Differential Temperature Control

[RafalBor]

Hi,
I've got complex plantroom controling various motors and valves.
I’m trying to find suitable script which would do as follows:

A-Output – supply to building from header - target temperature 52degC-55degC -max 55dgC 
B-Input – return from building to header - target 45deg C
 
If above 55degC then blend using proportional 0-100% (all valves are 24V/1-10V) - analogue control currently sits on KNX

If ever difference between a-b>10 or <10 then correct through blending
 
I’ve checked PID script but it will fit this  type of control
 
 My KNX groups i.e.:
Group for sensor A (temp) – 4/0/14
Group for sensor B (temp) – 4/0/2

Group for absolute valve control - 2/1/177
Group for absolute FB - 2/1/178

100% means valve open and feeding system 
0% means valve close and feeding bypass once temperature on output A over 55degC

I'd appreciate some guidance or ready script.

[admin]

Please provide a better description of the system you want to control. Maybe provide a simple schematic of what goes where

[RafalBor]

Script would control 3-port valve through 0-10V signal opening and closing bypass proportionally. Level of opening (expressed in %) will be determined on 3 factors:

Target temperature of flow (max 55degC)
Difference between flow and return 10 deg C (+/- 2 degree)
Target temperature of return (45 degC)

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I've attached screenshot of mechanical drawing

[Daniel]

Do you have mathematical calculation for this ?

[RafalBor]

No, I don't have and I believe none are required - is rather mechanical integration where script has to to open/close proportionally 0-10V valve (so scaling of valve opening)  in relation to target temperatures. But I believe you are after some time target date measuring opening of the valve through duration Tx in relation of raising/dropping temperature

when I draw that on the board and tested on the valve 50% bypass/50% system - this opening is blending water that it drops 2degC from target within 2-4min, 80% system/20%bypass seems to be most effective and allows better control extending time of 2degC drop to fixed 5min

I hope that helps

[admin]

I suppose you can use PID control and target either one of the temperatures or the difference between them. You can also add some additional logic where PID runs only when other system parameter is within limits.
Create a trend log for all values to check how the system behaves.