Validation of heat transfer data for fibre suspensions in coaxial pipe heat exchangers

Heat transfer data obtained previously in a small coaxial pipe heat exchanger were validated using a new, larger annular-area test unit with longer calming section and axial entry flow. Water and two suspensions of long-fibre softwood Kraft pulps with fibres having different coarseness (mass/unit fibre length) values were used. Data in both systems were obtained over a range of flow rates with the fibre concentration at 0.4% and compared. Heat transfer coefficient hc was found to decrease with increasing annular gap size for both water and fibre suspension flow at equal velocities. Various fibre dimensions and fibre characteristics are also shown to vary systematically with hc at the same experimental conditions. The effect of hydrodynamic annular entry length on heat transfer and pressure loss using water and fibre suspensions (concentrations 0.2% and 0.4%) are also described. It was found that that extending the central calming extension rod made no significant difference to hc values at the low fibre concentration of 0.2%, and only small differences at the higher concentration of 0.4%. The successful validation of data from the smaller-annulus coaxial pipe heat exchanger show that the relationships among heat and momentum transfer and fibre characteristics and specific properties of paper are also valid for both heat exchangers.

► Heat transfer to synthetic and natural fibre suspensions was studied. ► Frictional pressure loss of synthetic and natural fibre suspensions was studied. ► Heat transfer, pressure loss and fibre and paper properties are well correlated. ► Annular and pipeline flow results are similar in nature for the same fibre suspensions used. ► Obtained results clarify the key variables in turbulent fibre–liquid interactions.