ML 1100 PID limit ramp

[iraiam]

Hi,

I'm working on a system that was built and then upgraded, not engineered that I know of.

It's a high pressure hot water heat exchanger, the problem is with the PID control of the steam valves.

The water flow is extremely variable with high pressure hoses throughout the plant. The flow can vary from 9 to 100 GPM, which causes wide changes in the time it takes for the water to get though the heat exchanger, which makes it difficult to use the Ti portion of the PID instruction.

My current setup is to limit the PID control variable max limit with a scaled ramp based on water flow, this works but it appears the ramp I need is not linear, currently the water temp is a little low at low flow rates. I also have a ramp to change the Ti of the ML 1100 PID instruction based on water flow but it's currently not in use. The PID is currently running as just a P with a ramping CV limit.

I was thinking about using another PID instruction to limit CV max, but it's a ML 1100 so I don't think it does the Master/Slave PID setup.

Anyone have any other ideas?

 

[MarkNightingale]

Can you fit a tank, that will buffer the water flow going in to the heat exchanger?

Cheers

Mark

 

[iraiam]

There is a fresh water reservoir tank that the pumps draw from, but it's not heated. I'm not sure if the material the reservoir tank is made from will allow recirculating some heated water. I'll have to find out.