Lowering Heat Exchanger Supply Temperature
Lowering Heat Exchanger Supply Temperature
(OP)
If you were to lower the heating water supply temperature of a U-tube shell-and-tube heat exchanger (water to water) while maintaining the same delta T, the performance would decrease unless a new heat exchanger were selected for the new supply temperature, correct? I was discussing retrofitting an old heating water system with a new condensing boiler, and reducing the supply water temperature of the system to take advantage of the new boiler's efficiency, but I'm assuming that would also require replacing all of the heat exchangers which are already on the system. Can anyone provide additional resources on this topic? Thanks.





RE: Lowering Heat Exchanger Supply Temperature
I wouldn't calculate anything. Too many variables.
As for the winter heating season. I assume the the terminal units are sized for 180F? (depending on your location - which you did not provide). Ask the client if they have had any heating issues. If not, it will probably work at 160F. Rebalance the system for more flow to help offset the lower temp. With a 20 deg delta, this will get you near condensing effieicncies. After that, you can have the owner reduce the 160 a few degrees every design day until they dont meet setpoint, then raise it back up a few.
And clean the terminal unit fins, this will maximize the systems efficiencies.
knowledge is power
RE: Lowering Heat Exchanger Supply Temperature
RE: Lowering Heat Exchanger Supply Temperature
I replaced a central steam plant campus system in Northern Illinois with individual boilers. In most cases, central steam was immediately converted to hot water in each building, and the how water was used for heating. Many buildings were old with a historical status. What information I could find was sketchy at best. Many buildings were calling for 210 degree water on the old prints, and I did not replace any terminal units.
My system design was based at 180 degree on a design day. We actually had a -17 deg F day to test it out and had no heating complaints. My reset schedule went all the way down well into the condensing range, as I had hybrid systems with both condensing and non-condensing boilers.
As I'm sure you know, the efficiency of a condensing boiler is only realized when you are down at those condensing temperatures. I certainly seems like you could always be down in that range for a strict re-heat application. Sounds like you might have time to experiment. Have each building start resetting it's supply temperature at the heat exchangers and see how low you can go.
But to answer your question, if you lower your supply temp and keep flow the same, you will get less heat transfer. Even increasing flow may not get you the same exchange do the the approach temperature between the two fluids.
RE: Lowering Heat Exchanger Supply Temperature
I was working with a YMCA with reheat coils considering condensing boilers. Unlike my experience, they used one row coils. At the lower temp, the job could not be done. Had they selected two row as a minimum, it would have worked.
Early in my experience I missed a roof load when sizing a shell and tube steam to water HX. After some sleepless nights I asked the manufacturer how much could I get out of the selected unit. Shell and tube HX are somewhat "sloppy" and the selected unit could do half again as much.
You could get lucky with the original design's incremental sizing and safety factors, but unless you check every HX and downstream terminal, you'll likely encounter difficulty and participate in who pays for the consequences.