Document GKOdorQYjRmJqNXYkyjd73dr7

162 CHAPTER 10 Rf.pobt No. 1491--Further studies of ^wtnl ch&neteris* tics of plaster panels (ASHVE Transactions, VoL 59, 1953, P- 511). H. B. Nottoge, C. V. Franks, L. E. Hulbert, and L. F. Schu- tnim: ASHVE Rrurabch Repost No. 1492---Heat flow analy sis in panel heating or cooling sections, ease El--Floor dab on earth with uniformly spaced pipes or tabes at the dab-earth interface (ASHVE Transactions, VoL 59, 1953,j>. 527). W. S. Harris and E. h. Saitain: Performance of Hot Water Panel Beating Systems (University of Illinois Engineering Experiment Station Bulletin No. 453, 1959). 1962 Guide And Data Book C. M. Humphreys, H. B. Nottage, C. V. Franks, R. G. Huebacher, L. F. Schuiram, and D. W. Locklin; ASHVE REsju&ts Report No. 1387--laboratory studies on heat flow within concrete panel (ASHVE Transactions, Vol. 56, 1950, p. 175) L. F. Schutrum and T. C. Min: ASHAE Research Report No. 1598--Cold wall effects in a ceiling-panel-heated room (ASHAE Transactions, Vol. 63, 1957, p. 187). E. L. Sartain and W. 8. Harris: Performance of overed hot water floor panels, Part II--Room conditions (ASHAE Tb.v, actions, Vol. 63, 1957, p. 209) CHAPTER 11 ELECTRIC HEATING Resistors, Hearing Element*, Electric Hearing Units, Electric Space Heating Applications, Types of Electric Hearing Systems, Equipment and Installation Methods, Calculating Capacities, Power Considerations LECrnUCTTY as a source of heat represents thermal pads, sheets, blankets, aviators' clothing, and some radiant E energy in a refined form, easily applied to space heat panel heating installations. ing, and readily distributed and controlled. For many appli Paper or fabric incorporating a surface resistor on the back cations, including use to supplement heating systems of other side is being applied for railing or wall panel heating instal types, the compactness, simplicity, responsiveness, accuracy lations. of control, safety, and cleanliness of electric heating may Coiled resistors in quarts tubes, metal sheath elements, carry greater weight in choice of method than operating cost or special incandescent lamps with tungsten or carbon fila and initial investment. Electric heat often is more expensive ments and glass or quarts envelopes are designed to produce on a direct-heat-equivalent basis than heat from conventional maximum energy in the infra-red portion of the spectrum. fuels. Economical service requires careful application in the They are applied as radiant heaters. desigD of system, adaptation of building structure, choice of control devices, and in method of operation by the user. ELECTRIC SPACE HEATING APPLICATIONS Using the power-to-heat conversion constant, 1 kilowatt Complete electric heating systems are now finding wide equals 3,413 Btuh, resistance heating calculations are ex application not only in residences but also in many commer pressed directly in terms of kilowatt power requirements. cial and industrial establishments. Auxiliary electric space Definitions of Resistor, Heating Element, and other terms heating is often used where minimum initial cost is the domi for electric heating practice are given in Chapter 63 of the nant factor, or for convenience, for instance: (a) for local use 1961 Guide And Data Book. when the main heating system is shut down, (fc) for local use - All equipment, materials, and construction used in electric where the main heating system is inadequate, (c) for tem heating systems should comply with the requirements of the porary heating, (d) for special conditions involving separate National Electrical Code and with local codes. or remote control, (c) for isolated locations. Portable unite ELECTRIC HEATING UNITS readily satisfy some of these requirements. Compact electric heating units are sometimes installed in An electric heating unit is a frame, casing, or other sup main supply or branch ducts of central-fan steam and water porting means containing one or more heating elements, elec systems to provide the final temperatures and relative hu tric terminal connections or leads, and electrical wiring and midities required for comfort or process air conditioning- insulation, all assembled into a unit. There are many varie EIlectric heaters installed for use primarily in the heating ties of housing, material, and arrangement of heating ele cycle ran also be utilized for reheat in the cooling cycle. (See ments, operating temperatures, ratio of radiant to convective later section Central Warm Air Systems.) heat delivery, directional control for radiation, natural or In control of systems, the flow of air over the electric heat forced convection, automatic or manna! control, and electric' ers should be constant while the input to the heaters should load limiting features. Many specific types of units are dis be varied. This is necessary because of the' basic difference cussed in the section Equipment and Installation Methods. between electric heating and other methods such as steam- RESISTORS AND HEATING ELEMENTS heating. Steam is approximately a constant temperature source of heat at any given presure, and, consequently, a Electric resistors usually are composed of metal-alloy wire change in air flow over a steam coil does not greatly alter or ribbons, non-metallic carbon compounds in rod or other the coil surface temperature. Hence, the heat output of a shapes, or printed circuits. Heating elements may have ex steam coil varies in proportion to the air volume, but the posed resistor coils mounted on insulators, metallic resistors surface temperature of the coil remains about the same. With embedded within refractory insulation encased in a protec electric heat the energy input and output are constant, and tive metal sheath, or a printed circuit encased in glass sheets. if the electrical energy input remains constant, the tempera Fins or extended surfaces may be used to add heat-dissipating ture of the heating elements will vary inversely with the air area. Elements are made in many forms. Strip elements are flow. Therefore, face and bypass dampers should not be used used for clamping to surfaces for heat transfer by conduction in electric heating, because the reduction in air flow and the m some convection air heaters and some low temperature corresponding rise in heater temperature would endanger the radiant heaters. Ring and plate elements are common in elec operating life of an electric heating element. _ _.. tric ranges and many small air heaters. Metal or oxide con The majority of electric heaters are controlled directly by ductive films on glacR and ceramics have been used usually line voltage thermostats (or low voltage thermostats operating in panel form. Tubular elements may be immersed in liquids, contactors) to provide on-off control. Where a more refined ptey be used bare, may be formed into coils, or may be cast control is required a proportional-type step controller may into metal or plastic, and used in water heaters, ranges, and be used to energize or de-energize heaters in increments small mr heaters. enough to achieve a balance with littte cycling. The electrical Cloth fabrics, incorporating flexible resistor wires, are em input per step is normally determined by the permissible ployed for many low temperature purposes such as heating change in supply air temperature per step. High-Iumt thermo- 163 TP f i? ill 3 V jl I: