Oil-fired, gas-fired or electrically heated melting and holding furnaces for nonferrous metals; bale-out furnaces; low-pressure casting furnaces; re-melting furnaces for non-ferrous metals; apparatus for loading furnaces, namely, charging units in the nature of feeders specially designed for furnaces; heating and drying plants, namely, electric heaters for commercial use, gas fuel-fired heaters [, industrial dryers for heating ]; heat exchangers