Patent GB-712834-A (GB)
Title: Improvements in or relating to evaporating plants
IPC: B01D1/28
Filed: 1951-07-07
Abstract: <PICT:0712834/III/1> In evaporating plant of the type wherein the evaporated vapours are withdrawn from the evaporating chamber by a compressor and, after compression are introduced into a heat exchanger disposed in the evaporating chamber, the operation of the evaporator is automatically maintained steady, despite the formation of scale on the surface of the heat exchanger, by employing a heating surface of considerably greater area than that necessary to condense the compressed vapour when the surface is unscaled, and by connecting an open pipe to the vapour space of the condensate drainage compartment of the heat exchanger so that air can pass into and out of the heat exchanger.  The glands of the vapour compressor are preferably of the labyrinth type and, in order to prevent the ingress of air into the vacuum side of the plant, the vapour outlet from the compressor is connected to the gland at the suction end of the compressor so that there is a slight escape of vapour therefrom.  The evaporation of sea water and of natural land water is referred to. The compressor may be of the axial flow type, or rotary displacement type.  In the Figure the heat exchanger 2 of the evaporator 1 comprises tubes 3 passing between headers 4 and 5; the tubes being divided into two banks by rib 6. Compressor 10 discharges compressed vapour through pipe 13 to header 4.  The condensate formed as a result of the vapour condensing within tubes 3 is withdrawn from the drainage compartment 7 through pipe 24; and is used to assist in preheating the feed, which is supplied to the evaporator from tank 14 via heat exchangers 16 and 17.  The pressure within the tubes 3 will be slightly greater than atmospheric pressure, since the heat exchanger 2 is in communication with the atmosphere by a pipe 37. When the tubes are clean, air will be drawn into the heat exchanger 2; but as scale forms on the tubes, air will be discharged from the heat exchanger in order to compensate for the increase of resistance to heat flow across the tubes.  The temperature difference across the tubes will increase, and the vacuum in the evaporator 1 will rise. This action will continue until the compression ratio is at the limit of the capacity of the compressor.  When this point is reached it will be necessary to clean the scale from the tubes.  Since the pressure in heat exchanger 2 is substantially atmospheric, the discharge pipe 13 of compressor 10 and the gland adjacent thereto will be automatically subjected to a pressure slightly in excess of atmospheric pressure.  A pipe 37A is connected to a point on the discharge of the compressor, whereby vapour at a pressure slightly in excess of atmospheric pressure is admitted to the gland on the suction side of the compressor, thereby sealing this gland against ingress of air.  In order to vary, if necessary, the amount of heat supplied to the plant, additional heat may be supplied from auxiliary evaporator 29, which is supplied with distilled water from tank 31 and is heated by electric elements 30.  The vapour formed in evaporator 29 is led via pipes 36 and 13 to the heat exchanger 2 of evaporator 1.  A supplementary supply of steam may also be fed in at branch 13a.  An electric heating element 38 may, if required, be incorporated in the liquid space of evaporator 1. Specification 642,845 is referred to.