In an evaporator handling an aqueous salt solution, the overall coefficient U (kW/ m2 deg K) is .. these. Figure shows the flow and plate arrangement of an APV falling-film plate .. MINTON, P. E.: Handbook of Evaporation Technology. This Handbook describes the many types of evaporators and operating options manufacturing capabilities of companies within the APV Group. oncentration. This handbook describes the paraflow and its operating principle, The paraflow is the original plate type heat exchanger designed by APV to provide.

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Efficient Energy Utilization in Drying Drying of fruit, meat and various building and craft materials date back before the discovery of fire. The physical laws governing drying remain the same, even though the machinery to accomplish it has improved considerably! Today, dryers are in operation in most manufacturing industries including chemical, pharmaceutical, process and food. Products that are dried range from organic pigments to proteins, as well as minerals to dairy products.

Because of the spectrum of duties required, there is a great variety of dryers available. The correct choice depends on the properties of the feed material and the desired characteristics of the final product.

This handbook reviews many types of dryers and provides guidelines for their selection. Certain types of commonly used dryers are reviewed in detail. Since drying is an energy intensive operation, this handbook also provides information on techniques to improve efficiency. Solids fully dissolved Slurry: Suspended or dispersed Thixotropic: Thins with shear Dilatent: Thickens with shear Cohesive Paste: Just plain sticky Friable Cake: Dry to touch, breaks up Granules: Typically pigment powders, fillers, talcum powder Free Flowing: Usually over 50 microns, press filling applications Dustless: Dyes, bulk handling, Kaolin — still free flowing Granular: Coarser and less free flowing Wettable: For easy mixing into water — agro chemicals, dyes Agglomertated: Easiest mixing and dissolving — infant formula, creamer Coated: Time release, lecithinated, flavored Lump: Flavors, salts, sugars, HVP Hydrophobic: Fats, polymers, coated inorganics Thermoplastic: Protein, pigments, iron oxide, crystals Inert: Some involve the removal of water or other volatiles from pasty materials such as pigments, clays, synthetic rubbers and fine chemicals.

Others involve the drying of solutions or liquid suspensions such as whey, milk and coffee. To assist manufacturers in arriving at a reasonably accurate first assessment of the type, size and cost of equipment for a particular duty, this article describes the most widely used types of both batch and continuous dryers. Three basic methods of heat transfer are used in industrial dryers in various combinations. These are convection, conduction and radiation.

In the processing industries the majority of dryers employ forced convection and continuous operation.

Circulation evaporator – Wikipedia

This limitation effectively restricts them to applications involving somewhat modest production runs. Its main application is in operations such as the drying of surface coatings on large plane surfaces, since efficient utilization generally requires a line of sight between the material being irradiated and the heat source or emitter.

In all the dryers considered here, however, there is a radiant component in the heat transfer mechanism. This is due to the fact that there are no heat exchanger losses and the maximum heat release from the fuel is available for the process.

However, this method is not always acceptable, especially where product contamination cannot be tolerated. In such cases, indirect heating must be used. With forced convection equipment, indirect heating frequently employs a condensing vapor such as steam in an extended surface tubular heat exchanger or in a steam jacket where conduction is the method of heat transfer.


Alternatively, systems which employ proprietary heat transfer fluids can also be used. These enjoy the advantage of obtaining elevated temperatures without the need for high pressure operation, as may be required with conventional steam heating. This may be reflected in the design and manufacturing costs of the dryer. Furthermore, in addition to the methods listed above, oil- or gas-fired indirect heat exchangers can also be used.

In general, dryers are either suitable for batch or continuous operation. A number of the more common types are listed in Table 1, where an application rating based on practical considerations is given. In the following review, some of the factors likely to influence selection of the various types are discussed for particular applications. As evsporator result, the batch type forced-convection unit certainly finds the widest possible application of any dryer used today.

However, in special applications of this type of dryer that call for total air rejection, this figure is somewhat lower and is largely related to the volume and temperature of the exhaust air. Capital investment and installation cost are relatively low.

APV Dryer Handbook

The use of fan systems minimizes both power requirements and operating costs. In contrast, labor costs can be high. Comparative performance curves for Thruflo and conventional handbolk In such a plant, the drying cycles are extended, with 24 to 45 hours being quite common in certain cases.

This is a direct result of the low evaporative rate, which normally is in the region of 0. Following the recent trend and interest shown in preforming feedstock with regard to the design of extruding and tray-filling equipment for de-watered cakes — it is now possible to obtain the maximum benefit of enhanced evaporative rates by using through-air circulation dryers when handling preformed materials.

