Hi James - we met a few ...

Hi James - we met a few years ago at Nashville

your ideas for minimal water use are music to my ears.

However we have to be careful when we look to automated control of cycles. It's not as good as people suspect.

modelling a dynamic process such as a cooling tower where we are expecting to achieve the best control with a batch operation which is the usual conductivity control method (more on PID control later).

When we look at water level control with probes , blowdown cuts in and out on predetermined conductivity levels. Blowdown (purge) will start at there high limit and continue until the lower limit is reached. This is a function of continued evaporation and system concentration  - balanced by solids removal via purge (teaching how to suck eggs - sorry). 

If we now consider water levels in the system sump make up will be controlled on the upper and lower probes. De-concentration of there system water will only occur when the make up valve is operational. During the purge operational situation we have to move from high water to low water in order for the make up top activate. this volume of water removed as purge could equate to almost 20% of the system volume.

excess water use over theoretical is the biggest uncontrolled loss in a cooling system

PID control will overcome this - also float valve control of make up negates the above.

Do the maths!!!

regards

Des Prosser