Syphoning Effect on a plate and frame heat exchanger

[Gene7730]

Hi, I am working on a project to create a pressure differential in a plate and frame heat exchanger between the media and the product under static conditions. Please see attached image

The initial conditions on the media side is water at 190F, when the fluid that is being heated stops flowing, the idea is to close valves A and B, and open C to a seal tank creating a syphoning effect which will reduce the media pressure inside the heat exchanger to below atmospheric pressure.

The question is- If we have 190 F water at 40 psig, at a height of 75" from the seal tank, will the decompression of hot water from 40 psig --> 0 psig create nucleation vapor bubble near the seal tank's pipe that can break the syphoning effect and allow air to enter the heat exchanger? Will steam form towards the top of the heat exchanger due to this decompression effect since the pressure is lower at the top however the water is 10 degrees below boiling point.

Thank you

 

[georgeverghese]

What is the effect of the product stream when both flows are stopped ? - work out by heat balance what the equilibrium temp will be for both product and water sides based on the trapped mass inventory on each side - assuming this happens adiabatically or see if you can account for the heat gain /loss from ambient, and account for the metal thermal mass of the HX also.

The equilibrium temp will then tell you whether you will have vapor breakout on the water side at the top of the HX, which will be at (atmospheric press - 75inches of water) = 12psia, assuming that valve A is indeed leaktight.

 

[Gene7730]

Thank you for your response. The delta T on the heat exchanger under normal operations is about 3-4 degrees. The water is flowing much faster than the product. The stopping operation is supposed to take place within 3 seconds. This does not allow enough time for the water to cool. Assuming the temperature is 190 F and stays at this temperature in this 3-5 second interval to depressurize the heat exchanger, will the sudden pressure difference allow water vapor to bubble out to the point where the water surface tension is overcome breaking the siphoning effect creating a run away system. or will it be momentary at most where some air gets in/ if any but not enough to have a runaway system.

Will the sudden expansion delta P create steam displacing the water at the top of the heat exchanger creating a sudden surge of water out through the seal tank such that water keeps flowing out and lets air in?

 

[georgeverghese]

If the water side temp is still 190degF on shutdown at the top of the HX, water will only be 2 phase if pressure is less than 9.3psia < 12psia. So it will still remain in liquid phase at the top of this HX.

Water is an incompressible fluid; there is hardly any "expansion " as its pressure drops from 40psig to 12psia ( unlike a compressible gas), so there is hardly any flow out of the water side as it depressures.

 

[LittleInch]

You are though operating very close to the limit and a few degrees out and you're in trouble. Whilst water might not "boil" at the pressures you have, it will give off plenty of steam.

Having said that my reading of the OP is that the water is the element which is giving up energy, not the fluid on the other side of the HX??, i.e. the water usually exits at a lower temperature than it goes in.

You don't mention anywhere the fluid flow or velocity. Stopping a flow in 3-5 seconds has the potential for surge or "water hammer" and you could easily temporarily pull a vacuum at the top of the exchanger when you close valve A. If you close valve B just before you close valve A then this will probably go away.

Opening valve C doesn't create a "syphoning effect", it just creates a low pressure at the top of your HX. "syphon" implies flow.

Otherwise I agree with George, if the system is locked in at 40 psi and you then de-pressurize, the flow will be a small spurt and the "shock" won't be enough to do anything drastic.

At the end of the seal pipe the water pressure is atmospheric. Only if the end of the seal pipe comes above the water in the tank will you get any air coming in from the end of the seal pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Gene7730]

Thank you for your responses