Diverter or mixing valve

[JohnWeal]

Good evening

We have a requirement to put water at 70 deg C into the annulus of a concentric tube in tube heat exchanger (construction is 2 streams of 7 tubes which are each 6m long)

This is to heat digester sludge from 36 to 39 deg C.

The calculated mass flow rate is 24 kg/s for the maximum heat exchanger rating of 1005 kW.

The 1005 kw rating is of course sized for the extreme conditions which are a rare event. For this project it is ambient temperature of -10 deg C and a raw sludge feed temperature of 5 deg C.

Therefore, less kW are required in anything warmer than this and the control of the mass flow rate through a 3 way valve is th client specification.

I jave seen numerous circuits and numerous 3-way valve piping arrangements.

1) which is most advantageous, a diverter valve or mixer valve? I’m thinking a mixer valve keeps the same mass flow rate but varies the temperature and the diverter valve has the constant temperature but varies the mass flow rate.

What is generally the preferred way to control the heat transfer for sludge heating applications?

Best regards
John

 

[LittleInch]

I'm not sure about sludge in particular, but I would go for the mixer valve myself.

This leads to a more even heating along your tube and a less concentrated heating in one end of the tube.

The difference in heat requirement appears rather extreme to me - if you assume the sludge flow is the same, then heating it 3 degrees as opposed to 34 degrees implies a 10:1 turndown. Not many things work well at that sort of turndown and if you maintain temperature in the water your mass flow would be only 10% of what it is at max flow (unless I've misunderstood your figures).

"Sludge" doesn't sound like a great material to heat up so I would go for the longest exposure time to get even heating and not just a hot annulus where it is in contact with the tube.


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[robot6]

Maintaining velocity is important, because it reduces the thickness of the insulating wall film. (Remember the boundary condition which indicates that vfluid -> 0 at the wall, so there is a mass of fluid on the wall that does not move and acts as an insulator. Theoretically, of course.)

To limit this effect, you have to keep the velocity up and this suggests a mixing valve. This will also give you the best control over the rate of heat transfer.

High velocities are also used to minimize scale and, in the case of non-newtonian fluids, to reduce the pump power requirements (but of course this is more effective with some kind of shearing mechanism, such as a spiral inside the tube).

On the other hand, outlet control valves are often used to maintain pressure on the control side. This is important to make sure that process fluid, which is not on the control side, is kept out of the cooling fluid, which is typically something benign like water. There are other techniques to maintain pressure though, such as an outlet orifice and the like, but these cost pump power.