Damn haven't had my hand at this for a while, but they say it's like a bicycle, so here goes....
rule of thumb: The bigger the Delta in Temp between your transducers and your carrier, the more heat energy will be transferred.
what does this mean???.... simple really, flow-rate is everything. well almost.
but you will do good by making it your nr1 goal, to have as high a flowrate as is possible with what you have to work with...