Document a4qzY71G33bZkxe8ZyxKkpp0R

.^ American Society of Heating end .Ventilating Engineers Guide, 1936 Table 1. Drying Time and Conditions for Representative Materials Kind and Thickness of Material Temperature Deo F Dhttno Time Cores, Oil Sand, for molding.......... :........ Yi in.--1 in. thick Black sand with Goulac Binder about I g thick 6/10 of time for oil sand cores...............Lg in. thick Gypsum Wall Board--%. . . in. ,. /Start Wet thick...................... \Finish 150-190 110-150 145-155 160-180 300 480 480 700 150-180 90 140 110 70-90 130 150 350 275 350-190 150^190 140-180 120-180 90 70-300 T40-180 90 100-180 160-220 90-110 140-180 250-300 4--6 Hours 24 Hours 30 Minutes 2)4 Hours 4Yi Hours 10 . Hours 2-6 Hours 2-4 Hours 4 Hours 60 Minutes 8-16 Hours 1 Hour 2-4 Hours 4r-6 Hours 3-180 Days 2-14 Days Instantaneous Molds, Green Sand, C. I. Flasks (one surface only exposed) O 111. LlULtL.~.-----------------------------------------*---------* Sand, loose, 1 in., deep.,- ---------------------------------------------------------------------------------------- 600 700 75-140 ' 180 90-200 80-90 300 240 125 180-200 180-220 150-200 250-300 150-220 300 300-385 300-385 6 Hours 13 Hours 24 Hours 6-12 Hours 10-15 Minutes 1-2 Hours 12 Hours . 1-4 HourS 20-30 Minutes 20-30. Minutes Instantaneous 12-96 Hours 15-20 Minutes 2)4-3 Hours 24r-48 Hours 738 Chapter 41--Drying which can be used without injuring the material. In this case, theoretical analysis, according to Lewis, shows that in the early stages the time of drying is proportional to the square of the thickness and to the square of the total moisture lost, and the time moisture curve is a parabola. In the later stages, after all free water has been evaporated, the curve becomes logarithmic. The form of this drying curve will, in general, resemble that shown in Fig. 2. On the other hand, in drying material in which free water can flow by capillary action at a sufficient rate to keep pace with surface evaporation, the time is proportional, to the first power of the thickness and the rate will be governed by the drying conditions of the air and the area of surface exposed. DRYER DESIGN Methods of Calculations . A psychrometric chart applying to atmospheric pressure and plotted in units convenient for use in dryer calculations, is given in Fig. 3. The amount of moisture in the air, H is given in pounds of water per pound of dry air. With this choice of units, changes in H represent directly the moisture picked up by the air from the material being dried. The inclined Fig. 2. Typical Time-Moisture Curve for Materials in which no Capillary Flow of Free Moisture Occurs straight lines are adiabatic cooling lines for air in contact with water at its wet:bulb temperature, and also represent the cooling of air in the adiabatic drying of any stock, the sensible heat of which is negligible in comparison with the heat of vaporization of the water in it. The following nomenclature ancl explanation of terms will be used in the discussion of drying calculations: H = humidity of air, pounds of water vapor per pound of dry air. G = pounds of dry air supplied to the dryer per unit of time. 5 = pounds of stock dried per unit of time in a continuous dryer. 5' = pounds of stock charged per batch to a discontinuous dryer. 0 = time, Q = total heat supplied to the dryer. t = air temperature. t1 = stock temperature. f" = average stock temperature over short time interval, in a batch dryer. U* = wet-bulb temperature, s' = specific heat of the stock.