Document zQKgY9Z5kKM25M0rgEvdoYNNg

r American Society of Heating and Ventilating Engineers Guide, 1935 11 What natural evidence have we that air temperature alone is rit criterion of comfort and that radiant heat affects thebody more quickly? When standing in the sunshine on a cool spring day, a person feels perfectly comfortable, but when a cloud passes over the sun, he instantly feels much, cooler as the shadow reaches him. A shielded thermometer recording the temperature of the air ^ows no reduction in air temperature in so short a period, so that the person actually/feels a sensation of cold which an ordinary thermometer cannot register. This shoWs that light and heat rays are shut off simultaneously and travel at the same speediijt also proves that radiant rays affect the comfort of the body quicker than air temperature does. 666 Chapter 39 ELECTRICAL HEATING ,Resistors, Heating Elements Electric Heaters, Unit Heaters, Central Fan Heating, Electric Steam Heating, Electric Hoi Water Heating, Heat Pump, Control, Calculating Capacities, Power Problems, Electric Heating-Data ELECTRIC heating has a logical and a rapidly growing place in the heating industry because of its advantages of flexibility, cleanliness, safety, convenience, and ease of; control. - Electric heating practice has many basic principles in common with fuel, heating, but there are also important differences. . The advantages of good: building insulation, are even mqre important in electric heating than for. fuel heating, because the initial cost per Btu is usually higher. All heat is a form of energy. Fuels hold stored chemical energy which is released into heat by combustion. Electrical power is a form of energy which can be released into heat by passing it through a resisting material. Both fuel and electric heating have two divisions: first, the conversion of energy into heat; second, the.distribution and practical lise of the heat after it is produced. In converting the chemical energy of fuels into heat by combustion, there is necessarily a considerable variation' in thermal efficiency. This is not true, however, when converting electric power into heat, because 100 per cent of the energy applied in the resistor is always transformed into heat. In electric heating practice the engineer need not be concerned about efficiencies of heat production, but rather about efficiencies of heat utilization. DEFINITIONS Definitions of terms used in fuel heating are given in Chapter 41. The following terms apply particularly to electric heating: Electric Resistor: A material used to produce heat by passing an electric current through it. Electric Heating Element: A unit assembly consisting of a. resistor, insulated supports, and terminals for connecting the resistor to electric power. Electric Heater: A complete assembly of heating Clements with their enclosure, ready for installation in service. RESISTORS Solids, liquids, and gases may be used as resistors, but most com mercial electric heating elements have solid resistors, such as metal alloys, and non-metallic compounds containing carbon. In some types of electric boilers, water, forms the resistor which is heated by an alternating current of electricity. passing through it. One of the more common 667