3 replies to this topic

[CheE1986]

Hello All,

I am working on a pilot plant a we have a plate heat exchanger that is supposed to raise the temperature of a liquid from 70C to 104C. The heating media is saturated steam at 13-15 psig. When designing the pilot plant we sized the heat exchanger for a flow of 100 gpm, however due to operational problems the flow we are getting from the factory is only 30-40 gpm. As a consequence, we have more heat transfer area in the heat exchanger.
When we start up the pilot plant we pump a constant flow of 40 gpm, however when we start to raise the temperature of the juice up to 104C we have noticed a sudden drop of the flow to 20 gpm. I presume that the juice is boling inside the heat heat exchanger and the mass of vapor stops the incoming flow. The steam opening is controlled with an automatic valve that is connected to a temperature sensor at the oulet of the heated juice. Also, the HE has 1 pass of steam and 2 passes for the juice.

Please I do not know what to do to solve this issue and if I continue the operation in this way me heat exchanger will be damaged or it will scale. Does anybody know how can I solve this problem?

Thanks!

PD: I forgot to add that after the heated juice leaves the Heat Exchanger it enters into a flash tank for proper degassing.

Edited by CheE1986, 26 October 2012 - 06:49 PM.

[Pilesar]

Are you trying to boil your juice? If not, only heat it to a little less than the boiling point temperature. If you do not know the boiling point temperature of your juice at the pressure you are running, then that is your first step.

If your target temperature is lower than the boiling point, consider adjusting the heat transfer rate in the heat exchanger by controlling the condensate flow instead of the steam flow. Use a condensate flow control valve instead of a steam trap and leave your steam control valve in manual. This way, a level of condensate remains in the exchanger. The heat transfer rate of hot water is much less than for condensing steam and the effective exchanger area is reduced.

If you are purposely trying to boil your juice, the juice flow may be restricted by pressure drop in the exchanger or downstream piping. You will have to look at the entire system for the hydraulic analysis.

[TS1979]

CheE1986,

In your post, you did not give a key parameter - the operating pressure which determines if the juice will be flashing or not inside the heat exchanger. And from the pressure change you can also identify if the 20 gpm is still on the pump performance curve (if you have a centrifugal pump).

For troubleshooting, you should identify what conditions have been changed and what are not. In you case, you want to increase the juice outlet temperature by increasing the steam flow to the heat exchanger, so you only changed the steam flow. Do you try to see what will happen if you back to your previous operating conditions? Are the flow rate back to 40 gpm as previous?

[S.AHMAD]

The way I see your problem is due to higher pressure drop in the HE and outlet piping.
my suggestion is reduce the number of plates of HE that correspond to 40 m3/h since the plates for the H can be removed. If the problem is still occuring, you need to size the outlet line for 2-phase flow.