Well, your right about the messed up application part.
Air temperature is completely useless to determine to what point you can cool your water.
What you need to know is the wet bulb temperature. Wet bulb temperature is the lowest temperature to which you can cool water by evaporation and it is a function of temperature, relative humidity and barometric pressure. Its a complex calculation and although a hand held psychrometer is a simple instrument, psychrometric transmitters are complex and expensive.
While knowing the wet bulb temperatures is necessary for sizing a cooling tower, you don't need to measure it to control the tower water temperature. You don't even need a psychrometer. You can determine what the highest wet bulb temperatures for your location are with an internet search or by checking with the local weather bureau. It is futile to attempt to control below the wet bulb unless you have some secret way around the laws of physics (and if you do I want to talk with you about a money making opportunity
). So you size your tower to give you the necessary performance at the highest wet bulb temperature and control to that point.
In practical applications a cooling tower has an approximately 7 degree approach to the wet bulb temperature. Under certain ideal conditions a tower can do better. We are fortunate here to have just such a climate but we still use a 7 degree approach rule of thumb. Find the highest wet bulb temperature for your area and add 7F (4C) to that. For example, here in Salt Lake City where its high, dry, and cool, the highest wet bulb temperature on record in the last 40 years is 65F (18C). A typical annual high is 64F and it occurs in August. Using a 7 degree approach the tower system is sized for 72F (22C). We may see wet bulb temperatures approaching 32F (0C) in the winter but that is irrelevant, the system has to perform year round - we save a little energy in the winter but that is all. I suspect that for you in the UK things won't be so optimal. It is probably impossible for you to achieve 20C with a cooling tower during the warmer humid months, even here where conditions are nearly as good as it gets for towers, 20C year round is a stretch. All that means is that you determine your realistic control point for where you are and design the rest of the system around that.
You could use a PID loop to control your tower fan using only the water temperature. Make sure you include freeze protection to shut the fan down if the water temperature drops below 15C and shut down the tower feed pump if it drops below 10C so that you don't form ice in the tower in the winter. Cooling tower control however isn't rocket science and many towers use a staged system rather than PID - ie, there might be three for four fan speeds set in the drive and those speeds are selected by temperature from setpoint with minimum stage timers. This is probably the most common approach.
You know I never even thought of that on Peter hopefully the ambient air has a chiller unit nearby. I guess this points out an earlier discussion in understanding the process model.