Figure 2, shows how a high-performance dryer can produce lbs kg of dried material in a 24 gandbook period, at a handbooj figure of 0.

The significant improvement in performance can be seen Figure 3. The advantage is more apparent when it is seen that respective floor areas occupied are 55 ft2 5 m2 for the Thruflo dryer pictured in Figure 3, and ft2 22 m2 in the case of conventional units using transverse air flow. In reference to the drying curves for the processing of materials in solid, filter cake, or wet powder form, the ultimate rate-governing factor is the rate of diffusion of moisture from the wet mass.

This becomes increasingly so during the falling rate period of drying. This situation, however, can be improved by preforming the product to increase the effective surface area presented to heat and mass transfer. The logical extension of this technique is total dispersion drying, i. This produces rapid heat transfer with correspondingly short drying times. Batch type fluidized bed dryers have, therefore, evapkrator forced convection units in many cases — notably in the drying of pharmaceuticals and for the processing of certain thermoplastics.

When considering a fluid bed dryer, it is important to ensure that the feed material can be fluidized, both in its initial and final condition. It also should be remembered that standard fan arrangements are not equally suitable for a variety of materials of different densities.

Therefore, it is necessary to accurately determine the minimum fluidizing velocity for each product. If the feedstock is at an acceptable level of handbool content for fluidization, the fluid bed type of dryer provides many advantages over a batch type tray dryer.

Simplified loading and unloading results in lower labor costs — high thermal efficiencies are common and the drying time is reduced to minutes, as opposed to hours in conventional units. Current developments of this type of equipment include techniques for the simultaneous evaporation of water and the granulation of solids.


This makes these units ideal for use in the pharmaceutical field. The various batch dryers operate by means of forced convection. Good conditions are maintained for continued effective drying. Alternatively, and where the material is thermosensitive, implying handgook temperatures with consequently low evaporative rates, some improvement can be effected by the use of sub-atmospheric dryers, i. Several different configurations are in use and all fall into the category of conduction-type dryers.

The most usual type of heating is by steam, although hot water or one of the proprietary heat transfer fluids can be used. These units are comparatively simple to operate and when adequately insulated are thermally efficient, although drying times can be xpv.

They are especially suitable for applications involving solvent recovery and will handle powders and granules moderately well. However, with some materials, the tumbling action in double-cone dryers and the action of the agitator in agitated-pan machines can produce hqndbook degree of attrition in the dried product which may prove unacceptable.

Similarly, large rotary vacuum dryers are used for pigment pastes and other such materials, especially where organic solvents present in the feedstock need to be recovered. These units are normally jacketed and equipped with an internal agitator, which constantly lifts and turns the material. Here, heat transfer is entirely by conduction from the wall of the dryer and from the agitator.

Due to the nature of their construction, initial cost is high relative to capacity. Installation costs also are considerable. In general, there are limited applications for these dryers. A typical film drum dryer with a 4 ft 1. Where tonnage production is yandbook, the drum dryer is at a disadvantage.

Climbing and Falling Film Plate Evaporator

However, the thermal efficiency of the drum dryer is high in the region of 1. Drum dryers are usually steam heated, although work has been done to develope units for direct gas or oil heating. Completely packaged and evaprator of independent operation, these dryers can be divided into two broad classifications: In practice, these variants are necessary because of the differing characteristics of the materials to be dried and due to the fact that no universally satisfactory feeding device has yet been developed.

Where the feed is very mobile, rotating devices such as spray feeds are used.


It must be emphasized that the method of feeding the product to the dryer is of paramount importance to selection or design. There are, of course, certain materials which are temperature-sensitive to such a degree that their handling would preclude the use of an atmospheric drum dryer.

In such cases, special sub-atmospheric equipment may provide the answer, although the capital cost in relation to output generally would restrict its use to premium grade products. As an alternate, the spray dryer offers an excellent solution to a host of drying problems.

Many materials, such as dairy and other food products, which would suffer from thermal degradation if dried by other methods, can often be handled by spray drying due to the rapid flash evaporation and its accompanying cooling effect.

The continuous method of operation also lends itself to large outputs and with the correct application of control equipment, to low labor costs. However, the design of an efficient spray drying plant requires considerable expertise along with access to large scale test facilities, particularly where particle size and bulk density requirements in the dried product are critical.