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| method is slow and superficial, and is generally used with small transformers, when local conditions prohibit the use of other methods. |

| 2. External Heat. The windings are again left in the case with the oil removed and cover left off to allow air circulation. Externally heated air, (Fig. 4-7) is blown into the tank at the bottom through an oil valve for larger transformers or by means of a tube lowered to the bottom of the case. Baffles may be placed between the core and the case to force the heated air up through the windings. Care should be taken with the air, preferably heating it by electrical resistors to avoid smoke contamination if heated by open fire. Its temperature should not exceed about 125°C to maintain the 80°-90°C temperature about the windings and not scorch the insulation. If it is not expedient to leave the windings in the case, they should be removed and placed in another box with holes near the bottom and top for circulating the hot oil. |

| 3. Internal and External Heat. This is a combination of methods 1 and 2 above. The transformer should preferably be left in its own tank and the same precautions followed. The current circulated in the windings should be less than when method 1 alone is used. |

| 4. Heating in Oil. Here the windings are submerged in oil and the heat is generated as in method 1. The moisture is driven from the windings and insulation into the oil, and is removed from the oil by evaporation into the air or by circulating the oil through a filter, or both. The temperature of the windings should be maintained at 80°-90°C and the oil at as high a temperature as possible to keep a safe temperature in the windings. To bring the oil to a temperature sufficient to evaporate the moisture into the air, it may be necessary to reduce the cooling efforts either by insulating the case, by stopping or reducing the oil circulation, by lowering the oil level below the top radiator connections, or by reducing or cutting off the water in a water-cooled transformer. As much ventilation as possible should be provided, especially in the large transformers. Manhole covers may be raised and forced ventilation used. If ventilation is not sufficient, moisture will condense on the cover and drip into the oil, temperature of the oil should then be reduced and ventilation increased. |

| In all these methods, care should be taken to see that the windings and insula- |

| Figure 4-7 Method of using electric transformer dryer. |

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