Reboilers

Question:

            Sometimes, when I OPEN the channel head condensate drain valve, my reboiler duty increases. But, sometimes when I partly CLOSE off the same valve, my reboiler duty decreases. What’s happening?

Answer:

            There are two problems with draining condensate from the channel head:

1.      Blowing the condensate seal.

2.      Condensate back-up.

If you open the drain valve too much, steam and water will blow-out together. The high velocity steam, blowing though the tubes, will reduce the condensing heat transfer coefficient. Hence, the observed loss in heat transfer and reboiler duty.

If you close the drain valve too much, water will back-up and submerge the lower few rows of tubes, which will reduce the surface area exposed to the condensing steam, which also diminishes the rate of heat transfer.

Without some method of observing the steam condensate level in the channel head, the optimum position of your condensate drain valve can only be found by trial and error.