TABLE OF CONTENTS Ratio control paces (controls) the flow rate of one stream so as to maintain a specified ratio between that stream and the measured flow rate of another stream. The measured flow rate is called the "wild" flow, since (at least within the jurisdiction of this control loop) it is uncontrolled. The controlled flow loop is often called the "secondary" loop. Figure 10-1 depicts a ratio control system using both ISA (Ref. [10-1]) and SAMA (Ref. [10-2]) symbols.
Although there may be some applications for ratio control that are not flow-based, most applications ratio flow-to-flow. The broad definition of flow used here is any form of mass or energy transfer. Typical applications for ratio control include:
Blending two or more components. One ingredient may be set for the master production rate; other ingredients are then ratioed to this master ingredient.
Air-fuel ratio control for a combustion process. In a simple air-fuel ratio control system, the fuel flow may be controlled by a temperature or pressure controller. A measure of the fuel flow is then used to determine the set point of the air-flow controller. [1]
Controlling a product stream to feed rate, as a means of composition control. This is a common control technique for distillation towers.
In the most common configuration for ratio control, the measured value of the wild flow is multiplied by the specified ratio, controlled flow-to-wild flow. The product then becomes the set...
