Define the meaning of Open Kettles?

A number of foods can be satisfactorily concentrated in open kettles which are heated by steam. This is the case for jellies and jams, tomato juices and purées and for specific types of soups. High temperatures and long concentration times must be avoided in order to decrease or eliminate damage. It is also essential to prevent thickening and burn-on of product to the kettle wall as these slowly lower the efficiency of heat transfer and slow the concentration process. Though, when the process is under control, this type of evaporation is still highly suggested for small scale operations in developing countries. It is a quite extensively used system, mostly for jellies, jams and marmalades.