What kind of control use to level and temperature?

Hi friends

I need to control level and temperature in a water tank process.
for example:
50% level
40°C

According with the image attached, I just have a pump to feed water (hot and cold) to the process tank. This water comes through a heat exchangers.

I have done a simple level control and works fine, but how can I link with temperature control?



What kind of control should I use to do that? is it possible?

The PLC is a Honeywell HC900.

Valves are proportional, 4-20 mA
Pump speed control, 4-20mA
The temperature at heat exchangers is contant: hot=70°C cold 20°C


Thanks in advance

If everything stays constant and if you only have solenoid valves and not a way to throttle them, and depending on mixing action at the tank....

The simplest solution would be use just a proportional amount of each water. ie.

(C1 *V1) + (C2*V2)
-------------------- = concentration
V1+V2

in this case
C1= 70
C2 = 20
V1 is % of water added at 70c
V2 is % of water added at 20c

Not knowing volumes of the tank and how fast the water flows in
and if the tank is made in batches, or constantly flows into the reservoir.

would basically come out to 40% 70c + 60% of 20c = 100% at 40c

so if the tank is 100 gallons
40 gallons of 70c water and 60 gallons of 20c water would mix to around 40c water.

if it is batch process I would personally do trial testing and and just add water as a ratio opening each valve for X amount of time to allow for a 40/60 ratio of water adding to the tank. or use an orifice or by pipe sizes allow both valves to open at same time for with a flow ratio of 40/60

use the pump speed to control the level
use both valves for the temperature, if to cold open hot water, if too hot open cold water.
now a little sofisticated, when the temperature is just both valves open 50%.
now comes the trick: use a PID for both processes this way the valves will be set into correct position by the Integrator,

I'd create 3 PIDs:
1) Level PID: Output is pump speed, setpoint is 50%, input is present % level
2) Cold PID: Output is cold valve, setpoint is 40 deg C, input is present tank temp
3) Hot PID: Output is hot valve, setpoint is 40 deg C, input is present tank temp

There are other approaches (high select?) that other smarter people than me on this forum might suggest. I think the above approach is simple and can be tuned to give you good results.

This look like a class room project.
Only two PIDs are required.
There was a project like this many years ago about mixing hot and cold water and maintaining water level for a dormitory shower system.

There were a couple of ways to solve the shower system problem but I think this problem will be simpler because I don't see how there can be huge disturbances. This is also why I think it is a college project.

asteroide said:

Hi friends

What kind of control should I use to do that? is it possible?

Click to expand...

Yes. It is "probing control" approach. In this case, you don't need any proportional valves or pump with speed control. Good results can be reached use switch valves and constant seed fan pump.