SE537903C2 - Procedure for drying wood with warm air and a duct dryer - Google Patents

Abstract

SUMMARY The invention relates to a process for drying goods, wood, with hot air, where the drying is divided into at least a first and second sub-process (A, B), parallel or in series with each other, and the circulation streams (2) of the two sub-processes in the respective drying chambers are conditioned to substantially different water temperatures (TvA) and (TvB), respectively. For reduced energy input, the circulating air (2) in subprocess (A) is assigned a higher water temperature (TvA) than the circulating air in the second subprocess (B) and that enthalpy in the circulating air in the first subprocess (A) is recovered and transferred to the second subprocess (B) .

Description

The invention relates to a process for drying goods, wood, with hot air, wherein the drying is divided into at least a first and second sub-process (A, B), in parallel or in series with each other, and the circulating streams (2) of the two sub-processes in each drying chamber are conditioned to vasentlicien different water temperatures (TvA) and (TvB) respectively. For reduced energy input, the circulating air (2) in subprocess (A) is assigned a higher water temperature (TvA) than the circulating air in the second subprocess (B) and enthalpy in the circulating air in the first subprocess (A) is recovered and transferred to the second subprocess (B) . 9a / 9 45 ° C T (h) T (h) T ° C 70 I 9b r 6 - -n- ° C r- 77 -1E -IT 7 II_J1 IF! t11.II11.11 \ I SUBPROCESS A 100 (Zone I) E TE-11-17 HHH ,,, i1H ± 11I] 2 SUBPROCESS B 1 (Zone II) T, = ° C = 70 ° C Tt = ° C Tv The invention relates to a process for drying goods, wood, with hot air, where the drying is divided into at least one first and second sub-process (A, B), parallel or in series with each other, and the circulating streams of the two sub-processes ( 2) in each drying chamber is conditioned to substantially different water temperatures (TvA) and (TvB), respectively. For reduced energy input, the circulating air (2) in subprocess (A) is assigned a higher water temperature (TvA) than the circulating air in the second subprocess (B) and that enthalpy in the circulating air in the first subprocess (A) is recovered and transferred to the second subprocess (B) .

The present invention relates to a method for drying wood, in particular stacks collected in stacks, by rewinding with circulating air stream, the condition of which is assumed to be dry and wet temperature by the supply of water by means of water. to the circulating air stream and venting moisture therefrom. The invention also relates to a channel dryer operating with an externally closed drying channel through which wood in the form of batches of stacked wood at regular intervals, so-called traction intervals are advanced.

As heat-transferring and moisture-transporting drying medium, air is used which, with a certain temperature, humidity and flow rate, is circulated through the drying material with the aid of floats. The drying air is heated by means of an air heating device which comprises a heating coil. The drying air is passed through the goods so that moisture and water evaporate from the goods and are absorbed by the drying air. In order for the gradually dehumidified drying air circulating in the drying chamber to be able to absorb more water from the goods, it must be dehumidified, which is normally done by ventilation, whereby the air is sprayed out with cold relatively dry outdoor air - fresh air.

When drying wood, either batch dryers, so-called chamber dryers or progressive dryers, also called duct dryers or continuous walking dryers. In the chamber dryer, the drying takes place in batches in closed chambers. The dryer is filled with wood and the drying then continues until the entire batch is completely dried. In duct dryers or continuous traveling dryers, a stack of wood is loaded at a time and fed step by step through the dryer.

The drying climate is varied in zones along the drying channel. The drying climate is constantly varied and the drying process is adapted to the wood in the dryer, the desired poor moisture ratio and the desired final quality. The climate of the drying chamber is controlled by regulating a predetermined parameter such as dry and wet temperature. The dry temperature is controlled by regulating the emitted water from a heating coil, while the water temperature is controlled by regulating the degree of opening of the ventilation nozzle and any irrigation or steaming equipment. The temperature difference between a dry thermometer and a wet thermometer reflects the relative humidity, whereby a relatively small temperature difference means a relatively high humidity. In order for the wood to dry, it is necessary that the water that is bound in and around the cells is transported out of the wood. How fast this happens depends primarily on temperature and current psychrometer difference, ie. the difference between dry and water temperature, the moisture content of the wood and the air velocity.

