Document Dvb779V8Magdg4qBwDp3jR0k5

612 CHAPTER 41 1960 Guide tional blade type or glass mat eliminators. With water drainage provided for each cell tier, most of the stratification found in spray-type washers is avoided. An essential requirement of operation of this type of air washer is good distribution of water over the face of the cells. When compared with conventional spray-type washers, the water requirements are much lower, approximately 4 gpm per 1000 cfm for ordinary washing and evaporative cooling, and the pumping head is approximately one-half the bead of conventional spray-type washers. Humidification with Air Washers Air humidification can be accomplished in three ways with an air washer. These are: (1) use of recirculated spray water without prior treatment of the air; (2) preheating the air and washing it with recirculated spray water; and (3) using heated spray water. In any air washing installation the air should not enter the washer with a wet-bulb temperature less than 35 F. This is a precaution to eliminate the danger of freezing the spray water. Method l. Except for the small amount of energy added from outside by the recirculating pump in the form of shaft work, and for the small amount of heat leakage from out side into the apparatus, including the pump and its con necting piping, the process would be strictly adiabatic. Evap oration from the liquid spray would therefore be expected to bring the air immediately in contact with it to saturation adiabatically; and, since the liquid is recirculated, its tem perature would be expected to adjust to the thermodynamic wet-bulb temperature of the entering air. It does not follow from the foregoing reasoning that the whole air stream is brought to complete saturation, but merely that its state point should move along a line of con stant thermodynamic wet-bulb temperature as explained in Chapter 3. The extent to which the leaving air temperature approaches the thermodynamic wet-bulb temperature of the entering air, or the extent to which complete saturation is approached, is conveniently expressed by a ratio known as humidifying effectiveness or saturating effectiveness, and is defined in Equation 1 as tk - (1) where e* ** humidifying or saturating effectiveness, percent. <i = dry-bulb temperature of the entering air, Fahrenheit. tt B dry-bulb temperature of the leaving air, Fahrenheit. t' thermodynamic wet-bulb temperature of the entering air, Fahrenheit. The following may be taken as representative of the hu midifying or saturating effectiveness of a spray-type air washer for the arrangements stated: Arrangement 1 bank downstream 1 bank downstream 1 bank upstream 2 banks downstream 2 banks opposing each other 2 banks upstream Length Ft 4 61 6 8 to 10 8 to 10 8 to 10 EjtecitvbNESS--% 50-60 60-75 65-80 80-90 85-95 90-98 The humidifying or saturating effectiveness of a washer is dependent upon the essential items of design mentioned in the section Air Washers. Other conditions being the same, a low velocity of air flow is conducive to high humidifying ef fectiveness. Method f. The preheating of the air increases both the dry- and wet-bulb temperatures, and lowers the relative hu midity, but does not alter the humidity ratio (pounds of wa ter vapor per pound dry air). At a higher wet-bulb tempera ture, but the same humidity ratio, more water can be absorbed per pound of dry air in passing through the washer, assum ing that the humidifying effectiveness of the washer is not adversely affected by operation at the higher wet-bulb tem perature. The analysis of the process occurring in the washer itself is the same as that explained under Method 1. The final desired conditions are secured by adjusting the amount of preheating to give the required wet-bulb temperature at the entrance to the washer. Method $. Even if the heat is added to the spray water, the miring occurring in the washer itself may stiff be re garded as adiabatic. The state point of the mixture should move in a direction determined by the specific enthalpy of the heated spray as explained in Chapter 3. It is possible, by elevating the water temperature, to raise the air temperature, both dry-bulb and wet-bulb, above the dry-bulb tempera ture of the entering air. In each of the methods, 1, 2, or 3, the air leaving the air washer may require reheating to produce in the conditioned space the required dry-bulb temperature and relative humid ity. Dehumidification and Cooling with Air Washers Lowering of the wet-bulb temperature of an air-vapor mix ture can be accomplished by an air washer if the temperature of the spray water is lower than the wet-bulb temperature of the air. Moisture removal is obtained when the spray wa ter temperature is lower than the dew point of the entering air. In these cases the final dry-bulb temperature and rela tive humidity of the leaving air are dependent upon the de sign factors of the air washer. Both sensible and latent heat are removed in the praces of dehumidification by cold spray'water. Extraction of sensi ble heat occurs during the entire time that the air is in con tact with the spray medium. Latent heat removal takes place as condensation occurs. Therefore, the lower the spray tem peratures, the greater the amount of moisture removal per pound of dry air, all other conditions remaining the same. Where a limited supply of cold water is available, multiplestage washers may be used to great advantage. In such washers the cool water is pumped through the multiple spray systems in series and counterflow to the air flow. Such an arrangement brings the delivery air in contact with the coldest water, securing a maximum amount of cooling and saving water. When using cold well water or water from city water mains, care should be used to secure accurate data on the water tem peratures. Table 2 lists some approximate water main aver ages which may be used as a guide but should be verified from local records. This is particularly true with city water main temperatures. In the case of well water temperatures, Fig. 4 shows the approximate temperatures of water to be expected from wells at depths of 30 to 60 ft. Air washers for dehumidifying and cooling usually have separate recirculating pumps for each washer. These pumps deliver a mixture of cold and recirculated water under the control of a three-way valve. Evaporative Air Cooling and Humidification Table 2 .... Average Maximum Water Main Temperatures* 613 State Ala. Aria. City i State 84 80 83 82 City i State 78 N. Y. 77 62 57 City f State Texas CHr i Calif. 69 80 Ky. 75 La. Los Angeles... Pasadena-- Pomona........... 80 69 70 82 75 78 72 65 84 71 Me. Md. Mass. Shreveport.... 88 60 Boston............. 80 Cambridge.... 70 50 68 N. C. Yonkers......... 70 'Raleigh.............. 92 Winston-Salem 82 Utah Va. Wash. Wichita Falls.. 85 Fredericksburg 75 Lynchburg.... 73 Conn. D. C. Del. Fla. Bridgeport.... 75 66 73 76 72 84 83 \finn Flint................. 70 84 77 56 Ore. Ga. Mo. Jefferson City. 82 60 111. Chicago........... 78 Springfield....... 82 83 S. c. W. Va. Lakewood......... 82 Wis. McKeesport -- 82 Pittsburgh......... 86 Provme* Alta. B. C. Ont. Huntingdon___ 78 City f 64 50 82 S. D. Ind. Evansville....... 88 N. H. Manchester___ 76 P. E. I. Charlottetown. 48 Gary................ 75 N. J. Jersey City-- 63 Tenn. Chattanooga___ 84 Que. 61 Terre Haute... 82 Trenton........... 79 Nashville........... 90 Quebec............. 68 * TbeM tverftgea L&kea from rations city water main locations, with aome actual values slightly higher and tower rabies shown. Sms values f were supplied by H. EL Degler, Harley Company. Some were obtained from City Water Department records. The tugboat values given by the various authorities are usually those listed. Performance of an air washer, when cooling and dehumidi fying, depends upon a number of factore, among which are; (1) Air velocity through spray chamber area. (2) Spray nossle pressure. (3) Site factor (larger washers are more efficient than smaller ones). (4) Spray density (gpm/sq ft/spray bank). (5) Water temperature rise. (6) Air wet-bulb temperatures--entering and leaving. (7) Water-to-air weight ratio. (8) Water temperatures--entering and leaving. (9) Length of air washer in direction of flow. (10) Number and arrangement of spray banks. Where increase of overall beat transfer between the air ? * *