Convective Rapid Drying of Hollow Bricks

(FH) Ralf König, Diploma Engineer, D-Krumbach

Abstract
Ralf König: Convective rapid drying of common bricks
The rapid development taking place in our industrial society demands a maximum of flexibility and readiness for innovation from firms. This also applies to drying technology in the heavy clay industry. A revolutionary step in this field is the introduction of the rapid drying technique. This article will give a graphic explanation of the principle of rapid drying for checker bricks.

Today’s rapid industrial development requires every enterprise to carry out technological innovations with maximum flexibility and speed, according to its own situation. This principle also applies to the field of drying technology in the brickmaking industry. About 100 years ago, green bricks were still dried in drying racks called “hacks” (i.e., natural drying). Today, this natural drying process is completely outdated. It allowed only seasonal production, with drying cycles of 2–3 weeks; the drying racks, or open‑air drying yards, could only be turned over 10–12 times a year. Without a sufficient number of drying racks, such a drying process could not adapt to continuous kiln production.

The first development in drying technology was the so‑called “large‑capacity drying shed”, built on top of ring kilns or zigzag kilns, using the rising hot air from the kiln surface for drying. This reduced the drying cycle to 10 days.

Today’s chamber or tunnel dryers use waste heat from tunnel kilns for artificial drying. The drying cycle depends on the product type and raw material properties, ranging from 1 to 3 days. Another revolutionary step in this field was the introduction of rapid drying technology, i.e., a drying time of only 1–2 hours. This article provides a graphic illustration of the rapid drying principle for high‑voidage hollow bricks and discusses its investment prospects.

Origin of Rapid Drying

In the mid‑1980s, factories in the Federal Republic of Germany that manufactured industrial catalysts began to develop. These catalyst bodies had a cross‑section of 150 mm × 150 mm, a length of about 1.0–1.2 m, and a very high void fraction. At that time, many of the dryers in these newly developed factories came from Novokaram. With regard to drying quality and drying time, the best results were achieved only when the green bodies were subjected to through‑flow and cross‑flow air. If the required forced drying exceeds a certain level, other production parameters also play a role, such as the air velocity through the holes and over the surfaces of the green bodies, as well as the heat‑carrying state of the gas as the bodies move forward. It was found that in some cases, because the saturated water vapor pressure in the gas greatly exceeded that of the green body, one‑third of the dried bodies were eventually damaged by adsorbed condensed water.

Microwave or high‑frequency heating would be ideal methods for heating the airflow. However, practically insurmountable problems were encountered. Two representative issues are mentioned here:

a. In some regions, high‑frequency heating is used only for metal equipment components such as sensors and sensor sleeves; naturally, the drying boards that have carried green bodies cannot be reused.
b. High‑frequency heating generates considerable static electricity in the heating zone. Even the very thin water film on the green body or between the green body and the plastic drying board can cause the board to scorch or even be damaged due to the discharge rate.

Therefore, the method of intermediate preheating by means of heatable drying boards (to prevent condensation on the green bodies) proved successful in practice. In fact, the experience gained in Novokaram’s catalyst drying inspired the idea of developing a rapid drying chamber for perforated bricks. In recent years, Novokaram has conducted extensive drying tests, with products ranging from large slabs (50 × 30 × 300 cm) to ordinary perforated bricks of traditional length. It has been consistently found that convective drying can fully achieve the required results.

Basic Principle of Convective Rapid Drying

The most familiar example of convective drying is blow‑drying hair with a hair dryer. The basic principle is that the drying medium (usually hot air) passes over the item to be dried, evaporating and removing moisture. Since evaporation requires heat, the drying medium gradually cools down and absorbs more water during the process (see Fig. 1). The ability of air to absorb moisture is limited by a temperature‑dependent value – the so‑called “saturated water vapor pressure”. If this value is exceeded, the excess natural moisture condenses in the form of fog or condensate, which is particularly dreaded in drying. The state of the air in a drying chamber is usually expressed in terms of temperature (°C) and relative humidity (%). Incidentally, when using an h‑x diagram, these two parameters are fundamental values.

