WO1999001709A1

WO1999001709A1 - Dryer and method for drying

Info

Publication number: WO1999001709A1
Application number: PCT/BG1997/000006
Authority: WO
Inventors: Andrey Lukov Benev
Original assignee: Todorov, Todor Tenchov
Priority date: 1997-07-04
Filing date: 1997-07-04
Publication date: 1999-01-14
Also published as: AU3331197A

Abstract

The invention relates to dryer and method of drying materials making use of the principle for regeneration of the waste heat for drying. Dryer is comprised of zones (1) formed in the drying space through each of which passes a line for drying medium (5) and a line for heat carrier (13) both being formed as separate closed circuits connected to each other by an air cooler. Each zone (1) is provided with a heat exchanger (2) connected to the heat carrier line and each zone (1) is connected to the drying medium line (5).

Description

DRYER AND METHOD FOR DRYING

FIELD OF THE INVENTION

The invention relates to dryer and method for drying materials using the principle for regeneration of the waste heat for drying with a broad application everywhere when drying process is required, for example in ceramic, woodworking, textile, paper, food and other industries. BACKGROUND OF THE INVENTION

Many are the fields of technique where large quantities of material have to be dried before use or subsequent processing. The major part of dryers make use of a heated drying medium, as air for example, for drying materials. Generally this is joined up with consumption of large quantity of energy for heating the fresh drying medium and/or material. In order to utilise the heat accumulated in the exhausted drying medium, many dryers are developed where a portion of this heat is regenerated and supplied again in the drying process.

For example DE 3316418 provides a dryer including a line for the heat carrier, formed as a closed circuit and a line for the outgoing drying medium, which through a common heat-exchanger serves for the heat carrier heating. A method used in this dryer provides material to be dried is placed in separate zones of the drying space. Each zone is supplied with fresh drying medium, atmosphere air in the case. The outgoing drying medium through a common tubing is fed to a heat- exchanger where is cooled down, heating at the same time a heat carrier, which enters each zone in parallel and by another heat-exchangers mounted in the zones, heats the incoming fresh drying medium. Disadvantage of this method and dryer is that the quantity of so regenerated heat depends on the parameters and quantity of the incoming drying medium and is generally about 10 % of the total waste heat. Moreover, the outgoing drying medium is discharged in the atmosphere, that may cause environmental pollution.

Another dryer and method for drying, DE 33 21 178, are also known, where material to be dried is placed in separate zones of the drying space, but the drying medium, air in the case, circulates from one to other zone consecutively by means of a circulating air-ejector fan. In each zone the drying medium is intermediately heated up by the heat carrier, supplied through a common line in parallel for all the zones. The temperature difference between the heat carrier, and the drying medium is positive in each zone. This method and dryer are characterized in that the material is passed consecutively through the zones, and the drying medium is in a counter-flow to the material. The heat carrier is supplied from a central heating station and is not included in an independent circuit for the dryer. A separate recirculation circuit for a secondary heat carrier is also provided, heated by the outgoing drying medium and used for heating up the fresh drying medium. Disadvantage of this method and dryer is that the transferred heat by the outgoing drying medium is used for heating of fresh atmosphere air that is with lower heating capacity and may utilize only 10-12 % of the total waste heat, as shown in Keey R.B., Introduction to Industrial Drying Operations, Pergamon Press, 1978. The process is not harmless from the ecological point of view. In addition, if not provided with a device for controlling the heat carrier supply to each zone, the use of parallel heat carrier supply to all the zones only will result in equal heat dependent parameters of the drying medium, which decreases the possibility for picking up higher moisture quantity. SUMMARY OF THE INVENTION

It is an object of the invention to provide a dryer and method for drying one may achieve lower energy consumption in the drying process by the utilization of a considerable portion of the heat for evaporation of the material moisture, which is of particular effectiveness with the higher drying capacities.

Another object of the invention is to provide a dryer where the moisture absorption for equal time when compared to other dryers is higher, that results in considerable shortening of the drying process as well as in realisation of energy saving.

Yet another object of the present invention is to provide improvements of the process of drying from the ecological point of view.

Another object of the present invention of no lesser importance is to provide a dryer where by the absorption of higher moisture quantity ensures an easier to achieve "soft mode" of drying, where the material tensions are eliminated, preventing this way any crack formation. This is of particular importance for the ceramic and woodworking industries.

The next object of this invention is to provide reduction of the power and material consumption of the fan dryer, as a result of the higher moisture absorption. Another object of the invention is to provide reusing the condensed moisture with process requiring water, as is the case with ceramic industry.

