Wood Drying 2 | uWoodcraft.com

Because a wide range of kiln types and operating equipment is available, a comprehensive consideration of the operating characteristics of the various systems is beyond the scope of this guide. However, it is possible to outline a number of basic procedures which are relevant to most kiln drying operations. On mechanical or technical aspects of kiln operation it is recommended that the kiln or kiln equipment manufacturers are consulted. Preliminary Procedures Any pre-start maintenance procedures recommended by the kiln manufacturer should be followed. The normal procedures associated with examining and piling the timber should be carried out and appropriate drying conditions selected. When loaded in the kiln the correct alignment of any baffles against the timber load should be checked and any gaps which might allow short circuiting of air should be sealed. Kiln monitoring equipment should be checked and calibrated if necessary and the kiln started up. Warming Up Generally no harm will result from fairly rapid warming of the load to the first stage temperature of the particular drying schedule which is recommended for the timber species being dried. During warming, the temperature of the wood will lag appreciably behind that of the air and there will be some risk of condensation on the wood. To minimize this it is recommended that a suitably low air relative humidity is maintained by keeping the wet bulb temperature about 5°C below the dry bulb reading. Maintaining Humidity Air conditions in conventional kilns are controlled by applying heat to raise the temperature and by adjusting ventilation and moisture spray equipment to control relative humidity. Traditionally these conditions were controlled manually and considerable expertise was required to keep kiln conditions in line with a particular drying schedule. Automatic Humidity Control In most modern kilns, automatic air control systems are now standard equipment, often providing a range of control functions. Semi-automatic operation (i.e. control of a set temperature/relative humidity condition) may be the most appropriate approach when drying a timber species for the first time or when drying a timber which is known to have unpredictable drying properties. Some kilns are equipped for fully automatic control although at present this approach is generally considered to have its most successful application in the drying of timbers which have predictable drying properties. Modern fully-automatic systems, incorporating microprocessors and remote sensing equipment measuring weight or moisture content of the load can control the complete drying operation from warming up right through to the cooling of the dried load. With dehumidifier kilns the air conditions are primarily controlled by adjustment of the temperature setting, although some kilns may be fitted with humidification and ventilation facilities. In the setting of kiln air conditions it is important to appreciate that the quality of drying is very dependent on the relative humidity of the air, as indicated by the wet bulb depression. In practical terms this means that small deviations from the schedule dry bulb temperature have little effect on drying, provided the correct wet bulb depression is maintained. For example if the schedule conditions were dry bulb 50°C/wet bulb 47°C (giving a relative humidity of approximately 84%), drying could be carried out equally satisfactorily if the actual kiln conditions settled at dry bulb 52°C/wet bulb 49°C where the wet bulb depression of 3°C would again result in a relative humidity close to 84%. If however the kiln conditions settled at dry bulb 52°C/wet bulb 47°C, giving a wet bulb depression of 5°C, the relative humidity would be only 75% and could be too severe for satisfactory drying. With automatic control systems and under frequent air flow reversal the correct positioning of the sensing equipment is an important consideration. Kiln drying schedules are based on the condition of the air as it enters the load after re-heating and air sensing equipment for slow air speed kilns should be installed to enable control of the air conditions at whichever side the reheated air enters the load. This is perhaps not so critical with fast air flow side fan kilns where the sensing equipment can be positioned on the side opposite the fan. In this position the air is sensed half way along its passage through the timber load, whichever way the air is circulating, and differences between the air conditions at the inlet and sensing positions will normally be small. Kiln Records Many modern kiln control systems will automatically provide a record of the air conditions throughout the drying operation and have largely eliminated the need to make repetitive written records. However, monitoring of air conditions should proceed hand in hand with monitoring of the timber to assess its moisture content and the risk of degrade. Such information on the condition of the timber is difficult to obtain automatically and normally this is monitored by examining a series of withdrawable timber samples which are incorporated in the load. Inevitably this