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856 CHAPTER 78 1962 Golds And Data Book humidity ratio of the mixture of outdoor and recirculated air and the dehumidifier performance data can be used to calculate the humidity ratio of the air leaving the dehumidi fier. The difference between the humidity ratio of the air in the room and that of the dehumidified air entering the room represents the-effective dehumidification per pound of air. The rate of - internal moisture gain in grains per minute, divided by the effective dehumidification in grains per pound of air, equals the air quantity required in pounds per minute. The following typical example using arbitrary values shows a general method of determining the dehumidifying require ments. Sensible heat determination considerations are dis cussed in other chapters, and are purposely omitted here. Example I: A sorption dehumidifier is to be used to maintain indoor conditions of 73 F and 20 percent relative humidity, i.e., 24.1 grains per pound of dry sir, 30 F dew point, in a room 20 ft x 30 ft x 10 ft high, having a total wall, ceiling, and floor surface area of 2200 sq ft. Outdoor design conditions are 72 F dew point (113.4 grains per pound). Internal sources of moisture are: 4 occupants; an open nat ural gas burner using 15 cu ft of natural gas per hour; an open top water tank, having an area of 2 sq ft exposed surface, in which water is maintained at 87 F, with air movement over the water surface being 100 fpm. Determine the quantity and con dition of the dehumidified air to be supplied to the room. Solution; The internal moisture gain consists of items 1 to 5. .,, 1. From occupants: Cnina per Minute 4 X 1800/60 - 120 1800 grains per person per hour is obtained from Fig. 7 of Chapter 10 of the 1961 Guidb And Data Book, by interpolation between curves C and D. 2. From burned gas: 15 X 650/60 - 162 1 cu ft natural gas produces approximately 650 grains of moisture. 3. From exposed water surface: 2 X 20 = 40 Evaporation from water surface is assumed to be 20 grains per (minute) (square foot) at 87 'F water with air movement of 100 fpm. 4. From infiltration: X (118.4 - 24.1) 696 One air change, 6000 cu ft, assumed per hour (see Chapter 24 of the 1961 Guidb And Data Boos). 5. Moisture transmitted through room surface: X 3 X (0.783 - 0.176) = 67 Permeability assumed to be 3 grains per (square foot) (hour) (inch Hg vapor-pressure difference on two sides of wall). Total moisture gain from internal sources 1085 Let o be the air delivered to the room, pounds per minute. Let it be assumed for this problem that 85 percent of the air is recirculated and 15 percent is outdoor air. Enough air must be supplied to replace leakage from the system or to satisfy normal ventilating requirements for the occupants of the room as given in Chapter 10 of the 1961 Guidb And Data Book, whichever is greater. The amount b estimated from experience or obtained by test. Tib humidity ratio of the mixture of recirculated and out door air entering the dehumidifier b then: 0.85g{2U) + Q.15g(118.4) 05q + 0.159 ** 38.3 grains per pound entering dehumidifier. From the manufacturer's performance data for the de humidifier to be used it b determined that with 38.3 grains per pound of entering air the leaving condition b 6.5 grains per pound. : Effective dehumidification in the room b 24.1 -- 6.5 or yi grains per pound of supply air. n, 1085 grains per minute ................ I ben 9* --17.6 grains per pound, -- ol.8 lb air ^per xmminrmutt** minimum that must be supplied to the room to maintain 30 F dew point. Note that this value represents the minimum requirement for the arbitrary conditions set forth and that in practice safety margins should be added to the outdoor air percentage value and to the calculated internal moisture gain APPLICATIONS FOR ATMOSPHERIC PRESSURE DRYING Preservation of Materials in Storage The annual cost of corrosion damage in the United States has been estimated to be between $5 and 6 billion,' while that for all forms of deterioration excluding foods amounts to upwards of S12 billion annually A significant amount of this loss could be saved by mobture control with sorption dehumidifiers. The Armed Forces utilize to some extent dehumidified warehouses for special moisture-sensitive ma terials in long term storage. The success of the U. S. Navy Operation Mothball in preservation of the inactive fleet b well known. Tests by the Bureau of Supplies and Accounts of the Navy Department concluded that 40 percent relative hu midity would be an adequately safe level in