In the original single-controller arrangement, a drop in the steam supply pressure would first have to lower the tank temperature before the temperature sensor could even notice the disturbance. That may sound like a convoluted way to achieve the same result as the first controller could achieve on its own, but a cascade control system should be able to provide much faster compensation when the steam flow is disturbed. If that rate turns out to be insufficient to produce the desired tank temperature, the first controller can call for a higher flow rate, thereby inducing the second controller to provide more steam and more heat (or vice versa). The second controller then manipulates the valve opening until the steam is flowing at the requested rate.
![split second fuel controller split second fuel controller](https://c1552172.ssl.cf0.rackcdn.com/1053687_x800.jpg)
Instead of dictating how widely the valve should be opened, the first controller now tells the second controller how much heat it wants in terms of a desired steam flow rate. The pressure in the steam supply line could drop while other tanks are drawing down the steam supply they share, in which case the controller would have to open the valve more than Y% in order to achieve the same X% change in tank temperature.Ī cascade control system could solve both of these problems as shown in Figure B where a second controller has taken over responsibility for manipulating the valve opening based on measurements from a second sensor monitoring the steam flow rate. However, several factors could alter the ratio of X to Y or the time required for the tank temperature to change after a control effort. This arrangement works well enough if the steam supply and the steam valve are sufficiently consistent to produce another X% change in tank temperature every time the controller calls for another Y% change in the valve opening.
![split second fuel controller split second fuel controller](https://i.ytimg.com/vi/ZHUVbixPQ6A/hqdefault.jpg)
In Figure A, a traditional controller is shown measuring the temperature inside the tank and manipulating the steam valve opening to add more or less heat as inflowing water disturbs the tank temperature.
![split second fuel controller split second fuel controller](https://www.stealth316.com/images/arc2installkit.jpg)
Consider, for example, the steam-fed water heater shown in the sidebar Heating Water with Cascade Control. When multiple sensors are available for measuring conditions in a controlled process, a cascade control system can often perform better than a traditional single-measurement controller.