Some heating mantles are constructed by encasing the fiberglass mantle in an outer metal case that provides physical protection against damage to the fiberglass. If such metal-enclosed mantles are used, it is good practice to ground the outer metal case either by using a grounded, three-conductor cord from the variable autotransformer or by securely affixing one end of a heavy, braided conductor to the mantle case and the other end to a known electrical ground. This practice protects the worker against an electric shock if the heating element inside the mantle shorts against the metal case.

6.C.5.4 Oil, Salt, and Sand Baths

In the use of oil, salt, and sand baths, care must be taken to avoid spilling water and other volatile substances into the baths. Such an accident can splatter hot material over a wide area and cause serious injuries.

Electrically heated oil baths are often used to heat small or irregularly shaped vessels or when a stable heat source that can be maintained at a constant temperature is desired. For temperatures below 200 °C, a saturated paraffin oil is often used; a silicone oil should be used for temperatures up to 300 °C. Care must be taken with hot oil baths not to generate smoke or have the oil burst into flames from overheating. An oil bath should always be monitored by using a thermometer or other thermal sensing device to ensure that its temperature does not exceed the flash point of the oil being used. For the same reason, oil baths left unattended should be fitted with thermal sensing devices that will turn off the electric power if the bath overheats. These baths should be heated by an enclosed heating element, such as a knife heater, a tubular immersion heater such as a Calrod®, or its equivalent. The input connection for this heating element should be a male plug that will fit a female receptacle from a variable autotransformer (e.g., Variac) output line. Alternatively, a temperature controller can be used to control the temperature of the bath precisely. Temperature controllers are now available that can provide a variety of heating and cooling options.

Oil baths must be well mixed to ensure that there are no ''hot spots" around the elements that take the surrounding oil to unacceptable temperatures. This problem can be minimized by placing the thermoregulator fairly close to the heater. Heated oil should be contained in either a metal pan or a heavy-walled porcelain dish; a Pyrex dish or beaker can break and spill hot oil if struck accidentally with a hard object. The oil bath should be mounted carefully on a stable horizontal support such as a laboratory jack that can be

THERMITE REACTION EXPLOSION

An explosion injuring 27 people occurred when a thermite reaction was being demonstrated as part of a magic show at an engineering open house. The demonstration, which generated molten iron in a 2,500 to 3,000 °C reaction, was being carried out in a clay flowerpot above a beaker of water and sand to show the heat produced by the reaction when molten iron particles fall into water. Suddenly, the demonstration exploded, sending hot metal and water toward the audience.

The most likely cause of the accident was thought to be a physical vapor explosion, which can occur when a very hot liquid comes into contact with a second liquid. In this case, the water may have turned to steam so rapidly that an explosion resulted. The injuries consisted of minor burns.

raised or lowered easily without danger of the bath tipping over. It is also important that equipment always be clamped high enough above a hot plate or oil bath that if the reaction begins to overheat, the heater can be lowered immediately and replaced with a cooling bath without having to readjust the clamps holding the equipment setup. A bath should never be supported on an iron ring because of the greater likelihood of accidentally tipping the bath over. Secondary containment should be provided in the event of a spill of hot oil. Proper protective gloves should be worn when handling a hot bath.

Molten salt baths, like hot oil baths, offer the advantages of good heat transfer, commonly have a higher operating range (e.g., 200 to 425 °C), and may have a high thermal stability (e.g., 540 °C). The reaction container used in a molten salt bath must be able to withstand a very rapid heat-up to a temperature above the melting point of the salt. Care must be taken to keep salt baths dry, because they are hygroscopic, a property that can cause hazardous popping and splattering if the absorbed water vaporizes during heat-up.

6.C.5.5 Hot Air Baths and Tube Furnaces

Hot air baths can be useful heating devices. Nitrogen is preferred for reactions in which flammable materials are used. Electrically heated air baths are frequently used to heat small or irregularly shaped vessels. Because of their inherently low heat capacity, such baths normally must be heated considerably above the



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