The SH-27B is designed to work at flow rates of 2-10 ml/min.  You can use it at flow rates of less than 2 ml/min, however, the heater is robust and will boil solutions when used at very slow flow rates or in a stopped-flow condition.

As an alternative, the SF-28 in-line solution heater is designed to heat from zero flow up to 2 ml/min.