tubes each 4 mm in diameter formed of kraft paper were arranged side by side as heat exchange elements 2 and headers 3 and 3 39 as shown in fig3 were connected to both ends of the tubes 2 to provide a heat exchange unit 1 as shown in fig2 then 40 of such heat exchange elements were superimposed with a spacing of about 1 mm between elements and the resultant assembly was accommodated within a casing to provide a heat exchanger such as shown in fig4 having a total of 1000 kraft paper tubes using the heat exchanger thus assembled air was introduced through the tube side by suction at a rate of 4 m 3 per minute as a result the pressure drop was 10 5 mm aq for comparison a plurality of heat exchange units as shown in fig1 were laminated and placed in a casing in the same manner as above to provide a heat exchanger of the same size and the pressure drop was measured under the same conditions and found to be 18 mm aq nearly twice as large as that of the heat exchanger of the present invention thus a heat exchanger according to the present invention exhibits a very low pressure drop twenty five tubes each 6 mm in diameter and formed of kraft paper which had been subjected to a flame retarding treatment were arranged side by side as in example 1 to provide a heat exchange unit 1 as shown in fig2 subsequently in the same manner as in example.