A novel design for a double-spiral-coil counterflow heat exchanger, which satisfies the requirement for a double wall between the fluids for heat pump water heaters and some solar energy collecting systems, was built and tested. The temperature profiles for both fluids were found to be linear functions of coil length, even at flow rates as low as 1.5 gpm (0.095 1/s).

Based on the linear temperature profiles, a mathematical model was set up and an easy-to- use closed form solution was derived . The solution predicted the experimental fluid temperature profiles well, except at very low fluid flow rates where the calculated heat exchanger performance became conservative.

The performance of the conventional counterflow heat exchanger was calculated, based on the same amount of coil material and the same operating conditions, and compared with the novel design. It was found that the new design, for the tests performed so far, was at least SOHO better in terms of heat exchanger effectiveness.

Units: Dual