Shell and tube heat exchanger issues
Shell and tube heat exchanger issues
(OP)
Hello everyone. First time posting here.
I'm looking for some opinions and recommended courses of actions regarding two issues we are facing with shell and tube heat exchangers we run in series for our EO reactor heating/cooling needs. These HXs both utilize cooling and heating through the shell side, thermocycling 10-20k+ times a year. We run batch processes.
We run over 30+ reactions in, lets call it Reactor A, utilizing EO and usually some sort of fatty acid. The reactions are all exothermic. We try to maintain a temperature setpoint through the use of two shells and tube heat exchangers in series.
The flow is... reactor A -->HX 1-->HX 2--> back into reactor.
The temp setpoint differs from product to product. The product goes through the tube side, and utility goes through the shell side on both heat exchangers. We utilize 115 psi steam for heating, and cooling tower water for cooling which ranges from (5C-40C) depending on time of year. The utility feed, whether steam or cooling water, into both heat exchanger shells are in parallel.
The reactions are exothermic but sometimes we over cool and need to utilize heating. Therefore in a typical batch we may thermocycle anywhere from 10-20 times. There are three states in the programmed logic. COOL, OFF, HEAT. Therefore there are 4 transitions. COOL TO OFF. HEAT to OFF. OFF to COOL. OFF to HEAT. If the setpoint is 160 C, there is a +/- 2.5 C range from which the system will be in OFF state.
Depending on if we are heating, cooling, or off... certain valves will be opened and closed.
See attached photo. This shows one of the HXs while it happens to be in the HEAT state.
I will outline all three states below.
KEY:
Steam
CWS (Cooling water supply)
CWR (Cooling water Return)
CND (Condensate return)
1) When in the OFF position the CWR valve is open and all others are closed
2) When in the HEAT position (as seen in the attached photo) the steam valves are open, the CND valve is open, all others are closed.
3) When in the COOL position the CWS is open and CWR is open, all others are closed.
Our first issue is WATERHAMMER. This appears to occur when we go from HEAT to OFF. When we go from HEAT to OFF the steam and CND valve close simultaneously, and a millisecond later the CWR valve open since that is the default OFF state as mentioned above. It's at this moment that we witness waterhammer through the CWR line. It is vicious. One theory is that the steam/condensate hasn't evacuated the shell of the HX + is under vacuum from turning the heat off, and once the CWR valve is opened you are introducing the steam/condensate to the cold water in the CWR line which creates thermal shock water hammer.
Our second issue, which is likely related to the waterhammer described above, is damage to the tubes and tube sheet.
My questions/Suggested corrective actions would be the following.
1) Would it make sense to delay the closing of the CND return line and delay the opening of the CWR line when transitioning from HEAT state to OFF state? In other words, instead of the steam and cnd valve closing simultaneously, we would have the steam valve close first once high temp set point is reached, then leave the CND valve open to let it drain for 10-15 seconds, then close the CND line, then open the CWR line.
2) Should we have a vacuum breaker on the CWR/CND common line that exits the HX. Perhaps placed next to the TT on the picture I've attached. I've determined that we have no vacuum breaker on our system. Could we be creating vacuum when we go from HEAT to OFF and holding condensate in the shell, which then allows for waterhammer to occur after opening the CWR line. Could this be what is lifitng our PSV every 2-3 months? Could this be what's causing damage to our tubes, and tube sheet?
3) Since we use steam through this shell, should we have an impingement plate on the utility inlet?
I was thrown this project/investigation not to long ago, but this waterhammer has apparently been an issue on these HXs for decades here. No one has been able to implement a solution which works.
Are my suggested courses of actions reasonable? Are there any other recommendations?
Thank you
365CHE
I'm looking for some opinions and recommended courses of actions regarding two issues we are facing with shell and tube heat exchangers we run in series for our EO reactor heating/cooling needs. These HXs both utilize cooling and heating through the shell side, thermocycling 10-20k+ times a year. We run batch processes.
We run over 30+ reactions in, lets call it Reactor A, utilizing EO and usually some sort of fatty acid. The reactions are all exothermic. We try to maintain a temperature setpoint through the use of two shells and tube heat exchangers in series.
The flow is... reactor A -->HX 1-->HX 2--> back into reactor.
