Báo cáo khoa học nông nghiệp " Improvement of operator skills and technology in small rural sawmills in Vietnam. " doc

Ministry of Agriculture & Rural Development Collaboration for Agriculture & Rural Development Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam. Kiln Drying in V ietnam by Gerry Harris, Peter Vinden and Philip Blackwell October 2009 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 2 of 79 INDEX INDEX 2 PART 1 - VIETNAM FACTORY VISI TS AND KILN ASSES SMENT 5 Introduction 5 Background 5 Summary of results . 6 Temperature Monitoring & Controls 9 Air Flow 12 Baffling 12 Racking Practises 17 Findings 18 Wood fired Boilers 19 APPENDIX - FACTORY VISIT REPORT S 21 Factory Visits A - CAXE Environmental & Thermal Engineering Co Ltd 21 Director Mr Bui Anh Viet 21 Factory Visits B - Hoang Thanh Company 23 Factory Visits C - Troung Tai Compa ny 28 Factory Visits D - CHANH HUNG DRYING UNIT 30 Factory Visits E - Nong Lam University 37 Factory Visits F - Thien Hung Company Technologies (THC) 42 THC Factory 43 THC Kilns 44 Factory Visits G - Forest Science Institute of Vietnam – Sth. Vietnam Co - Op 50 Factory Visits H - Truong Thanh Wood Processing 57 PART 2 POTENTIAL FOR SOLAR KILNS IN VIETNAM 65 Introduction 65 Backgroun d 65 The efficiency of solar drying 68 Climatic Data 70 Calculat ion Method for Fuel Savings 70 Insolation 71 Kiln types 73 Concl usions 78 REFERENCES 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 3 of 79 SUMMARY AND RECOMMENDATIONS This report is in two parts. Part 1 summarises a number of visits to furniture manufacturers and sawmills w here audits were undertaken of the operation of the kilns. The kilns varied in sophistication from being state of the art t echnology to low cost, low tech, locally built plant. However, without exception improvements could be mad e to the operation of each kiln that was audited. This report summarises where improvements can be made to each of the kilns visited. T his form s the basis of a “Wood drying improvement program” that needs to be implemented . This is an urgent need given that most of the kiln dried ma terial is destined for furniture manufacture. As a general comment most kilns in Vietnam operate as ovens. Whilst the primary objective of reducing the moisture content of timber to below fibre saturation point (28%moisture content ) is achieved, there is a high variability in final moisture content. This inevitably results in down - stream problems associated with the quality of joinery or furniture that is manufactured from the timber . Part 2 evaluates the potential for solar kiln drying in Vietnam. There i s great potential for solar kiln drying technology in Vietnam for furniture manufacturers and small sawmillers particularly in the HCM City region and Dak Lak Province, where air - drying is prevalent and where final drying of timber to equilibrium moisture content is a requirement of further processing. It is recommended that: 1. A “Wood drying improvement program” is established by the Department of Forestry comprising customised advice and generic training on site at each processing facility where drying is undertaken. Some nominal charge should be made for the provision of this service. It is antici pated that the service would take approximately 1week and could involve 2. Further research is needed to develop drying and processing methodologies specifically f or plantation grown acacias for use in furniture production in Vietnam. This could involve the training of a Masters research student jointly in Vietnam and Australia, with the objective of training the person for implementing the “Wood drying improvement program”. 3. The development of new d rying technology should focus on: Solar kilns using Australian technology in the first instance, to develop demonstration models for solar drying in Vietnam . This could be followed up by local manufacture. Comparative inv estigations are needed to determine the best locations for solar drying (using the regression analysis models identified in this report) throughout Vietnam. 4. A model should be developed for the creation of modern drying plants for clusters of small - scale sa wmills and/or clustering existing sawmills into larger Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 4 of 79 units to improve processing and drying techniques. Funding is needed to support the building of appropriate kilns. Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 5 of 79 PART 1 - VIETNAM FACTORY VISI TS AND KILN ASSESSME NT Introduction The CARD Team visite d a number of furniture factories located in the Central and South Vietnam, with kiln drying facilities. The facilities at these factories ranged from very basic, small capacity, in - house built kilns to very large capacity modern units. The Team also visit ed a number of kiln manufactures in order to gain an understanding of the expertise in Vietnam. All kilns assessed by the Team utilised wood waste as the main heat source, either as direct fire units, suppling heat to the kiln chamber via flue gasses duct ed through the kiln or as a fuel supply to boilers which inturn supply heat to the kiln via heat exchangers in the form of steam. The Team also inspected the drying facilities at Nong Lam University which had research and commercial drying facilities. Thes e facilities were originally established and designed by Prof. Cac in the 1990’s (see CAXE Section in appendix A for further description). This design appeared to be the basis of many of the kilns assessed by the Team throughout Vietnam. Background The pro cess of timber or ‘lumber’ drying, also know as ‘timber seasoning’ and is the process of removing water from the timber to a target moisture content (TMC) suited to the purpose of use. The moisture content (MC) of timber in service is usually in the range of 8 - 15%. In freshly felled green logs the wood cell walls and the inside the cell (i.e. lumen) are fully saturated with water. When drying starts the water inside the lumen is lost first, then the water from the cell wall starts to is evaporate (Bootle, 2005) . This process will continue (over time) until eventually it reaches a moisture content that is in equilibrium with the surrounding environment. The point at which there is only moisture left in the cell walls is referred to as the ‘fibre saturation point’ (fsp) Timber is dried mainly:  To reduce its weight for the purpose of transport,  To increase strength – dry wood is considerably stronger than green wood,  To produce a product that is s table and durable and  That can be painted / stained and glued and  Can be machined and finished to an acceptable standard (ATSM, 1997) . From a commerc ial perspective this needs to be done as cheaply and efficiently as possible and to achieve this goal the timber needs to dried as fast as possible without too much degrade (Wa lker, 2006) . The purpose of drying timber (from a commercial perspective) is to:  Dry the stack of timber as fast as possible – without excessive degrade Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 6 of 79  Dry to the desired MC and within set limits (e.g. ± 2% MC)  Produce timber that is relatively free of i nternal stress, e.g. relieve drying stresses by conditioning at the end of the schedule  Produce timber that is relatively free of degrade e.g. checks, collapse and stains (Walk er, 2006) . Therefore from a value adding point of view it is essential that the drying practice is a controlled and continually monitored process to ensure the highest value can be obtain from the timber being produced. Summary of results The Team found t hat there was a general lack of understanding of the basic principles of drying lumber by the workers and kiln operators. Kiln operators often used the rear of the kiln as a place of residence. Kiln condition appeared to be controlled on a set and forget b asis. In particular there appeared to be a lack of understanding, by the operators, as regards the relationship between dry bulb (DB) and wet bulb (WB) temperatures, airflow and timber racking and the overall effect that these parameters have on the condi tions inside the kiln and the resultant non - uniformity of drying of the lumber if they are not correctly monitored and controlled. More specifically it was often found that WB sensors were disconnected (possibly by the operators) from the kilns control sys tem (fig. 1). This meant that the kilns were solely controlled on DB temperature alone, in effect creating a ‘hot box’, with no humidity control. The operators appeared to have made this decision to control the kilns only by DB temperature, purely to simpl ify the process through lack of understanding of the actual relationship between DB and WB temperatures. It was reported at one facility that as they had dried using the same schedule (i.e. using DB and WB temperatures) that the operator knew from past exp erience when to change condition based on the DB temperature alone. Where sensors were correctly installed they were only used for manual control, that is, the control of kiln conditions relied basically on the operator monitoring the temperature (i.e. WB and/or DB) and making a ‘uneducated’ decision to open or close the appropriate valve or vent, which was set and forgot until next time he happened to check the reading. Humidity control was generally based on a random basis depending on the gut feeling of the operator. It was reported at one facility that the humidity control consisted of “turning the stop valve, mounted on the external wall, ‘ON’ until water ran out from under the door”. Only one kiln drying facility had fully automated controls ensuring 2 4 hour control of the drying conditions inside the kiln. Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 7 of 79 The use of baffling to restrict by - passing around stacks was nonexistent. Also it was observed on one occasion that boards were rotated 90° to the airflow in order to stack more material inside the k iln (fig. 4a) in order to fill up the remaining space inside the kiln which did not allow for a full length board. This had the effect of blocking any cross flow between the boards due to the placement of stickers at regular intervals. Further it was obser ved that kilns were generally stacked to ‘maximum capacity’ with little regard to leaving sufficient ‘plenum’ space between the edge of the stack and the side of the kiln wall (fig.4b). This has the effect of creating uneven airflow through the stack, from top to bottom. On some occasions the plenum space was filled up with surplus material that required redrying (fig. 4c). This practise only leads to reduced efficiency of the kiln. With respect to timber stacking it was found that there was a lack of use o f dedicated material for ‘stickers’. In general boards were racked out using the same material that was being dried, by placing the components or boards at right angles in a ‘crosshatched’ fashion (fig. 4d) to create the gap between boards in order to allo w airflow. This meant that often the gap was too wide, consequentially reducing the efficiency of the kiln. Figure 1. Disconnected wet bulb sensor. Figure 2. Typical kiln controls – DB sensors and fan switches Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 8 of 79 Figure 3. Analogue readout inserted t hrough wall. Figure 4a. Boards stacked at right angles to airflow. Figure 4b. Minimum plenum space. Figure 4c. Excess components stacked in plenum space. Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 9 of 79 Figure 4d. Typical example of ‘crosshatch’ stacking. Temperature Monitoring & Controls The drying conditions inside a kiln are, in general, monitored by the measurement of the dry bulb temperature (DBT) and wet bulb temperature (WBT). These parameters can in turn be used to determine the relative humidity (RH) (i.e. the amount of moisture in th e air). These three (3) parameters can be used to determine the equilibrium moisture content (EMC) or the dry effort of the environment inside the kiln. DBT and WBT are set according to predetermined conditions which are known as ‘schedules’ or recipes. Di fferent schedules are used for specific species and are determined through extensive research and consultation with industry. And example specific schedule maybe DBT = 60° and WBT = 55° giving and EMC = 12.5%. If the kiln is controlled only on DBT (i.e. no humidity control) may result in the EMC falling as low as 2 - 3%, generating an extremely harsh and dry environment which may lead to drying degrade in the product. The practise of drying lumber, monitoring only the DBT is not recommended as it questionable as to whether the practice will result in acceptable drying quality, especially for high valued products such as fro furniture. This practice can only lead to moisture content variations in the final product and contribute to drying stress and degrade. Th is is due to the potentially low EMC’s that may be maintained in the drying chamber, resulting from the lack of humidity. In high value product such as in furniture where components are require to be laminated into larger components, it is essential to hav e uniform moisture content, both within and between components. Any variation in MC (between pieces or boards) will result in uneven shrinkage, resulting in defects such as splitting or cracking in the finished product. Further if the boards are ripped or machined, the inherent drying stresses will result in unacceptable distortion in the finished product. The Team found that WB sensors were not used and they had been deliberately disconnected (fig. 1). This meant that the kilns were solely controlled on DB temperature alone, in effect creating a ‘hot box’, with no humidity control. The majority of sensors that were correctly installed were only of a ‘manual’ control type Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 10 of 79 comprising of and analogue readout with a stem that was inserted through the external w all (fig 3). This would have the effect of conducting heat away from the sensing element and through the opening in the wall, giving an incorrect reading. Kilns that did have sensors correctly installed were generally only used for ‘manual’ control. This meant that the control of the kiln conditions relied solely on the operator monitoring the temperature (i.e. WB or DB) and opening or closing the valve to the steam or water sprays in order to control DBT and/or WBT or opening or closing the vents to contr ol the humidity. Venting or dumping of humid air, was generally only performed via ‘manual’ control (fig 5), where the operator was required to open or close the vents according to visual monitoring of the WBT. This may led to an over exhausting of moist a ir and resulting in an overly dry environment in side the kiln, as the vents set then left until the operator retuned from other duties Humidity was generally controlled by turning on a stop cock to provide steam or water to the kiln. It was reported at one facility that the humidity control consisted of “turning the stop valve, mounted on the external wall, ‘ON’ until water ran out from under the door”. For the production of ‘high - value’ components with correct moisture content and minimal degrade, it is recommended that kilns should be, at a minimum, fitted with semi - automatic controls. Only one kiln drying facility was observed by the Team to have been fitted with fully automated controls. The