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Calculate BTUs of Variable Speed Heating Loop
Hello all, after browsing this forum I believe I've come to the right place for help.
I have a large primary secondary heating system and with five secondary loops, each having a VFD controlled pump.The secondary loops are mostly 4" lines well over 1000' long. The secondary loops are essentially primary loops further downstream, so the flow rate is consistantly relevant to pump speed. A PLC monitors the secondary loops supply and return temps and controls the VFDs to maintain a specified delta tee. The PLC logs the power consumption of the pumps but now I'm looking for an economical way to get a general idea of the BTU-hours on each seconday loop. After doing mind numbing research I realized doing calculations with water is a lot more complicated than electrical calculations. The difficult part is getting the gpm, installing flow meters would run into a lot of $$$. Can pump head in a closed loop be calculated by (pump out psi - pump in psi) then take that number to the pump-curve chart to get gpm? Say I know my gpm at 100% pump speed. Is there a formula to factor in speed to get gpm at other speeds? I don't need a very accurate figure just a general idea. I'm not afraid of complicated equations I just need to get steered in the right direction. Sorry if this got long.