The temp setpoint differs from product to product. The product goes through the tube side, and utility goes through the shell side on both heat exchangers. We utilize 115 psi steam for heating, and cooling tower water for cooling which ranges from (5C-40C) depending on time of year. The utility feed, whether steam or cooling water, into both heat exchanger shells are in parallel.
The reactions are exothermic but sometimes we over cool and need to utilize heating. Therefore in a typical batch we may thermocycle anywhere from 10-20 times. There are three states in the programmed logic. COOL, OFF, HEAT. Therefore there are 4 transitions. COOL TO OFF. HEAT to OFF. OFF to COOL. OFF to HEAT. If the setpoint is 160 C, there is a +/- 2.5 C range from which the system will be in OFF state.
Depending on if we are heating, cooling, or off... certain valves will be opened and closed.
See attached photo. This shows one of the HXs while it happens to be in the HEAT state.
I will outline all three states below.
KEY:
Steam
CWS (Cooling water supply)
CWR (Cooling water Return)
CND (Condensate return)
1) When in the OFF position the CWR valve is open and all others are closed
2) When in the HEAT position (as seen in the attached photo) the steam valves are open, the CND valve is open, all others are closed.
3) When in the COOL position the CWS is open and CWR is open, all others are closed.
Our first issue is WATERHAMMER. This appears to occur when we go from HEAT to OFF. When we go from HEAT to OFF the steam and CND valve close simultaneously, and a millisecond later the CWR valve open since that is the default OFF state as mentioned above. It's at this moment that we witness waterhammer through the CWR line. It is vicious. One theory is that the steam/condensate hasn't evacuated the shell of the HX + is under vacuum from turning the heat off, and once the CWR valve is opened you are introducing the steam/condensate to the cold water in the CWR line which creates thermal shock water hammer.
Our second issue, which is likely related to the waterhammer described above, is damage to the tubes and tube sheet.
My questions/Suggested corrective actions would be the following.
1) Would it make sense to delay the closing of the CND return line and delay the opening of the CWR line when transitioning from HEAT state to OFF state? In other words, instead of the steam and cnd valve closing simultaneously, we would have the steam valve close first once high temp set point is reached, then leave the CND valve open to let it drain for 10-15 seconds, then close the CND line, then open the CWR line.
2) Should we have a vacuum breaker on the CWR/CND common line that exits the HX. Perhaps placed next to the TT on the picture I've attached. I've determined that we have no vacuum breaker on our system. Could we be creating vacuum when we go from HEAT to OFF and holding condensate in the shell, which then allows for waterhammer to occur after opening the CWR line. Could this be what is lifitng our PSV every 2-3 months? Could this be what's causing damage to our tubes, and tube sheet?
3) Since we use steam through this shell, should we have an impingement plate on the utility inlet?
I was thrown this project/investigation not to long ago, but this waterhammer has apparently been an issue on these HXs for decades here. No one has been able to implement a solution which works.
Are my suggested courses of actions reasonable? Are there any other recommendations?
Thank you
365CHE
Talk To Other Members
RE: Shell and tube heat exchanger issues
We have a similar application with heating/cooling cycles, albeit with reactor jackets instead of heat exchanger. Potato, Pot(ah)to, in this case - it's the same problem in a different skin.
As soon as you close the steam and condensate valve, you have uncondensed steam remaining in the shell. You immediately charge water, cooling the steam it comes into contact with and creating vacuum pockets that initiate water hammer. A few points:
1. Waiting to introduce water will not help the hammer much. You will need to wait a long time (probably longer than your process can handle) to allow ALL of the remaining steam to condense before introducing water. Even if you can wait that long, you will have a vacuum inside the shell. The opening of the water valve will still send a slug of water through the system, which will also hammer your tubes/tubesheets.
2. Though you may not see it as badly, going from water to steam will likewise cause hammer. Instead of a huge "BANG", you will typically get a phase-front hammer at the steam/water boundary as the water is pushed out. This will usually sound more like a crackling or gravel in mill rather than a singular bang. This is due to the limited, but continual, surface area of the steam as it pushes the water out. This type of hammer is still damaging to equipment and should be avoided.
So, what to do? In both cases (Steam->Water or Water->Steam), you really shouldn't have either medium in contact.
The solution we have found is to purge the system between cycles. Used plant compressed air. If you have a dump valve (open drain) use air to blow remaining steam to ground before opening cooling. If going from cooling to heating, use air to blow remaining water either to drain or back through the CTW return line, depending on the geometry of your Hx.
