Improved performance of secondary heat exchanger for latent heat recovery from flue gas using mini-tubes

• A new type heat exchanger was proposed for efficient heat utilization.
• The heat transfer rate in mini tubes were enhanced remarkably.
• Non-dimensional correlation of enthalpy change against tube length was proposed.
• The equations correlate well with the measured data independently of conditions.
• Volume of HE could be reduced remarkably by mini-tube (e.g., 5% of d = 5 mm for 1 mm).

A new type of heat exchanger, in which flue gas flows inside thin tubes and cool water is on the shell-side, was proposed to develop the performance and compactness of shell and tube type heat exchangers for latent heat recovery from flue gas. The experimental heat transfer characteristics of single tubes were systematically investigated to determine the effects of tube diameter (1.0–5.0 mm) and length (7–100 mm). Furthermore, a correlation between the non-dimensional bulk mean temperature and the ratio of effective tube length to the thermal entrance region was proposed and was correlated well with the measurement data. Prediction of the heat exchanger performance using this correlation was possible. As a result, it was elucidated that the using mini-tubes is remarkably effective to reduce the size of heat exchanger due to enhancement of heat transfer coefficient and enlargement of heat transfer surface. The volume with 1 mm inner diameter of tubes was approximately 5 percent of that with 5 mm in diameter.