A heat transfer device comprising:- (a) a first heat transfer member defining a heat exchange surface with which a single heat transportable medium contacts in use and via which heat is transferrable between the single heat transportable medium and a single working medium in a fluid or gaseous condition; and (b) a device body for containing the single heat transportable medium; wherein:- (i) said body defines at least a first chamber, a second chamber and a third chamber which are in fluid communicable relationship; (ii) said body is configured to allow the single heat transportable medium to pass from said second chamber to said first chamber and then to said third chamber, or from said third chamber to said first chamber and then to said second chamber; and (iii) said second chamber is sandwiched between said first chamber and said third chamber in said body; (iv) at least one of said chambers is made of at least a pair of oppositely facing plates (21, 22, 23, 24) generally arranged in parallel to each other; and (c) a conduct for a single working medium, characterised in that, the conduct is connected to the first heat transfer member so that the single working medium is provided on the side of the heat transfer member positioned opposite to the first chamber, and wherein the conduct is adapted for a liquid or gaseous working medium; wherein said first heat transfer member is in direct contact with or part of said first chamber such that heat transfer between said first heat transfer member and said first chamber can take place, but not in direct contact with said second chamber. (a) a first heat transfer member defining a heat exchange surface with which a single heat transportable medium contacts in use and via which heat is transferrable between the single heat transportable medium and a single working medium in a fluid or gaseous condition; and (b) a device body for containing the single heat transportable medium; wherein:- (i) said body defines at least a first chamber, a second chamber and a third chamber which are in fluid communicable relationship; (ii) said body is configured to allow the single heat transportable medium to pass from said second chamber to said first chamber and then to said third chamber, or from said third chamber to said first chamber and then to said second chamber; and (iii) said second chamber is sandwiched between said first chamber and said third chamber in said body; (iv) at least one of said chambers is made of at least a pair of oppositely facing plates (21, 22, 23, 24) generally arranged in parallel to each other; and (i) said body defines at least a first chamber, a second chamber and a third chamber which are in fluid communicable relationship; (ii) said body is configured to allow the single heat transportable medium to pass from said second chamber to said first chamber and then to said third chamber, or from said third chamber to said first chamber and then to said second chamber; and (iii) said second chamber is sandwiched between said first chamber and said third chamber in said body; (iv) at least one of said chambers is made of at least a pair of oppositely facing plates (21, 22, 23, 24) generally arranged in parallel to each other; and (c) a conduct for a single working medium,