It is possible that the energy consumption of a drying plant during drying can be reduced by dividing the drying plant into a number of sub-zones and, as far as possible, recovering the heat content from one zone to another zone which has an instantaneously greater heat demand.

Usually a heat exchanger is used in which the ventilation is carried out in such a way that leaving the air (with a higher temperature) from one zone may give off part of its enthalpy to the incoming supply air (with a lower temperature) to another zone. For efficient energy utilization, in the case of channel dryers, it is advisable to arrange the drying channel in two drying zones in the form of a preheating zone and a main drying zone. With the help of heat exchangers, the heat energy is transferred in the exhaust air, which is ventilated out from the main dryer, to the preheating zone in which the wood is preheated before it is fed into the main drying zone for controlled and controlled drying.

In another case it is possible to mix from the main dryer emitting exhaust air with fresh air from the surrounding atmosphere and use the resulting mixture as supply air in the preheating zone with the result that the preheating zone works with colder drying air on the main drying zone. The disadvantage of such a process with a preheating zone with a moderate drying shape for sawn troughs Or that the risk of drying cracks becomes great because the initial part of the drying process takes place at a relatively low temperature. At this stage of the drying, when larger drying stresses in the wood easily give rise to crack formation.

Another complication that can occur when drying wood Or aft s.k. thermotolerant molds due to relatively low temperatures in the preheating stage can grow on the wood and form spores during drying. The wiglet grows and gives a grass-black discoloration of the wood, which in any case meant that the wood had to be discarded. By initially maintaining a relatively high water temperature of about 50 ° C, which means that the dry temperature of the air will vary between 55 and 70 ° C, the living conditions of fungi and mold can be limited. A few hours at these high temperatures Or in this part sufficient to control delta.

Relatively high energy costs are expected in the future and the demand for drying processes with low energy consumption will therefore increase sharply. Furthermore, the requirements for the quality of the wood will also increase, whereby wood with surface defects in the form of cracks or discolorations due to mold cannot be accepted.

According to the first object of the present invention, it is therefore possible to provide a method for drying trawl with hot air which is possible in duct dryers to streamline the drying process and thereby achieve a reduced energy consumption. A second object of the invention is to provide a process for drying trawl with hot air which makes it possible to avoid wood with surface defects, discolorations or mold. A third object of the invention is to provide a duct dryer which can perform drying with low energy input and it is possible to avoid wood with surface defects, discolorations.

These objects of the invention are solved by a method of the kind set out in claim 1 and a duct dryer according to claim 9.

The basis of the invention is to let a dryer work in the opposite way to what has hitherto been the usual way of drying goods, which especially when it comes to wood meant that 2 in a series of successive process steps move from a relatively cold drying climate to a warm drying climate. . According to the principles of the invention, however, the conversion takes place, namely to move from a relatively warm drying climate to a cold drying climate during the process steps. The many benefits of drying goods, sarkilt timber, according to this principle will be set forth5 in the following.

An interesting application of the invention consists of a timber drying channel with longitudinal circulation which is divided into tv5 by a liftable gate delimited series-connected drying zones, where the subsequent second zone is regulated at a dry temperature below the water temperature in the first zone, and the heat access for the second subprocess is thanked enthalpy, in the form of heat energy from hot moist air from the circulating air in the first zone, is transferred and paralyzed to the second zone via a heating coil arranged in said zone. In addition to energy efficiency, such an arrangement has significant advantages which are explained in more detail below.

An application of the invention in a drying plant with tv5 series-connected processes with different water temperature levels provides - in addition to a substantially reduced heat consumption - an increasingly significant advantage when it comes to drying high-quality wood in a long-circulation dryer. In a long-distance circulation dryer, the condition of the drying air becomes highly dependent on the evaporation from the wood in the drying channel. Thanks to a high temperature in the first zone, cracks and mold can be avoided. Due to the fact that no or only a limited air exchange takes place in the first zone, a large part of the energy can be recovered, transferred and used in the subsequent second drying zone. Another advantage is that the heat from the first drying zone stored in the timber in the timber can be used in the subsequent second drying zone.

The invention will be described in more detail below with reference to the accompanying drawings, in which: Fig. 1 schematically shows a process according to the invention in a first embodiment in which the drying is divided into two sub-processes A, B and the use of a heat theater recovery system as heat between the sub-processes. via a heat-carrying liquid medium, and Fig. 2 schematically shows a process according to the invention in a second embodiment where the drying is divided into two sub-processes A, B and a heat recovery system in which a heat theater recovery system acts as heat between the sub-processes of the two sub-processes. .

