5.2 Plant layout, building design and equipment selection
5.3 Materials handling
5.4 Log breakdown, peeling and particle reduction
5.6 Hot pressing
5.7 Primary finishing
Historically, the mechanical wood products industry developed from what were makeshift operations to the present day sophisticated plants currently being installed around the world. However, in some instances the old influence of complacency with respect to energy matters is still very much in evidence in both mill design and operation.
It is rather unfortunate that the ad-hoc addition of equipment in earlier years was rarely carried out with the advice from either equipment manufactures or plant designers; in any event, energy costs at the time were not considered of significance to warrant the degree of caution as is the case nowadays. It is the legacy of past actions that today's plant designers, managers and operators must rectify if energy is to be conserved to any significant degree.
Indeed, it is only recently that planners, design engineers, consultants and equipment manufacturers have come to recognize the important role that reduced energy costs can play in the ultimate profitability of a manufacturing unit. Gone are the days of a labour intensive industry based on comparatively cheap raw materials and energy, with a fairly low capital requirement.
Spurred by the increasing costs of raw material and labour, mills have been prompted to improve their efficiency and productivity and to meet the dictates of market competition by better product quality. This has been achieved by increased expenditure on capital equipment and by adopting a higher degree of mechanization, although, at times, at the expense of an overall increase in energy consumption. Equipment designers and manufacturers for their part, have met the demand for new and improved process technology by considerable developments in plant design, though not all in the direction of energy efficiency. The basic concept, that energy can be saved, but at cost, prevails.
The extent to which energy saving equipment is incorporated in a design largely depends on the size of the mill, the suitability and cost of the equipment, as well as the potential energy savings in terms of return on investment.. However, regardless of the ultimate benefits of incorporating such plant, it is not uncommon for a supplier to omit them from his supply package when tendering against competition. Also investors may wish to disregard the long-term savings that such equipment or energy design features would have on the overall plant operation, in favour of short-term savings, or by virtue of limited finance, or an acute shortage of foreign currency.
Yet, it is the basics that count when designing with energy in mind a good clean layout, ease of production flows, avoidance of bottlenecks, provision of sufficient buffer zones so as to minimize plant idle time, correctly sized motors, process plant that is well balanced and suited to the raw materials and product range, and close attention paid to the design and selection of service plant.
Due to the wide variations in design and energy needs of each mill within the mechanical forest industries, comment with respect to the relationship of energy and design shall be of a general nature and is restricted to the major energy consuming centres of sawntimber, plywood and particleboard production. No attempt has been made in this document to comment on the latest design technology nor to predict future trends in energy based design.
5.2.1 Wood-yard and plant layout
5.2.3 Equipment selection
The fast and efficient handling of raw materials in the receiving yard and the proper routing of the finished product is an important part of a manufacturing concept. Storage yards should be located as near to the mill as possible and correctly laid out so as to enable inventories to be easily handled. Sufficient storage space should be planned for so as to cater for the stock needed to allow the plant to run continuously with an uninterrupted supply of raw materials, with allowances for seasonal bad weather hampering supplies, etc. It has been cited that approximately 12.5 percent of delay time in most sawmills can be attributed to their running out of logs.
The wood-yard should also be designed so as to facilitate rapid and convenient separation of species and space permitting, should be stored as to the size and grade. Such segregation plays a significant role, not only in the uniformity of the final products, but in the energy consumed during handling and processing.
Although the site generally influences the type of mill building to be constructed, it is far better to consider economy of operation firstly rather than the initial cost and to have excess manufacturing area rather than too little. In designing the mill layout each item of equipment needs to be individually considered, allowing sufficient space for its operation, maintenance, inspection and rapid access to trouble spots.
Depending on the climatic conditions of the country in which a mill is located, the buildings themselves may well give rise to energy losses, particularly those in which heating or air conditioning is required for worker comfort. It is not uncommon to find in buildings more than 10 to 15 years old minimal insulation, poorly designed despatch doors and ill-fitting windows, all of which contribute to lost energy.
Despatch doors should be self-closing and designed to suit the largest vehicles and loads most likely to use them. Roofing, being a major cause of heat loss, should be adequately insulated or fitted with false ceilings and the provision of down-draught fans would assist in the redistribution of accumulated hot air to the working levels below.
Mill layout and equipment selection go hand in hand, in that the needs of the equipment with respect to the processes, flow direction, ease of operation and maintenance, etc. must be blended into the overall plan, with the ultimate objective of maximizing the productivity of each machine and minimizing handling.
On selecting equipment with energy in mind, due consideration should be given to:
(a) its relationship to other equipment in the process and to be balanced accordingly;
(b) its energy demands per unit of production must be acceptable;
(c) to be properly sized to meet production demands as well as having adequate capacity to cater for surge requirements, yet not to operate well below its rated capacity;
(d) to be robust in construction, reliable and permit ease of maintenance so as to ensure a minimum of downtime;
(e) to incorporate a correctly designed waste disposal system so as to avoid accumulation of residues which would otherwise be detrimental to both equipment and the overall plant operation.
It has been estimated that on average between 10-20 percent (94) of operating time within the mechanical wood products industry is comprised of idle time, mainly attributed to mechanical troubles, interrupted flow of raw materials, congestion, etc. which could well have been avoided at the planning and design stage. It is axiomatic that a well-balanced mill that is running with a maximum throughput, with a minimum of idling time and waste is utilizing its energy to the maximum effect and it is towards this end that the designer must select and size his materials handling and process equipment.
