Shell and tube heat exchanger failure

dfru

I am working on a building which has a mechanical equipment room which is supplied with 175 psi water at 315 degrees F, from a central plant a few blocks away. The high temperature hot water loop in the mechanical room circulates this hot water through the tube side of three shell and tube heat exchangers. One is for domestic water, one is for building heating, and the third is for swimming pool heating. The tubes in the domestic water heat exchanger have repeatedly failed/leaked. Examination indicates that there is likely velocity erosion in the tubes. Water through the tubes is controlled by a motorized valve which shuts off water flow when the shell side of the system is satisfied. I do not believe that there are currently and bypasses in the system.



My guess is that when the tube side of the pool and/or the heating hot water heat exchangers is off (valves closed), the system is trying to push all of the central plant hot water through the tubes of the domestic hot water heater, creating excessive velocity, and ultimately erosion.



Does this make sense?

If so, how should I modify the system to prevent this? (I believe that the central plant system is very large, and is controlling many other buildings - so I suspect that the flow from the central plant cannot be modulated in response to the changes in the building)

If this is correct, why would the domestic hot water heater tubes be the only ones failing (the other heat exchangers would see high velocities when the domestic water heater system is satisfied)?



Any thoughts or recommendations would be appreciated.

jumper

constant flow valves

There are valves which limit flow. CV can stand for constant velocity. Generally the heat exchanger manufacturer provides information about what the recommended flow is. It can't be too low either because that that'll impair heat transfer. Also the motorized valve actuator should be very slow. That several blocks of water wants to keep moving.

Gordan

A couple of ways to solve this...

The heat exchangers could each be plumbed into the external loop with some sort of hydraulic separation (simplest would be closely-spaced tees.) Each of them would have its own circulator that would be sized for appropriate flow through it. This decouples the flow in the external loop from the flow through the heat exchangers (and the flow through each of the exchangers from that through the other two.)



You could also plumb a pressure bypass valve around the three exchangers. Not sure about PBVs rated at that kind of pressure and flow (I'm sure they exist, though.)



Lastly, but should probably be examined first as an option, is there a possibility for maintaining a constant pressure differential through variable pumping in the external loop? Addressing the issue at its source?

10uta

Reverse

My question is why we are having the heating medium in the tubes? If the hot pressurized water can travel through the shell, we can simply control the temperature the respective systems while the heating loop operates as normal and since the heating, DHW, and pool water systems are not connected, we wouldn't be increasing the flow on one heat exchanger if the other two are satisfied.

dfru

Reverse

I agree, but I am dealing with an existing system which is already set up with the central plant water flowing through the tubes for each system,