The effect of a change in flow rate of a 2% w/w caustic soda solution on the circulation cleaning of a plate heat exchanger fouled with diluted tomato paste has been investigated at a temperature of 90°C and fixed circulation cleaning times of 5 and 10 min.

For both circulation cleaning times a transition point was noted in the range 2.75–2.86 1/min (Reynolds number range 6600–6300, shear stress range 0.77–0.82 N/m2). Below the transition point the effect of flow rate on cleaning efficiency was minimal, but at flow rates above the transition point there was a marked increase in cleaning efficiency as the flow rate (and hence shear stress at the surface) increased.