Application is an industrial gas fired burner feeding circulated heated air through a series of heat exchangers and back to the burner. We have a temp sensor on the burner output connected to honeywell UDC3500 which controls the gas valve via open close relays. On the burner inlet we have a recirculaton fan which has to be protected from over temp on the return leg. So for instance as heat exchangers reach setpoint dampers close and more heat is returned to burner. We have fitted a second temp sensor just prior to fan and linked this back to same honeywell controller. My question is this a candidate for cascade control, i have some doubts but would welcome some comments.

 

A cascade is used to reduce the effect of disturbances that are acting on the process input, in other words: on the MV.

So, if the air temperature before the burners is changing a lot, then a cascade would make sense. The secondary controller would then have the task to keep the total heat input (gas + air) at the target set by the master temperature controller.

H. Eder
www.act-control.com