Heat Exchanger Temperature Control

[CPTingo]

Hi there! I'm pretty new to the engineering world, and I'm wondering if anybody has any experience with temperature control in heat exchangers. I'm looking into installing a shell and tube heat exchanger to use hot oil at 500 F to heat water to 170 F. The hot water will be used for cleaning tanks, and occasionally as a raw material, but the water will not be used continuously. Therefore, I'm wondering what the best way to design a system will be so that water can not stagnate and boil in the heat exchanger. If anybody has any advice or good resources, I would be very appreciative!

 

[itdepends]

Temperature probe on the water outlet line
Ensure you have a continuous flow of water- if you can't guarentee that because you don't use it all the time- put a recirc line in so the discharge temp probe always sees the actual temp
Control the flow of hot oil to the heater to give the required water discharge temp, return the cooled oil to the supply tank

Assuming your water is non fouling- run the oil on the tube side so that you limit the thermal mass of your controlling stream (gives better discharge temperature control) and you can clean the tubes).


As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[LittleInch]

In a very basic sense you need to control either hot oil flow or water flow or both via s simple control system.

In essence,

To regulate temperature, vary the hot oil supply rate to control on outgoing temperature. There will be some lag to this, but if flow is reasonably steady then it should be manageable. Alternatively you could have some sort of mixer valve mixing very hot water with cold water to arrive at fixed temperature.

When flow stops, the system needs to stop hot oil flow, but maintain water flow for a while to reduce the heat energy left in the hot oil in the HX. This is similar to what some electric showers / heaters do which allow some water to pass through whilst the current is off.

How much water you can work out once you know the relative volumes of oil / water and flow rates.

Where you put that cooler water is up to you, but otherwise you are correct, the water would rise in temperature and pressure is contained which would need to be dealt with via relief valves etc.

In any event you will need a steam relief valve to cope with accidental blockage or loss of flow / pressure - the pressure relief line needs to be directed away from anyone.

This all sounds rather basic information - you're using temperatures and products which can kill people if you get it wrong so my advice is to consult an experienced engineer or vendor who already has this all worked out.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[25362]

Another item sometimes used is a three-way control valve on the hot oil stream.

 

[MJCronin]

I believe that a shell & tube heat exchanger is not the best choice here. It is unwise and difficult to try to make that large temperature "jump" with water "on the fly" as is necessary in an S&THX

I believe that you would be better served by an atmospheric water tank heated by a hot oil jacket or internal coils. The hot water tank should be sized for the expected water usage necessary to clean the largest tank.

The new tank should be vented to atmosphere and have a new transfer pump to be used to clean the tank and recirc the water to promote heat transfer.

If steam is available, a neater and less expensive solution would be a direct steam injection heater; I have specified PICK HEATERS for many years...

http://www.pickheaters.com/

Please respect us and the help you have been given...... and complete this thread. Give us details and the reasoning of your final solution...

MJCronin
Sr. Process Engineer

 

[CPTingo]

Thank you all for your advice,

MJCronin, a hot water tank is a fine idea, but for our application it would have to be quite a large tank, and we simply don't have the space to accommodate it. Thank you for the suggestion, though.

Otherwise, it looks like we'll be pursuing a system similar to what LittleInch has suggested. We will measure the water outlet temperature and adjust the hot oil inlet with a control valve. When hot water use is no longer needed, we will close the hot oil inlet and run cold water through the exchanger to cool off the hot oil to a safe temperature. Oil will be on the tube side, and a relatively low amount of water will be necessary to cool the oil.

For safety, thermocouples will gauge the temperature of the water inside the heat exchanger; if the water gets too hot and is approaching boiling (in the event of a failure of the previous control system), then another control valve will open to run cold water through the system, and set off an alarm. For a total failsafe, a pressure relief valve will be vented to a safe location.

I'll be consulting with one of our senior engineers about the project, but I just thought I would get a wider range of opinions from y'all. Thank you!

 

[Compositepro]

Just take into consideration the thermal expansion and stresses created by rapidly heating and cycling parts of the heat exchanger.

 

[EdStainless]

It should probably be a U-tube heat exchanger, that way the tubes are not restricted in tubesheets as they heat and cool. It is also handy to have the oil inlet and outlet near each other. Your hot oil control valve should be on the HX outlet so that you can keep the HX full of oil even when cold.
You probably want a small water flow through it even when it is not in use to prevent fouling.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MJCronin]

Hot is the "Hot oil system" heated ? Steam ? Electric ? Is saturated steam available ?

It is hard to beat the quick response and low capital cost of a PICK heater.....

If no space is available for an inexpensive tank with coils, how will you have space for your desired large heat exchanger ?

I suggest that you contact STHX vendors and get a quick preliminary size/orientation/shape for this new HX asap ! For equipment layout purposes only..... ask for a budgetary price quote and schedule too

I believe that you will be quoteded a large TEMA BEU or AEU unit - multipass on the tube side (perhaps 6 or 8 passes)... and I believe that will be long and expensive.

Will the new STHX fit somewhere ? How bad is the CAPEX. ? Don't forget the CAPEX and OPEX for the new HW forwarding pump ! ( This is because of the increased pressure drop from your new multi-pass HX) Is there power available ?

and on and on.....

MJCronin
Sr. Process Engineer

 

[CPTingo]

The hot oil system is heated with natural gas.

Actually, this new system is to replace an old steam operated heat exchanger that our plant purchased used many years ago. Our plant has low pressure steam at 15 psi (also heated with natural gas), but the steam is primarily used for tank heating in jackets. The reason we're interested in getting an oil-driven heat exchanger is (1) to take the load off of the steam system, which is taxed already, and (2) because the higher temperature of the oil allows for a smaller heat exchanger (plus, our hot oil system has plenty of capacity, and can easily accommodate addition of a new system).

Our new heat exchanger will likely be installed right where the old one stands now, and will utilize a lot of the same piping. A heated storage tank would need to be able to handle hot water additions on the order of 5,500 gallons on a fairly regular basis, so would need to be quite large.

I've consulted with a vendor, and they've recommended an AEU type HX with a 4 pass U-tube bundle. All in all, it will be on the order of 11' long and 1' in diameter. The CAPEX looks pretty favorable, and because of the similarities to the new system to the old system, the OPEX isn't all that different.

 

[georgeverghese]

Nothing stated on operating pressure in this water stream - hot oil at 500degF looks very high - if operating pressure for water is low, film temps on water side would be high and maybe you might be vapor trapped in the passes. Have you checked for pass arrangement and vapor trapping.
Without much information here, looks like it would be better to stick with old 50psig steam ( 300degF sat temp)
Also, with high film temps, salts in the water will most likely drop out and scale, and in time, you'll lose heating capacity and have corrosion induced tube leaks. If your hot oil press is higher than water, then hot oil is going to get into hot water - the least you can say is you wouldnt get much cleaning done with oily hot water!!
Take a cold hard look at what you've got - cant help much more within a forum context

 

[elridyeng]

500 F = 260 C
170 F = 76
Flash Evaporation Will occurred i think !
use Low Pressure Steam it may help you without any problems

 

[shahzadmeo]

You can also put a bypass of oil stream.....it may easy and not expensive.