5.3.1 Log handling, washing and barking
5.3.2 Raw materials for particleboard manufacture
5.3.3 Conveying systems
5.3.4 Importance of buffer stocks
5.3.5 Mobile equipment
As may be seen from figures four, six and eight, materials handling in the mechanical wood processing industry consumes a large proportion of its overall energy requirements and in some sawmilling operation may account for up to 50 percent of their non-thermal energy consumption. The wide range of raw materials used naturally leads to an equally diverse variety of materials handling equipment from front-end loaders, chain link, belt, screw and pneumatic conveyors, etc.
Hence, it is essential that care and attention be paid to the systems' design and equipment selection in order to effect energy savings.
The degree of mechanization of raw materials handling and the production process obviously has a direct influence on the plants' energy consumption and in countries where manual labour is both readily available and relatively inexpensive, consideration should be given to labour intensive operations so as to minimize both capital expenditure and high energy costs.
Not only should log handling equipment be designed to handle the wide variations in lengths and diameters likely to be delivered to the mill, but also to be robustly constructed in order to withstand the considerable demands of wear and tear and the inevitable surges in log supply. Additionally the plant must be flexible to allow the operator to select logs of specific size or species according to product demand. In fact it is the ability to be able to presort and supply logs to uniform diameter and species that leads to increased production and reduced energy consumption during the sawing and veneer peeling operations, by enabling the operator to select the most appropriate saws and peeling angles.
As mentioned previously, log storage should be located as near to the infeed decks as possible in order to maintain an even supply of logs for processing. Provision should also be made for buffer stocks of at least 30 minutes operating capacity, thereby ensuring the smooth operation of downsteam equipment in the event of a breakdown of handling equipment or snarl-ups causing a log shortage.
Due to the introduction of contaminants brought into the system with the wood supply, the provision of barkers, shower pipes, stone traps and magnetic separators into the design of the wood handling system should be viewed as a safeguard against wear on cutting surfaces and the possible damage to the process plant itself.
Generally, Cambial and Ring barkers are favoured in the industry and are considerably more energy efficient than hydraulic barkers Whichever barking system is chosen sufficient capacity must be incorporated to cater for surges and adequate provision made for the collection and removal of the bark, either for disposal or for fuel.
The diversity of raw materials used for particleboard manufacture from logs, sawmill off-cuts, veneer trimmings to planer shavings demand an equally large assortment of handling equipment. However, it is essential that whatever handling system be adopted, that it permits the maximum degree of presorting. Energy consumption will increase dramatically and product quality adversely affected if there are no provisions for the control of uniformity of size, species, moisture content, flow, etc., to the reduction equipment and particle dryers.
It is equally important that the raw material stocks be adequately protected from the elements, especially in the case of the dry, smaller sized materials such as sander dust, planer shavings and veneer waste; damp material not only increases power consumption in their conveying and screening but has a marked effect on the quantity of fuel needed to dry it.
Conveying accounts for a major portion of the electrical energy consumed in the sawmilling industry and other wood-processing plants, for which reason the correct design and selection of conveying equipment is essential if a mill's energy consumption is to be kept to a minimum. Conveying lengths are to be kept as short as possible, bends to a minimum and both the handling plant and motors to be correctly sized - to undersize conveying plant would result in production bottlenecks and the inability to handle surges, yet to oversize would result in wasted power. The provision of control devices and automated transmissionn systems do lead to energy savings and a reduction in handling systems running idle.
However, it is the choice of the conveying system which largely determines the overall power demand, and in spite of considerable advances in the energy saving designs of present-day conveying plant, the fundamental differences remain largely unaltered.
- Pneumatic conveyors consume up to 10-20 times the power required by mechanical conveyors, due to the large volumes of air needed to transport a disproportionately small amount of materials, added to which the need for dust extracting imposes additional demands on energy. Therefore, unless conveying materials over distances in excess of 300 metres, when the capital and operating costs become more economically attractive (37), or in need of flexibility of design, mechanical conveyors are to be considered the better alternative.
- Mechanical conveyors, though less expensive to operate compared to pneumatic conveyors, can prove to be high in capital cost and their use' relative to manpower, should be examined carefully when considering handling sawdust and other such residues. During recent years the energy efficiency of belt conveyors has markedly improved, due largely to the advances in belting materials incorporating woven polyester fabrics which render them virtually stretchless, with better tracking characteristics and reduced tension requirements.
- Screw conveyors, being low in power needs and maintenance, are well suited to conveying chips, particles and waste over small distances whereas chain conveyors, designed for impact loads and for use in handling bulky loads such as logs and wood waste, are best kept to distances of less than 100 metres and operated at speeds below 30 metres per minute in order to avoid excessive wear and power needs. Roller conveyors, by virtue of being gravity assisted when located on a sloping frame, have minimal energy requirements and are ideal for log decks and for handling slabs, edgings, etc.
In order to maximize the operation of all processing plant it is considered essential that buffer storage areas be a feature of plant design and located ahead of major items of equipment, which, combined with surge areas on the downstream side, would allow a smooth and continuous flow of raw material in the event of a minor plant breakdown or operational problems, which would otherwise leave plant idling through want of material and needlessly consuming power.
Statistically, from three to five litres of fuel are consumed by mobile equipment for every cubic metre of finished product in the mechanical wood-based industry and consumption can be expected to double in the event of handling partial loads, inadequate maintenance and when left idle. Hence, the merits of using mobile transport inside and outside the mill, compared to other means of conveyance should be thoroughly investigated.
