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230 CHAPTER 16 1959 Guide Where close control of relative humidity is desired, heating coils or electric heaters for reheat may have to be added. The use of air-conditioning psychrometric techniques is only re quired. for accurate humidity control. Cooling equipment is usually rated on the basis of the over all room-to-refrigerant temperature differential instead of using the intermediate average coil-surface temperature. If the tentatively selected unit cooler does not' possess the proper capacity, it may be possible to adjust the fan speed to a new air volume. At the new air volume and the new overall differential, the cooler may he able to deliver the necessary cooling capacity. If not, the procedure must be re peated with another size of unit cooler, the final selection be ing based on the proper balance between unit cooler and the condensing unit to maintain the Btu per hour heat removal and temperature difference at design room temperature.' Procedures for rating and testing room coolers are given in an ASRE Standard1 which establishes four groups of con ditions (numbered I to TV) under which units may be rated. Many manufacturers establish and publish their ratings in accordance with this standard in which forced circulation air coolers are classified according to air side surface conditions as (1) dry coil, (2) sprayed coil, and (3) spray (no coil); or according to type of air delivery to room as (1) free delivery fan, or (2) pressure fan. Natural convection air coolers are classified according to type as (1) external baffle, (2) built-in baffle, and (3) without baffle. Arrangement and Operation The refrigerant is usually supplied to the evaporator through a thermostatic expansion valve, thus obtaining dry expansion in the evaporator. In other designs, float-valve feeding is used as a' means of metering the refrigerant into the evaporator. The refrigerant circuit in the coil may be of the continuous-flow type from inlet to outlet, or it may have means for internal recirculation of the refrigerant. The coils having the dry-expansion or continuous flow from irdet-tooutlet circuits may be of the upward feed or the downward feed arrangements. Upward flow is generally regarded as pro ducing more refrigerant holding capacity and more-effective use of the Surface. However, with some refrigerants and in some systems, the downward-feed system has been found to produce better oil-return characteristics. Many direct-ex pansion installations use liquid-vapor heat exchangers to in crease the coil utilization without undue risk of returning liquid refrigerant to the compressor. Where two or more evaporator coils are to be attached to a single condensing unit, and different evaporator tempera tures are desired, a' back pressure valve may be installed to limit the minimum evaporating temperature of the warmer coils. This valve also finds application where fluctuation in the evaporator temperature prevents accurate control of air temperature and humidity. Except in product precooling, the objective of air distribu tion is to absorb the heat load as it comes into the room while maintaining a desirable degree of ventilation-air movement through hygroscopic cargoes that are not encased by vaporbarriers. In cold storage practice, this generally requires the securing of as uniform an air distribution as possible to every., part of the product zone, with employment of diffusion of air over the product and the prevention of direct blasts against the product. These requirements must be recognized when selecting the unit coolers and their outlets and when selecting the locations for the unit coolers. It is important to note that the quantity of air in motion in the refrigerated space is not only that passing through the coils. A quantity of air many times in excess of the air handled by the units is always set in motion by the induction effect of the moving cooled air. Therefore, in evaluating the velocity for any given area, the total air set in motion must be considered. It is a function of the type of outlet, the discharge air. velocity, and the unit location relative-to restrictive walls and product. Unit location is also important from the standpoint of oc cupant comfort, low-velocity outlets being preferred for floor type units. High-velocity outlets are acceptable in fur storage vaults, ice cream, hardening rooms, and other spaces where air motion is not an important factor. In general, unit air coolers should not be suspended in front of door openings, or close to them where moist warm air will be drawn directly into the unit each time the door is opened, thereby causing excessive frosting and loss of capacity. Better performance will be obtained by placing the unit air cooler where the air will be discharged toward the door. If the shape of the space is such that this location would result in excessive air velocity over the product, then the location of the unit air cooler should be changed so that the air is discharged parallel to the wall in which the door is located. . Control of Unit Air Coolers : . Although most unit air coolers ean be adapted to any con trol cycle, continuous fan operation is recommended to avoid stratification and wide fluctuations in space temperature. Should the unit be completely self-contained, control of the direct-expansion refrigeration unit may be obtained from the temperature of the recirculated air and from suction pressure. In the case of multiple unit systems supplied with refrigerant .or chilled water from a central source, a valve in the supply to each cooling coil may be controlled thermostatically from space temperature. REFERB4CES 'joint Committee of the American Society of Refrigerating Engineers, American Society or Heating and Ventclatino En gineers, Refrigerating Machinery Association, National Elec trical Manufacturers' Association, and Air Conditioning Manu facturers' Association: ASRE Circular No. 13-42.* * Joint Committee of the American Society of Refrigerating Engineers, American Society of Heating and Ventilating En.GiNEEsa, Refrigerating Machinery Association, National Elec trical Manufacturers' Association, and Air Conditioning Manu facturers' Association: ASRE Circular No. 16.