Document OyBoaw55E7zBpdydwQYGdexM

264 CHAPTER 19 1959 Guide / L-A1R-M1YIMG ^ | ROOM UNIT \*m*t ! matmc coZr 1 ! 11 hu.oa| | H PROCATtft woudr r v' 1 lMM A SUPPLY Mr n< COLO AIR SUPPLY TO OTHER ROOM UMTS TO OTHER ROOM UMTS OOLLCft PUUP G? ^------- 1 wrcR '"`COOLING con. aA. * OUTDOOR AIR fig. 7.... Schematic Arrangement or Dual-Duct System outdoor air is favorable it is used increasingly instead of return air. The warm-air supply-duct temperature is controlled by a thermostat, T2, Fig. 7, in the air leaving the heating coil. Thermostat T2 is reset by thermostat T1 in accordance with variations in outdoor-air temperature. Whenever, out-' door-air temperature is not low enough to accomplish the required cooling, the cold-air supply-duct temperature is regulated by the cooling coil. The final temperature in the conditioned areas is controlled by the individual air-mixing units. Air-mixing room units under control of the room thermo stat draw warm and cold air from the ducts in accordance with the room requirements, affording individual room con trol without zoning. Fig. 8 shows schematically one type of room unit designed for dual-duct systems. The warm and cold air pass through the three-way miring damper which delivers air at the temperature required by the room. Some units are placed under windows, while others are mounted in the ceilings from which air may be delivered to the room through ceiling, wall, or window outlets. Some units are a-tsn provided with volume regulators to maintain a steady flow condition. This system is best suited for multi-room or multi-zone installations, and may be of either high- or low-velocity and high- or low-pressure type with substantially the same con trol methods. It is often applied to high-velocity systems with long distribution mains, and consequently high-static fan pressures; although extremely high static pressures are seldom justified. With two supply mains, the system static pressures vary as a result of changing demands for heating or cooling. Thus volume regulation often becomes necessary. Extreme variations of flow to different rooms may be some what limited by static-pressure regulators and volume dam pers at selected points along the mains. There are some, in stallations where volume control at each room supply is desirable to maintain close control regardless of static-pres sure changes in the ducts, and incidentally to facilitate bal ancing of the distribution system. Air-capacity requirements are generally governed by sum mer cooling loads, and general practice is to supply the cold air to the cold-air duct at 25 to 30 deg below the established room temperature, this being satisfactory with suitably de signed room outlets. Fan and Coil Units Another type of room convector used in connection with central cooling and heating plants is known as the Fan and Coil Unit-(see Fig. 5, Chapter 16). These units, like induction units, are located around the periphery of a building, usually under windows, and are equipped with fans and a water-type heating-coolmg coil. They normally take air for ventilation directly from out doors, and have a manual damper for adjusting the quantity of outdoor air within certain limits. Positive closing of this damper when the fan stops, can be obtained by adding a damper operator actuated by the fan-motor switch. Hot water supplied during the heating season frequently is varied in accordance with outdoor temperature. Cold water for use during the cooling season should be supplied at a fixed tem perature low enough to provide the proper amount of dehumidification. These units sometimes are used with a primary-air sys tem that supplies the outdoor ventilation air and handles the latent load. In such cases the primary air unit is con trolled as explained under the section Unitary-Central Sys tems, and may be of either the low- or high-pressure type. The combined heating-cooling coil of each unit may be controlled by a valve in the water supply line to each unit, actuated by a thermostat in the room or recirculated air of the unit. The thermostat is of the heating-cooling type. EVAPORATIVE COOLING In climates where, on the hottest days, the outdoor wetbulb depression is relatively great, it may be possible to replace mechanical refrigeration, or other cooling sources, and use the evaporative cooling effect. A well-designed air washer using recirculating sprays will reduce the entering dry-bulb temperature to within a degree or two of the en tering wet-bulb condition. Thus, it may be possible that, with air entering at 100 F dry-bulb, 60 F wet-bulb, a leaving condition of 62 F dry-bulb, nearly saturated, ran be ob tained. Under some conditions of latent and sensible heat load, this evaporative cooling may be adequate. At times when the outdoor wet-bulb temperature is not low enough to permit the use of straight evaporative cool ing, it is still possible to use precooling convectors with re frigeration, well water, or a cooling tower, as the basic source of sensible heat removal to reduce the wet-bulb tem perature of the air before it enters the spray chamber. Where internal heat loads are high, this scheme may be more economical than one using return air. Where the required supply-air dew point is too low to permit straight evapora tive cooling, and where the sensible heat load is not too great, intentional partial saturation may be employed. In this case the low dew point of the outdoor air is utilized by permitting some of this air to pass through the humidifying sprays untreated, or to bypass the humidifier. All of these fig. 8....Room Air-Mixing Unit for Dual-Duct System Central-Systems for Air Conditioning 265 remarks with regard to evaporative cooling are based on the assumption that all of the supply air will be taken from outdoors. Provision should be made in most cases for the return qf some air from the conditioned spaces for control purposes, as well as for economy of fuel in winter. PRECOOUNG Where sufficiently cold cwater from wells or streams is available, a saving in refrigeration may be obtained by the use, in location ahead of the dehumidifier, of precooling coils through which the cold water is circulated. The resultant cooling of the air decreases the load to be cammed by the dA^nmidifior and refrigeration