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Commissioned Reports - Drying

Date: 2006
Title: Scoping study – Microwaves (231934)
Authors: Report commissioned from BRE
Full Report: PDF

The difficulties encountered when drying and treating Sitka spruce are recognised. The implementation of technological processes capable of minimising these effects would have significant impact on the UK Sitka spruce market, allowing it access to product ranges previously restricted due to the cell wall behaviour of the dried wood. The first stage of the project considers the potential of microwave drying, such that a full assessment of the latest microwave drying system will be considered on UK spruce. The second stage of the project considered the microwave pre-treatment. The need for treatment depends upon the intended product application, defined as the use class. Typically Sitka spruce is limited to areas where there is a limited risk of decay (up to use class 3). By improving the means of uptake of secondary treatments (e.g. modifications, preservative treatments) it may be possible to allow Sitka spruce to enter more diverse product areas. This project would increase market innovation for utilising UK Sitka spruce.

Date: 2005 & 2008
Title: An assessment into the use of higher airspeeds during conventional kilning to improve the
        drying of spruce (231943, 231944)
Authors: Report commissioned from BRE
Full Report: PDFPDF

The objective of this proposal is to investigate how increases in airspeed during the drying of spruce can be utilised to reduce drying times and improve the economics of applying this in an industrial setting. Airflow is an extremely important factor during the drying process. The circulating air within the drying kiln imparts heat from the exchangers to the timber and removes moisture from the wood surface. In most UK softwood sawmills the airspeeds used are in the region of 3 m/s. This project will investigate how increasing the airspeeds affects the drying times and the resulting quality of the timber at the end of the schedule. This will be coupled with an assessment on how this process affects the economics involved in applying this research in industry.

The UK softwood sawmilling industry is constantly investigating new methods to improve their processing technology and subsequently the quality of material produced.  The use of higher airflow speeds has been discussed by the UK sawmillers at several of the latest BRE dissemination meetings.  It was felt that the benefits that may be possible by increasing air speeds would be easily incorporated on existing equipment with an acceptable economic outlay. 

Date: 2004
Title: High temperature drying of Sitka spruce (228852)
Authors: Report commissioned from BRE
Full Reports: PDF, PDF

This project investigated the high temperature drying of UK spruce. Previous trials on UK spruce by research organisations in France, Finland and the Netherlands showed that UK material could be dried to a moisture content of 18% in approximately 63 hours with a quality similar to, or better than that achieved using conventional drying methods. Although the results from this previous work were very promising, several areas of high temperature drying still required further investigation before the UK industry could be confident that the system was commercially viable. These included:

1. Investigate the possibility (using experimental trials) of further reducing drying times whilst
    retaining or improving final dried quality of UK material.
2. Undertake a full scale industrial trial with the most promising kiln schedule developed in the
    experimental phase, and assess the drying time and wood quality.
3. Provide an indication of the construction costs of an industrial size high temperature kiln and a full
    breakdown of the energy requirements to dry spruce in such a facility.
4. Undertake a programme of bending and stiffness assessments on conventionally dried and high
    temperature dried spruce to clarify situation on possible strength reductions due to subjecting
    timber to high temperature.

To date four high temperature drying trials have been undertaken at the premises of HB Koeltechniek, in Almelo, Holland. The drying times recorded from these trials range from 76 hours to 56 hours. After some discussion, it was felt that the drying times could be further reduced, although the variation in final moisture content of the charge would be likely to increase. During distortional characteristic assessments on high temperature dried material, the average twist values were found to be either very similar too or lower than those exhibited by conventionally dried material. In comparison, bow and spring values tended to be slightly variable, although in all instances well within acceptable limits. Results from strength and stiffness assessments undertaken on material from each of the trials were found to be quite variable. Certain batches of material dried using high temperature showed large reductions in strength and stiffness, whilst others remained fairly stable, when compared to timber dried conventionally.

These results indicate that the schedule type, and also possibly length, does have an influence on the structural properties of the timber being dried. Due to the variable strength and stiffness results recorded from the material dried in the Netherlands. It was decided that further strength and stiffness tests would be undertaken with material dried using high temperatures in the Windsor/Nardi project. This would clarify whether the reduction in the structural properties was due to the high temperatures used during the drying process (similar temperatures are reached by both systems) or whether the drying system itself is causing some of the reductions.

Data and written comments supplied by Michel Reipen of the TNO Institute in Holland indicate that high temperature drying is more energy efficient and therefore more economical to run that conventional drying kilns. Further work will hopefully clarify the strength and stiffness issues which have been highlighted in this report and also provide a strategy for future work.

