Cascading PID loops

Can someone help me understand cascading PID loops?
We are controling a steam valve through a heat exchanger this way, and I am having trouble getting a steady output to the valve.
Any help would be greatly appreciated!
Thanks.
Ed

For me this is one of the best sources for PID.
http://www.controlguru.com/pages/table.html

A cascaded control loop just means that the output of one loop becomes the setpoint for another loop. In your case, for example, the output of a water temperature control loop (measured variable is exchanger output temp and setpoint is manual) could be a steam flow rate. The secondary loop would control the steam flow rate by manipulating the valve (measured variable is steam flow, output is valve position, setpoint is from the temp control loop).

Cascade loops can be handy, but tuning can be problematic. Any tendency to hunt is exacerbated by the second loop.