Fig. 1,971.—Water cooled transformer with external cooling coil. In this arrangement the cooling coil is placed in a separate tank as shown. Here forced circulation is employed for both the heat transfer medium (oil) and the cooling agent (water), two pumps being necessary. The cool oil enters the transformer case at the lowest point and absorbing heat from the transformer coils it passes off through the top connection leading to the cooling coil and expansion tank. Since the transformer tank is closed, an expansion tank is provided to allow for expansion of the oil due to heating. The water circulation is arranged as illustrated.
Ques. How much circulating water is required?
Ans. It depends upon the difference between the initial and discharge temperatures of the circulating water.
Fig. 1,972.—Interior of General Electric water cooled 140,000 volt transformer showing cooling coil.
Ques. In water cooled transformers how much cooling surface is required for an internal cooling coil?
Ans. The surface of the cooling coil should be from .5 to 1.3 sq. in. per watt of total transformer loss, depending upon the amount of heat which the external surface of the transformer case will dissipate.
For a water temperature rise of 43° Fahr., 1.32 lbs. of water per minute is required per kw. of load.
Transformer Insulation.—This subject has not, until the last few years, been given the same special attention that many other electrical problems have received, although the development of the transformer from its original form, consisting of an iron core enclosed by coils of wire, to its present degree of refinement and economy of material, has been comparatively rapid.