fed to the burner from becoming overly rich (too much fuel) or lean (too much air) as operating conditions change. Variations on this cross-limiting architecture are widely employed within the air/fuel ratio logic of a broad range of industrial combustion control systems.