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240 CHAPTER 16 1960 Guide use of air-conditioning psychrometric techniques is only re quired for accurate humidity control. Cooling equipment is usually rated on the bass 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 be 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 ASHRAE Standard* which establishes four groups of conditions (numbered I to IV) under which units may be rated-. Many manufacturers establish and publish their rat ings in accordance with this standard in which forced circula tion 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-togutlet 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 utilisation 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 wanner, 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 recognised 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 tire 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 can 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.witb refrigerant or chilled water from a central source, & valve in the supply, to each cooling coil may be controlled thermostatically from space temperature. REFERENCES 1 Joint Committee of the Ambrcain Society or Refrigerating Engineers, American Society or Heating and Ventilating 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 gineers, Refrigerating Machinery Association, National Elec-, trical Manufacturers' Association, and Air Conditioning Mannfacturers' Association: ASRE Circular No. 16.* * ASHRAE Standard Method of Rating Air Coolers for Re frigeration, Standard 25-56 (American Society of Heattng, Re frigerating and AiB-CoNDrnojtraa Engineers). * 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). 'Standard for Room Air Conditioners {Underwriters' Labo ratories, Inc., UL4S4, September 1957, 2nd ed.). * ASHRAE Standard Methods of. Rating and Testing Air Conditioners, Standard 16-56 (American Society of Heating, Refrigerating and Air-Conditioning Engineers). * Cooling Load Estimate Farm for Room Air Conditioners (Air-Conditioning and Refrigeration Institute Standard 120-56). w Chapter 34, Air Conditioning Refrigerating Data Book-- Applications Volume 1956-57 (American Society of Heating, Refrigerating and Air-Conditioning Engineers). * Circvlert No. 13-42 Mid Not IS have bcD combined la ASHBAE Standard No. 1S-SS. U*liidi af Batins end Tflint Air Conditioner!. CHAPTER 17 ELECTRIC HEATING Resistors, Hearing Eferaenfs, Electric Hearing l/ntfs, Electric Space Heating Applications, Types of Electric Heating Systems, Equipment and Installation Methods, Calculating Capacities, Power Considerations LECTRICITY as a source of heat represents thermal ployed for many low temperature purposes such as heating E energy in a refined form, easily applied to space heat pads, sheets, blankets, aviators' clothing, and some radiant ing, and readily distributed and controlled. For many appli panel heating installations. cations, including use to supplement heating systems of other Special incandescent lamps with tungsten or carbon fila types, the compactness, simplicity, responsiveness, accuracy ments and glass or quarts envelopes are designed to produce of control, safety, and cleanliness of electric heating may maximum energy in the infra-red portion of the spectrum and carry greater weight in choice of method than operating cost applied as radiant heaters. and initial investment. Electric heat often is more expensive on a direct-heat-equivalent basis than heat from conventional ELECTRIC SPACE HEATING APPLICATIONS fuels. Economical service requires careful application in the design of system, adaptation of building structure, choice of Complete electric heating systems are now finding wide application not only in residences but also in many commer control devices, and in method of operation by the user. Using the power-to-heat conversion constant, 1 kilowatt cial and industrial establishments. Auxiliary electric space heating is often used for convenience, for instance: (a) for equals 3,413 Btub, resistance heating calculations are ex local use when the main heating system is shut down, (6) for pressed directly in terms of kilowatt power requirements. Definitions of Resistor, Heating Element, and other terms local use where the main heating system is inadequate, (c) for temporary heating, (d) for special conditions involving for electric heating practice are given in Chapter 1. separate or remote control, (e) for isolated locations. Porta All equipment, materials, and construction-used in electric heating sykems should comply with the requirements of the National Electrical Code and with local codes. ble units readily satisfy some of these requirements. Compact electric heating units are sometimes installed in main supply or branch ducts of central-fan steam and water ELECTRIC HEATING UNITS systems to provide the final temperatures and relative hu. midities required for comfort or process air conditioning. An electric heating unit is a frame, casing, or other sup Electric heaters installed for use primarily in the heating porting means containing one or more heating elements, elec cycle can also be utilized for reheat in the cooling cycle. (See tric terminal connections or leads, and electrical wiring and insulation, all assembled into a unit. There are many varie ties of housing, material, and arrangement of heating ele ments, operating temperatures, ratio of radiant to convective later section Central Warm Air Systems.) In control of systems, the flow of air over the electric heat ers should be constant while the input to the heaters should be varied. This is necessary because of the baric 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, a cussed in the section Equipment and Installation Methods. change in air flow over a steam coil does not greatly alter RESISTORS AND HEATING ELEMENTS 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-metaliic carbon compounds' in rod or other shapes, or printed circuits. Heating elements may have ex electric heat the energy input and output are constant, and 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 beating, 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. Bing and plate dements are common in elec energizes or deenergizes 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 fonned into coils, or may be cast sometimes used to de-energize the heaters if the heater tem into metal or plastic, and used in water heaters, ranges, and " peratures become excessive for any reason such as a reduction . air heaters. in air flow. Heaters must be de-energized on fan shutdown. \ ClotH'fabrics, incorporating flexible resistor wires, are em (See Chapter 43 fpr general information on controls.) 241