Date: 2002
Title: In-use performance of commercially kiln dried Sitka spruce (PPD21, CV6321, PT7620)
Authors: Report commissioned from BRE
Full Report: Awaiting electronic version

This study consisted of an assessment of UK spruce processed and dried from 7 of the UK's largest softwood mills.  Each mill sent two lots of material (approx 130 pieces in each lot) from each of their processing lines for drying quality assessment.  This included moisture content uniformity, twist, bow, spring and cup.  The study then compared results form each of the mills to identify differences in drying practices and instigate improvements.

Date: 2002
Title: Review of Cold Water Dehumidification for Timber Drying (211915)
Authors: Report commissioned from BRE
Full Report: PDF

This report reviews instances where possible energy savings can be made during the wood drying process. Wood drying is an energy hungry process. The economics of which form a significant cost of the overall sawmilling process.  This review investigates instances in the kiln drying process where energy savings are possible.  These include energy recovery systems specifically designed for use on wood drying kilns (systems which recover heat from the moist/hot air vented during the wood drying process), the use of frequency inverters on kiln fans, general kiln maintenance and insulation capability.  There is a great potential for energy saving systems to be incorporated onto UK wood drying kilns, not only for the cost benefits, but to aid the environment and pre-empt government directives on energy usage.

The main conclusions and recommendations drawn from this review were:

1. A simple kiln check and maintenance regime will detect breakdowns and other potential problems
    and ensure that they are quickly rectified, thus saving energy and money. 
2. When choosing a new kiln, thought should be given to the type of insulation being specified, and
    its ability to withstand temperatures with which the kiln will be used now and in the future.  
3. The addition of a speed controller to the kiln fan system when purchasing a new kiln can provide
    significant energy savings during drying. 
4. From the evidence reviewed, it should be possible for significant savings to be made using a heat
    recovery system on softwood kiln drying operations based in the UK.
When minimising the overall costs of timber drying, effort should be directed into energy efficiency and the avoidance of drying defects.  As a guideline these goals are achieved by the proper selection of kiln type and in the longer term adequate kiln maintenance.

Date: 2002
Title: Drying trials (204988, 201435)
Authors: Report commissioned from BRE
Full Reports: PDF, PDF.

This work generated results from a kiln trial to assess a temporarily installed atomised spray system to control humidity.  The trial was carried out at the sawmill of James Callander and Son, Falkirk to compliment work previously conducted under the EU DRYCON project.  The report compares the results obtained from task 1.of the DRYCON project (Baseline Drying Quality), with a kiln schedule used regularly by James Callander and Son.  This report forms an output for the Forestry Commission, under the title ‘Optimisation of kiln drying UK softwood using conventional drying capability’. This optimisation trial encompassed three main objectives:

1. To significantly reduce the number of hours used to dry a similar load of timber, as assessed in the
    previous trial 
2. To retain the distortional quality and moisture content spread determined in the first trial.
3. To assess the performance of a new atomised spray system temporarily installed in the kiln for the
    introduction and control of humidity

Processed construction grade timber was assessed for distortion and moisture content before and after kiln drying.  This included monitoring drying conditions, moisture content and wood temperature throughout the drying cycle.  Selected battens were also assessed for drying degrade and other important growth characteristics.  Comparisons were made between the kiln schedule times and heating-up periods of the two assessments.  A further assessment was made to compare the current assessment results with records on previous drying runs supplied by James Callander and Son. 

The results from this study were encouraging, the main conclusions being:

1. Material dried in the current assessment showed more uniform moisture content values than that
    dried in the first trial, with far fewer battens exhibiting moisture contents over the 24%. 
2. From an assessment of the first and current kiln schedule, it has been shown that the atomised
    spray system reduces the heating-up time by approximately two hours (this could be expected to
    reduce further with colder ambient conditions).  A comparison between the drying time of the
    current assessment and records kept by the sawmill show a mean reduction in the overall drying
    time of 4 hours.  These figures may vary as further data is collected.
3. The atomised spray system retains a high humidity throughout the heating-up phase far more
    accurately than the old spray system.  Thus, the heating system does not have to wait for the
    humidity to ‘catch up’, resulting in quicker heating-up times.
4. The distortion values for both trials were very similar for battens in their 'green' state, although
    battens from the current trial showed an increase in twist of approximately 1 mm. 
5. Moisture content for both ‘green’ and dry battens were very similar.
6. Material from the current kiln trial showed an increase in all distortion values when compared to
    material from the first trial.
7. In particular, twist values were greater by 2 mm when all the test material was considered.  
    Contributing factors may be the higher percentage of boxed pith battens present in the current
    trial or the harshness of the drying regime below fibre saturation point.
8. The overall drying time for the current assessment using the atomised spray system was greater
    than that in the first trial.  One significant reason for this was that in the early part of the current
    drying schedule there was very little drying taking place for approximately 10 hours.
9. Material dried in the current trial showed a much higher percentages of battens with boxed pith, 
    than were recorded in the first trial.
10. A comparison of the charts from the first kiln trial (using the old spray system) and those 
    recorded using the atomised spray system has shown that no major differences in humidity 
    between the two systems could be identified.