Another object of the present invention is to provide dryers, which are simple, inexpensive, and which may be constructed with available drying capacities like tunnel dryers or joined up chambers, situated arranged or in an arbitrary manner on the premises.

Another object of the present invention is to provide the dryer which may be automated and controlled. These and some other objects of the invention are attained in the dryer and method for drying materials.

The dryer is comprising the drying space with formed at least three zones. Through each zone passes a line for the drying medium and a line for the heat carrier, realised in separate closed circuits joined up to each other by a air-cooler. Each line has an inlet and outlet manifolds. Each zone is provided with a heat- exchanger. The heat exchangers are joined up in series downstream to at least one of inlet manifold of the heat carrier line. A heater is included in the heat carrier line downstream the air-cooler to heat the heat carrier before its initial feed to no matter which heat exchanger. The drying medium line is provided with a circulating fan and at least one recirculating fan. The zones are joined up consecutively downstream to the inlet manifold of the drying medium line. This line is adapted for initial feed of drying medium to the zone from where the exhausted heat carrier is taken away. Regeneration of the most part of exhausted heat without environment pollution is provided by the closed circuits of the drying medium and the heat carrier. The step- wise increase of the temperature in the zones ensures the higher moisture absorption per unit time.

According to one embodiment, at least three zones are formed like separate drying chambers. The drying medium line comprises two inlet manifolds and one outlet manifold. The separate chambers are joined up in series downstream to the first inlet manifold of the drying medium line by consumption control devices. Each chamber is joined up in parallel to the second inlet manifold of the drying medium line by an incoming control device. Each chamber is joined up by outgoing control device to the outlet manifold. The chambers are joined up to each other by at least one by-pass and each by-pass connects at least two non-neighbouring chambers by control device.

According to another embodiment of the invention the heat carrier line comprises two inlet manifolds and one outlet manifold. Each two neighbouring heat exchangers are connected in series downstream to the first inlet manifold of the heat carrier line by consumption control devices. Moreover each heat exchanger is joined up in parallel to the second inlet manifold of the heat carrier line by an incoming control device and to the outlet manifold by an outgoing control device. The heat exchangers are joined up to each other by at least one by-pass, and each by-pass connects at least two non-neighbouring heat exchangers by a control device. These embodiments make possible the control of the drying process to be effected with predetermined modes in each zone - switching off the heat exchanger and working without intermediate heating in the zone. So the dryer may be easily changed over for different mode of operation. According to one next embodiment, the dryer is provided with a heat pump and at least a part of heater for the heat carrier, or at least a part of pre-heater for the drying medium is a condenser of a heat pump, the evaporator of which is joined up in the circulation circuit for the heat carrier prior to the condenser, or in the circulation circuit for drying medium after the condenser. This contributes for the maximum regeneration of the waste heat for evaporation of material moisture.

According to the invention, the drying method includes placing the material in the drying space, through which circulates the drying medium, passing consecutively through the separate zones of the drying space, where the material moisture content is consecutively changed. The drying medium recirculates in each zone with intermediate heating. The exhausted drying medium is cooled down to the point of moisture condensation and the heat exchanged during the condensation is used for heating up of a heat carrier, which is fed consecutively in each zone of the drying space. The heat carrier warms up the drying medium in each zone where the temperature difference between the heat carrier and the drying medium is always positive. The heat carrier and the drying medium move in separate closed circuits in counterflow. The heat carrier is fed firstly in the zone with the lowest material moisture content and from where the exhausted drying medium is taken away. The drying medium is firstly fed in the zone with the highest moisture content of the material and from where the exhausted heat carrier is taken away.

The flow in a closed circuit, particularly of the drying medium, leads to a more efficient utilization of the heat for moisture evaporation from the material, as well as to improvement of the process from ecological point of view. Moreover, by means of heat carrier is provided increasing of the drying medium temperature in each zone when compared to that in a preceding zone. This, in its turn, leads to increased drying medium saturation point and absorption of more moisture. On the other hand, the saturation of the drying medium with more moisture provides more uniform moisture field in the material, thus reducing the material contracting force that eliminates the formation of undesirable cracks. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a connection diagram either for the heat carrier line, or the drying medium line for the dryer;

Fig. 2 shows another connection diagram either for the heat carrier line, or the drying medium line for the dryer; Fig. 3 shows a connection diagram where the heat exchanger inlet and outlet can be changed;

