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.696 CHAPTER 34 1965 Guide Ar.d Date -Book . Air-Cooling arid Dehumidifying Coils 607 transfer surface. In air-cooling coils, the air usually flows at right angles to the tubes. In a coil having only one row in the direction of air flow, the relative directions of flow would all be at right angles. In coils having more than one row in the direction of air flow, the mAdia in the tubes may be variously circuited as illustrated for water coils in fig. 8. It is the usual practice to.designate a coil that is circuited and employed as in fig. 8A, as a parallel-flow coil. Figs. 8B and 8C show the general - arrangements ' that are-termed counier-fidw and cross-flow respectively. Cross flow is common in steam heating 1 coils, the temperature, within the tubes''being substantially uniform, and the mean temperature 'difference the. same whatever the direction of flow, relative to' the air. The parallel and counter-flow arrangements'as illus trated in figs. 8A and-8B are common:in brine.or water fig. 7 .;.. Arrangement for Depth Control , / figl 8 ...: Row of Media in Tubes in Relation to Air Row coils, with the counter-flow arrangement being the preferred arrangement to secure the advantage, of highest possible mean' temperature difference. 'Mostdirect-expansion coils also follow the gtaeral scheme of counter flow but the cir cuiting requirements for proper loading' sometimes result in a combination of the other flow arrangements with counter flow in the same coil. Cross flow is more difficult, to control in the volatile refrigerant coils because of the.problem of equalising loading of the parallel circuits. Applications ' A, typical arrangement.of coils is shown in fig. 9. All of the air should be filtered to prevent dirt, insects, and foreign matter from accumulating on.the cods. The cooling coil and humidifier (when used) must be provided with a drip panto catch the condensate formed during thecooling cycle and the excess, water from the humidifier. The drain connec tion should.be on the downstream side of the coils, made of ample rise;,.(not . less tiimi - lK-in.'f pipe), provided with accessible clean-outs, a "deep-seal trap, and discharge to an indirect waste or storm sewer, so that there will be no possi bility that sewer gas may enter the system. Precautions against freezing must be taken with the humidifier piping and drain connections as well as with steam or water coils. Suffi cient insulation should be provided on such components as the drip pan, unit casing, water piping, etc., to prevent sweating. Face-and-bypass dampers have proven to be satisfactory rnpana of capacity reduction for cooling coils using non volatile refrigerants. The use of such face and bypass damper arrangements, on volatile .refrigerant coils poses difficult refrigerant control s problems which are most severe when a change of setting at the thermostat imposes a rapid change in position of the dampers. .On many installations, the coil fig. 9 .. * Typical Arrangement of-Cooling Coib In' a . Built-up Central System bfintc is divided into 2, 3, or 4 sections and each section is con nected to ah individual compressor or to a modulating com pressor or compressors which will adjust to the load as each succeeding coil section is cut out by the thermostat. The design features'of the coil (fin spacing, tube spacing, type of fins), determine at which air velocity condensed moisture will be blown off the coil. Water blows off some coils below 400 fpm face velocity, while this does not occur on others atface velocities in excess of 500 fpm. The carryover of water, blown off the coils into air duct work external to the air-conditioning unit, is always undesirable and should be avoided. However, water blow-off from the coils is not in itself necessarily harmful, provided arrangements are included in the unit design to catch and dispose of the condensate. Where there is a likelihood of water being carried over from the airconditioning unit into external air duct work, eliminator plates should be installed on the down-stream side of the coils if no other means for condensate disposal are included. Usu ally, in a central station type air-handling unit or a factoryassembled self-contained air conditioner, eliminator plates are not used; other means for condensate disposal are included in the unit design to cover the maximum rated air quantity. When a number of coil sections are stacked one above another, there is a tendency for the condensate to be carried out into the air stream as it drips from one coil to the next. In such cases, drip troughs, as shown in fig. 10, are used to collect the water and conduct it to the drain pan. The enclosure around a coil or filter hank should be cor rosion resistant and should have adequate access doors to provide for changing of air filters, doming Qf coils, ad justing expansion valves, oiling motors, etc. Sometimes cooling and dehumidifying coils are sprayed with water to increase the rate of heat transfer and to provide outlet air approaching saturation. This arrange^ ment requires a collecting fawk and recirculating pump to maintain water circulation whenever the apparatus is in operation. Fig. 11 illustrates this arrangement with a by pass around the coil. This bypass is often used to facilitate thermostatic control.. It is advantageous to direct only re turn air through the bypass, rather than a mixture of re turn and outdoor air, in order to hold the humidity ratio under thermostatic control. `It is considered good practice to support stacked coils in dividually in an angle iron frame to facilitate their removal for repair or replacement. The piping arrangement must be made to allow for such removal. COIL SELECTION In the selection of a coil it is necessary to consider various factors: 1. The duty required: cooling, dehumidifying,: and the ca pacity required to maintain balance with other system compo- fig. 10 .... Coil Arranged with Drip Trough neats such as compressor equipment in the case of direct expan sion coils. 2. Temperature of entering air--dry-bulb.only if there is no dehumidincation; dry-'and wet-bulb if moisture is to be re moved. 3. Available cooling media arid operating temperatures. 4. Space and dimensional limitations. . 5. Arr quantity limitations. 6. Allowable frictional resistances in air circuit (including coib). 7. Allowable frictional resistances in cooling madia piping sys tem (including coib). ' 8. Characteristics of individual coil designs. 9. Individual installation requirements, such as, for example, the type of automatic control to be used, corrosive atmosphere, etc. 10. Coil air face velocity. ; The duties required may be determined from informa tion in Chapter 24, 25, 26, and 27. There may, or may not, be a choice of cooling media, as well as operating tempera tures depending upon whether the installation is new-or is in combination with present sources of heating or . cooling. Space limitations are dictated by the requirements 'of indi vidual cases. The air quantity is influenced by a number of considerations. The air handled may be limited by the use of installed ventilating ducts for air distribution' or may be determined by-requirements for satisfactory air distribution or ventilation. The resistance through the air circuit influ ences the fan horsepower and speed. This resistance may be limited to allow the' use of a given size of fan motor,- or to keep the operating expense low, or it may be limited because of sound-level requirements. The allowable friction- through the water or brine'circuit may be dictated by theheadavailable from a given sue of pump and pump'motor. Selection of -fig. 11....Recirculating: Spray System'for Cleaning Coils