If it is a vertical Hx with CTW return at the top, you'll have to dump water to ground to clear the Hx. This does waste water, but you could have a separate dump tank/valve for the water and pump return if you really wanted to recover the water. Your diagram provided suggests the CTWR is located on the bottom of the Hx because it is tied in with the condensate return line. If true, you should be able to blow most of the cooling water back through the CTWR line and not dump it to the ground.
This is the best solution we have found to combat hammering. It's pretty cheap to implement (compressed air line to the system, a few valves added, and some programming), and it'll save tons in repair headaches down the line.
RE: Shell and tube heat exchanger issues
Thank you for taking the time to read through my post and provide your insights.
I'd like to add some further context and clarification to make sure we are on the same page.
"As soon as you close the steam and condensate valve, you have uncondensed steam remaining in the shell. You immediately charge water, cooling the steam it comes into contact with and creating vacuum pockets that initiate water hammer."
We do not immediately charge water after the steam and condensate valve close. We never actually go directly from HEAT to COOl or COOL to HEAT. There is always an OFF state that remains over a 5C range. When in the OFF state the Cooling water return valve opens. In other words if reaction temp goes 2.5C below the setpoint than HEAT will turn on. This means that Steam and CND lines are open, rest are closed. Once the temperature enters into the OFF range, then we transition from HEAT to OFF. At this time the STEAM and CND valve close simultaneously, and a millisecond later the cooling water return valve open. The Cooling water supply valve does not open.
I think you are entirely correct that there is still uncondensed steam remaining in the shell. I think this is the case for two reasons.
1) Once the temperature transmitter provides the DCS with a reading which is within the OFF state range, the Steam supply valve closes. The Condensate return valve closes at the SAME time. This leaves no time for any steam/condensate to be evacuated from the shell of the HX.
2) We have no vacuum breaker on the common line exiting the shell of the HX. Going from HEAT to OFF will lower the temperature in the shell creating a vacuum which will hold the remaining condensate up in the shell. Then once the cooling water return valve opens a millisecond later, the high pressure (cooling water in the downstream portion of the cooling water return line) rushes to the low pressure area (the steam/condensate) in the shell and causes thermal shock water hammer.
"Though you may not see it as badly, going from water to steam will likewise cause hammer."
When we go from OFF to HEAT the following occurs. The CWR remains open (since this is open as default in the OFF state). Lets say that time T=0 is right when we've gone from OFF to HEAT. Therefore we can say that at time T=0 seconds, the CWR valve is open (remains open from OFF state). Also at T=0 the steam valve opens. Steam then blows through the shell into the CWR line. Yes you read that correctly. At T=15 seconds the CND valve opens. At T=20 seconds the CWR valve closes. Therefore there is a 5 second interval where both the condensate return and cooling water return line are both opened, then the CWR closes and just the CND remains opened until HEAT goes to OFF state. Oddly enough, I don't recall witnessing the hammer when this transition between OFF to HEAT occurs. I could be wrong about that, but I've defintely witnesses the problem occur when the opposite occurs... HEAT to OFF. I know because I witnesses it happen in person, noted the exact time, then looked at our PI Trends and saw which valves opened and closed at that moment which told me which transitional state had occurred.
Lastly, you are correct in your observation. We do run both STEAM and Cooling water through a common line into the shell of the HX through the TOP. The condensate or cooling water return exit the shell of the HX through a common line at the bottom. The valves tie off of these common lines in and out of the HX as shown in the picture I provided. In the past I've typically seen cooling going into coils of a reactor through the bottom, and steam go into the coils through the top. When you're cooling, some of the water will evaporate and naturally want to travel upwards hence why it's best to put the cooling water in at the bottom. Steam on the other hand goes in the top that way once it condenses, it will naturally flow down the coils to the bottom. Why our system was set up with both supply going into the top I'm not sure of. Legacy I presume.
I think your idea about blowing the lines clear could be a good solution. We'd likely use nitrogen instead of air to prevent any possibility of air getting into the tube side then into the reactor. EO is some extremely explosive stuff after all.
Thoughts?
RE: Shell and tube heat exchanger issues
Good Luck,
Latexman
RE: Shell and tube heat exchanger issues
Thanks for the input.
RE: Shell and tube heat exchanger issues
A few things: You are running 115 psig in the shell side? If so, then as soon as steam and the condensate valve are closed, you are trapping 115 psig steam in the shell. Then, as the CTW return valve opens immediately afterwards, that steam is blowing into the CTW return line (unless your return header pressure is >115 psig, which I very much doubt). This is the source of your water hammer.
