A tray liner of flexible construction relies on the rigid airline or railroad car seat tray underneath it to create a horizontal clean and sanitary surface useful for supporting food and drink utensils. The register unit receives a number input at least a first time and a second time, and upon determining that a number input the first time and a number input the second time correspond to a same input number, registers the same input number as a valid facsimile number. All the surfaces of the scanning lens are rotationally symmetrical, and the compensation lens has an aspherical surface. A device for letting off residual steam and residual water from the heating unit includes a heat-exchanging and heat-storing condenser having a residual line section coupled to the residual fluid line, a fresh water line section coupled into the fresh water line and a heat storing medium that is connected for conducting heat between the residual line section and the fresh water line section.