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Ratio control is used to maintain the flow rate of one stream with relation to another. Two feed streams can be combined together, creating a mixed flow and this flow usually has physical properties or desired compositions. The flowrate of one of the streams feeding the mix flow can change freely. As this is the case, many factors can affect the flowrate, such as feedstock variations, the action of another controller in the plant, product demand etc. On the other hand, this may be the stream that has to kept the same during normal operation.
The second stream is the one that feeds the mix flow (controller feed). There is also a final control element (FCE) in the controller feed stream which receives and reacts to the signal given from the output of the controller, within the ratio control. Compressors and speed pumps can also be contained within a ratio control. The freely flowing stream sends a signal to a relay which can be one of two types, ratio relay or multiplying relay. A ratio relay is not usually available to operators and is only entered into the mix once. A multiplying relay is the opposite as it can be adjusted and is accessible to be changed.
Ratio control requires for a received signal to change linearly with the flow rate. Hence, the signals within the freely flowing stream should increase and decrease in a straight line manner. Any non-linear signals can be converted to linear signals by adding a function block between sensors and ratio relays. A flow fraction controller is the mixing of an additive into a process stream. This functions similarly to a delay but differs by using the additive to vary the mixed product flow over time. Having a flow fraction controller over a relay means there is more control over how much of the additive enters the stream and does not require high levels of permission as does a normal relay controller. A ratio controller can also be included within the larger cascade architecture and is known as the multiplying relay.
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