plant. In normal practice the water, after passing through the precooling coils, may be further utilized in the refrigeration plant condenser. The economic advantages of this scheme are apparent, and it is frequently used. SENSIBLE COOLING WITH UNWETTED COILS Under favorable atmospheric conditions where a large wet-bulb depression exists and the dew point of the outdoor air is sufficiently low at all times, acceptable cooling may be obtained by removing only the sensible heat from the out door air delivered to the rooms. Under this condition of a great wet-bulb depression, a temperature-reducing coil may be located in the air stream and supplied with water from a cooling tower. When humidity control is desired, sprays to saturate or partially saturate the air may be used down stream from the unwetted coil. Saturation or partial satura tion after the coil will reduce further the dry-bulb tempera ture and the air quantity required. This system has very definite application in hot dry climates. SaECTlON OF TYPE OF SYSTEM Low buildings with large floor areas may be divided, into sections or zones with separate central air-supply systems to facilitate temperature control. In the case of large de partment stores it may be possible to provide a single con ditioner, with a fan delivering the conditioned air to local mixing fans which supply the various departments or spaces. This application is limited by the practicability of running the large conditioned-air ducts to the various recirculating fans. Each vertical section of the building also may be sup plied by a separate fan delivering conditioned air to local mixing fans. In many cases the most economical and satis factory scheme may be to employ a hot water or steam reheater in each branch duct. Where vertical sectionalizing is not indicated, the building may be divided into horizontal groups, each handled by a central system and adequately zoned. In some large buildings, apparatus rooms for the systems may be located in the basement and attic and on intermediate floors. ' In high buildings the necessity for horizontal sectionaiizing may be suggested by the size of air-supply and return risers, and by the extent to which they encroach upon usable space. Each story should be cut off by doors from other stories, as otherwise the cool air tends to collect in the lower story and the warm air is forced to the' upper story. Balconies and large lobbies in theaters and. similar high rooms frequently justify the use of separate zoning fans, to counteract the tendency of the heavier, cooler air to collect at the lower levels of these spaces. Fans operate at full capacity continuously in many sys tems, and therefore should be selected for good efficiencies. In winter when higher temperature differentials are used, it is sometimes practicable to deliver smaller air quantities than when cooling. In f-limatftfl where winter temperatures fall below freezing, the tempering coils should be of the steam-distributing type; or if they are heated by a liquid, this liquid should contain some antifreezing substance such as ethylene glycol. If hot water is used in cold climates, the control of air temperature should be obtained by means of face and bypass dampers rather than by throttling the valves, to prevent damage due to freezing. If zone reheaters placed in supply-duct branches are em ployed, they should be of such type as to be heated over the entire surface so that no temperature stratification can occur in the delivered air. Steam-distributing-tube coils or me chanically-circulated water coils are satisfactory in such cases, and throttling valves may be used. Refrigeration equipment must be carefully selected to satisfy the particular requirements of each installation. For some small plants the evaporator may be placed in the air stream, when type of refrigerant and nature of occupancy permit. In many cases, chilled water coils are required by considerations of safety. Where low temperature and rela tive humidity are necessary, brine, often of calcium chloride, may be indicated. Condensing requirements must have economic analysis. Wells, public water service, cooling towers, and evaporative condensers present possibilities for consideration. Condenser water may have a secondary use for roof sprays in hot weather, and is usually suitable for lawn sprinkling. Most health department rules in cities prohibit any connection from refrigerant condensers that might permit the water to be used for drinking or lavatory purposes. Practically without exception, air cleaners should be pro vided for both outdoor and recirculated air. Control of temperature and of relative humidity by auto matic means is vital, if comfort and economical operation of air-conditioning equipment arc to be attained. The insulation of ductwork may be necessary to prevent condensation of moisture either inside or outside of ducts, housings, and fan encasements or to prevent a change of temperature in ducts passing through hot or cold spaces. In general, the apparatus should be arranged to have a straight-line air flow. Each change in direction increases air resistance and, in addition, elbows and offsets may cause eddy currents resulting in stratification. The usual order of equipment location, beginning at the outdoor-air intake, is: weather hood or louvers, outdoor-air dampers, return-air connection, filters, tempering coils, cooling coils or sprays, bypass connection with or without prebeaters, reheaters, fan and distributing ducts. Screens at the intake prevent the entry of large foreign matter, birds, etc. A hood or louver at the outdoor-air intake prevents the entry of rain and snow. Since in most climates there are many days during which use of 100 percent out door air unheated or uncooled may be economical, the areas of all air-passing and treating apparatus should be large enough for such a volume, and the exhaust or spent-air equipment should be capable of discharging out of doors all of the air admitted. The bypass connection normally connects the return-air duct system with tbe apparatus casing between the condi tioner and the supply fan. Usually the bypass opening is sized to handle about 50 percent of the fan capacity where a variable bypass is used though extreme load variations