Fig. 4 a, b show a part of the dryer in which a heat pump is connected. DETAILED DESCRIPTION OF THE INVENTION

The drawings show examples of the connection diagram of the dryer according to the invention. The dryer is comprising a drying space, divided into zones 1, designated by Roman numerals, six pcs. Each zone 1 is provided with recirculating fan (not shown) and heat-exchanger 2 (Fig. 2). An inlet and outlet of the drying space are joined up respectively to an inlet 3 and outlet 4 for drying medium, as said inlet 3 and outlet 4 are included in a drying medium line 5, dividing it into at least two manifolds, inlet 5A and outlet 5C respectively. A fan 6 and an air- cooler 7 are joined up in series to said line 5 after the outlet 4. A pre-heater 8 is included after the air-cooler 7. An inlet 9 for fresh drying medium and an outlet 10 for exhaust drying medium may be included in said line 5, just before the pre-heater 8. The heat-exchangers 2 (Fig.) are joined up in series to one other. An outlet 11 and an inlet 12 for the heat carrier in a heat carrier line 13 are connected respectively to the inlet and outlet of the drying space, dividing said line 13 into at least two manifolds, inlet 13A and outlet 13C respectively. A circulating pump 14 and said air- cooler 7, are also joined up in series in line 13, just after outlet 11. A heater 15 is mounted after said air-cooler 7 and is joined up to the inlet 12, to which the heat exchanger 2 located in the last zone 1 is connected. An expansion vessel (not shown) may be joined up in parallel between the air-cooler 7 and heater 15 of the heat carrier line 13.

According to the connection diagram of Fig. 1 and Fig. 2 some zones 1 may be eliminated from the drying process, thus making it controllable in respect to time and parameters. For the sake of clearness explanations are made for the heat carrier pipe line only, but it obvious becomes that the same connection diagrams may concern the drying medium pipe line too, with the references respective given in parentheses. According to Fig. 1 every two adjacent heat exchangers 2 (1) are joined up in series by consumption control device 16 for controlling heat carrier consumption. The heat carrier line 13 (5) comprises two inlet manifolds 13A (5A) and 13B (5B), and one outlet 13C (5C), all of them parallel, with a common outlet and inlet for the manifolds 13A (5A)and 13B (5B) of the heat carrier line 13 (5). The inlet manifold 13 A (5 A) comprises said heat exchangers 2 (1) and said consumption control devices 16. The heat exchangers 2 (1) are joined up in parallel to the inlet manifold 13B (5B) and to the outlet manifold 13C (5C) by the means of inlet 17 and outlet 18 control devices, mounted before and after each heat exchanger 2 (1) respectively downstream the heat carrier. Heat exchangers 2 (1) are joined up with at least one by-pass 19. Each by-pass 19 connects at least two non-neighbouring heat exchangers 2 (1) by means of other control devices 20. Fig. 1 shows six joined up heat exchangers 2 (1) by six by-pass 19. Each by-pass 19 connects five heat exchangers 2 (1) in this case. It is clear that one by-pass 19 may connect for example only the first and the last heat exchangers 2 (1). Fig. 2 may show another connection diagram of the line 13 for example. Here each by-pass 19 connects only two non- neighbouring heat exchangers 2 (1). By the above described connection diagrams thus can be reduced the duration of drying process by switching off no matter which of heat exchangers 2 and the respective zone 1 can be used, for example, for thermal-moisture treatment of the material, or for other purposes. Thereby if the dryer is a tunnel one, it would not be necessary to disturb the movement of the transport system in the dryer. The line of the drying medium 5 reasonably can be formed following the pattern of the heat carrier line 13. In this case all the zones 1 have to be separated by dense fixed partitions, or be formed as independent chamber dryers.

The above described embodiments make possible the control of the drying medium flow in each zone, as well as the elimination of its supply, controlling this way the drying process more completely. A combination with the above described embodiment of Fig. 1 and Fig. 2 for the heat carrier line 13 and the drying medium line 5 provides a group of chambers connected to each other in an arbitrary manner on the premises.

Fig. 3 shows an embodiment in which allows the inlet and outlet of each heat exchanger 2, or chamber 1 to change their positions. In such a case an additional duct 21 having at least one control flow device 22, joins up the inlet and outlet of each heat exchanger 2 or chamber 1. Each inlet 13A (5A) and outlet 13C (5C) manifolds and by-pass 19 are joined up to the additional duct 21, so that between their connections with duct 21 are disposed the flow control devices 22 are set up. The zones 1 can be joined up in an arbitrarily succession due to the presence of the additional duct 21.

