The good heat transfer is achieved by excellent liquor distribution on the tubes from the distribution plate above the tube bundle. Steam is introduced into the tubes at the bottom. This results in complete countercurrent flow and desuperheating of the incoming steam by the down flowing condensate. In order to minimize any kind of scaling the bottom tube sheets can be inclined to prevent any gathering of crystals on the tube sheet. If necessary, both sides of the heating surfaces can be washed.