utilisation of a fully automated control system ensures that kiln conditions were controlled on a continuous basis around the clock, regardless of the kiln operator being in attendance. This would ensure maximum output of lumber at correct moisture content with minimal degrade. As has already been reported WB senso rs tended to be disconnected. However where wet bulb system were fitted these installations were considered to be satisfactory, with appropriate water baths and sensors etc, utilised. However the Team observed on most installations, that excessively ‘thick ’ fabric was employed as the ‘wick’ material (fig 5a). This had the effect of reducing the evaporation and airflow over the sensor and inturn giving an incorrect WBT reading. The wet bulb wick should be made from thin smooth material that readily evaporate s moisture from its surface. Therefore it is recommended that a thinner type material be utilised that is able to cover the entire bulb of the sensor and be capable of maintaining a continuous flow of water. This will ensure that a correct reading of WBT i s returned. One option is to employ a ‘boot lace’ type material (fig 5b) which ‘snugly’ fits over the sensor bulb. The wet bulb sensor should also be located so that the air to be measured can freely flow over the wick and not placed in a corner, resulting in reduced air flow. The wick should also be kept clean and changed on a regular basis. [...]... site Kiln Drying in Vietnam Page 23 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure B3 Example of cross-hatch racking Figure B4 Example of cross-hatch stacking – note min plenum space Kiln Drying in Vietnam Page 24 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure... Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure D4 Kilns loaded by hand Figure D5 Incorrect alignment of boards Figure D6 Simple solution to bypassing using canter-levered layer of boards (see arrow) Kiln Drying in Vietnam Page 32 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure... Page 34 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure D12 Manual vent controls Figure D13 Vents in ceiling of kiln & fan compartment Figure D14 Heat exchanger & steam humidification outlet Kiln Drying in Vietnam Page 35 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam.. . Vietnam Page 26 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure B10 DB sensor mounted on internal wall & WB sensor disconnected Figure B11 Manual vent control Figure B12 Boiler and controls Kiln Drying in Vietnam Page 27 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Factory... Improvement of operator skills and technology in small rural sawmills in Vietnam Figure C4 Direct fire heat source vents Figure C5 Manually operated butterfly vents Kiln Drying in Vietnam Page 29 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Factory Visits D - CHANH HUNG DRYING UNIT The Chanh Hung Drying Unit has 9 kilns of the Prof Cai type... making the cost of fuelling kilns from wood waste very expensive Kiln Drying in Vietnam Page 19 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure 13 Wood waste used to fuel boiler Figure 14 Truck being loaded with wood waste - US$2/t Kiln Drying in Vietnam Page 20 of 79 Project Report 027/06VIE Improvement of operator skills and technology. .. This in turn can result in an increase in drying time of up to 50% Kiln Drying in Vietnam Page 12 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure 6 Typical kiln schematic showing baffles at ceiling and floor level It was considered that the Type I style kiln had an inherent air by-passing problem between the heating coils and the... system, bypassing the stack and flows back to the fans (fig 7a) Figure 7a Type I kiln showing bypassing Kiln Drying in Vietnam Page 13 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure 7b Use of hinged baffles to block air by-passing Figure 7c Simple solution to bypassing using extended layer of boards (see arrow) Kiln Drying in Vietnam.. . consideration be given to shortening the width of the stacks Kiln Drying in Vietnam Page 16 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure 8c Bypassing around stack – need for baffling Figure 8d Long pathway for airflow through stack Racking Practises The racking out of timber involves placing small strips of timber know as ‘stickers’... be maintained at a minimum of 400mm Figure D1 Kilns Kiln Drying in Vietnam Page 30 of 79 Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Vietnam Figure D2 Kiln door and overhead opening mechanism Figure D3a Kiln with side mounted fans and heat exchangers mounted at 45° Figure D3b Side mounted fans & close-up of fan Kiln Drying in Vietnam Page 31 of 79 . Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 5 of 79 PART 1 - VIETNAM FACTORY VISI TS AND KILN ASSESSME NT Introduction The. drying conditions inside the kiln. Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 7 of 79 The use of. clean and free from decay and staining fungi (ATSM, 1997) . Project Report 027/06VIE Improvement of operator skills and technology in small rural sawmills in Viet n am. Kiln Drying in Vietnam Page 18 of 79 Figure