This trial has indicated that the use of an atomised spray system can improve the performance of a wood-drying kiln.  Although the increases are not great, they do help in the overall tuning of the drying regime, by obtaining the best drying possible from the equipment in use.

Within the stacking, stickering and loading report, methods used by individual sawmills were assessed and evaluated in 8 commercial kiln trials around the UK. This enabled the best practices from each sawmill to be included in this guidance document. The sawmill site assessments have identified a number of areas where improvements could be made. However in general, the working procedures of the eight sawmills assessed were of high quality and none exhibited any serious kiln drying procedural problems.

Date: 2000
Title: Best Practice Guidance for kiln-drying British Grown timber (81-295)
Authors: Report commissioned from BRE
Full Reports: PDF

This study assessed the UK softwood sawmills practices for stacking and stickering prior to drying.  A guide (aimed at the guy's doing the job) on how best to do the job was written.
This best practice guidance presents recommendations and guidance on the stickering, stacking and loading of sawn timber for kiln drying.  This work was jointly funded by the Forest Practice Division, Forestry Commission and the European CRAFT project: DRYCON (BE-S2-53 60).

The main objective of task 1 was to determine the effect of stacking, stickering and loading on air flow and the subsequent quality of dried timber.

The stacking, stickering and loading methods used by individual sawmills were assessed and evaluated in 8 commercial kiln trials. This enabled the ‘best practices’ from each sawmill to be included in this guidance document. The sawmill site assessments have identified a number of areas where improvements could be made. However in general, the working procedures of the eight sawmills assessed were of high quality and none exhibited any serious kiln drying procedural problems.

The main recommendations of this study are that:

  • Dunnage or bearers should be as small in cross-section as possible, whilst still allowing easy entry of forks for loading and un-loading
  • Stickers should be of adequate length to support the timber, but not excessively long to hinder close loading
  • Sticker spacing should be in the region of 500 mm to 600 mm
  • Parcels of battens should be made up of timber containing all the same lengths and dimensions
  • Spaces between parcels should be reduced and excessively large gaps between the timber load and kiln frame blocked, to direct air through the kiln load
  • Saw blades and chipper canter blades should be changed regularly to produce a smooth surface finish on battens, thus ensuring a uniform and constant air flow through the load 
  • All baffles, curtains and flaps should be well maintained and regularly checked for correct working
  • Kiln operators should have appropriate training and guidance
  • Operatives should be allocated an adequate amount of time with which to correctly load the kiln, so as to ensure consistent and uniform drying of the load

Date: 2000
Title: Improve Kiln Drying (216175)
Authors: Report commissioned from BRE
Full Report: PDF
Interim Report: PDF

This project specifically targeted the improvement and optimisation of conventional drying techniques using current drying equipment for the drying of UK spruce. Most of the equipment in use by the UK softwood sawmilling industry has the capability of drying timber using higher temperatures, which, in theory, should reduce drying times. However, a number of industrial kilns are limited in their maximum temperature capability due to the low thermal capacity of the insulation material used during construction. This effectively limits the maximum temperature achievable during drying.

The main objective of this project was to investigate how increases in drying temperature (and changes in other drying parameters) affect drying times and the final quality of kiln dried UK spruce. From the analysed information, a series of kiln drying schedules were produced to supplement those already in use and provide additional information to improve drying times and quality. During the production of the final report, it became apparent that the results from this project would only provide a small section on improvements required to improve the drying process. The kiln drying process is multi-faceted, consisting of a wide range of processes, which, if not undertaken correctly, either individually or collectively, will have a detrimental effect on the quality of the dried timber. For this reason, the report has been written as a form of guidance manual on the improved drying of UK spruce using conventional kiln drying technology.