You have to
1. Depressurize before opening the CTW return valve.
2. Purge (or dilute, as Latexman says) with N2. Your point about EO and no O2 in the line is good. The main point is that you CAN'T have pure steam in the shell side, even if fully depressurized to atmospheric pressure, or you WILL have hammer when introducing water.
Lastly, when going from OFF to HEAT mode - blowing cooling water out with steam typically doesn't produce the same type of hammer as dumping water into a steam-filled vessel. We do the same thing on some of our jackets, and I will typically hear a "crackling" sound, but no banging or pipes shaking. Even though it's not obvious, there is hammering occurring.
Another point: vacuum breakers won't help collapsing steam hammer - the required relief rate is enormous due to the near-instant collapse of steam. Your only option is preventing the hammer in the first place.
RE: Shell and tube heat exchanger issues
The diagram you've posted shows no check valve on the CWR line out of the HX, so is it possible for CWR from the CWR main return header to back up into the HX when this valve opens, resulting in explosive decompression of the shell? Why do you have to open this CWR anyway right after the steam valve is closed - can we leave the steam inventory to cool down slowly, since you've still got EO going through the tubes, with the CWR valve closed for a minute or 2?
RE: Shell and tube heat exchanger issues
What would be the difference between the two scenarios you outlined? They seem to be the same process to me, that being we blow steam against liquid water.
In the first scenario (HEAT to OFF), the steam in the shell, that hasn't been evacuated, blows into the CWR line and contacts the cool water downstream of the CWR valve.
In the second scenario (OFF to HEAT), the steam from the STEAM LINE blows any remaining cooling water out through the CWR line.
Are these not similar conditions? That being, steam is blowing against water? What would cause the first scenario to experience massive water hammer, but second scenario only minor crackling?
The biggest obvious difference to me is that in the HEAT to OFF transition, you still have live hot pressurized steam making contact with 'COOL' cooling water. While in the OFF to HEAT transition the cooling water in the shell (which is blown out by the steam into the CWR line) will be warmer water.
As noted previously, we never go from HEAT TO COOL or COOL to HEAT directly. Therefore I don't think scenario two applies much because when going from OFF to HEAT we will already have been in the OFF state for at least a few minutes with the CWR line open. In that time I would imagine most of the water will have evacuated the shell through the CWR line. By time HEAT is called, the shell should have evacuated through the CWR line unless it was unable to evacuate due to vacuum or some type of back pressure perhaps?
As for your suggestions.
1. Depressurize before opening the CTW return valve.
2. Purge (or dilute, as Latexman says) with N2. Your point about EO and no O2 in the line is good. The main point is that you CAN'T have pure steam in the shell side, even if fully depressurized to atmospheric pressure, or you WILL have hammer when introducing water.
1a. What type of instrument or process change could be implemented to accomplish the depressurization of the shell?
2a. I would plan to use N2. I would program the logic to blow N2 when going from HEAT to OFF & COOL to OFF.
- When going from COOL to OFF cooling water will remain (if it hasn't drained through the open CWR line in the OFF state). I would blow that through the CWR line.
- When going from HEAT to OFF Steam/Condensate will remain in the shell. I would have to depressurize the shell first as steam pressure> N2 pressure ,then blow with N2 to drain.
Thoughts?
RE: Shell and tube heat exchanger issues
'
Thanks for your comment. I don't believe we have a check valve on the CWR line although I could be wrong about that. There isn't one on the P&ID and I don't recall seeing one walking the system down.
"The diagram you've posted shows no check valve on the CWR line out of the HX, so is it possible for CWR from the CWR main return header to back up into the HX when this valve opens, resulting in explosive decompression of the shell? Why do you have to open this CWR anyway right after the steam valve is closed - can we leave the steam inventory to cool down slowly, since you've still got EO going through the tubes, with the CWR valve closed for a minute or 2?"
As TICL4 pointed out, it is more likely that the steam inside the shell transverses into the CWR line as opposed to the cooling water rushing into the shell. This assumes that the steam/condensate pressure inside the shell is at higher pressure than the cooling water return header which I think is an accurate assumption.
