as additional flushing agent during the flushing step alternatively the unevaporated flushing agent can be separated from the liquid product in a further evaporation step or if the presence of the flushing agent in the liquid product exiting the flow tube is not detrimental to the concentration process the flushing agent can simply remain in the liquid product in certain situations it may be desirable during the flushing step to have the liquid product and or vapor product exit the flow tube at a point which is located before the end of the flow tube the flushing agent which is evaporated in the flow tube can be discharged separately from or together with the other evaporated components if necessary or desirable the evaporated flushing agent may be separated from the other evaporated components in a separate step after the evaporated components have exited the flow tube the process of the present invention avoids pressure changes in the flow tube wherever possible such pressure changes normally occur as described above when crusts form on the inner surfaces of the flow tube in a preferred embodiment of the present invention the conditions in the flow tube are maintained so as to prevent crust formation in this.