kind of monitoring requires a certain amount of manual recording and it is recommended that record sheets are kept of the behavior of the timber during a kiln operation. These not only help to keep track of events during the drying operation, but can also be valuable later. Particularly in terms of modifying schedules to advantage, assessing the effectiveness of treatments which have been applied to correct drying degrade or evaluating the performance of a kiln. Completion of Drying and Cooling Towards the end of the drying schedule a reassessment of the condition of the load is normally carried out. The purpose of this is to recalculate and check the moisture content of the load and to ascertain whether a casehardening relief treatment is necessary before either applying a conditioning treatment or allowing the kiln to cool. A few timber species can be withdrawn from the kiln immediately after completion of a schedule or conditioning treatment. However, generally this practice is not recommended, particularly when high kiln temperatures have been used. The cool outside air heats up and becomes drier as it passes over the hot timber and this may renew casehardening stresses. During cooling, the relative humidity should be kept as high as possible and a wet bulb depression of not more than 5°C is recommended until the kiln temperature is within 20°C of the outside temperature. At this point the load can be removed from the kiln. Kilns may not always have facilities to allow such accurate control of conditions during cooling. An alternative method is to raise the humidity for a short time before cooling and then switch off all kiln systems, allowing the kiln to cool without further attention until, as before, it is within 20°C of the outside temperature. This approach may result in a very slight increase in the moisture content of the timber. Kiln Maintenance and Testing A regular program of day to day and long term preventive maintenance should be worked into the operating timetable for each kiln. It is recommended that the kiln and kiln equipment manufacturers should be consulted about the maintenance program which are suitable for particular categories or models of kiln. The high operating temperature and humidity conditions inside kilns require that a number of fairly basic maintenance procedures are carried out on a regular basis. Many of these may not require the attention of a specialist technician. Normally it should not be necessary for a newly installed kiln to be tested. However, older kilns may develop faults and often these can be diagnosed by simple tests. Usually the first sign of a fault will be the occurrence of poor or irregular drying which cannot be attributed to poor kiln practice or low quality timber. Tests using a number of hygrometers positioned strategically within the kiln can be carried out to evaluate whether temperature/humidity conditions are suitably uniform within the kiln. Lack of uniformity may often be attributed to fairly straight-forward causes such as poorly fitting doors/door seals or inadequate insulation. Inadequate or non-uniform air flow rate within the kiln is also a common cause of poor drying and velometers or hot wire anemometers can be used to check flow rates over the timber at appropriate locations. Selection of Kilns Capital and running costs are of prime importance when selecting kilns and kiln equipment, but choice may be controlled by a number of other factors, in particular the types of kiln load which will normally be dried. Also choice may often be predetermined by existing site facilities For example, if spare heating capacity is available, it may be best when expanding kilning capacity to select new plant which can be incorporated into the existing heating system. In general, kiln plant should be selected to be versatile and to anticipate possible future changes in volume, species and thickness of the timber to be dried. Intending purchasers of kilning equipment are advised to seek the recommendations of more than one kiln manufacturer, together with independent advice if necessary. Obviously, satisfactory kiln installations depend upon individual requirements and limitations. Nevertheless, if possible, the following information should be available to assist selection. Volume, thickness, lengths and widths of species to be dried in a given time. Approximate initial/final moisture content and standard of drying required. Existing heating arrangements and fuel supply. Possible alternative fuel supplies. Available electrical supplies. Kiln loading and unloading facilities, including any handling equipment which will be used before, during and after kilning. Available space or other layout considerations The capital cost per kiln unit volume may vary according to capacity, and tends to differ between suppliers. Usually, kiln manufacturers will advise which of their products has the most economic capacity for a particular application. If heat pump kilns are being considered the manufacturers should be consulted to ensure that: The kiln has sufficient capacity at all recommended temperatures and humidities. Electrical power consumption is acceptable.