What are the effects of boiling in a heat exchanger?

[TheHypnotoad]

I'm working on a heat exchanger design, with water as the coolant. I'm running an analysis on this thing, and it looks like it's going to get hot. Like, 900F hot. So, of course this is going to be steam by the time it reaches the outlet. I'm really not a fluids kind of girl, but it's on me to tell the experimental engineer whether this test is going to end in tears, and if it's going to be a problem, I want to try to talk him into using a different coolant.

So, my question for forum! What happens to a heat exchanger when boiling occurs in the coolant flow? Should I be expecting sudden pressure increases? Losses in the maximum mass flow rate? Exploding heat exchangers?

Some more information:
-Fluid enters at around 200psi
-Coolant is not immediately vented at the outlet, but enters a manifold and is piped a few feet off to be vented

Thanks for your input!

 

[russ_watters]

Welcome to PF

Sounds like you are saying this is an open system - the coolant gets discarded? The pressure in the system will be very low since it is open, and any backpressure will be due to dynamic effects (the steam rushing out at probably supersonic speed) unless there are valves or other devices to create backpressure.

What you're describing is a boiler - turning water into steam is the main function. But whether you want steam or not depends on what the coolant is meant to do after it leaves the boiler. If you don't want steam, you'll need to increase the coolant flow rate until it can remove the heat without boiling.

Seems like an odd way to run a heat exchanger, but without knowing more details it is tough to know why you are doing this.

 

[TheHypnotoad]

Welcome to PF

Sounds like you are saying this is an open system - the coolant gets discarded? The pressure in the system will be very low since it is open, and any backpressure will be due to dynamic effects (the steam rushing out at probably supersonic speed) unless there are valves or other devices to create backpressure.

What you're describing is a boiler - turning water into steam is the main function. But whether you want steam or not depends on what the coolant is meant to do after it leaves the boiler. If you don't want steam, you'll need to increase the coolant flow rate until it can remove the heat without boiling.

Seems like an odd way to run a heat exchanger, but without knowing more details it is tough to know why you are doing this.

Thanks for the response, Russ! You're right, it's an odd way to run a real heat exchanger . . . in reality, it's going to be an active cooling system. Eventually, they'll run the system with a combustible gas as the coolant, and they'll burn the gas to heat the panel. But for an initial test, they wanted to run something less combustible, like water. So yes, it's a bit of an odd setup to start out.

So it sounds like all that will happen here is as the water boils, the steam will rush out and I will be left with no coolant, unless a back pressure is applied, right? You asked if this was a boiler, but really, the point is to keep the thing under a certain temperature, and if I have no coolant, that's not going to fly. Sounds like the test engineer and I have some talking to do. . . .

And thanks for the welcome :) looks like a neat little forum around here

 

[Sakha]

You might have troubles with the Leidenfront effect, take it into consideration.