Using a shell & tube heat exchanger as a steam generator.

LarryC

Would it be safe and make sense to use an appropriately rated shell and tube heatexchanger to make steam?



The application would be as follows.  The HX is designed for low pressure steam (less than 150 psi) on the shell side and used for heating the fluid in the tubes.  I am thinking of running it in "reverse". 



Using a concentrating solar collector to heat up a suitable heat transfer fluid, pump the HT fluid thru the tubes of the Hx and supply water to the condensate output of the shell.  The generated steam would need to go thru a moisture seperature to create dry steam, and then the steam would be used to drive a small steam engine to generate electricity.



Obviously overpressure safety reliefs would be required.  Yes, this is an off grid application.  Intended steam pressure would be around 80-90 psi.  The thought is to run the heat transfer fluid thru a phase changing salt to store heat during the day and to use the stored heat plus solar collected heat to run the boiler when there is enough energy to run the steam engine for several hours.



HX would be something like a Bell & Gossett SU85-2 or similar.



Some additional thoughts are mounting the HX vertically or tilting it up at a steep angle to maintain half of the tubes under water to maximize steam generation and modifying the shell to add an external sightglass.



Assuming the math pencils out, would there be any major issues besides the steam being generated would be wet.



Thanks.



<a href="http://www.genemco.com/catalog/pdf/HDAF869bellgossettspecs.pdf">http://www.genemco.com/catalog/pdf/HDAF869bellgossettspecs.pdf</a>

Al Corelli_2

PV

Wouldn't it be more efficient to use photovoltaic cells to make electricity?

Al Corelli_2

dupe post

Steve Whitbeck

heat exchanger.

Why not use a plate and frame. Low mass fast heat up. much better heat exchange. But I could be all wet.

Steve Whitbeck

opps

Charlie from wmass

It is like using a

12 volt generator to power your car down the road by way of a v belt to the main engine pully and a trunk full of batteries. The shell is designed for steam to go in and condense. There is no room for it to expand. I am not saying that electricity could not be made this way, just that it would be unsafe and inefficient. I love steam but there needs to be more thought given here to the 1700 times expansion rate.

Mark Eatherton

Al...

The typical PV panel is around 10% efficient BEFORE inverter losses.



A typical solar thermal collector can be between 60 and 70% efficient. So, even if your system losses accounted for roughly half the collected energy being lost, you;d still be able to produce 3 times as much energy as the typical PV system.



I'm not sure that it is possible to raise the pressure required to drive a turbine with a generator on it, but thermal solar is significantly more efficient than PV, which confuses me, because our gubernmint is spending so much money on a relatively inefficient technology...



Guess I shouldn't be too surprised tho...



ME

LarryC

First reply.

Thanks for the responses.



From my understanding of boilers, the two basic forms of tubed boiler design are fire on the outside of tubes full of water to make steam inside the tubes, or running heat thru tubes to boil the pot of water.  The first method confines the steam to pipes, which are easier to make strong enough to hold the force of expanding steam.  Problem with the second method is that the boiler needs to be able to hold all of the force generated by the expanding steam. 



In a previous life I used to operate Pressurized Water Reactors (PWR) nuclear power plants for the US Navy.  Our boilers or Steam Generators are vertical shell and tube heat exchangers that used a inverted U shaped tube bundle.  The hot water that circulated thru the core of the nuclear reactor was pumped thru the tubes.  Secondary or steam plant water was pumped into the shell side, circulated around the tube bundles, where it boiled off into steam, went thru a moisture separator to dry out the steam and then it went off to the turbines.



My thought is to do the same basic idea.  Mount the HX vertical and pump 400 - 500 F working fluid thru the tubes, while maintaining a water level about half way up the tube bundle.  That might give enough of a steam chest to minimize the amount of water carry over coming out of the steam exit.



Safety issues include maintaining a correct water level while steaming, over and under pressure safeties, etc.  Materials would have to be compatable for all working fluids.



