Abstract: A heat exchanger comprising a casing through which pass a plurality of spaced-apart tubes. Heat exchange takes place through the tube walls between a first fluid within the tubes and a second fluid outside them but within the casing. The rate of heat exchange is enhanced by electrohydrodynamic effect by means of an electrode comprising a sheet-form first part which encompasses the tubes, and connected second parts which run lengthwise through the spaces between the tubes. The first part may be mesh-like and the second parts may be mesh-like and/or rod-like. The electrode is excited to high voltage and the casing and tubes are grounded. The effect of the second parts is to make the electric field around the individual tubes more uniform than would be the case if the electrode consisted of the first part alone.