In the embodiment shown in Fig. 4a the dryer includes a heat pump 23, whose condenser 24 is at least a part of pre-heater 8 for the drying medium and whose evaporator 25 is joined up in the circulation circuit for the heat carrier just before the air-cooler 7.

In the embodiment shown in Fig. 4B the condenser 24 of heat pump 23 is at least a part of heater 15 for the heat carrier.

The operation mode of the dryer is as follows: The material which has be dried is placed in zones 1, where is blown out with drying medium, air for example, that is moved transversely to each zone 1 by recirculating fans (not shown). Some of it passes in the next zone 1 by the action of fan 6, while the remaining part is recirculated in the preceding zone 1. In each turn the air transfers a portion of its heat to the material to be dried, absorbs moisture and cools down, after which is heated again by heat exchanger 2. Circulation of the drying medium in a closed circuit is achieved with the help of fan 6. After the air-cooler 7 the drying medium temperature is raised up by several degree in pre-heater 8, avoiding this way condensation of moisture in the tubing of line 5. The heat carrier, passing through

5 the joined up in series heat-exchangers 2, moves in counterflow to material and drying medium, its temperature being gradually decreased. This ensures relatively constant temperature difference between heat exchangers 2 and the material in each zone 1. In the course of its consecutive flow through the zones 1 the air is gradually heated up. It is moistened to, but thanks to the change in its temperature dependent

10 parameters, however it does not reach the saturation point so that in each subsequent zone 1 it can absorb more moisture. The air and the heat carrier are brought into contact again in air-cooler 7, as a result of which a process of heat exchange accompanied by moisture condensation take place and the latent evaporation heat is transferred from the drying medium to the heat carrier. Due to

15 non-reversible heat losses during the actual heat carrier transfer process, the initial temperature level of the heat carrier in air-cooler 7 can not be restored completely, and that is why the heat carrier is heated up additionally before entering heat exchanger 2 of the last zone 1 by the heater 15. Its circulation in a closed circuit is carried out by means of circulating pump 14. By controlling of devices 16, 17, 18 and

20 20 the different pipe lines for heat carrier may be set up in accordance with the dryer capacity and the requirements of the technological process in any case.

The method for drying may be illustrated by the following practical example: Wet oak elements have been dried in a dryer consisting of eight chambers. Consecutively on every two days it is charged with new material per one chamber.

25 Each chamber collects 10 m3 thick elements, located together with air clearance between them. Hot air passes through the chambers, in this case drying agent medium which circulates through two fans with flow rate of 10 000 m3 per hour. One part of the air is drawn off from each chamber and sent to other chamber through a separate fan with flow rate of 600 m3 per hour. The circulating air in the chamber is

30 heated in this case through a hot-air heat exchanger with ribbed pipes. Through each heat exchanger a heat carrier circulates, in this case hot water with temperature higher than the temperature of the air in the chamber and flow rate of 1.4 m3 per hour. The water also is sent from the chamber to other chamber in counterflow of the air. The elements have been dried for 12 days and the regime of drying is

35 changed on every two days. The continuity of the first and the last regime is at least 8 hours, during which a complete circulation is realized of the air in the correspondent chamber without drawing off part of the air. Consecutively grows the temperature of the dry thermometer in each charged chamber from 30 to 80 C and the relative humidity being 95 % in the first and the last regime, 80 % in the second, 70 % in the third and diminishing to 55 % in the fourth and the fifth regime. The drawn off air from the chamber with the highest regime of drying is sent to air-cooler where it is cooled through the water passing through all chambers. The air is sent after that again to the chamber for drying where a second regime of drying is already established. The cooling water is heated from the hot air and the condensed moisture and finally heated in a steam air-heater up to 92 C and put again in the dryer, firstly in the chamber with last regime and secondly in the chamber with regime next to the last etc. and is delivered from the chamber with first regime for new cycle of circulation.

