Document re1YvkNR1QzgvdQaKnk9exY9G

608 CHAPTER 34 1965 Guide And Data Book the sir face velocity is determined by economic evaluation of initial and operating costs for the complete installation as influenced by: (1) heat transfer performance of the specific coil surface type for various combinations of face areas and row depths as a function of the air velocity, and (2) air ride frictional resistance for the complete air circuit (including coils) which affects fan sue, horsepower and sound-level re quirements. The performance of an extended-surface cooling coil depends upon its correct choice and matching with other original equipment components, and upon proper application and maintenance. Coil ratings are based on a uniform face velocity. Inter ference with uniform air flow through the coil will affect performance. Such air flow interference may be caused by air entrance at odd ftnglog or by inadvertent blocking of a portion of the coil face. To obtain rated, performance it is necessary that the air quantity be adjusted on the job to correspond with that used when selecting the coil, and that it be kept at this value. The most common causes of a reduction of air quantity are the fouling of the filters and collection of dirt or frost on the coils. These difficulties can be avoided by proper Hpgjgn and regular servicing. There are a number of ways in which coils may be cleaned. A common method is to wash them with water. They can sometimes be brushed and cleaned with a vacuum cleaner. In bad cases of neglect, especially on restaurant jobs where grease and dirt have accumulated, it is sometimes necessary to-remove the coils and wash off the accumulation with steam, compressed air and water, or hot water. The best practice, however, is to keep the filters serviced, and to inspect and wash the coils at regular intervals. In the selection of coils, sufficient surface area must be in stalled to transfer the total heat load from the air to the cool ing media, under the required temperature conditions and .mass flow rates of-both air and refrigerant. The coil total heat capacity should also be in baianco with the capacity of related equipment, such as compressors. . , In the case of dehumidifying coils, it is also important that the proper amount of surface area be installed to obtain the ratio of air ride sensihle-to-total heat which is requiredTor maintaining the air dry-bulb wet-bulb temperatures in the conditioned space. The method for calculating the sensible .and total heat loads, and the leaving air conditions at the coil to satisfythe sensible-to-total heat ratio required for the con ditioned space is given in Chapter 27. The same room air conditions can be maintained with different air quantities (including outside and return air)-through the coiL However, for a given total air quantity with fixed percentages of.outride and return air, there is only one set of airconditionS'leaving . the coil which will maintain the room airconditions. Once the - air quantity and leaving air .conditions at the coil have been selected, there is only one combination of face area, row depth and air face velocity, for a given coil surface design and ar rangement, which will maintain the required room air condi tions. Therefore, in making final coil selections, it is necessary to recheck the initial selection to assure that the leaving air conditions, as' calculated by coil selection procedure, will match those as determined from the cooling load estimate. This may involve a reselgction with changes in air face ve locity, coO rise' and coil depth.. Coil ratings and selection procedures are usually presented in one of two wayB: I. Banc data meVi<xL Go& performance parameters are pub lished in the form of tables or charts from which the coQ row depth is calculated, after determining the required coil sensible and total heat capacities and other design variables from the job , conditions. The initial selection will generally indicate a nonintegral row depth requirement. It is frequently necessary to recheck and reeelect the coQ to more closely m*t/-h the required air and refrigerant conditions with the integral row depth actu ally installed, particularly for dehumidifying coils. The method is generally used in selecting coils or coil hmfei for field assembly since there is a vast number of eise and row depth combinations available. 2. Unit rating method. Performance for specific combinations of coil face area and row depth are presented in tables or charts. This method provides a direct selection of specific coils to match the required capacity under the job conditions. It is frequently used in selecting coils for central station type air-handling units and* also employed in determining performance for factoryassembled self-contained air conditioners. With either method (1) or (2)," coil selections are quite flexible. It is possible, \with a given type of coil surface, to choose various combinations of coil face area, row depth, air velocity, air quantity, etc., for the same duty. . The proper selection of coils requires on-understanding of ,the requirements of each case, and should be. based on mi economic analysis of the plant Hwtign as.a whole. While no general rule can be established .for the selection'of. cooling coils, it is possible, nevertheless, to point out-the limits of usual practice and to indicate the influence of the variables involved in the coQ selection. -Application Range Dry surface (sensible cooling) coils, and dehumidifying coils (which accomplish both cooling and dehumidification), are usually rated within these lmiit8:. Entering Air Dry-Bulb -- 65 to 100 F. Entering Air-Wet-Bulb * 60 to 85 F. . Air Face Velocity 300 to 800 fpm (sometimes as low as 200 and as high u 1500). Volatile Refrigerant Saturation Temperature -- 30 to 55 F at coil suction outlet (refrigerant vapor superheat at coQ suction outlet is 6 . . deg or higher). . Entering Water Temperature = 35 to 65 F. Water Quantity * 1.2 to 6 gpm per ton (equivalent to a water temperature riseoffrom4 to 20deg). Water Velocity -- 1 to 8 fps. For special applications, the range in design variables tabulated above may be exceeded. Dry Cooling Coils Since no air dehumidification is obtained for thin applica tion, dry coil selection is made on the of dry-bulb tem peratures involving sensible-heat transfer only, the 'same as with heating coils. Dehumidifying Coils ` - ':" / -. The ratio of air ride sensible-to-total heat removed varies in practice from about 0.6 to 1.0, i.e., sensible heat is from 60.to 100 percent of the total, depending upon the application. (See Chapter 27.) For a given coil surface and arrange ment, the required sensible heat ratio may be satisfied with. wide variations in and combinations of air face velocity, re^ fngerant temperature and coil depth, although general rules as.to their values may be misleading. The maximum coil air face velocity should be limited to a value which will prevent water carry-over into the air duct work. Although dehumide fying coils for comfort application are frequently selected in the range of 500 to 600 fpm air face velocity, central station type air-handling units are available which.operate.at air face velocities of 700 fpm or higher without carryover of moisture into the duct work. Refrigerant temperatures ordi narily vary between 35 and 50 F. Water velocities normally range from 2 to about 8 fps. M Air-Cooling and Dehumidifying Coils 609 The performance of dehumidifying coils is determined by a sene of tests carried on under laboratory conditions. Coil stings are prepared from such laboratory test data to fadli- tatecofl selection. There are several forms or methods used by manufacturers in publishing coil ratings, such as: (1) entering and leaving air conditions, (2) effective coil surface temperature, and (3) basic data selection using heat transfer performance parameters. The selection of dehumidifying coils for factory-assembled self-contained air conditioners is generally accomplished in conjunction with laboratory testing. The current industry Btepdards call for ratings at 33.4 cfm per thousand Btu.-of capacity and this is approximately. 400 cfm per ton of refrigeration. The use of an entering air condition of 80 F dry-bulb and 67 F wet-bulb is representative of the entering air conditions actually encountered in many com fort operations because,' while the indoor'conditions: are asiially lower 67 wet-bulb, the introduction of.out door air will usually bring the mixture of air to the cool ing coil up to an approximation of the 67 wet-bulb entering air fpnditinn at design conditions. The of dehumidifying coils for field-assembled projects and for central station type air-handling units is usually accomplished by use of coil rating tables. The prac tice of selecting coils from the load-division indicated by the calculation has worked out satisfactorily for the usual human comfort applications. Additional design precautions and refinements are being used for more exacting industrial applications and, for improved results on all types of air con ditioning in the more humid areas. One of these refinements is the use of a separate cooling coil to cool and dehumidify the ventilation air before admixture with recirculated air. This procedure care of one of the main sources of moisture in the uffnat application. Provision for reheat is required for poTT industrial applications, and is used for better results on some commercial and comfort applications. In choking the operating results obtained from dehumidi fying coils in various air-conditioning installations, it is necessary to keep in mind the influence of the climatic condi tions of the various areas encountered. -The majority of prob lems are encountered at Hghtload conditions when the cooling requirement is considerably ! than at design conditions. In the hot, dry climates, where the outdoor dew points are so constantly low that