Printed on recycled paper COMPARISON OF NATURAL CONVECTION HEAT EXCHANGERS FOR SOLAR WATER HEATING

The use of natural convection, side-arm heat exchangers in solar domestic water heating systems is becoming a popular method of reducing system cost and electric demand. In this paper, we compare thermal performances of three natural convection heat exchangers mounted along the outside of the water storage tank and a commercial wraparound-tank heat exchanger to two forced-flow heat exchangers. The external natural convection heat exchangers are 1) a commercial, single-wall, coil-in-shell design, 2) a double-wall, two-pass, tube-in-shell exchanger and 3) a single-wall, single-pass, tube-in-shell design. Performance is discussed in terms of pressure drop, heat exchanger effectiveness, and overall heat transfer coefficient. Measured data were either obtained in our laboratory or from previously reported studies. Heat transfer area appears to be the most significant factor in determining thermal performance. Because its heat transfer area is three times larger, the commercial coil-in-shell heat exchanger has natural convection flow rates nearly three times greater than the two tube-in-shell designs.