large warehouses under relatively stable temperature conditions. These relative humidity maximums are selected to control deterioration of materials. Others have indicated that 60 percent rb b low enough to control microbiological attack. With storage be low 30 to 40 percent relative humidity, no undesirable effects on metals or rubber type compounds have been noted. Some organic materials such as sisal, hemp, and paper may lose flexibility and strength, but regain these characteristics with the regain of moisture content. Preservation of materials by dehumidification b effectively accomplished on shipboard, warehouses and caves by the use of sorption equipment. Commercial storage operations also rely on similar equipment for applications which include beer fermentation rooms, meat storage and penicillin processing, as well as storage of machine tools, candy, food products, furs, furniture, seed, paper stock, and chemicals. Reference should be made to Chapters 23, 46, and 50 for recommended conditions of temperature and humidity for storage of various materials. Process Dehumidification The requirements for dehumidification in industrial processes are many and varied. Some of these processes are: 1. Metallurgical processes, in conjunction with the controlled atmosphere annealing of metals. 2. Conveying of hygroscopic materials. 3. Film drying (see Chapter 30). 4. Candy, chocolate, and chewing gum manufacturing (see Chapter 33). 5. Manufacturing of drugs and cbemicab. 6. Manufacture of plastic materials. 7. Manufacture of laminated glass. - 8. Packaging of moisture-sensitive products. 9. Assembly of motors and transformers. 10. Solid propellent mixing rooms. 11. Manufacturing of electronic components such as transis tors and micro-wave components. In the heat treating operation for annealing of wire and strip in steel mill operations, low moisture content of the nitrogen and hydrogen gases in the range of --40 to --100 F b required. The presence of moisture would result in discol oration and possible surface corrosion. - In the powder granulation of-, vitamin concentrates,` a minute ipchumidification fay Sorfaent Materials 857 quantity of mobture will prevent proper compression to . form. Dryer temperatures over 145 F cannot be utilized !tthey destroy the enzymes of the product. The problem is fitber aggravated by the fact that at 100 F, the dryer temoSature determined as best suited to and safest for the orotiuct, sufficient mobture is not removed on humid summer days. Complete shutdown on these days may frequently result. Application of a sorbent dehumidifier to automatically jmd continuously extract all excess mobture will provide gjgady, year-round production at operating speeds and temperatures. In the production of certain vitamin con centrates, extracts from fruits and vegetables are dried and blended while in powder form. The blend b compressed into tablets and packaged for shipment. Drying b accomplished *itfa a batch-type tunnel dryer. The blended powder granu lations, in treys, are placed on racks in the dryer, and hot dry air at approximately 100 F b circulated throughout. Excess mobture must be extracted from the powders to as sure satisfactory compression in the tabletting machines. The mobture content of the powders before drying b 30 per cent, while after drying, it b only 5 to 6 percent. One com plete cycle of operation takes approximately 24 hr. Without fhfrmcn-l dehumidification, it might take 48 hr, even under favorable conditions. Satisfactory yeast drying requires the slow, even removal of moisture at pre-determined rates. Otherwise the delicate outer shells of these living organisms might be ruptured, de stroying their use. Economic operation of the dryer itself necessitates a constant substantial production rate. For these reasons, sorption dehumidifiere will assure product quality and production rates and permit year round satisfactory operation. Yeast, in order to grow, must have energy and nitrogen sources. The propagation of yeast begins in the laboratory with the isolation and controlled growth from a single pure yeast culture. After growth through planned fermentation, the yeast b mechanically separated and b used for additional laboratory fermentation or for seeding Uie final trade fermenters. After fermentation, the batch b put into centrifugal separators which concentrate the yeast to a cream of about 16 percent solids. For active dry yeast production, thb cream b next filtered.