Fuel savings may be achieved by the careful selection of the plant most suited to the materials to be handled and the duties to be performed, as well as using diesel rather than petrol engines. Yet, in spite of the advances in the design and construction of engines, transmission systems and the like, with reported energy savings of up to 40 - 50 percent compared to traditional power units, fuel consumption is still very much determined by the quality of service, driver skills and the layout and distances of the routes used.
5.4.2 Veneer peeling
5.4.3 Peeler-log conditioning
5.4.4 Particle reduction
It has been considered appropriate, for the purposes of this document to examine the influences of design on the energy involved in the processes of sawing, veneer peeling and particle reduction as a group.
All three processes are high in power demand and involve the reduction of wood to a size and form suitable for further processing. In each case the efficiency of the operation, product quality and energy consumption per unit of product are largely determined by the characterists of the raw materials i.e. size, species, hardness, moisture content, cleanliness etc., as well as the equipment and the skill of the operators employed.
Considerable progress has been made in recent years in the materials, design and construction of saws, which, combined with improved teeth geometry and wear resistance has enabled the use of higher speeds, extended blade life and sharpness which has led to a reduction in waste and energy consumption.
By incorporating flexibility of log feed to the headrigs in the design of the sawmill operation, together with the provision of a secondary primary saw, the sawyer is now able to saw according to log size and density. Additionally, the combined use of scanners, automatic saw setting, motor load meters, etc. has also resulted in better sawing control, recovery and energy efficiency. However, operator skill, good saw maintenance and the efficient removal of sawdust and debris still remain major factors in determining the power needs of the sawing operation.
Sawing power requirements are largely dependent on several interrelated factors, namely: (62)
- wood species, density, hardness, moisture content, knots, log face width;
- saw type, speed, saw diameter, style, set, number of teeth, plate thickness;
- feed speed, bite, kerf, width, gullet capacity, depth of cut, sharpness of teeth;
- operator skill, level of maintenance and cleanliness of overall operation;
all of which must be carefully taken into account at the design selection stage. Should any one of the above-mentioned fall outside acceptable standards, power consumption would be adversely affected.
It is in the design, selection and maintenance of the lathe knife, according to the characteristics of the logs to be peeled, that primarily influence the energy consumed during the peeling operation. The material composition, hardness and grinding bevel of the knife are a function of the predominant wood species, density, moisture content, grade and degree of conditioning. These factors, together with blade thickness determine the bevel angle, which may range between 17 - 23° (5).
The overall efficiency of veneer production may be improved by mechanical lathe charging devices, with geometric chuck centering, hydraulic clamping systems, telescopic spindles, et. all of which contribute towards a faster changing of peeler blocks and lathe operation.
Not only does a high moisture content in the peleer log facilitate peeling and produce better quality veneer, but there are marked savings in the heat consumed during conditioning and power during the peeling operation. Therefore, in hot dry climates mill designers should incorporate facilities to either allow for a fast turn round in log supply or for the logs to be kept wet by way of storage in freshwater ponds or tanks, or by spraying the log piles with sprinklers.
Conditioning of the logs by controlled exposure to a moist and heated environment, so as to obtain a uniform moisture profile and to plasticise and soften the wood, considerably facilitate peeling, improves veneer quality and recovery and reduces power consumption. Additionally, as a result of the higher quality veneer, minimal degrade, uniform sheet thickness and moisture profile further energy savings are gained through better controlled drying, reduced pressing times and surface finishing needs.
The energy consumed in both heat and log handling will be determined by the type of conditioning system adopted, the operating temperature relative to that outside and the size and density of the logs. Hot water vats, although ten percent slower (5) in conditioning time than steam conditioning, do have the advantage of a more even heat distribution as well as the hot water transporting the logs toward the exit. Heat may be supplied in several ways - steam, dryer and press condensate, or even boiler blow down (32) by way of heat exchangers, coils located alongside the walls or bottom of the vat or directly through spurge pipes.
Drive-in conditioning chambers, being above ground, tend to be cheaper to build, and condition batches of logs in a steam heated environment. However, by virtue of their batch operation, higher heat energy is required compared to hot water vats and the chambers need to be well insulated and fitted with heat retaining curtains in order to minimize heat losses. Conditioning tunnels operate on a continuous basis whereby the logs move through heat graduated zones, separated by baffles, so that the zone nearest the exit is the coolest and thereby minimises unnecessary heat losses. (5)
Energy consumed in particle reduction is directly related to the characteristics of the raw material, the equipment employed and the degree of reduction, particle geometry and size required. Energy consumption is also very much influenced by knife cutting angles, disc configuration, speed of rotation and the like, added to which operator skills and the level of maintenance play an essential role. As in the case of all other process plant, it is by selecting the equipment most suited to the raw material and finished product that the most efficient power to product ratio is obtained.
Irrespective of the type of reduction equipment to be used, the ability to control the wide variations in material sizes, species and moisture contents by way of designing flexibility into the materials handling systems, will be compensated by reductions in energy demand. Improved performance and power consumption may also be achieved by way of variable speed belts, screw conveyors, metering rolls and surge bins so as to regulate the flow of raw material to the equipment. It is equally important that stone traps and metal detectors be provided so as to avoid undue wear and damage to knives and refiner plates.
Chippers need to be selected according to the predominant types of wood supplied and whether they be delivered in the form of logs, slabs, trim or peeler blocks, all of which influence power requirements. Conveying systems must be designed to allow for a uniform and controlled flow of wood to be fed endwise into the chipper so as to avoid the risk of blockage in the chipper throat and to keep energy consumption within acceptable limits.