As for your second question, we run exothermic reactions capable of running away quickly. I believe the idea behind opening the CWR line immediately was to equalize the temperature quicker. If we kept the steam inside the shell and allowed it to cool over time, the reaction may need to call for COOL quicker than it otherwise would which would mean more thermocycling per batch. Wouldn't allowing it to cool contained within the shell also create vacuum? Then once we open the CWR line water hammer would still occur, but now as you described with the cooling water rushing INTO the shell and hammering?
I think the suggestion of blowing the shell clear of steam/condensate or cooling water between thermocycles would eliminate this concern either way.
Thoughts?
RE: Shell and tube heat exchanger issues
The different between the two is that you are starting with a shell full of steam when going from HEAT to OFF. When the hammer occurs in the CTW return line, it starts in the CTW return line, but has a chance to slam all the way back into the HX as the steam begins collapsing. Think of it like this:
When the CTW return line opens, steam begins to blow into the CTW line. However, when it hits the water, it collapses and creates a vacuum. Even though the shell is still 115 psig, that particular zone of steam collapse is 0 psig. This zone is then filled by water/steam mixture, with the phase interface closer to the HX. This happens over and over again in an incredibly short timeframe. The interface may actually start moving into the CTW return line before reversing direction - it depends on the relative pressures and volumes, and line sizes involved. The reason the interface eventually travels back towards the HX is that it is a fixed volume of steam, which begins to deplete (lower pressure) as the front wave collapses. Macroscopically, this results in a huge slug of water slamming back into the heat exchanger even though it is higher pressure. All of this happens in an incredibly short timeframe, resulting in a fast-moving slug of water that results in the hammering.
When you go from COOL to OFF to HEAT, the HX is full of water. The steam/water interface behaves the same, but the steam is kept at a constant 115 psig supply, and is enough to constantly push the interface through the HX and out the CTWR line. The main difference between these two once the HX is clear of water is that the shell is kept at a constant 115 psig supply pressure due to the open steam valve.
To answer your other questions:
1a. Put a "dump" valve off of the CTW return and condensate return line combined lines. Also put an N2 line on the CTW supply line. When you transition from HEAT to OFF, open the dump line. After a certain time period, open the N2 line (to avoid blowing steam back into the N2 line. Be sure to analyze this piping for:
1. The bending stress on the blowdown piping created by the blowdown
2. The acoustic characteristics of the blowdown (will be dangerously loud if not designed properly)
3. The blowdown area for personnel exposure risk.
2a. Transition from COOL to OFF should open the N2 line and the CTW return line. Minimum blowdown times should be evaluated for each transition.
RE: Shell and tube heat exchanger issues
Your explanation on the difference between the two scenarios makes absolute sense.
I'm going to put together a visual to make sure I understand your suggestions.
A suggestion of another engineer here at our site: When we go from HEAT to OFF, instead of closing STEAM supply and CND return simultaneously, we instead leave the CND return line open to allow condensate to drain through the steam trap. We would leave it open for 20-30 seconds before it's closed, then open CWR line.
My thoughts on their suggestion. Wouldn't the issue here be that not all the remaining steam in the shell would condense and exit through the steam trap prior to the CWR line opening in any reasonable timeframe? We would still have some steam remaining in the shell and would experience the same thermal shock water hammer issues once the CWR line opens.
Similarly, but slightly different, the user georgeverghese above provoked the question: Once we go from HEAT to OFF can we close the STEAM supply and CND return, but hold off for a minute or two before opening the CWR line. The reasoning being to allow the steam to cool slowly.
My thoughts are that the same situation above would occur. (Not all the steam would condense and evacuate the shell through the steam trap in the time before the CWR line opens.)
I will be posting a PFD with my interpretation of your suggestions in a little while.
RE: Shell and tube heat exchanger issues
It looks like I can't attach more than one document per reply so I'll post the two separately. The first document (shown as an attachment to this reply) shows a P&ID of the HX system as a whole. I've whited out irrelevant or potentially confidential information. This P&ID is meant to give an overview of the system. The second attachment, which will be attached to my next reply is a more simple PFD of the HX system with my interpretation of your suggestions highlighted in red pen.
RE: Shell and tube heat exchanger issues
RE: Shell and tube heat exchanger issues
Update: Looks like the first attachment will not upload properly. After I submit post, I can't seem to open it up so I doubt you can either. When I click the link I see "P". It's only a 1.65 MB PDF so I'm not sure what the issue is. The limit appears to be 2 MB.
