that a line is provided connecting the collection vessel 2 or neutralization and collection vessel if the heat exchanger block is not made from graphite with a nozzle disposed at the flue gas outlet region of the heat exchanger block 1 and or a nozzle 10 is provided for injection of fresh water therefore additional condensate and or fresh water can be sprayed into the heat exchanger block from above resulting in more effective flushing away of deposited dust fig6 which also illustrates a heat exchanger block assembly similar to that shown in fig1 includes a demister 11 provided at the upper end of the heat exchanger block 1 although only shown for countercurrent operation in fig6 the demister can also be used in cocurrent operation as is shown in fig2 this is advantageous for the following reasons the demister 11 separates condensate droplets from the flue gas stream leaving the heat exchanger block since those droplets usually form with small dust particles acting as condensation nuclei those fine dust particles together with the droplets are separated from the flue gas outlet stream when passing the demister for example by providing a demister constructed for separation of droplets with a diameter of 10 u03bcm dust particles with a diameter below 10 u03bcm can be separated from.