1. A cleaning device, including in combination: an upright cylindrical heat exchanger housing, heating means in said housing, first heat-exchange means for liquid, in said housing and heated by said heating means, said heat-exchange means having an inlet and an outlet, a cylindrical wall surrounding said housing and spaced outwardly therefrom and defining between itself and said housing an annular chamber; said outlet of said heat-exchange means leading tangentially into an upper portion of said chamber lying below the upper end thereof to provide a liquid-free expansion space thereabove, said chamber having an outlet conduit near its lower end lying athwart the path of flow of said liquid around said annular chambeR, circulating pump means having an inlet connected to said outlet conduit from said chamber and an outlet connected to the inlet of said first heat-exchange means, upper and lower closure means connected to said wall and to said housing for sealing said chamber, whereby said pump can circulate liquid in a sealed system from the outlet of said first heat-exchange means into said chamber tangentially, swirling in said chamber in a downward helical path to said outlet conduit and back from there through the pump to the inlet of said first heat-exchange means in a continuous cycle, pressure relief means for said closed system in said upper closure means at the upper end of said expansion space to prevent too high a pressure buildup, and second heat-exchange means for a fluid to be heated and comprising, a helical coil in said annular chamber substantially narrower than said chamber so that its full circumference is in contact with said helically downwardly swirling liquid, and having an inlet end at the lower end of said chamber and an outlet end located at the upper end of said chamber, for heat exchange between said liquid and said fluid, whereby said fluid is heated by the heat imparted to said second heat-exchange means by countercurrent flow of said liquid rather than being directly heated by said heating means, thereby giving improved control of the temperatures imparted to said fluid.

5. A cleaning device, including in combination: an upright cylindrical housing having a lower end and an upper end, a gas burner in and at the lower end of said housing, a first heat-exchange coil for the flow of liquid therethrough, located in an upper portion of said housing and heated directly by said burner, said coil having an inlet at a lower end thereof and an outlet at an upper end thereof, an insulated cylindrical wall surrounding said housing, concentric therewith and spaced outwardly therefrom and defining between itself and said housing an annular chamber, said coil outlet being connected to said chamber through a sealed tangential opening at the upper end of said chamber inducing circular flow of said liquid in said chamber, said annular chamber having an outlet conduit with a radially extending opening lying near the bottom of said chamber, so that said liquid swirls helically downward in said chamber from said tangential opening to said radially extending opening, top and bottom walls joining said cylindrical wall and said housing and sealing said annular chamber, while leaving a central exhaust opening for the burned gas from said burner, said top wall being spaced above said tangential opening to provide an expansion space, a circulating pump having an outlet connected to the inlet of said first heat-exchange coil and an inlet connected to said outlet conduit from said chamber, whereby said liquid is circulated by said pump in a closed, pressurized system into said coil inlet, through said coil outlet to said chamber and back from there to said pump in continuous cycle, and a second heat-exchange coil for a fluid to be heated, having an inlet end at the lower end of said chamber and an outlet end at the upper end of said chamber and located entirely in said chamber spaced from the walls thereof and only about half as wide as said chamber, for full circumference heat exchange between said liquid and said fluid, whereby said fluid is heated by the heat imparted to said second coil by countercurrent flOw of said liquid rather than being directly heated by said burner, thereby giving improved control of the temperatures imparted to said fluid.