Other important factors in achieving energy efficiency are the use of correct disc speeds, number of knives and their ability to retain their sharpness. Although the use of blowing vanes fitted to the chipper disc may be seen as a way of utilising the disc energy to transport the chips, they do in fact absorb extra power and therefore necessitate the use of larger motors. It is far better to ensure that the discharge chutes are angled in the same direction as the chips' trajectory (36) and that less energy demanding bucket elevators are employed, than resort to the blowing vanes as a means of conveyance.
Rather than using synchronous motors to drive the chippers, power saving may be gained by using induction wood rotor motors with a belt drive, whereby the flywheel provides the release of energy needed to chip through logs larger than usual, and up to 15 percent slow-down (87) in designed disc speed may be tolerated before plugging occurs. However, if sawlogs, or tree lengths are the normal form of wood supply envisaged then the use of synchronous motors would be preferable.
A departure from the conventional disc chippers has been the use of drum and spiral chippers with lower knife wear and power consumption (63).
Impact mills, being of rugged construction and with minimal maintenance demands are well established as a means of reducing off-cuts, oversized chips, veneer trim, etc. to particles. Power consumption is relatively low but is influenced by the degree of reduction required, the settings of the hammers and size of perforations.
Flaker energy requirements, whether of the ring, disc or drum type, are dependent largely on raw material flow, reduction required and the speed and sharpness of the cutting knives.
Disc refiners may be of the single or double-rotating disc type, operating either at atmospheric or raised pressures and temperatures, with provision for pre-steaming, gravity or screw feeding. The choice of refining system, disc plate size and pattern, according to the raw materials and product required, are major determinants in the refiners' power demand.
Excessive power consumption and the risk of motor overload may be avoided by incorporating metering equipment in the design of the approach flow system, so as to provide a uniform flow of feed stock to the refiners.
5.5.1 Kiln drying of lumber
5.5.2 Air drying and pre-drying
5.5.3 Veneer drying
5.5.4 Particle drying
5.5.5 Heat recovery
By refering to figures five, seven and nine it may be seen that thermal energy is by far the greatest user of energy in the sawmilling, plywood and particle-board manufacturing processes, accounting for approximately 80-90 percent of the industries' energy consumption. Yet it is only recently that steps have been taken in making kilns, dryers and hot presses more energy efficient.
Although the principles involved in the removal of moisture from wood are fundamentally the same, each drying system shall be examined separately as to the way in which their design influences energy demand.
Kiln drying is a high energy consuming process, accounting for approximately 70-90 percent of the sawmill's total energy needs, and if insufficient attention is given to the kiln's design and operation the drying process will lead to a substantial waste of energy. Kiln drying takes its energy from two main sources - electrical energy to drive the circulation fans and heat for the drying operation.
Kiln design. To accomodate the volume and provide sufficient energy to dry peak loads during adverse climatic conditions, kiln capacity needs to be carefully considered at the design stage; - an overloaded kiln demands substantially more energy and creates a production bottleneck for the rest of the sawmill.
Of the two main categories of kiln, batch kilns allow an operator a greater flexibility of control over the charges, especially when the kiln is made up of several chambers. However, progressive kilns are generally cheaper to install, do not need to be loaded and unloaded and consume less electrical energy and some 10-35 percent less thermal energy than batch kilns (26).
Kiln location. Heat losses may be minimized and transport costs reduced if the kilns are grouped together and sheltered from prevailing winds, as well as being located as near to the sawmill as possible, yet downwind so as to avoid the effects of the corrosive exhaust on the process plant.
Construction. The energy aspect of kiln design requires that the structure be both weather tight and air tight and constructed of materials that are resistant to moisture, corrosion, decay, insect attack and fire. In view of the fact that between 10-20 percent (15) of total heat consumption is normally lost through the kiln structure it is imperative that the quality and thickness of insulation used be such that heat losses are kept to a minimum. Although insulated aluminium chambers are ideal for kiln construction, their cost, compared to using wood, may be considered prohibitive in some mills.
Due to the corrosive nature of the exhaust, circulating fans, ventilation equipment, motors and fired heaters need to be corrosion resistant if the kilns are to be operated efficiently with the optimum use of drying energy.
Kiln heating may either be by indirect means, transmitted by steam, hot water or thermic oil or by direct firing, whereby the combustion gases or hot air from a source exterior to the kiln are directed into the kiln. Although direct firing has been proven to give greater heat efficiency compared to the indirect method, the need to vent the combustion gases does constitute a heat loss.
The fuels most commonly used are oil, gas or sawmill waste. In the event that sawmill residues are used and deemed to have little resale value, it is well to consider at the design stage the adoption of high temperature drying, which would greatly improve the overall energy efficiency of the kiln by way of increasing the kiln's potential capacity and reducing the amount of air needed to be circulated and as a consequence lower the demand for electricity.
Air circulation is essential to the drying process as it transmits the heat energy to the timber surfaces and carries the evaporated moisture outside through the vents. Air velocities of between 1.8 - 3.0 metres (11) per second are considered normal for conventional kilns, however, the lowest velocity that will permit the effective drying of the charge should be designed for, as power consumption increases by 30 percent relative to increase in air velocity. (26)
The provision of baffles in the design of the kiln ensures that the maximum amount of freshly heated air supply reaches all areas of the charge, without passing directly to the exhaust zone. Care must be taken to correctly locate the exhaust zone and for the dampers to be designed so as to avoid uncontrolled venting and loss of heat.
The use of micro-electronic frequency changers to provide electronic speed variation of the fans, direct drives to reduce transmission losses, reversible or 180° adjustable fans with aerofoil type blades, all contribute towards power savings.