In Fig. 1, the present invention is shown in a first embodiment as a schematic arrangement using a liquid medium to transfer heat between sub-processes.

The drying Or is divided into tv5 subprocesses A and B respectively, whereby drying in subprocess A takes place at relatively high water temperature TvA = 50 ° C while drying in subprocess B takes place at relatively low water temperature TvB = 20 ° C. According to the invention, the drying in the sub-process B 3 takes place at a dry temperature TtB which is below the water temperature TvA for drying in the sub-process A, i.e. With the aid of a heat theater extraction system, heat can be transferred between the two sub-processes A, B via a heat-carrying liquid medium, whereby it should be understood that enthalpy is mainly transferred from sub-process A to sub-process B. A condensation heat exchanger 7 arranged in the first sub-process A and sub-process B a heating coil 8. A closed line circuit 9 is arranged to circulate a heat-carrying medium between the condensate heat exchanger 7 in sub-process A and the heating coil 8 in sub-process B by means of a circulation pump (not shown). circulating drying air in subprocess A and B, respectively, whereby loop 9a absorbs heat from condensed water vapor in drying air in subprocess A and transmits via heat 9b this heat to the heating battery 8 in subprocess B. The heat consumption for subprocess B is thanked by the enthalpy obtained from from the sub-process A. Som called harovan, no or only a limited air exchange with the surrounding atmosphere is normally needed in sub-process A. The combination of these! Dada arrangements provide an extremely low heat consumption for the drying process as a whole and can be avoided thanks to the relatively high temperature in sub-process A . It should be understood that if necessary, the second drying zone for sub-process B can be equipped with an additional effect which can be constituted by a heating coil.

Fig. 2 shows the present invention in a second embodiment as a schematic arrangement as opposed to heat between sub-processes by switching the different air streams of the processes in a heat exchanger of, for example, counter-flow or cross-heat type. As described above, the drying is divided into two sub-processes A and B, respectively, whereby drying in sub-process A takes place at relatively high water temperature TvA = 50 ° C, while drying in sub-process B takes place at relatively low water temperature TvB = 20 ° C. According to the invention, the drying in the sub-process B takes place at a dry temperature TtB which is below the water temperature TvA for drying in the sub-process A, i.e. TtB When applying the invention in a long-circulation channel with series-connected zones for drying wood Or by means of a liftable port 100 the drying channel divided into two adjacent zones I and II which are shown indicated by dashed contour lines in Fig. 1. As described above for sub-process A, zone I is regulated at a high water temperature, e.g. 50 ° C, and zone II as described above for sub-process B at a low water temperature, e.g. 20 ° C. Zone ll Or challenge as a channel, where the circulating air flows through the wood in countercurrent to the timber's transport direction and cocurrent in the first zone I. It should be understood that the specified direction of the circulating streams can only be exemplary and in practice varied. The circulating air in zone I is heated in a conventional manner in heating batteries 5. The drying air in zone I is dehumidified by passing a condensing heat exchanger 7 placed in the circulating stream in zone I, where the condensate formed gives off heat to the heat-carrying liquid medium passing through the condensing heat exchanger. The heat emitted from zone I is transferred via a liquid medium to the heating coil 8 in zone II. Since the drying in sub-process B takes place at a dry temperature TtB which is below the water temperature TvA for drying in sub-process A, ie. It is to be understood that an embodiment of the invention may comprise a combination of the embodiments of the invention described herein with reference to Figs. 1 and 2. heat-carrying liquid medium, as well as a heat theater extraction system such as Over-for heat from sub-process A to sub-process B by alternating or mixing the different air streams of the sub-processes. Furthermore, it should be understood that the invention is applicable to any drying system which can be divided into at least two sub-processes A, B, connected in series with each other and heat transfer. The invention is not limited to what is described above and that shown in the drawings, but can be changed and modified in a number of different ways within the scope of the inventive concept stated in the appended claims. 6

Claims (9)

