Document bNOQ0k9r5z47KrGkZ5eXbX2y

11** 892 CHAPTER 38 1956 Guide steam is normally used for heating the liquid absorbent. When it is de sirable or necessary to use gas or electricity, an auxiliary low pressure steam boiler is usually added to the equipment. Concentrators operating on a simple boiler principle have not as yet been commercially practical. It should be noted that the solution concentration phase is the reverse of the absorption process. During concentration, the aqueous vapor pres sure of the solution is greater than that of the surrounding.air, while during dehumidification, the reverse is the case. Utilization of this principle per mits winter humidification by heating (instead of cooling) the solution pumped to the contactor. Water is thereby evaporated into, rather than condensed out of, the conditioned air stream. This requires dilution of the liquid absorber externally to the contactor. CALCULATION OF MOISTURE LOAD Calculation of the dehumidification required to maintain' lower than normaJ'moisture content in a given room begins with determination of the 3 KJ ? .V. Cooling coil and Fio. 5. Liquid Absokbent Equipment in Which Solution Cooleb AND CoNTRACTOB ABE COMBINED rate of moisture gain in the room from all sources. It is common practice, when maintaining a low humidity ratio, to recirculate a large percentage of the air in the room through the dehumidifier, and to add only enough out side air to meet the needs of the problem. The humidity ratio of the mixture of outside and recirculated air and the dehumidifier performance data can be used to calculate the humidity ratio of the air leaving the de humidifier. 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 requirements. Sensible heat determination considerations are discussed in other chapters, and are pur posely omitted here. Example 1: A solid absorbent dehumidifier having performance characteristics as shown in Fig. 6 is to be used to maintain inside conditions of 73 F and 20 percent rela tive humidity, 24.1 grains per pound of dry air, 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. Outside design conditions are 72 F dew-point (118.4 grains per pound). Dehumidification by Sorbent Materials 893 Internal sources of moisture are: 4 occupants; an open natural gas burner using 15 cu ft of natural gas per hour; an open top water tank, having an area of 2 sq ft ex posed surface, in which water is maintained at 87 F, with air movement over the water surface being 100 fpm. Determine the quantity and condition of the dehumidified air to be supplied to the room. Solution: The internal moisture gain consists of items 1 to 5. 1. From occupants: 4 X 1800/60 = 1800 grains per person per hour is obtained from Fig. 7, Chapter 6, by interpolation between curves C and D. 2. From burned gas: 15 X 650/60 = 1 cu ft natural gas produces approximately 650 grains of moisture. Crains per M1in2u0te 162 3. From exposed water surface: 2 X 20 = 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: ST^Tilz X (118.4 - 24.1) = W x lo.oo One air change, 6000 cu ft, assumed per hour (see Chapter 11). ---------- 5. Moisture transmitted through room surface: oqnn X 3 X (0.783 - 0.176) = Permeability assumed to be 3 grains per (square foot) w(haolul)r.) (inch Hg vapor pressure difference on two sides of 40 696 67 Total moisture gain;from internal sources 1085 Let 5 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 outside 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 6, which ever is greater. The amount is estimated from experience or obtained by test. The humidity ratio of the mixture of recirculated and outside air entering the dehumidifier is then: 0.85?(24.1) + 0.15?(118.4) 0.85? + 0.15? 38.3 grains per pound entering dehumidifier. For the dehumidifier whose performance is shown in Fig. 6, for 38.3 grains per pound in entering air, the leaving humidity ratio will be 6.5 grains per pound. Effective dehumidification in the room is 24.1 -- 6.5 or 17.6 grains per pound of supply air. TMTh, en ? = 11078-.56 gg--rraaii:n--nss -pp-ee--rr--mp--oi-nu--un-td=e 61.6 lb air per minu. te m. inim. u. m tha,,t m. ust be. , supplied to the room to maintain 30 F dew point. Note that this figure represents the minimum requirement for the arbitrary condi tions set forth and that in practice, safety margins should be added to the outside air percentage figure and to the calculated internal moisture gain. VAPOR TRANSFER TO DEHUMIDIFIED SPACE The walls enclosing a dehumidified space are subjected to a vapor pres sure differential. The pressure of the vapor outside the walls tends to force moisture' through the walls into the dehumidified zone of relatively low vapor pressure. As this.process can be an unnecessary load on the