Achieving Balance in the Flow State

The starting point for considering rapid drying is that the drying time of green bricks in traditional dryers is always determined by the bricks that dry slowest. This is directly related to the position of the green bricks in the dryer (see Fig. 2). For example, bricks on the outside dry much more slowly than those closer to the fan inside. Thus, as the drying air from the middle passage flows further, its flow velocity gradually decreases, its temperature drops, it becomes more saturated, and its moisture‑absorbing capacity declines. Even when the bricks on the inside of the dryer can be removed, the drying system must continue to operate until those poorly positioned bricks are also dry – even though most of the bricks in the dryer did not need the extended drying process.

Therefore, the first step in rapid drying is to balance the air flow conditions across the entire cross‑section of the direct air circulation. In this way, the drying process of each green brick is independent of its position in the dryer – i.e., it should be the same at any time during drying.

Increasing Air Velocity

As long as suitable climatic conditions exist, air velocity has a very specific influence on the drying rate. An increase in air velocity speeds up the drying rate accordingly. Low velocities produce a uniform laminar flow – an example of a relatively uniform flow in nature is a quietly flowing large river. Increasing the velocity makes the flow more turbulent. An analogy in nature is a mountain stream rushing through a gorge during snowmelt.

The implication of turbulence in drying is that there is a stationary air layer on the surface of the green body, the so‑called boundary layer. This layer hinders drying and becomes thinner during the drying process (see Fig. 3). Fast‑moving air particles absorb water particles much more easily than slower‑moving ones.

After increasing the air velocity, the drying rate rapidly accelerates, and the moisture content of the gas increases by more than 5%. Of course, at higher air velocities, the primary condition to be observed is that the continuous flow state of the gas must be uniform in order to achieve satisfactory results. That is, the green bodies over the entire cross‑section must be exposed to the airflow, and the air velocity must be the same. This is easier said than done, and under the conditions at the time, this experimental study took more than a year.

Ratio of Cross‑Flow to Through‑Flow

Due to recent new regulations on thermal insulation, the void volume has become larger. This means that the inner walls of the holes are becoming increasingly thinner. Such thin hole walls have their advantages, and they pose few problems in drying, because apart from the different wall thickness, only a slight difference arises – the amount of moisture generated is different (see Fig. 4). If the difference in moisture content is very small, the difference in shrinkage is also small, and the risk of drying cracks appears very low.

On the other hand, because surface area plays a decisive role in convective drying, these high‑voidage hollow products have a large internal surface area – about three times the external surface area. Thus, for a given moisture content, the larger the surface area, the easier the drying.

The ratio of cross‑flow to through‑flow for perforated bricks must satisfy a certain proportion. This proportion depends on the height A of the gap between the top surface of the lower green body and the bottom surface of the upper drying board, and the width B of the gap between two adjacent bricks (as shown in Fig. 6). However, due to the limitations of fan arrangement in convective dryers and tunnel dryers, the appropriate flow ratio cannot always be achieved – or fully achieved. Successful rapid drying requires three conditions: the flow conditions across the entire cross‑section should be the same (same air velocity for cross‑flow and through‑flow); the air velocity should not fall below a certain value; and the rates of cross‑flow and through‑flow for each brick should be consistent.

Experience in the Field of Rapid Drying

Over the past two years, Novokaram has conducted continuous research at its factory, gaining important information in the field of aerodynamic modelling. In addition, theoretically based conclusions have been confirmed. Based on these fundamental principles, a large‑scale demonstration plant for the rapid drying of clay hollow products was built, and subsequently, three different brickworks were equipped with the rapid drying method. The relevant drying characteristic parameters are listed as examples below.

Rapid Drying and Drying Cracks

It is often erroneously claimed that drying cracks are a direct consequence of shrinkage. As briefly described in this article, drying cracks are not the direct result of shrinkage. Drying cracks are caused by differential shrinkage within the green body, which in turn depends on different moisture distributions. In rapid drying, the green bodies should be uniformly exposed to the air so that the moisture differences generated are very slight. With this background in mind, it is easy to see why rapid drying does not necessarily attribute drying cracks to high drying sensitivity.

A comparison of bricks dried by traditional methods with those dried very rapidly confirmed the above conclusion. At the same quality level, the quality of rapidly dried bricks is higher.

