Document NGroM4NndnYyepeJLvXZqooDb

American Society of Heating and Ventilating- Engineers Guide, 1930 In some operations, time controllers-are used which automatically alter the conditions according to a predetermined schedule and finally bring the drying operation to a close. With other dryers regulation is obtained in accordance with the loss in weight of samples of the material suspended on sensitized scales within the dryer, the drying being automatically terminated when the samples reach the desired degree of dryness. Gener ally, however, the only function of the control instruments is to maintain the. desired temperature or humidity and any change in the conditions must be, made by the operator by re-setting the instruments. The.higher the temperature, other things being equal, the faster will the moisture in the material transfuse to the surface and the faster will the drying proceed. Hot water passes through the material more rapidly than cold water because of its increased vapor pressure as well as its decreased viscosity. Increasing the rate of surface evaporation by greater circulation at low temperatures produces faster drying only when a corre sponding increase in the rate of transfusion is obtained.. Provision must be made in every dryer to dispose of evaporated moisture as it comes from the material. In most dryers this is done by discharging part or all of the moisture laden air to the atmosphere, replacing it with an equal amount of outside air. In some types, par ticularly of the blower style, the surplus moisture is often removed by some form of condenser such as cold.water coils or sprays. It is a mistake to attempt to obtain a practically saturated condition of the leaving air, as this is apt to slow down the speed of drying and cause an undesirable amount of condensation in the moist air ventilators or ducts returning to the blower. A saturation of from 50 to 75 per cent in the leaving air is usually as high as can be obtained without sacrificing speed. Operating the Dryer No matter how well the dryer may be designed, a considerable part .. of the success of a drying operation depends upon the operator. He must watch the material and study the results in a constant effort toward' improvement. Good dryer operation includes regular sampling and testing of material, watching the conditions within the dryer, and keeping systematic and complete records of all essential data such as the move ment of material, the length of the drying period, the temperatures and . humidities at critical points in the dryer, and the extent to which the ' equipment is kept fully occupied. Standard schedules of drying time and drying conditions for material of different kinds are also useful in esti mating production and regulating the operation of the dryer. Moisture Content Calculations Moisture content is usually expressed as a percentage of the original combined weight of the material and its contained water. Thus, if the moisture content were 20 per cent, the dry material would weigh 80 per cent of the total weight. In drying some types of material, notably lumber, it is customary to use the bone-dry or water-free weight of the material as the divisor, the weight of the water being calculated as a percentage Of this dry weight. Regain, or- absorption of hygroscopic. ' moisture after drying, is almost always expressed by the latter method, as a percentage of the water-free weight of the material. 482 . Chapter-29^--Drying by Evaporation Table 1. Drying Time and Conditions for Representative Materials - Kind and Thickness of Material Temperatube Deo. Fahil Drttnq Time Bedding---- Cereals.----Cocoanut Coffee------------ ---------------p------------------ Cores, Oil Sand, for molding...---------,,J4 in. - 1 in. thick Black sand with Goulac Binder 1 about 6/10 of time for oil sand cores 3 in. " 8 in. " 16 in. " Feathers------------------------Films, Photographic........ Fruits and Vegetables...... Furs------------------------------- Glue-----------------------------Glue Size on Furniture.... G_ ypsum hWiailil nBoarad................................... -........... \/FSitnaisrthWet Gypsum Blocks...................... -............................................ -....... Hair Goods............... ................................................ -.................... Hats, Felt--------- ----------------------------- ------- -------------------------Hops.................................................................................... -........... Hides, Thin leather..;................. ,...... ,--..................................... Ink, Printing-........................... ........... ................................ --...... Knitted Fabrics......--1................................................................. Leather, Thick Sole........................................... .......................... Lumber, Green Hardwood................................ ........................ Lumber, Green Softwood............................................................ Macaroni--................-- ........................................................... Matches----------------------------------------- ---------- ---.....-............... Milk and other Liquid Foods (Spray Dried)............ ............ Molds, Green Sand, C. I. Flasks (one Surface only exposed) 8 in. thick--........................ :--------.-------------13 in. " .............................................. ........... Nuts._................................... -............... -................... Paper, Glued...................................... -............. --- Paper, Treated--.......... _........................................ Rubber........................................................................ Sand, loose, 1 in. deep............................................ Shade Cloth.......................................................... -- Soap_________________________________________ Starch........ ........................................ .--................... Stock Feed, Mixed............... .......................... :----Sugar............................ _............................................ Tannin and other chemicals (Spray- Dried),-- Terra Cotta (air-drying in conditioning room). Wall Board........................................... ................... 150-190 110-150 145-155 160-180 300 480 480 700 150-180 90 140 110 70-90 130 150 350 190 350-190 150-190 140-180 120-180 90 70-300 140-180 90 100-180 160-220 90-110 140-180 250-300 4--6 Hours 24 " 30 Minutes, 2)- Hours " 2-6 " 2-4 4 60 Minutes 8-16 Hours 1 Hour 2-4 Hours 4-6 3-180 Days 2-14 " Instantaneous 600 700 75-140 130-300 140-200 80-90 300 240 125 180-200 180-220 150-200 250-300 150-220 200-250 6 Hours 13 " 24 " 6-12 " 10-15 Minutes 1-2 Hours 12 1-4 " 20-30 Minutes 20-30" " Instantaneous 12-96 Hours 12-24 " 483