Given the right climatic conditions and available yard space the air drying of lumber, down to 25-30 percent moisture content prior to kiln drying, can result in energy savings of up to 30 percent (26). Also, consideration should be given to pre-drying, whereby drying to 30 percent moisture content at temperatures of some 40-45 percent (84) in shed-type pre-dryers, fitted with heaters with either natural or forced air circulation, can lead to greatly reduced kiln drying costs.
Veneer drying is comparable in many aspects to timber drying and accounts for some 60 percent of the total energy consumed in the plywood manufacturing process and is therefore an area where considerable savings may be made at the design stage.
In the case of mechanical dryers the veneer sheet is supported on mesh belts or driven rollers and passes through a series of zones in which air velocity, temperature and humidity may be individually adjusted to achieve maximum drying and energy efficiency. Obviously the dryer must be sized so that it is kept as full as possible to ensure optimum thermal efficiency.
Drying temperatures of between 90-160°C are considered normal, but the use of higher drying temperatures are giving rise to a reduction in drying time and increased capacity. To cater for the higher range of temperatures, systems are now being designed for the use of thermic oil in indirect-heated dryers, whereas in the case of direct heating the high temperatures may be obtained merely by reducing the amount of fresh air make-up used to bring the firing temperatures down to that which its required.
Additional savings can result from kiln designs that permit a high initial heat at the green zone and then graduated towards lower temperatures prior to the veneer's exit. Temperature gradients as high as 300-170°C have been used successfully (36).
Air circulation in the older design of drying chamber is achieved by using centrifugal fans to either direct the air flow parallel to the sheet surface or across the sheet width. In the case of cross-flow ventilation the dryer may be divided into several zones so as to allow for individual control of both temperature and air velocity, thereby substantially improving drying and energy efficiency. Whereas longitudinal air flow systems normally allow only one or two separate zones, compared to the former with six to ten.
In comparison, impinging jet dryers operate on the principle of forcing hot air at high speeds to impinge upon the veneer surface. The considerable velocity of air involved and the localised turbulence generated, causes the boundary layer of the moist air to be dispersed (46), which would normally inhibit the efficient transfer of heat, and therefore gives rise to greater drying efficiency and economy.
Process controls. Mills should avail themselves of the process control systems that are currently available and being constantly upgraded. Whether semi-automated controls be selected to allow for a certain degree of operator control, or fully automated systems, the flexibility of dryer operation would greatly improve together with considerable potential savings in both heat and power consumption.
Heating systems, fresh air make-up, exhaust dampers, recirculating air fan speeds and settings, all may be automatically controlled in accordance with the conditions of the dryer and the extent to which moisture is to be removed from the lumber or veneer. In the case of veneer dryers, automatic moisture detectors help to optimize the feed speed of the veneer sheets all of which leads to better drying efficiency relative to energy input. However, in view of the expense involved, the advantages and overall savings in energy and production costs need thorough analysis before their installation is considered, assuming of course that the availability of funds and skilled maintenance personnel are not limiting factors.
The need to dry particleboard furnish to levels of three-eight percent moisture content, as required for use with liquid resins, necessitates the use of heat energy which represents some 60-65 percent of the total plant's process energy requirement. The drying process is continuous, in which the particles are normally dried in suspension by gases derived from the combustion of oil, gas or residues.
Three categories of dryer are in use today: the flash, tube and drum dryers. Although the tube and single and three-pass rotating drum dryers are currently the most commonly used, the use of flash drying and the shift from the three-pass to the two-pass dryer is leading towards improved energy efficiency in the drying process.
The two-pass dryer originated in design from the single and three-pass dryers and comprises of an inner horizontal flash tube in which the particle moisture is evaporated at high temperatures and air velocities and then passes to the outer tube, of larger volume, where the remaining moisture is removed at lower temperatures and air velocities. This arrangement permits longer retention times and an overall gain in thermal efficiency compared to its predecessors. On the other hand, flash dryers, currently using dry fine particles, require lower drying temperatures.
Uniformity of furnish supply is essential to the efficiency of the drying operation, and must be a feature of process design. Fluctuations in particle size, moisture content and feed rate, if uncontrolled, will have an adverse effect on the dryer's performance, giving rise to either over or under-dried material, excess energy demand and an inferior quality end-product.
As indicated previously, pre-sorting at the wood handling stage, the provision of variable-speed conveying, surge bins, as well as moisture sensing and control equipment before and after the dryer will afford the operator a greater degree of control over material input and the drying process.
Dryer heating is mainly by direct firing of oil, has or finely ground residues to attain air temperatures up to 870°C. Although tube dryers are designed -for indirect heating with steam, hot water or oil, which eliminates the risk of fire, the heat transfer efficiency is considerably less compared to direct firing.
Although oil and gas are the main sources of heat, due to their ease of use, the economic advantages of using sander dust and screening fines, etc., have been made possible by recent developments in burner design and is leading to conversion of old burner systems and the use of waste-based firing in new installations.
Air systems. The drying process necessitates the use of large volumes of air in both the heating and conveyance of the particles through the dryer, involving approximately 4 m3 of air for every kilogram of water evaporated at velocities in the order of 1200-1800 m per minute, (65) at a high cost in energy.
Continual advances in dryer design are aimed at reducing the volumes of air involved, and the variation of tube volumes within the dryers enables the velocities to be graduated so as to achieve better control of drying and energy economy. However, the most significant energy savings may be made by attention to the design and selection of the fans, cyclones and ductwork-conveying lengths to be kept short, bends to a minimum and fans and cyclones to be correctly sized for the air volumes involved. The advantages of using either a positive or negative air system needs to be studied, especially as the former demands less power.