PATENT REQUIREMENTS A method of drying hot air trawl in a duct dryer, wherein the drying is divided into at least one first and second sub-processes (A, B) in separate chambers arranged in series with each other, wherein the wood is allowed to migrate from the first to the second sub-process and where the circulating air (2) of the two subprocesses in each drying chamber is conditioned to substantially different water temperatures (TvA) and (TvB), respectively, it may not be that the circulating air (2) in the first subprocess (A) is assigned a higher water temperature (TvA). the circulating air (2) in the second sub-process (B) and that heat in the circulating air (2) in the first sub-process (A) is recovered and transferred to the second sub-process (B) and that the second sub-process (B) is assigned a dry temperature (TtB) which is lower than the water temperature (TvA) in the first sub-process (A). Method according to claim 1, wherein heat in the circulating air (2) in the first sub-process (A) is recovered and transferred to the second sub-process (B) by heat-exchanging circulating air (2) in the sub-processes (A, B). . A method according to claim 1 or 2, wherein the circulating air (2) in the first sub-process (A) is dehumidified by cooling water vapor bound in the circulating air and causing it to condense in the drying chamber of the first sub-process (A). Method according to claim 3, wherein the drying chamber of the first sub-process (A) is equipped with a condensation heat exchanger (7) with which the circulating air (2) in the first sub-process (A) can be caused to condense out, the drying chamber of the second sub-process (B) is equipped with a heating coil (8) with which the circulating air (2) in the second sub-process (B) drying chamber can be heated, and that a heat-carrying medium is circulated between the condensate heat exchanger (7) in the first sub-process (A) and the heating coil (8) in the second sub-process ( B). A method according to claim 1, wherein heat in the circulating air (2) in the [Ada subprocesses (A, B) is recovered and transferred by mixing exhaust air drums from the two subprocesses (20) with each other and passing a heat exchanger common to the processes (21). ) for preheating the supply air (26) for the second sub-process (B) and after passing the heat exchanger (21) constitute supply air for the first sub-process (A). 7 A method according to claim 4, wherein heat is transferred between the two sub-processes (A, B) by using a heat exchanger (14, 21) of, for example, countercurrent or cross-heat type to exchange air currents comprising supply and exhaust air (25, 26; 16, 19) between the sub-processes. A method according to any one of claims 1 to 6, wherein a heat recovery system is used which on the one hand converts heat from the first sub-process (A) to the second sub-process (B) via a heat-carrying medium of, for example, a liquid in which the circulating air (2) in the first the sub-process (A) is dehumidified by cooling and circulating water vapor bound in the circulating air in the drying chamber of the sub-process (A), and a heat theater extraction system which converts heat from the first sub-process (A) to the second sub-process (B) by alternating the different sub-processes. air drums comprising supply and exhaust air (25, 26; 16, 19) between the sub-processes. A method according to claim 7, wherein enthalpy in the circulating air (2) in the two sub-processes (A, B) is recovered and transferred by mixing (20) exhaust air streams from the two sub-processes and passing a heat exchanger common to the processes (21) for preheating the supply air (26) for the second sub-process (B) and after passing the heat exchanger (21) to provide supply air for the first sub-process (A). Duct dryer comprising an outwardly closed drying channel through which wood in the form of batches (1) of stacked wood at regular intervals, so-called pulling interval, is fed so that a set of timber with palastat moist wood is fed into the channel at the same time as a set of timber with ready-dried wood is fed out of the channel, the sets of wood being coated and passed through drying air (2) circulating in the drying channel, marked by the channel dryer. 100) with which the drying channel, when the door is closed, is delimited in a first and a second drying zone (I, II) which each form a chamber which works with circulating drying air, the door (100) in the tip making wooden sets (1 ) is allowed to migrate from the first to the second drying zone, that habit chambers comprise equipment (5, 7, 8; 5, 14, 21) which is possible to control and regulate the drying climate p5 in such a way that each chamber forms a first and a second chamber, respectively. sub-process (A, B), and which the control and regulating equipment is arranged to assign the circulating air (2) in the first sub-process (A) a higher water temperature (TvA) than the circulating air in the second sub-process (B), on the one hand allowing heat in the circulating air (2) in the first sub-process (A) to be recovered and transferred to the second sub-process (B) and that the water temperature (TvA) in the first sub-process (A) is higher than the dry temperature (TtB) in the second sub-process (B) 9 9 7 ° C