One thought that just occurred to me was to swap a longer but same diameter shell with a shorter tube bundle to increase the steam chest volume. 



The logic behind using an existing heat exchanger is that the engineering and construction is already done.  If the shell side is rated for MAWP of 150 psi, then we should have enough of a safety margin for system that would use 100 psi steam.  I am assuming the max pressure inside the tubes would be less than 60 psi.



My limited understanding of thermodynamics is that heat travels from hot to cold so the amount of heat flow is primarily controlled by delta T between the heat transfer fluid and the saturated steam, transfer area, and conductivity of the material.  Assuming there is not a condition of excessive buildup of steam clinging to the surfaces of the tubing due to excessive nucleate boiling.



Does my reasoning have any significant flaws?

Al Corelli_2

Thanks Mark

Thank You

LarryC

Anybody else have any comments?

Does anybody else have any other input?  Am I pursuing a very expensive scrap metal sculpture?



Thanks,

icesailor

Low Vs. High pressure:

It's my vague understanding that 15# steam pressure is considered "Low Pressure Steam. Anything over that is considered "High Pressure Steam."

It seems to me that what you are trying to do with your reverse heat exchanger is a potential bomb, waiting for the right conditions to prove that water has a very high expansion rate if and when the pressure drops. Sort of like the steam in those boilers that feed the propulsion and generating systems that you worked on while in the Navy.

Theoretically, it may make sense. On paper, less so. But in real world application, I would worry about what might happen if the pressure went below the boiling point.

I saw something here the other day that gave me pause to contemplate. "If you drive up to a steam power plant, and you see a stream of people running away from the building, follow them."

I would.

LarryC

Safety concerns.

Ice,



I agree with you 100% about the safety concerns.  That is why I suggested using an appropriately rated heat exchanger to start with.  I also mentioned the need for safety valves for both overpressure and underpressure situations.  Safely maintaining correct water levels is a major hurdle to solve too.



I am thinking this method is a heck of a lot safer then their original plan of using a non rated tank and just using a copper coil to pump heat transfer fluid thru it.  I understand their desire to create a functional steam boiler to drive a steam engine and I am trying to visualize a safe manner to do so.



As far as the 1700 times volume liquid to gas expansion concern, that is why I am thinking of using an oversized shell to hold the tube bundle to provide room for expansion along with room for the steam to dry out.

Jim_47

This brings back memories

From 2000 - 2005 I was in charge of a facilities plant that used High temp hot water to create steam. There is a lot of math and engineering in the design of these systems.

High temp hot water was delivered to my building at 375'F and 270psi. 18inch welded pumps delivered the HTHW. not sure of the flow rate.

The Steam generator was in fact a shell and tube exchanger. I had 5 of these beasts to maintain.

As someone else here mentioned safeties safeties safeties.  The rate of expansion is controlled by the flow of hot water into the steam generator. Double safety blow off's piped high above the boiler room were ther to insure the rate of expansion was kept safely.  These steam generators had a 8 inch steel steam exhaust to the manifold. I was asked to maintain 21 psi steam pressure.

As for a home brew unit, I would have to say "DONT TRY THIS AT HOME" To much could go wrong in your design and equipment.

Once during a safety check one of the high pressure safeties blew off. I looked around and of the 4-6 guys in the boiler room before, I was the only one left taking the boiler out of automatic and  bringing it back down. nearly had to change my pants. 

Good luck and let me take out th insurance on you and your family if you do decide to nest one of these in your basement

LarryC

How did they control water level in the steam generators?

Jim,



How did they control the water level in the steam generators?



This is definitely not happening in my house.  I am just trying to determine a safer way to generate 90 psi steam than welding heating coils into an existing tank.  I am thinking the quantity of steam they will be needing is no more than one BHP (Boiler Horse Power) or 34.5 lbs of steam at 212 F.



Larry C