Previous snowmelt application thread

Rocky_3

Ok, so we have determined it is ok to bring -20 glycol solution back to a bank of Munchkins doing direct return snowmelt. If the glycol is a 50/50 mix of propylene, then it increases the head resistance by 50% and cuts the heat transfer rate by 30%. So if the Munchkin requires 40 gpm across its heat exchanger at a 20 degree delta t, (a 400,000 BTU boiler) this 40 gpm produces a corresponding head loss of 22 feet of head across the boiler's heat exchanger (from the OEM manual). Then by using a 50/50 mix of glycol, the head loss is now 33 feet of head for that same 40 gpm? (22 feet of head + 50%=33 feet) But if the heat transfer rate is cut by 30% due to the glycol, then I am only gonna get 280,000 BTU out of this boiler (70% of original 400,000 BTU). So, if I am only gonna get 280,000 BTU, do I then only need to move 28 gpm across the heat exchanger? If that is the case, according to the OEM manual's flow/head chart, 28 gpm only produces approximaely 8 feet of head if pure water, or 12 feet of head for a 50/50 mix. So, how screwed up is my thinking? Am I totally wrong? If I'm correct, I need to know before I buy the pump, because a pump that will move 40 gpm at 33 feet of head is a MUCH different pump than one which will move 28 gpm at 12 feet of head. Which pump do I need? Are my assumptions correct, or, am I as usual COMPLETELY out in left field smoking crack? Please enlighten me. I would really appreciate it.
Warm regards from not-so-chilly Fairbanks,
Rocky

ALH_4

Flow Rates & Head Loss

I dont think you want to limit yourself to a 20°F DT with snowmelt. First, use this formula for your required flow rate: GPM=BTU/(500*.85*DT) The .85 accounts for the reduced heat capacity of the 50% glycol concentration. At a 40°F DT, I get about 24 GPM. Figure your head loss at that flow rate, and you can size your boiler circulator.