Vacuum heat transfer

As discussed above, under high vacuum heat transfer occurs primarily by radiation, Fiberglas acts to block this transfer but in the process adds a small amount of solid conduction to the heat transfer. If one uses a thin piece of... 

Vacuum Radiation Furnaces. Vacuum furnaces are used where the work can be satisfactorily processed only in a vacuum or in a protective atmosphere. Most vacuum furnaces use molybdenum heating elements. Because all heat transfer is by radiation, metal radiation shields ate used to reduce heat transfer to the furnace casing. The casing is water-cooled and a sufficient number of radiation shields between the inner cavity and the casing reduce the heat flow to the casing to a reasonable level. These shields are substitutes for the insulating refractories used in other furnaces. 

Radiative Heat Transfer Heat-transfer equipment using the radiative mechanism for divided solids is constructed as a table which is stationary, as with trays, or moving, as with a belt, and/or agitated, as with a vibrated pan, to distribute and expose the burden in a plane parallel to (but not in contacl with) the plane of the radiant-heat sources. Presence of air is not necessary (see Sec. 12 for vacuum-shelf dryers and Sec. 22 for resubhmation). In fact, if air in the intervening space has a high humidity or CO9 content, it acts as an energy absorber, thereby depressing the performance. 

Horizontal-Tank Type This type (Fig. ll-56a) is used to transfer heat for melting or cooking diy powdered solids, rendering lard from meat-scrap solids, and drying divided solids. Heat-transfer coefficients are 17 to 85 W/(m °C) [3 to 15 Btu/(h fF °F)] for drying and 28 to 140 W/(m °C) [5 to 25 Btu/(h fF °F)] for vacuum and/or solvent recovery. 

Heat is transferred by radiation, condurtion, and convection. Radiation is the primaiy mode and can occur even in a vacuum. The amount of heat transferred for a given area is relative to the temperature differential and emissivity from the radiating to the absorbing surface. Conduction is due to molecular motion and occurs within... 

Vacuum Insulation Heat transport across an evacuated space (1.3 X lO"" Pa or lower), is by radiation and by conduction through the residual gas. The heat transfer by radiation generally is predominant and can be approximated by... 

Final vacuum versus water temperature, water cost, heat-transfer performance, and product quality. 

Equipment commonly employed for the diying of sohds is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Diyer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum diyers, furnaces, steam-tube diyers, and rotaiy calciners. 

Description A tray or compartment diyer is an enclosed, insulated housing in which solids are placed upon tiers of trays in the case of particulate solids or stacked in piles or upon shelves in the case of large objects. Heat transfer may be direct from gas to sohds by circulation of large volumes of hot gas or indirect by use of heated shelves, radiator coils, or refractoiy walls inside the housing. In indirec t-heat units, excepting vacuum-shelf equipment, circulation of a small quantity of gas is usually necessary to sweep moisture vapor from the compartment and prevent gas saturation and condensation. Compartment units are employed for the heating and diying of lumber, ceramics, sheet materi s (supported on poles), painted and metal objects, and all forms of particulate solids. 

Design Methods for Batch Vacuum Rotary Dryers The rate of heat transfer from the heating medium through the diyer wall to the sohds can be expressed by... 

Vacuum diyers are usually filled to 50 to 65 percent of their total shell volume. Agitator speeds range from 3 to 8 r/min. Faster speeds yield a shght improvement in heat transfer but consume more power. 

The inerts will blanket a portion of the tubes. The blanketed portion has very poor heat transfer. The column pressure is controlled by varying the percentage of the tube surface blanketed. When the desired pressure is exceeded, the vacuum system will suck out more inerts, and lower the percentage of surface blanketed. This will increase cooling and bring the pressure back down to the desired level. The reverse happens if the pressure falls below that desired. This is simply a matter of adjusting the heat transfer coefficient to heat balance the system. 

Another example of pressure control by variable heat transfer coefficient is a vacuum condenser. The vacuum system pulls the inerts out through a vent. The control valve between the condenser and vacuum system varies the amount of inerts leaving the condenser. If the pressure gets too high, the control valve opens to pull out more inerts and produce a smaller tube area blanketed by inerts. Since relatively stagnant inerts have poorer heat transfer than condensing vapors, additional inerts... 

J. E. Troyan s series of articles on plant startup has a cause/effect table on instrumentation in Part II. This article also has troubleshooting hints for distillation, vacuum systems, heat transfer, and filtration. Here is the table on instrumentation. 

In the above example, 1 lb of initial steam should evaporate approximately 1 lb of water in each of the effects A, B and C. In practice however, the evaporation per pound of initial steam, even for a fixed number of effects operated in series, varies widely with conditions, and is best predicted by means of a heat balance.This brings us to the term heat economy. The heat economy of such a system must not be confused with the evaporative capacity of one of the effects. If operated with steam at 220 "F in the heating space and 26 in. vacuum in its vapor space, effect A will evaporate as much water (nearly) as all three effects costing nearly three times its much but it will require approximately three times as much steam and cooling water. The capacity of one or more effects in series is directly proportional to the difference between the condensing temperature of the steam supplied, and the temperature of the boiling solution in the last effect, but also to the overall coefficient of heat transfer from steam to solution. If these factors remain constant, the capacity of one effect is the same as a combination of three effects. 

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