I have been having a debate with a coworker regarding heat exchanger fluid temperatures and while I thought I firmly grasped the principles, I was not arguing my point very well.
I am looking at an air to water coil for an air handler. I was selecting the coil based on a leaving air temp of ~60F (it's an industrial space with a lot of ventilation). I am using condensing boilers so I sized for 180 leaving water temperature with 140 return on the coldest day which gives a 40 degree temperature delta. On the air side, I am bringing in 5 degree air (EAT) at a rate of 16000 CFM on one of the air handlers. My coworker wants to lower the temperature to 140 supply and 100 degree return, his reason being that it would still be a 40 degree temperature delta on the water side so the thermal output would be the same. I believe the hotter supply temperature will result in more rapid heat transfer but I didn't really have any good argument against his logic. He was simply relating the two rules of thumb equations for air and water: [Btu/hr]=1.08*CFM*deltaT and [Btu/hr]=500*gpm*deltaT to prove his point. My argument was that this does not account for the heat transfer taking place from one side of the coil to the other, but I was not able to adequately illustrate my side to the point that he would agree.
First of all, am I even right about operating the system at 180 degrees being a more efficient solution, and, if so, how do I fit the heat exchanger heat transfer into the system of equations?