Design of Vapour Condensers for Pans and Evaporators
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The function of the condenser is, self evidently, to condense the vapour being driven off the syrup or massecuite in an evaporator or vacuum pan. This is done in order to create a vacuum in the pan or final effect evaporator. The vacuum pump's function is to remove the incondesible gases that find their way into the vapour stream. The incondensible gases come from the following sources:
- leakage of air into the vessels;
- inherently in the juice;
- air in the heating steam
- air in the cooling water
Direct Contact Condensers
In this kind of condenser the condensing medium (cold water) is in direct contact with the vapour. The amount of cold water required is calculated from an energy balance around the condenser
A number of different designs are discussed by Hugot (pg 798 3rd ed). Single perforated tray rain-type condensers are discussed in Moult JM and Smits JH, Single Tray Rain Type Condensers, Proc S Afr Sugar Technol Assoc 1979, pg 98.
This design consists of a single perforated plate tray, with a number of so-called chimneys to allow incondensible gases to pass from below the tray to above to be drawn off by the vacuum pump.
10% of the water flowing into the tray leaves the tray via the circumferential holes and 10% overflows the tray. This is to ensure the walls of the condensor are continually wetted. It is understood that most of the condensation happens on the walls. The remaining 80% of the inflowing water leaves via the perforated plate tray bottom. The cicumferential holes and the plate perforations are sharp edged orifices.