RE: Shell and tube heat exchanger issues
From Heat to Off:
Heat exchanger with 115 psig steam expands into CWR sytstem piping until steam pressure equals CRW system pressure with some possible overshoot to just a lower pressure. Assume CWR sytem flows to a pump suction tank so that it can absorb sudden increase in liquid volume with little increase in CWR pressure. At this time gas (steam) occupys shell of heat exchangers and downstream CWR piping at a distance in piping depending on the expanded volume of steam. At the same time, steam is condensating in the CWR piping due to large diffrence in temperature. This causes a vaccuum in the CWR system back to the heat exchanger so pressure falls lower than equillibrium pressure of when steam became fully expanded. Now due the difference in pressure between CWR and empty heat exchanger shells and downstream piping, liquid rushes back the other way with very high velocity. When it fills up the heat exchangers the fluid comes to a instantaneous halt and causes water hammer. The water hammer pressure wave travels from the heat exchangers back to the CWR system.
In order to eliminate the issue at the source I would put a automatic bleed valve on the heat excanger to depressurize before opening the CWR valve.
RE: Shell and tube heat exchanger issues
a)Suggest a small auto sequence operated vent valve to depressure the shellside steam to atm at a safe location (size to depressure as quick as you can to minimise exothermic temp excursion on the tubeside EO) before opening the CWR valve. The vent release location needs to be in a safe area free of ignition sources just in case there is EO contamination of shellside contents through some pinhole tube leak. This is perhaps similar to @TiCL4's suggestion to dump shellside contents to drain??
What is your safety practice for venting /dumping vapors that may be contaminated with EO? With EO being so unstable, presume you dont have a common vent collection header ?? Is this why the current design made no attempt to depressure / dump the shellside before opening the CWR valve? Else you need some higher integrity design on the HX to considerably reduce the likelihood of EO contamination of the shellside due to tube rupture or leaks through tubehole to tubesheet roller expanded joints...
b)Looks like you need a check valve on that CWR line for more reasons than one.
RE: Shell and tube heat exchanger issues
RE: Shell and tube heat exchanger issues
In your opinion, could the automatic bleed/dump valve on the heat exchanger be utilized without the need of a N2 supply as TiCl4 had suggested? I would route the dump line to ground. Both HXs are well off the ground level currently. We already utilize a significant amount of N2 throughout our plant and our process engineer is not too wild about using more for this purpose.
RE: Shell and tube heat exchanger issues
Suggest revising this scheme to 2 separate parallel banks of exchangers of 2 each, with cooling only on one bank and heating only on the other. And switching the EO to either bank as required for heating or cooling operations. Keep each bank on a minimal trickle flow when not in operation. The cooling bank is prone to fouling since this is open loop cooling, so a spare HX here would be better to enable offline cleaning. This way, you wont need to dump or vent shellside steam potentially contaminated with flammable EO. For the bank which goes off, the residual EO on tubeside may be purged out into the EO recycle stream to reactor with some N2.
RE: Shell and tube heat exchanger issues
RE: Shell and tube heat exchanger issues
I agree with you that this scheme was terribly designed from the start. It's been this way for at least a decade from what I've been told. The reaction pressure is usually fairly low around 35 psig. There usually is a momentary 50 psig spike for each batch. Our steam and cooling water pressures are higher in the shell side than the EO moving through the tube side. When there is failure at the tubes or tube/tube sheet interface, we usually see a spike in the hydroxyl levels in our product upon quality checks, indicating that water has entered into the TUBE side. Additionally, we load the reactors with an excess of fatty acid prior to the slow addition of EO. The EO reacts almost immediately upon entry to the reactor.
I also pitched the idea of two separate parallel banks of HXs. They want to try simpler solutions before spending any significant amount of capital.
RE: Shell and tube heat exchanger issues
See attached P&ID.
RE: Shell and tube heat exchanger issues
Recirculation pumping rate also has an effect on temp stability; the larger the recirculation flow, the more stable the reactor temp would be.
RE: Shell and tube heat exchanger issues
There actually is a temp transmitter on the bottom of the reactor (not shown in the drawing). That temp transmitter controls whether HEAT, OFF, or COOL is activated for both HX A and B. The three way valve downstream of HX A is not involved in the logic. If they want to skip HX B, they can activate the 3-way and go REACTOR --> HX A --> REACTOR --> REPEAT
I Don't believe the TIT you are referring to is involved with the logic.