Residual Moisture and Drying Time

Our initial target was a drying time ≤ 2 hours. The residual moisture after drying depends on the drying cycle, product specifications, and raw materials, generally ranging from 0.5% to 2.5%. It should be noted that extending the drying process by just a few minutes in rapid drying can significantly reduce the residual moisture. In the same plant, the traditional drying time was about 32–48 hours, with a residual moisture content of 1.0%–2.5%. There was no difference in fired quality between rapidly dried products and those dried by traditional methods.

Optimum Drying Curve

As with conventional convective drying, a drying curve adapted to the raw material must be found for rapid drying. The rapid drying curve can be conceived as a compressed version of the traditional drying curve – in this view, rapid drying is merely “fast‑motion” conventional drying.

Process of Rapid Drying

If the green bodies have been steam‑treated, it is important – as in ordinary drying – to transfer them from the extruder to the drying chamber in the shortest possible time. The higher the green body temperature, the more intense the early drying – i.e., the green bodies already begin drying at a higher temperature, without a gradual heating phase in the drying chamber, thus avoiding wasting valuable time.

The ratio of cross‑flow to through‑flow during drying has already been emphasised. This ratio critically depends on the accuracy of the setting of the bricks in the workshop. However, higher setting accuracy would mean higher investment. Therefore, a special experimental study was conducted to see whether a reasonably accurate setting pattern could still be accepted. The test results showed that the tolerances of conventional setting and unloading devices are acceptable for the whole process and have no adverse effect on the ratio of cross‑flow to through‑flow. This means that, under current technological conditions, conventional setting devices can be used.

Advantages of Rapid Drying

When introducing a new design, every entrepreneur immediately asks about its advantages – and convective rapid drying is no exception.

What are the advantages of convective rapid drying compared to traditional convective drying? The most fundamental and important aspect is quality. Especially the reduction in time requirements is a priority. In many different clay brick factories, rapid drying tests were carried out without setting complex drying curves, and the fired bricks obtained good or very good quality. When compared with bricks produced by traditional methods, the bricks selected for rapid drying were at least as good as those dried by traditional methods – even without necessarily knowing whether the drying curve was adapted to the available raw materials.

Another very important advantage is the reduced investment required for building a rapid drying plant. As shown in Fig. 7, the entire rapid drying chamber occupies considerably less space in the production building. This means that for the same output, the production floor area is reduced, or alternatively, output is increased – achieving a saving effect. In addition, the rapid drying process is simplified, transport routes are shortened, and the necessary conveying equipment is simplified, which also contributes to lower capital investment.

Finally, a few technical data should be mentioned. In traditional drying chambers, the heat consumption is around 3200–3600 kJ/kg H₂O. The electricity consumption depends on the water‑removal characteristics of the raw material itself. According to records from different brickworks, the electricity consumption is 5–11 kWh per tonne of fired material.

Example of Rapid Drying Production

Fig. 7 is a schematic layout of the production process in a brickworks where the traditional drying process has been replaced by a rapid drying system.

Similarly, in other brickworks, the green bricks are cut, placed on drying boards, then transferred to drying cars. The drying boards are placed on the drying cars, which then pass through the rapid drying chamber. The drying car stays for a certain time in each compartment – i.e., different conditions prevail in each stage. The air velocity varies in each compartment, but the drying principle of cross‑flow and through‑flow and the travel speed of the drying car are the same in all compartments. When the drying car enters the circulation, half of the drying process has already been completed. During the pre‑drying stage, as it moves from one compartment to the next, the temperature continuously increases while the relative humidity continuously decreases. The rapid drying chamber described here has 10 sections in each direction. If one superficially thinks of a traditional tunnel dryer, one would naturally consider this as having 20 drying zones.

After the drying car leaves the rapid drying chamber, the subsequent steps proceed as usual. The dried bricks are removed from the drying car, placed on tunnel kiln cars, waiting to be loaded into the kiln, and then fired. The unloading and packaging of the tunnel kiln are not affected by the rapid drying.

Source of the Article

This article was written by the author Ralf König, Diploma Engineer (D‑Krumbach), and originally published in the International Brick and Tile Industry (ZI‑China Issue), 1996–1998, Chinese combined edition, Bauverlag GmbH. It is posted here for learning and reference purposes only. The copyright belongs to the original author and the original publisher.

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