I would like to set up a temperature control scheme for direct steam heating using feedforward control with feedback trim. The controller is a Allen Bradley SLC505. The goal is to control the outlet temperature. I will use the energy equation to calculate the steam flow required to achieve the outlet temperature. So controlling the steam flowrate is the feedforward part. Then to narrow in on the temperature setpoint I want to fine tune using a normal PID loop with feedback control (error of actual vs setpoint). To set this up do I use a first PID loop which is based on flow control and use this output as a feedforward or bias input into a second PID loop which is temperature control. Or do I do the reverse, temperature first then flow? Or is there a simpler way?
Thanks in advance for your help.

You could master/ slave the PID read it in the AB book programming book its pretty helpful.

This might help http://www.jashaw.com/pid/cascade.html

Mickey

Excellent. You need to have a model of the system. This model predicts how the plant temperature will respond to the input. The inverse of the model is used to calculate the BIAS. Now even with the gains set at zero you should be able to get close. If your model is good you should be within 5%. I can usually get much much closer than that. Now calculate the gains for the PID. You should know that if you are using a feedforward or BIAS it will work best if you do make a step jump in changing the set point.

Temperature control using PID and pole placement

If you are doing temperature then you should check this out. If is much simpler, requires less processing power and is much more robust when the load changes.
When the load changes using the BIAS approach the model will change and performance will degrade. In the link below the system gain is changed by significant amount and yet the temperature is still maintained.

Temperature control using Sliding Mode Control.