request heat ratio control using hot and cold feeds

[ae861616]

hey,
have a problem at the moment, trying to get the temperature of the water in a tank to a required temperature and level using two feeds, a hot feed and a cold feed.
ive never done ratio control before so looking for a bit of info as to how to go about it.
ive tried search for info but came up hopeless
any info or links would be much appreciated

thanks