Process controls. To safeguard against over or under drying, which would result in the unnecessary waste of heat energy or the risk of fire, drying temperatures must be closely controlled in accordance with the type and moisture content of the furnish i.e. green planer shavings are usually dried at 650-750°C (65), whereas inlet temperatures of between 260-310°C would he more applicable for fairly dry furnish. This may be readily achieved by present day process controls which have the accuracy, sensitivity and quick response needed to rapidly adjust to changes in the make-up of the particle furnish.
Temperature control, speed of dryer rotation, particle feed rate, dwell time and final moisture content may now be controlled and regulated according to the particle size and moisture content entering the dryer, thus, resulting in a better control of the drying process and a more efficient use of both dryer and fuel
Consideration should also be given to the viability of providing storage space or cover to allow for the natural drying of the raw materials in order to lower the dryer's energy needs - if economically justified and climatic conditions permit.
Heat recovery does afford the mechanical forest industries with a means to reduce its drying energy costs. However, the value of energy saved is largely dependent on the differential temperatures between exhaust and ambient temperatures, the efficiency of the recovery system and its capital and maintenance costs.
In the case of lumber drying the recovered heat would normally be used to heat the incoming fresh air supply to the kiln by way of a heat exchanger. Additionally, it may be used for pre-drying of lumber, or in the event of a multi-chamber batch kiln system, it could be recovered from one chamber using a high-temperature drying process to provide the greater part of the heat required by another chamber drying at lower temperatures.
The high temperatures involved in particle drying make heat recovery economically attractive in large-scale installation which are largely dependent on oil or gas as a fuel source. It is estimated that between 20-60 percent (14) of the thermal energy used in drying may be saved by the recirculation of the dryer's flue gases and that up to 40 percent (106) could be recycled. (Care being taken to avoid surface condensation and the risk of corrosion.) Likewise, boiler flue gases, conveyed by way of insulated ducting, may be used to indirectly heat the make-up air.
The principles behind hot pressing in the production of plywood and particleboard are similar, in that the application of pressure brings about contact between veneer/particle and resin, and heat accelerates the curing. It is imperative that at the design stage all other items of equipment be balanced and synchronised to the capacity of- the press so as to ensure its continuous operation.
The number of press openings normally range from 5-25 for plywood and 14-18 for particleboard manufacture. In each case heat is applied to the platens by way of hot water, steam or thermic oil. The use of thermic oil, although requiring more pumping energy to achieve the degree of circulation needed to attain high platen temperatures, has enabled presses to operate at up to 315°C and thereby resulting in reductions in the pressure and pressing times normally needed to effect curing.
As loading and unloading become more mechanized, and automated in some larger mills and opening and closing times shorter, the overall pressing cycle times are being lowered and heat losses reduced. Further improvements may be made to the press's heat efficiency by the application of insulation to the platens and ail heated surfaces.
Advances in the development of resins has also led to greatly reduced press times by as much as 50-60 percent, with considerable energy savings. However, in order to effectively evacuate the vapours given off by the resins during pressing, large volumes of air are required. The energy consumed during ventilation may be reduced by designing the ventilation systems with close-fitting enclosed hoods and by only handling the quantity of air strictly needed to effectively rid the working environment of the noxious vapours.
Pre-pressing of both the ply and particleboard mat at low pressures, by either platen-type or continuous pre-presses, helps in the consolidation of the laid-up veneer panels and particleboard mat. Of course, a pre-press is mandatory when a caulless system is used in particleboard production.
Pre-pressing does reduce the overall hot press cycle time by facilitating loading, and as a direct result of the reduced ply and mat thickness, the width of the openings may be reduced. By the application of heat to the upper platen of the pre-press further reductions may be made to the hot-press cycle time, all of which leads to lower thermal energy requirements in pressing, though a somewhat higher power demand.
Present day primary finishing consists of semi-automatic process lines incorporating trimming, sophisticated wide-belt sanders, computerised cut-to-size sawing systems combined with dust extraction and waste collection; it all represents a high energy consuming centre in the manufacture of mechanical wood products.
Also, advances in the design of wide-belt sanders, although now producing a better and faster surface finish, have done so at the expense of power - with consumption now increased to one kilowatt per centimetre of belt width. This may be partly off-set by the advances in the design of the upstream equipment, such as veneer lathes, dryers, formers and hot presses, all affording a greater degree of control on product quality and, as a consequence, resulting in less dimensional variation and degrade and therefore less finishing needs.
Semi-automated systems are now being adopted to maximize the utilization of all primary finishing plant, which, combined with strategically located surge areas or holding stations, ensures the smooth flow the materials through the finishing department with minimal interruptions due to minor breakdowns and operational problems.
Trimming saws with thinner blades, increased tension, higher speeds and with wear resistant tungsten carbide cutting edgers to be able to handle the latest resin binders, are becoming more energy efficient. The advances in automatic computerised cut-to-size sawing systems have led to considerably less trim and, as a result, less energy is absorbed in recycling large quantities of waste within the system.
Yet, the need for better environmental control and the removal of sander dust, trim, etc. has meant that the power needed to remove waste has increased over the years. So as to achieve optimum energy efficiency in waste removal, attention needs to be paid to the correct selection of exhaust fans and dust collection equipment and to the design of ductwork and cyclones, with provision for skirting and purpose designed housing to localize collection.
