why we control the water flow by using valves on the return lines?

why we not control by the supply (water inlet)?

 

> why we control the water flow by using valves on the return lines?

> why we not control by the supply (water inlet)?

I assume you are asking about something like a heat exchanger

You can put the valve on either side provided the liquid doesn't boil through low pressure or cavitate (another form of boiling).
Try running your process through a valve sizing program first of all with the inlet conditions then with the return conditions.

 

I agree with Roy Matson, and I also presume the original poster is referring to a heat exchanger of some type. I would like to add that if you throttle with the inlet valve of a heat exchanger there is a chance that insufficient fluid will get past the inlet control valve to keep the vessel full. So, generally the control valve is placed on the outlet in order to keep the vessel full when throttling flow through the vessel.