* * ASRE Standard Methods of Rating and Testing ForcedCirculation and Natural Convection Air Coolers for Refrigera tion (ASRE Circular No. 25-44). * Room Air Conditioners (Air-Conditioning and Refrigeration Institute Standard 110-56). * National. Electrical Code (National Board of Fire Under writers Pamphlet No. 70, Paragraph 195). * American Standard Safety Code for Mechanical Refrigera tion (American Standards Association, B9-1-1953), TStandard for Room Air Conditioners (Underwriters' Labo ratories, Inc, UL484, September 2957, 2nd ed.). * Methods oj Rating and Testing Air Conditioners (American Society of Refrigerating Engineers Standard 16-56). * Cooling Load Estimate Form for Room Air Conditioners (Air-Conditioning and Refrigeration Institute Standard 120-56). " Air Conditioning Refrigerating Data Book--Applications (American Society of Refrigerating Engineers, 1950-57, Chapter * ASRE has eoatuned Cimdaro No. I3HJ and No. IS in ASBB Standard No. IMJ. Method* of Aotifif end Tooting Air Conditioner*. CHAPTER 17 ELECTRIC HEATING Resistors, Heating Elements, Electric Heating Units, Electric Space Heating Applications, Types of Electric Heating Systems, Equipment and Installation Methods, Calculating Capacities, Induction and Dielectric Heating, Power Considerations ELECTRICITY as a source of heat represents thermal into metal or plastic, and used in water heaters, ranges, and energy in a refined form, easily applied to space heat air heaters. ing, and readily distributed and controlled. For many appli Cloth fabrics,, incorporating flexible resistor wires, are em cations, including use to supplement heating systems of other ployed for many low temperature purposes such as heating types, the compactness, simplicity, responsiveness, accuracy pads, sheets, blankets, aviators' clothing, and some radiant of control, safety, and cleanliness of electric heating may carry greater weight in choice of method than operating cost and initial investment. Electric heat often is more expensive bn a direct-heat-equivalent basis than heat from conventional fuels. Economical service requires careful application in the panel heating installations. Special incandescent lamps with tungsten or carbon fila ments and glass or quartz envelopes are designed to produce maximum energy in the infra-red portion of the spectrum and applied as radiant heaters. design 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 for convenience, for instance: (a) for applying to resistance electric heating practices will be found local use when the main heating system is shut down, (6) for in Chapter 1. local use where the main heating system is inadequate, (c) All equipment, materials, and construction used in electric for temporary heating, (d) for special conditions involving heating systems should comply with the-requirements of the separate or remote control, (e) for isolated locations, (/) for National Electrical Code, with local codes, and be listed as cases where minimum initial cost is the dominant factor. approved by Underwriter's Laboratories Inc., or other recog Portable units readily satisfy some of these requirements. nized certifying agency. Compact electric heating units are sometimes installed in ELECTRIC HEATING UNITS main supply or branch ducts of central-fan steam and water systems to provide the final temperatures and relative hu An electric heating unit is a frame, casing, or other sup midities required for comfort or process air conditioning. porting means containing one or more heating elements, elec Electric heaters installed for use primarily in the heating tric terminal connections or leads, and electrical wiring and cycle can also be utilized for reheat in the cooling cycle. insulation, all assembled into a unit. There are many varie In control of systems, the flow of air over the electric heat ties of housing, material, and arrangement of heating ele ers should be constant while the input to the heaters should ments, operating temperatures,.ratio of radiant to convective be varied. This is necessary because of the basic difference heat delivery, directional control for radiation, natural or between electric heating and other methods such as steam ' forced convection, automatic or manual control, and electric heating. Steam is approximately a constant temperature load limiting features. Many specific types of units are dis source of heat at any given pressure, and, consequently,^ cussed in the section Equipment and Installation Methods. change in air flow over a steam coil does not greatly alter RESISTORS AND HEATING B.EMENTS the coil surface temperature. Hence, the heat output of a steam coil varies in proportion to the air volume, but the Electric resistors usually are composed of metal-alloy wire surface temperature of the coil remains about the same. With or ribbons, non-metallic carbon compounds in rod or other electric heat the energy input and output are constant, and shapes, or printed circuits. Heating elements may have ex if the electrical energy input remains constant, the tempera posed resistor coils, mounted on insulators, metallic resistors ture of the`heating elements will vary inversely with the air embedded within refractory insulation encased in a protec flow. Therefore, face and bypass dampers should not be used tive metal sheath, or a printed circuit encased in glass sheets. in electric heating, because the reduction in air flow and the Fins or extended surfaces may be used to add heat-dissipating corresponding rise in heater temperature would endanger the area. Elements are made in many forms. Strip elements are operating life of an electric heating element. used for clamping to surfaces for heat transfer by conduction Automatic regulation of the electrical input is usually ac in some convection air heaters and some low temperature complished through a proportional-type step controller that radiant heaters. Ring and plate elements are common in elec energizes or de-energizes heaters in increments small enough tric ranges and many small air heaters. Metal or oxide con to prevent excessive cycling. .The electrical input per heater, ductive films on glass and ceramics have been used usually or step, is normally determined by the permissible change in in panel form. Tubular elements may be immersed in liquids, supply air temperature per step. High-limit thermostats are may be used bare, may be formed into coils, or may be cast sometimes used to de-energize the heaters if the heater tem- 231 y