dehumidifying is not generally a problem, the light-load condition does not pose any special problems. In the hot, humid climates, where the outdoor dew points are generally high and close to the dry-bulb tem peratures, the light-load condition has a higher proportion of. moisture'and a correspondingly lower proportion of sensible heat. These' climatic conditions result in higher dew points in the conditioned spaces during the light-load conditions unless some of'the special'for controlling the'inside dew points are used. In the geographic locations where warm weather occurs with both high and low dew points, such as around the Great Lakes, the bight-load operating conditions with the higher percentage of moisture loading will be encountered less frequently and consistently than in the south coastal areas. Care should be taken to avoid freezing at light loads. In general, freezing occurs when the coil surface temperature falls to 32 F. With usual coils for comfort installations, . this does not occur unless the evaporating temperature at the coil outlet is about 20 to 25 F. The exact value de pends on the design of'the coil and the amount of loading. Although it is not customary to choose coil and 'condensing units to balance at low temperatures.at.peak loads,'there is danger of/this occurring when the load decreases. This condition is further aggravated if a bypass is used, causing less air to be passed through the coil at light loads. > HEAT TRANSFER AND AIR FLOW RESISTANCE . . The rate of sensible heat transmission from air passing over a clean tube, with or without extended surface, to a fluid flow ing within it is impeded by three resistances. The first is that from the air to the surface of the tube and is usually called the external surface or air-film resistance. The second- is the .re-, sistance to the conduction of heat through the fin and tube roeteb Finally .there is another surface or film resistance to the flow of heat between, the internal surface of the metal and the fluid in .the tube. For some applications, an additional thermal resistance is included to account for surface fouling. For the applications under; consideration both the resistance of;the metal to heat conduction, and the internal surface or film resistanee are usually low as compared with-the air-side surface resistance. Economy in space, weight, and cost makes it advantageous to decrease the external surface resistance, where it is proportionately large, to approach that of the metal and that from the tube to refrigerant. This may be ac complished by increasing the external surface by means of fine. Water spray, sometimes applied to the same surface, while not necessarily increasing the overall heat transfer much,- may serve other purposes such as air and coil cleaning. , The transfer of sensible heat between the cooling medium and the air stream is influenced by several variables: 1. The temperature difference. 2. The design and surface arrangement of the coiL 3. The velocity and character ofthe air stream. ' . 4. The velocity and character of the medium in the tubes. The driving force is usually taken as the logarithmic mp>n temperature difference for cooling without dehumidifi cation. The rating of cooling coils for combined cooling and dehiiTnidifiirattnn is discussed later in this chapter. With water coils there is a rise in water temperature and with volatile refrigerants there is often an appreciable pressure drop and corresponding change in evaporating temperature through the refrigerant circuit. The problem is further complicated by the fact that the refrigerant is evaporating in part of the cir cuit and superheating in the remainder. In the case of volatile refrigerants, a cooling coil is tested and rated in conjunction with a specific distributing and liquid-metering device, and the are stated for a given superheat condition of leaving vapor. ' The design and surface arrangement of the coil include such items as materials, type, thickness,- height, and spacing of the fins, and the ratio of this surface to that of the tube, the use of the staggered or in-line tube arrangement,. and provisions to increase the air turbulence such as the use of configurated instead of flat fine. Staggered tubes increase the total heat transfer, as opposed to the in-line arrangement, end configurated fins may be more effective than flat. This design and surface arrangement has a great effect on the air- film heat-transfer resistance. The velocity of the air usually considered is the coil face velocity. This bears a varied relation to the actual velocity over the surface, depending.upon the individual .coil design, As long as a fired design of .coil i9 under consideration, face velocities may be used, but they may be unsatisfactory- in comparing different designs, as it is the actual surface ve locity that is significant. The air volume is usually based on standard air at 70 F and a barometric pressure of'29.92 in. mercury. I