the inner surfaces on which deposits may build up include the inner wall of the housing and the heat exchange surfaces . by rendering the heat exchange surfaces movable and by arranging at least some of the heat exchange surfaces close to the inner wall of the housing , the movement of the heat exchange surfaces may not only knock off any deposits on such surfaces but it may also cause contact between the inner wall of the housing and the heat exchange surfaces so that also deposits from said inner wall are removed . the heat exchange surfaces may have any suitable shape . hence it is possible to have heat exchange plates . preferably the heat exchange surfaces are constituted of a heat exchange tube . one way of constructing the heat exchange surfaces such that they are movable is by arranging bellows or a sliding joint at suitable locations . one such suitable location is the place at which or in the vicinity of which the heat exchange surface is connected to the inner wall of the housing . another suitable way to render the heat exchange tube is the form of a coil , in which the loops are not welded together but are loose . by causing a water - hammer effect in such a coil or by vibrating or impacting such coil , the tube starts to move and due to such movement and to any collisions that may take place between the tube loops and between the loops and the inner wall of the housing , an efficient way of removing deposits is attained . to facilitate the impact of a shock or vibration on the heat exchange surfaces the coil of a heat exchanger tube is suitably interlaced with another heat exchange tube having the shape of a coil . the latter tube may be securely fixed to the inner wall of the heat exchanger housing . another possibility is to construct this latter tube as loosely as the former coil . by causing a shock or a vibration to at least one of these coils , this one will start to move and will contact the other one so that an improved removal of deposits is obtained . the coolant to be used in the present invention can be any conventional coolant and is suitably steam or water . the water - hammer effect is created by a sudden change in the flow rate of the coolant , so that an increase in pressure is obtained which is capable of exciting the heat exchanger surfaces to vibration with a sufficient amplitude for cleaning without causing damage to the heat exchanger itself . the change in velocity ( δv ) required to achieve the desired change in pressure ( δp ) is given for a liquid as δv =- δp /( ρc ), where ρ is the coolant density and c is the coolant sonic velocity . since the heat exchanger according to the present invention advantageously can be employed in cooling hot synthesis gas emanating from the gasification of coal , the present invention further relates to a process for the production of synthesis gas containing carbon monoxide and hydrogen , which process comprises a ) subjecting coal to a gasification reaction with oxygen and steam to yield synthesis gas and slag ; b ) cooling the synthesis gas entraining fly slag droplets in a heat exchanger by passing said synthesis gas through the housing of the heat exchanger and by passing a coolant through a passage provided by movable heat exchange surfaces in the housing without bringing the coolant into direct contact with the synthesis gas ; and the deposits that are removed from the inner surfaces of the heat exchanger may be entrained further by the cooled fluid . it is also possible that they fall under the influence of gravity . to the case of coal gasification the fallen slag particles are preferably taken up in a slag bath and processed further . the movable heat exchanger surfaces may be moved continuously or intermittently . fig1 gives a diagrammatic view of a heat exchanger with a movable coil - type heat exchanger tube . in fig2 another embodiment of such a heat exchanger is depicted . fig3 shows an embodiment of the heat exchanger according to the invention in which the coil - type heat exchanger tube can be moved mechanically . fig4 gives a diagrammatic view of an embodiment of the present heat exchanger in which two coils are interlaced . fig1 shows a heat exchanger housing 1 in which a heat exchanger tube 2 has been helically arranged as a coil along a wall 3 of the housing 1 . the housing 1 provides a passage for a hot fluid via an inlet orifice 4 and an outlet orifice 5 . the tube 2 can be connected to a source of coolant via an outlet connection 7 . the tube 2 is only connected to the wall 3 at the spots where the tube penetrates the wall to arrive at the inlet connection 6 and outlet connection 7 . the loops of the coil are all loose . by suddenly changing the flow rate of the coolant through tube 2 the loops will start to move and deposits on the tube 2 and the wall 3 will be knocked off . in fig2 a similar heat exchanger as in fig1 is shown . this heat exchanger is provided with a housing 21 , with a wall 23 and a heat exchanger tube 22 . the heat exchanger tube 22 has been arranged such that the hot fluid is additionally cooled by a wall of heat exchanger loops 24 through which coolant is passed . in the embodiment shown the loops 24 are part of the heat exchanger tube 22 . it is evident that it is also possible to arrange two tubes , one as a wall of loops 24 welded together and one as a loose coil - type heat exchanger tube . from the figure it is apparent that the inlet 25 and outlet orifice 26 for the hot fluid and the inlet connection 27 and outlet connection 28 for the coolant are similarly arranged as in fig1 . fig3 shows a heat exchanger housing 31 with an inlet 32 and an outlet 33 for the hot fluid to be cooled . the heat exchanger further comprises a heat exchange tube 34 in the form of a coil with inlet connection 35 and outlet connection 36 for coolant . the heat exchanger is further provided with a sliding packing joint 37 by means of which the heat exchange tube 34 can be moved . a rapper or vibrator can also be used to cause a movement to the heat exchange tube 34 . fig4 shows a heat exchange housing 41 with a heat exchange tube 42 in the form of a loose coil , together with a heat exchange tube 43 that also has the shape of a coil . however , the loops of the latter tube 43 are welded to the wall of the housing 41 by means of welds 44 . the heat exchanger is further provided with an inlet 45 and outlet 46 . coolant is introduced into heat exchange tube 42 via an inlet connection 47 and into heat exchange tube 43 via inlet connection 48 . coolant is withdrawn from the tubes via outlet connections 49 and 50 , respectively . heat exchange tube 42 is constructed such that it can rotate relative to the other . the embodiments of the figures described above are illustrative to the present invention and are not set forth to have a limiting effect on the claims hereinafter presented .