5.8.1 Boiler plant
5.8.2 Steam and condensate system
5.8.3 Electrical power
5.8.5 Compressed air
The importance of services to a mill, whether they be steam, air, water or lighting, should never be underestimated, particularly as they may have a large influence on the energy bill. Yet it is an unfortunate fact that, regardless of the expense and effort spent on the design and operation of process plant, it can be squandered by disregard to the basics of good design and lax attention paid to the operation and maintenance of service plant.
It is a sobering thought that in the light of the current high fuel costs a boiler may consume the equivalent of its capital cost in fuel in a period of only two to three months, yet in most instances boilers are selected with overall capital cost savings in mind, rather than the long term gains in operating costs.
One cannot over-emphasise the importance of properly sizing a boiler so as to supply good quality dry steam at the correct pressure, yet cope with peak loads and maintain optimum performance in spite of fluctuations in demand. To oversize a boiler would result in its operating at a fraction of its rating, with disproportionate radiation losses and an overall drop in efficiency. Conversely, efficiency would also be affected if it must cope with peak loads well above its maximum continuous rating.
Ideally two smaller boilers are preferable to one large partially used unit, which would allow flexibility of operation to cater for load variations, as may occur due to changes in seasons or production capacity demands. However, cost invariably prohibits such a luxury in the smaller mills.
Regardless of the type of boiler or fuel used a certain amount of excess air is permissible in order that complete combustion is achieved, otherwise the unburnt fuel would constitute an energy loss. But, should the excess air surpass the exceptable level, the burners would become overloaded and heat would be absorbed and carried out of the stack at high temperatures, thus representing wasted energy which could otherwise have been used to produce steam or hot water. Hence, vigilance in both boiler operation and regular checking of the combustion efficiency is necessary.
Whereas the larger and more modern boilers are provided with automatic combustion controls, together with flue gas analyzers, the smaller and older designs may not be equipped with such facilities, whereupon the skill of the operator and the use of portable combustion testing kits (being readily available) are needed to fine tune adjustments so as to maintain the correct air to fuel ratio. However, not all fuels readily lend themselves to ease of handling and good mixing with air, for which reason it is important that their flow to the combustion chamber be well regulated and uniform so as to attain optimum control efficiency.
Regrettably, it is not uncommon to find mills with either inadequate or maladjusted boiler controls and a general lack in the level of attention and maintenance that needs to be afforded to such instruments and controls. The provision of reliable and well designed controls to regulate combustion air and meter both fuel and steam are to be regarded as essential adjuncts to a boiler.
Adequate training should be given to boiler operators and the process controls regularly serviced and maintained by suitably qualified personnel. As the boiler size and rate of fuel consumption increases, so does the need or frequent efficiency checks and improved metering of fuels, steam and hot water.
Burners, fans, dampers, brickwork and insulation, all should be subjected to regular maintenance to maximize boiler operating efficiency. Those mills which do not have staff specifically trained in boiler servicing may need to contract maintenance engineers to undertake the work.
All heat transfering surfaces are to be kept free of accumulated deposits of scale and soot, which would otherwise greatly hamper the transmission of heat and lower efficiency. Hence, both water treatment plant and soot blowing equipment should be of good design and kept in good working order.
It must be appreciated that the boiler is only part of the overall heat raising and distribution system, equal importance must also be given to the steam and condensate system.
Piping should be correctly sized and laid out, with distances and bends kept to a minimum. Adequate provision is to be made for air trapping and venting, with the prompt removal and recovery of condensate from the steam system. When one considers that, in a steam system operating at seven bar, condensate contains 25 percent of the heat required to generate steam, its loss would represent a drop in steam utilization efficiency to 75 percent or less (93). Hence, it becomes obvious that condensate collection is a necessity, which also helps to reduce the costs involved in heating and treating boiler feedwater.
The capacity of the boiler feed tank must permit the maximum use of returned condensate. Not only should it hold sufficient water to allow for at least one hour's steam production at maximum rating, but is to be well lagged so as to keep heat losses to a minimum - for every 6°C increase in feed water temperature, approximately one percent of fuel is saved (93).
All steam and condensate pipework and other exposed heated surfaces are to be correctly lagged with an economic thickness of insulation. Although this may be regarded as an expensive operation initially, it should be considered a worthwhile long-term investment with pay-back periods being usually less than two years.
It is axiomatic that the judicious use of electricity will bring about the greatest savings in a mill's electrical energy. Under normal conditions motors are generally over 90 percent efficient in converting electrical energy to mechanical energy, and transformers are 95 percent efficient in converting distribution voltages to working voltages. Hence, it is by minimizing or eliminating all unnecessary use of electricity and ensuring that driven equipment and process plant are well-designed, operated and maintained that any real reductions in power consumption can be made.
In fact it is at the selection stage, that particular attention needs to be paid to the electrical drive system in order to achieve maximum performance and economy of operation. Motors are to be matched to the duties required and work environment, and correctly sized so that they run as near to full load as possible, yet allowing for spare-capacity to cater for short-term overloads. Energy efficiency ratings and power factors are to be carefully reviewed and, although energy efficient motors tend to be expensive initially, they are proving to be long-term investments.
When purchasing motors it is recommended that those with service factors of between 1.00 and 1.15 be chosen, being more rugged and better able to cope with demand surges. Generally, synchronous motors are found to be more economical than induction motors with capacitors, however, this depends on speed and voltage. In the case of larger size motors the use of higher voltages should be considered, resulting in a reduction in both size and heat losses.
