Experimental characterization of a spiral heat exchanger for waste water heat recovery from partially filled sewage pipes

This paper focuses on the experimental characterization of horizontal, spiral heat exchangers used for heat recovery from sewage pipes. Such non-intrusive heat exchanger configurations are appealing for this application, but they often exhibit reduced heat exchange performances compared to more compact designs. To compare the performance of this configuration with that of an ideal counter-flow heat exchanger, a representative test setup was constructed. As the sewer pipes are generally not completely filled, tests were conducted using three different fillings (100%, 50%, 33%) and a filling parameter z was introduced in the heat transfer equations. Correction factors to be applied in the Logarithmic Mean Temperature Difference (LMTD) design method for this specific spiral configuration were derived from the test results. It was found that the filling of the sewer pipe has minimal impact on the correction factor, which ranges between 0.78 and 0.83. The average value of 0.81 is recommended for design purposes.