Direct Heating/Drying

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General

Direct heating and drying refers to combustion products mixing directly with the process environment (typically process solids and a forced "air" stream). Because radiation transfer is rapid, typically at high temperature, and ceases upon reaching a boundary (the outer layer of process matter), it is often undesirable and unnecessary. Therefore, natural and forced convection heat transfer engineering may dominate dryer design.

There are a wide variety of process dryers, kilns, calciners, ovens, etc. that incorporate an even greater range of combinations in forced convection, radiation, and conduction (through the material) heat transfer principles to satisfy the product requirements.

In all cases, however, the heat energy supplied to a system must perform the following four tasks:

  1. Heat the dryer feed to the "light" component's vaporization temperature
  2. Vaporize and/or free the liquid/byproducts above the solids' surface
  3. Heat the solids to the final desired temperature, and for the desired duration of time
  4. Heat the vapor to the final desired temperature.
    Process Uses

Numerous factors, including production throughput, local steam, natural gas and electricity prices, emissions restrictions, and equipment cost considerations, often result in similar solids being dried in very different ways.

However, common direct drying/heating operations and their typical product/process applications include:


1. Bringing variable water-weight percent feeds to a desired initial processing concentration.

Mined raw materials and/or prepared mixes fed to cement, gypsum, ceramics, and lime processes require crushing, sizing, and drying. Rotary dryers, impact dryers, drum dryers, and others are used to handle large volume, variable composition slurries. Water removal to organize/homogenize process streams for inorganic chemicals manufacture is also common.

2. More complete drying of slurries containing finer solids within certain size/weight specifications is performed using spray dryers, thin-film dryers, and drum dryers.

Within the Stone, Clay, Glass and Cement manufacturing sector (SIC 32), fine dry powders are desirable for handling, packing, and/or to produce a more consistent product. Specific products include kaolin clay, fluid cracking catalysts and ceramics that may also use this step to introduce property enriching additives/binders to the material

Emulsion PVC and PVP polymer processes often employ spray drying to rapidly remove water without degrading product

Milk/dairy powders


Organic and inorganic dry soaps, detergents, dyes and pigments


3. Pre-heating/drying materials


Metals fabrication and/or scrap metal industries use direct heat to remove volatile impurities (oils, plastics, paints, etc.) and/or to reduce energy demand of central furnace operations

Large kilns, calciners, and ovens (primarily in SIC 32) also benefit from preheated feeds, often containing preheat sections as part of the primary unit (tunnel kilns, etc.)

Coke processes may preheat coal feeds to reduce moisture content

Glass and mineral wool industries utilize many preheat techniques to reduce energy demands or increase throughput on central furnaces systems


4. Drying and heating meant to relieve chemically bound light components and/or otherwise modify solid structure. Rotary kilns, shaft kilns, kettle calciners, flash calciners, brick ovens/houses, tunnel kilns, regenerative kilns, and others are included in this grouping.


Kilns and ovens used for bricks, ceramics, etc. where residence times in hot and dry conditions may last hours to days to obtain desired final qualities in appearance and structure

Kilns and calciners used to produce/process gypsum, plasters, cements, limestone, etc. where energy not only thoroughly removes any remaining water, but also frees intimate impurities, and forces various reactions often resulting in the release of carbon and sulfur oxides. Along with those operations in SIC 32, both the pulp & paper and beet sugar industries use these lime kiln technologies


5. Drying to remove water (and/or other solvents/chemicals) added, left, or produced during processing.

Starch, stalk and husk dryers, and fruit peel and feed dryers, used in beet and cane sugar manufacturing, grain mill products, and other SIC 20 manufacturing sectors

Convection dryers in textile manufacturing

Veneer and other lumber/wood-furniture dryers

Pulp dryers, coated and tissue paper dryers in SIC 26

Dryers including conveyor and tray dryers used in non & cellulosic fibers (rayon, acrylics, etc.) processing, polymer rubbers manufacture, for pharmaceuticals, and latex.


6. Granulators, fluidized bed systems, rotary dryers, and tower dryers often used for producing finished grains, sugar, and fertilizer.

Integrating for Cogeneration

Addressing heat transfer, either more packing media, or extending the height of the column (or both) may be necessary to maintain normal operation (with retrofit systems). Pressure drop and thus back-pressure imposed on the generating system will be a key design element. Special consideration to ensure that no process water enters back into the DG unit's exhaust system is also crucial for practical implementation.

Many industrial facilities may not have a constant hot water demand, however two profiles may describe the demand well (e.g., normal production operation, and "clean-down" or full capacity day shifts with part capacity night shifts). In these cases, a bypass-recuperator option on a turbine-based cogeneration scheme can be integrated with a variable flow water tower to switch between profiles. Assuming precise hot-water energy requirements are known, a recuperator with bypass can be designed to maintain total system efficiency by diverting some or all of the exhaust past the (turbine) recuperator to boost the hot water delivery to the desired level.


Currently Available Systems

There are currently two off-the-shelf small industrial cogeneration systems available in the marketplace to generate hot water. The first system is a microturbine-based solution that works like the indirect liquid heating system. An air-to-water heat exchanger is used with the turbine exhaust gases to heat water.

The second system is a standard reciprocating engine cogeneration system. These system use liquid-to-water heat exchangers on the water jacket cooling fluid, the lubricating oil system, and sometimes on the aftercoolers. Some of these systems also use an air-to-water heat exchanges on the engine exhaust.

For more information on each application: