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djelectric
Posts: **5**Member

in Controls

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.

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.

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## Comments

681MemberBTU's = CFM X Delta T X 1.08

10,276MemberTo @djelectric 's question. Yes and no. That is, if you know the pump curve at a specific rpm, and the pump head gain (psi out - psi in) then you can determine flow remarkably accurately -- assuming the pump is in decent shape. Unfortunately, the flow does not vary directly with rpm; the whole curve shifts. However, that said... some manufacturers publish curves with output at different rpms, and it is quite safe to interpolate between curves (it isn't entirely accurate, but close enough). Lacking that, with a little experimentation with a pump, one can measure the actual flow at some particular rpm and at several different head gains, and simply shift the full rpm curve to fit those points (I would take at least four points -- shutoff head, fully open flow (no head gain) and two others in between). Depending on the size of the pump, I've done this several times with two pressure gauges, a nice globe valve, some appropriate pipe, and a couple of buckiets (little pumps!) to 55 gallon drums (bigger pumps) to in one case a couple of 500 gallon tanks (pretty good size pump) and a stopwatch.

Improvise, Adapt, and Overcome...

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch

5Member11,255Membertrainer for Caleffi NA

The magic is in hydronics, and hydronics is in me9,263MemberThe 15 psi differential pressure is the head pressure that pump is seeing at that flow rate of that loop so it’s irrelevant of the feet of pipe, and elbows etc. that’s all in the differential pressure reading.

However one gauge as Bob noted is the best way to do it. So you eliminate gauge in accuracies.

5Member11,255MemberOther methods have you assume pipe length and use a factor to add for fittings, valves, etc.

trainer for Caleffi NA

The magic is in hydronics, and hydronics is in me5MemberThanks @hot_rod , I saw your screenshots come from idronics, and coming to there website I discovered loads of information . Thanks for all the replies in clarifying the GPM process to me.

5,626Member9,263MemberThere is a lot of variables with EL calculations mostly pipe wall condition, and fitting connections.

5Member11,255MemberIdeally a certified, calibrated differential pressure gauge would give the most accurate pressure reading. Gauges like that can run upwards of a thousand bucks.

trainer for Caleffi NA

The magic is in hydronics, and hydronics is in me