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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

Replacing the terminal units along with the boiler will probably not be worth it to your client. (depending on the client and project, which you provided no details). I like your idea of replacing the boiler with a condensing boiler for effieicncies (if the boiler is at its life expectancy - cast irons can go pretty long) Provide an outdoor air reset control to get those high efficiencies in the swing seasons.

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

(OP)
Thanks cdxx139. I intentionally left the post vague because I was just looking for more general resources, but it's a large campus system that feeds over 20 buildings, with the load at each building being a heat exchanger (which transfers heat to the building-side where it goes to the terminal units or air handlers). Supply temp from the boilers is currently around 170F, although documentation seems to indicate most heat exchangers are sized for 220. This is a tropical climate so winter heating loads are minimal, the system is mostly needed for reheat after dehumidification (which I assume it fairly constant throughout the year). Also heat exchangers have different deltas, ranging from 20-40 and some currently operating even higher. Replacing heat exchangers and terminal units/air handlers would be a huge cost and logistics issue. I'm now looking to save operating costs by converting the system to variable flow. Thanks again.

RE: Lowering Heat Exchanger Supply Temperature

In a tropical climate where these coils are strictly re-heat, you shouldn't need anywhere near 170 degrees. Certainly not 220. Let me qualify that by saying unless the coils are very small compared to what would normally be sized.

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

ByonYrAur is on target.

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.

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