In this case for example chamber No 3 is blocked for repairs while in chamber No 1 have been dried 10 m3 of coniferous planks, seasoned at free air. In chambers NoNo 2, 4,5,6,7 and 8 have been dried elements for parquet which total to 60 m3 or 5 m3 per day and the moisture delivered from them totals to 1800 kg per day. The coniferous planks have been dried for six days or 1.67 m3 per day and they produce moisture of 300 kg per day. As the process in chamber No 1 has shorter in half regime of drying, the chamber is situated every day to other regime or removing the material and on charging with new one, i.e. on every two days per one chamber. For example if the chambers at the beginning were charged consecutively as per numbers, their order of switching to other regime in direction of the air circulation can be as follows: first day: chambers NoNo 2-1-4-5-6-7-8; second day: 2-4-1-5-6-7-8; third day: 8-2-4-1-5-6-7; fourth day: 8-2-4-5-1-6-7; fifth day: 7-8-2-4-5-1-6; sixth day: 7-8-2-4-5-6-1; seventh day: 6-1-7-8-2-4-5 and etc.

The heat consumption for drying in this case is 1885 kJ/kg, evaporated moisture which is about two times smaller than in common dryer.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above article without departing from the scope of the invention, it is intended that all matter contained in the above description are shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention which, as a matter of language, might be said to fall there between.

Claims

1. Dryer, comprising zones formed in the drying space, through each of which passes a line for drying medium and a line for heat carrier, each line comprising at least one inlet and an outlet manifolds, and each zone provided with a heat exchanger joined up to the inlet of said heat carrier line, and said zones are joined up to said drying medium line, which line is provided with a circulating fan, characterizing by the fact that: a)said lines for drying medium (5) and heat carrier (13) both being formed as separate closed circuits joined up to each other by an air cooler (7), b) a heater (15) is joined up in series downstream the air cooler (7) in said heat carrier line (13) with, c) all of said heat exchangers (2) are joined up in series to said inlet manifold (13A) of said heat carrier line (13), d) each heat exchanger (2) in the zones (1) being separate for a given zone (1), or a part of a common heat exchanger (2) at least for two adjacent zones (1), and e) said drying medium line (5) is provided with at least one recirculating fan.

2. Dryer according to Claim 1, characterized by the fact that: a) each two adjacent heat exchangers (2) are joined up in series downstream the heat carrier by consumption control devices (16) for controlling the heat carrier consumption, forming a first inlet manifold (13A) of the heat carrier line (13), b) each heat exchanger (2) is joined up in parallel to a second inlet manifold (13B) of the heat carrier line (13) with an inlet control device (17), c) each heat exchanger (2) is joined up in parallel to the outlet manifold

(13C) of the heat carrier line (13) with an outlet control device (18), d) said heat exchangers (2) are joined up each other by at least one by-pass (19) for heat carrier, and e) each by-pass (19) is joined up by other control devices (20) to at least two heat exchangers (2).

3. Dryer according to Claims 1 and 2, characterized by the fact that: a) said zones (1) are at least three and all of the zones (1) are separated by dense dividing partitions, thereby forming separate chambers, b) each two adjacent zones (1) are joined up in series downstream the drying medium by consumption control devices (16) for controlling drying medium consumption, forming a first inlet manifold (5 A) of the drying medium line (5), c) each zone (1) is joined up in parallel to a second inlet manifold (5B) for the drying medium line (5) with an inlet control device (17'), d) each zone (1) is joined up in parallel to the outlet manifold (5C) for the drying medium line (5) by means of outlet control devices (18'), e) said zones (1) are joined up each other by at least one by-pass (19') for drying medium, and f) each by-pass (19') is joined up by other control devices (20') at least two zones (1).

4. Dryer according to Claims 1 to 3, characterized by the fact that at least a part of heater (15) for the heat carrier, or at least a part of pre-heater (8) for the drying medium is a condenser (24) of a heat pump (23), and the evaporator (25) of said heat pump (23) is joined up to the line for heat carrier (13) just before the air cooler (7).

5. Method for drying, where material is placed in zones of the drying space, through which circulates the drying medium passing consecutively through the separate zones, in which the material moisture content is changed also consecutively, and the drying medium is intermediary preheated, and the heat carrier for the drying medium preheating is supplied to each one of the zones, and the temperature difference between the heat carrier and the drying medium is positive in each zone, characterized by the fact that: a) the exhausted drying medium is cooled down to condensation of its moisture, and the heat transferred during condensation is used for preheating of the heat carrier; b) the drying medium and heat carrier are flowing in a counterflow in separate closed circuits, and the drying medium recirculates in each zone (1), and the heat carrier is fed consecutively in said zones (1) starting with the zone in which the material is with lowest moisture content and in which the exhausted drying medium is discharged from, and c) the heat carrier is heated up before its initial supply to a zone (1) over the exhausted drying medium temperature, and said drying medium is fed firstly in the zone (1) with the highest moisture content of the material and from which the exhausted heat carrier is discharged.