S7 PID for dryer.

I need to write a PID loop for a seed dryer. The dryer has a gas heater with one valve that regulates the flow. The temperature inside the dryer should be 90 degrees in celcius. Maby anyone has anny suggestions what parameters are best for such dryer? P.S i don't want to use a PID wizard here, i need to write it manualy. Just i'm a little bit lack of experience.

Check this site http://www.chem.mtu.edu/~tbco/cm416/zn.html
The ziegler-nichols method is useful to get the initial parameters of your PID loop.
Also they've got a cool PID simulator

Well thanks for that but i need help how to write PID loop in S7 300. I understand how to tune it but i dont know how to write it Can anyone please help me?

Hi,
try googling for "discrete pid controller".

Don't know if this helps, but I wrote PID temperature control in the S7-224XP using a combination the wizard and manual control. I used manual control with dithering to get temperature say to within 10 degrees of setpoint, then switched to automatic control of the PID loop. If temperature increased to high above setpoint I switched to manual control to switch off all heat input.

Hi

Exactly why do you have to construct a S7-300 PID controller from scratch by writing it yourself? Siemens have several options. There are the basic PID controller blocks they provide with every STEP7 installation: FB41, FB42, FB42, FB58 and FB59. If these are too restrictive then they sell add-on software called "Standard PID" which has a series of other blocks with even more detailed parameters and access to the internal workings of the loop. And I think, although I've never seen or used it, they do yet another software option "Modular PID". With this I think you can select components of a PID function such as the P, or the I, or a filter, or a limiting function, or a switch etc and assemble exactly the loop control model you need.
In my experience with Siemens it's not often they don't have the answer. More usually they have 10 answers and you have to spend ages working out which one is the right one for your requirements!

Ken