The manual throttle, or hand expansion valve, is a manually operated needle valve (see example in Figure 4.10). The needle position is fixed, and the mass flow through it depends on the pressure difference across the valve. The hand expansion valve is normally used as a supplementary safety valve installed in a bypass line. It is also commonly used to control the flow rate through oil bleeder lines. The manual throttle is a non-regulating valve and should not be used as an expansion valve with a brazed plate heat exchanger evaporator, because any changes in operating conditions would instantly change the evaporation process inside the rapidly responding plate heat exchanger. Regulating the flow manually would require immediate adjustment, which is not practically feasible.
