Document 8VjDO4aVNJzXJXJ62YGQYgj6Z

~ 'Mi! 614 CHAPTER 29 1950 Guide spacesthat are otherwise usable can be charged against the system as an operating cost. In general, the apparatus should be arranged to have straight line air flow. Each change in direction is the cause of air resistance and, in addi tion, elbows and offsets may cause eddy currents resulting in stratification. The usual order of equipment location, beginning at the outside air intake, is: weather hood of louvers, outside air dampers, return air connection, filters, tempering coils, cooling coils or sprays, by-pass connection with or without reheaters, reheaters, fan and distributing ducts. - Screens at the intake prevent the entry of large foreign matter, birds, etc. A hood or louver at the outside air intake prevents the entry of rain and snow. Since in most climates there are many days during which use of 100 per cent outside 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 by-pass connection normally connects the return air duct system with the apparatus casing between the conditioner and the. supply fan. Usually the by-pass opening is sized to handle about 50 per cent of the fan capacity .where a variable by-pass is used, though extreme load variations may require a larger size. It is at times good, design to locate a reheating coil in the by-pass connection to permit using some by-pass air when heat ing is required. Since the relatively high resistance of the cooling coil or spray is to be balanced by the heating coil and by-pass connection, enough heating surface can be provided to raise the temperature of the by-pass air to the point where the mixture of by-passed air and conditioned air will have the required temperature. When a variable by-pass is used, a damper working in opposition to the by-pass damper should be placed across the face of the dehumidifier, for unless the resistances of the two are carefully balanced at all operating points, the proper mixtures of air will not be obtained. Outside air that has not been dehumidified should not be by-passed around a cooling coil or spray dehumidifier if accurate control of the delivered relative humidity is desired. Where the by-pass is made a part of the dehumidifier or conditioner and is located on the top or side of it, the return air connection should be arranged so that stratification of return air is insured, baffles being provided to accomplish this purpose if necessary. Where return air and by-pass air connections are taken off a return duct system, it may be necessary to install a back-draft damper between the return air connection and the by-pass connection. When the by-pass damper is at maximum opening it may be much easier for outside air to pass through the return damper, into the return duct connection and through, the by-pass, than for return air to pass through the by-pass con nection into the fan. Air tends to take the path of least resistance and, if the dehumidifier resistance is high, and if the return duct resistances are low, this situation is apt to occur. A recirculating air fan, instead of a .back-draft damper, may be required for this case, if the failure of return air to reach the dehumidifier or conditioner is a serious matter under reduced load conditions. .; LOCATION OF APPARATUS In general, the outside air intake, preheaters, and return air connections precede the conditioner, while, the by-pass, reheaters and fan follow it. In the case of a.blow-through system,,where the fan is located ahead of the conditioner, the leakage of air at the conditioner is outward, mstead-of inward, and may be accompanied by water leakage. Central Systems for Air Conditioning 615; The location of the complete apparatus assembly, including the dehumi difier, will be dependent on the type of building, spaces available, structural, characteristics, etc. The type of conditioner used may limit the location under certain conditions. Where cooling coils employing chilled water or brine as the cooling agent are used, there are few limitations with regard to location other than those of pumping power, working pressures, piping costs, etc. Where spray dehumidifiers are used, very definite limitations present themselves, and these may require certain extraneous equipment to make the system workable. If several spray type diehumidifieis are located on different levels, thus involving different water pressures, a surge or storage tank, to which the return water from each dehumidifier can be taken, is required. Should the water level in the pan of the dehuinidifiers be. low in relation to that of the surge tank, return water pumps will be' required, and these pumps will have to be operated until the water supply lines are drained in order to prevent flooding of-the lower dehumidifiers. Where spray dehumidifiers are on the same level, equalizing lines between the pans may be required if a storage tank is not provided. . It is exceedr ingly important that water-tight drained floors shall be provided,under all overhead cooling systems, since water condensed out of the air generally will be present and may damage the interior finish of the rooms below the apparatus. All of the various pieces of equipment from the outdoor air intake through the fan usually are.connected together by sheet metal casings. Frequently the building, structure or specially constructed walls or partitions may be used to form a portion of the casing. - In any case the casing or connection must be sufficiently sturdy for the required duty. Sheet metal work must be well braced not only to prevent vibration under pulsations in air flow, but also to withstand the abuse of normal usage. Casings should be braced wherever access doors are installed, and all.large panels should be adequately reinforced by structural steel. Accessibility for Service Each apparatus layout is to be made with accessibility in mind. Where cooling convectors are used, space for removing and repairing or replacing them should be provided. Adequate space is to be provided for the servic ing and replacement of eliminators. Whether these accompany sprays or wetted coils, filters must be so located that the proper cleaning, replace ment or routine servicing can be accomplished without difficulty. Free access to the bearings of all moving machinery is a necessity. Provision should be made for the complete removal and replacement of any parts of the apparatus that are subject to wear, deterioration or damage, whether it may be filter, fanwheel, motor, pump, impeller or heat transfer surface. DESIGN PROCEDURE The customary design procedure is outlined herewith. For simplifica tion the procedure is set up on the basis of a year-round system. For systems designed only for winter or summer, the unrelated parts may be omitted. 1. Selection of design conditions (inside and outside). a. Summer. b. Winter.