1128 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



filtering elements which hold back the undesirable solids and discharge 

 a clear product into the common filtrate manifold on which they are 

 mounted. There are eight leaves in each pressure tank providing sixty 

 square feet of filter area. The filter cycle is begun by filling the mix tank 

 with unfiltered solution or electrolyte from the storage tank. Sufficient 

 filteraid for precoat is rapidly mixed in by agitator. This mixture is forced 

 through the filter leaving a deposit or coat of filteraid on the intake 

 surface of the leaves. The purpose of the coating is to provide a filter 

 medium which is dense enough to prevent the passage of suspended solids 

 in the solution and yet is easily removed and replaced. The appearance 

 of the liquid on returning to the mix tank gives an indication when an 

 adequate precoat has been deposited on the leaves. Once the coat has 

 been applied, the filter is put on the line and clear filtrate is discharged 

 to the storage tank. The filtration stage terminates when the filter tank 

 pressure approaches a maximum operating value, at which time the 

 leaves are washed, a new coat is applied, and the cycle is repeated. 



The storage reservoir is fitted with one or more heavy duty vertical 

 sump type pumps to transfer the electrolyte in the processing area. All 

 pump parts in contact with chemical solutions are constructed of either 

 cast iron or a stainless steel alloy known in the industry as FA-20. All 

 pumps are fitted with graphitar bearings. Each pump is connected di- 

 rectly to its electric motor and the entire assembly may be quickly re- 

 moved from the storage tank. 



All piping associated with solution handling is either steel or saran- 

 lined steel. The Saran-lined pipe has flanged type joints and the unlined 

 steel pipe has welded joints except where disassembly is provided for and 

 here welded-on flanges with neoprene gaskets are installed. Flexible neo- 

 prene joints are provided in the solution riser and return pipe to insulate 

 the plating machine from the circulating pumps. To control solution 

 flow to the processing area, air operated diaphragm valves have been 

 installed. 



The two-pass heat exchanger selected for the process consists of a 

 steel shell, but all parts in contact with corrosive solutions, including 

 the tube bundle, are constructed of a chemically inert carbonaceous me- 

 terial. The heat transfer rate of this graphite base material is the highest 

 of all non-metals and is higher than that of most metals. For example, 

 the thermal conductivity oi 18-8 stainless steel is approximately one-fifth 

 that of the graphite base material. 



Operating temperatures for all solutions are specified and equipment 

 to control these temperatures has been provided. The temperature of 

 each solution is measured with bulb type expansion thermometer. The 



