Patent GB-551989-A (GB)
Title: An improved dew-point hygrometer
IPC: G01N25/68
Filed: 1941-09-03
Abstract: 551,989. Automatic control systems. AKT.-GES BROWN, BOVERI & CIE. Sept. 3, 1941. No. 11271. Convention date, Sept. 9, 1940. (Class 38 (iv)] [Also in Groups XX and XIII] In hygrometric apparatus employing a vessel through which a cooling fluid passes and having an external surface adapted to reflect radiations from a source to a receiver such as a selenium cell or photo electric cell which exercises control of the cooling of the vessel to determine the dew point of the ambient gas due to the deposition of dew on the reflecting surface of the vessel, the cooling fluid being arranged to be heated electrically a number of dew-point elements are employed the temperature of the elements being adjusted to prescribed values by means of cooling and heating devices whereby the automatic regulation of the humidity is obtained - for apparatus suitable for industrial drying processes. Fig. 1 shows the use of the hygrometric apparatus in a drying system. The material 16 to be dried is arranged in a ventilator 15 having a heating-coil 17 and valves 18, 19, operated by a motor 20 on the air inlet and air outlet. Two hygrometers 21, 22 are arranged as shown with one light source 23 and cells 24, 25 and , are adjusted to the lower and higher limiting dew points, say 80‹ C. and 85‹ C. by the heating coils and resistances 29, 30 connected to a programme regulator 32 which in turn is connected to the mains 31. The regulator 32 maintains the dew point temperatures of the elements 21, 22 for say one hour and then reduces the temperatures to say 60‹ C. and 65‹ C. for the remainder of the drying period say two hours. During the heating up in the ventilator 15, the cells 24, 25 will be fully illuminated and the valves 18, 19 closed by the control motor 20 which receives impulses from the regulator 24. When the dew-point temperature of element 21 i.e. 80‹ C, is reached the cell 24 reacts to the diminished radiation and through the regulator 34 and control motor 20 causes the valves 18, 19 to open slightly. If the temperature still rises until it effects the element 22 and cell 25 the valves are still further opened until the temperature drops and the dew point of the mixture of gas and water vapour in the ventilator is maintained between 80‹ C. and 85‹ C. The regulator 32 at the end of the hour readjusts the elements 21, 22 to the lower temperatures 60 ‹C. and 65‹C. and the drying process continues for the remainder of the period. The temperature of the element 21 is measured by means of a thermo-couple 33 or thermometer. The arrangement shown in Fig. 3 is used to record directly the variations of the dew point. A series of elements 35 are in communication with each other by means of a pipe 36 through which liquid flows and which is heated when passing from one element to the next by, means of heating coils 37. A beam of light from a source 38 is directed on to the surfaces of the elements 35 from which it is reflected on io an opaque plate 40 which will show the temperature variations. by means of a scale owing to the degree of reflectivity of the surfaces of the elements 35 due to the absence of or degree of deposition thereon of the dew. The elements may be arranged to transmit the rays instead of reflecting them and the cooling ,liquid may be replaced by cooling air.