-The resulting com pressed yeast is then dried, ground, vacuum packaged, and placed in refrigerated storage until shipment. Water vapor trapped in sealed refrigeration systems can cause erratic operation of the refrigerator due to the freezing of the mobture at the place of refrigerant expansion. It also speeds chemical reactions leading to the breakdown of the refrigerant and the oil. Prevention of thb water vapor entrap ment begins with the basic equipment manufacturer. The oil and refrigerant must be dry so that mobture will not be added when the unit b charged. The parts which make up a refrigeration system must be dry when they are assembled. These parts must then be quickly sealed to prevent the rein troduction of moisture. For further information, see Chapter 59 of the 1961 Guidb And Data Book. Condensation Prevention Many applications require mobture control to prevent condensation. Cold cargo in a ship's hold will cause cargo sweat when the ship reaches moist climates and ship sweat will occur when the mobt atmosphere in a cargo hold is cooled by the hull and deck plates, when the ship goes from a warm to a cold climate. In pumping stations and sewoge lift stations, the piping will cause condensation especially in the spring when the weather warms and water in the pipes b still cold. Dehumidification b also used to prevent mobture from the air to drip into oil and gasoline tanks and into open beer fermentation tanks. Electronic equipment b often cooled by refrigeration and dehumidifiers are required to prevent internal condensation of mobture. Electronic and instrument compartments in missiles are purged with low dew-point air prior to launching to prevent malfunctioning due to condensation. Wave guides and radomes are also usually dehumidified, as are also telephone exchanges and relay stations. The D.E.W. Line installations with regard to our national security, depend to a large extent for the proper operation of their components on the prevention of condensate on interior surfaces. Independent Humidity Control in Air-Conditioned Spaces Comfort air conditioning generally requires the main tenance of summer temperatures between 75 and 80 F and relative humidities of 45 to 55 percent. While these can nor mally be obtained most economically with refrigeration sys tems, there are numerous cases in which the latent heat load b larger than the cooling load where the use of sorption sys tems in conjunction with refrigeration will give the optimum results. Examples of thb are assembly halls, restaurants, night clubs, fallout shelters, and other places of assembly where the internal latent heat load b high. There are also large buildings and hotels located in high humidity areas which depend on dehumidification systems for proper com fort air conditioning. In general, installations involving the use of panel cooling require the circulation of a relatively highly dehumidified air for the maintenance of ventilation requirements and the prevention of condensation on the panel surfaces (see Chapter 3). There may be some need for dehumidification in areas that are heated by panel surfaces where insulation b good and water load is high. An example of this might be laundry and cooking areas in the home. There are sometimes instances where a given space must be held at a lower humidity than b available from an existing air-conditioning system. An example would be a special treatment room for some very hygroscopic material. A de humidifier, independent from the main air-conditioning system, would be used to handle thb bad. Testing Many test procedures require dehumdification with sorp tion equipment. Frequently, other means of dehumidification may be used in conjunction with sorbent units, but the low mobture content requirements can only be obtained by liquid or'solid sorbents. Some of the typical testing applica tions are: 1. Wind tunnels. 2. Spectroscopy rooms. 3. Paper and textile testing (see Chapters 25 and 26). 4. Bacteriological and plant growth rooms. 5. Dry boxes. 6. Environmental rooms and chambers (see Chapter 61). Many of the aerodynamic tests carried out in wind tunnels are possible only because high altitude mobture conditions are readily simulated by the use of sorbent dryers. Fogging conditions which would otherwise occur would interfere with observations and photography of the. test sections being analyzed. The sorbent dehumidifiers for supersonic and sonic wind tunnels are some of the largest in existence. At one wind tunnel installation, over 2,000,000 cfm of air b dried to a dew point of less than --40 F by using more than 2,000,000 lb of solid desiccant. In spectroscopy rooms, the control of