Energy losses may also be reduced by selecting starting gear which is designed to give the highest possible power factor and optimum performance, and for transformers to be located as near as possible to the users of heavy loads. In certain instances the oversizing of electrical conductors can also prove to be economically advantageous by reducing energy losses.
All mills, in their own interest, should aim towards improving the overall power factor of their electrical plant and that all effort be made to keep power demand peaks as low as possible; by so doing, electricity charges can be reduced and penalties imposed by the utility companies may be avoided.
On an operational standpoint power consumption can be monitored with the aid of a recording demand meter, and, after the major user items of equipment have been identified, steps may be taken to effectively lower the demand peaks. This may be achieved by reducing consumption, staggering start-ups and re-scheduling plant operation so as to spread the load more evenly. A thorough examination should also be undertaken of all motors and driven plant to ascertain that they are all correctly matched and that speeds are not in excess of that required to maintain efficient production.
All motors and process plant should be operated as near to capacity as possible, with idlling time kept to the barest minimum. As poor housekeeping and plant maintenance adversely affects power consumption, standards must be kept high. But, above all, electrical plant must be switched off when not in use.
Low power factors may also be improved by the installation of power factor correction equipment, using synchronous motors wherever applicable and by fitting capacitors to induction motors. Should capacitors be already installed, it may prove beneficial to have them checked, as they tend to deteriorate with age.
During the past five years developments in the design of lighting have been such that the new energy efficient lamps consume some 50 percent less watts, without loss in lighting levels, compared to their predecessors.
However, lighting systems are generally inefficient users of electricity, with incandescent lamps only converting ten percent into light, 20 percent for fluorescent, and even the more efficient metal halide and sodium lamps are only 24 and 33 percent efficient respectively. As electricity accounts for some 70 percent of total lighting costs over a five year period, it is essential that efficiency of design and operation is sought when selecting lighting systems, rather than initial cost.
Lighting levels must conform to established standards appropriate to the work activities and plant layout. Although it is wasteful to over illuminate, the failure to meet minimum standards will not only affect worker performance, but constitute a safety hazard. As recommended lighting levels range from 30 lumens per square metre for a stockyard, to 1500 lumens per square metre for fine benchwork, the importance of adopting the most appropriate and efficient lighting sources and fittings may be readily appreciated.
All lamps deteriorate with age and as a consequence their efficiency dramatically drops. Therefore, with energy savings in mind, capital costs should be equated against long term operational costs when choosing lighting systems. This may be illustrated by the fact that although a flourescent tube is more expensive than an incandescent lamp, it is three times more energy efficient with 12 times the life expectancy.
Of the countless designs of light fittings, reflectors and diffusers on the market, only those which will maximize a lamp's optical and energy efficiency should be purchased. The fittings should be so positioned so as to direct the light towards the area of activity and facilitate cleaning in order that their efficiency does not become marred by dirt. As fluorescent lamps are subject to a decrease in light output and efficiency when surrounding temperatures increase above 25°C, care must be taken in their location. (a 65 watt bare fluorescent lamp, with a 100 percent relative light output at 25°C, will only emit 80 percent light at 45°C and 75 percent at 55°C). (80)
When visiting wood-yards in daylight, or inside buildings where all work activity has long ceased, it is not uncommon to find all the lights left on. To overcome such human frailties, it is recommended that light switches be well labelled and conveniently located and time switches or programmed controllers, with limited manual over-ride facilities, be incorporated in the lighting system.
Yet one must not overlook the greatest potential for saving energy, and that is to make the most use of natural light; maximum use should be made of windows and skylights so as to allow as much daylight as possible to fall on the work areas. Additionally, all walls and ceilings should be painted in light colours to reflect both natural and artificial light.
Compressed air is one of the most expensive of all the services to provide and the compressor one of the single largest users of electricity. Yet it is not unusual to find in the majority of mills air leaks, poor plant selection, improperly designed piping layouts, the use of excessive line pressures, etc.
The reciprocating and rotary-vane compressors are the types most used in the forest industries. When a compressor runs at 100 percent load factors the power costs are approximately 82 percent of its total running costs, yet, when the load factor drops to 50 percent the power costs are hardly affected, at 77 percent of running costs (13). Hence, regardless of whatever compressor is chosen it must be so sized as to operate at near to full load at all times.
This may be achieved by selecting several smaller units to operate steadily at or near to the rated maximum capacity as possible, with just one unit used to handle load variations. The provision of relays to automatically shut-down a compressor if it runs for more than say five to ten minutes in an unloaded condition, will allow the other compressors to operate at a continuous full load condition.
If one considers that 15 percent more power is needed to produce air at seven bar rather than at 5.5 bar, it stands to reason that the selection of air operated equipment which function at lower pressures will prove to be cost effective. Additionally, consideration should be given to the use of smaller compressors located near to the point of end use, rather than the installation of a centralized air system, so as to reduce line losses.
Attention must be paid to the design of the air intake to the compressor so that it is located in a cool, dust and moisture free location, and kept as short and straight as possible. For every 4°C increase in air inlet temperature a compressor's energy consumption will rise by approximately one percent and for every 25 millibar of pressure lost at the inlet, two percent overall performance shall be lost. (13) It is equally important that piping layouts be correctly dimensioned and well laid out with the minimum of bends, reductions and other such restrictions to air flow, so as to keep pressure drops as low as possible.
Recent developments in heat recovery systems have let to the economical recovery of the heat generated by the air compressors for use elsewhere in the mill, with a claimed recovery of up to 80 percent of the inlet power. However, the economics of such systems very much depend on the size of the compressor plant and the amount of heat actually generated.