Manufacturers of Timber Drying Kilns for over 40 years 01621 785

Pallet Drying ISPM-15

Our range of pallet drying & heat treatment kilns can hold batch loads of 150 to 2500 pallets. The kilns can be loaded directly by fork lift truck or by using rail through loaded platforms. The kilns with rail through loading platforms are supplied with doors at both ends.

All control systems have fully automatic programs that meet the ISPM-15 heat treatment regulations. The heat treatment programs are controlled by the actual wood temperature and are designed to use the minimum amount of fuel.

When drying or heat-treating pallets, there are four control options:

  1. Automatic heat treatment program with print-out of core temperatures
  2. Time-based drying program, including heat treatment and printout of core temperature.
  3. Moisture content-controlled program incorporating heat treatment, and print-out of wood moisture plus core temperature.
  4. Automatic Drying-by-Weight with heat treatment. Please see article below outlining the advantages of this option.

Two pallet drying / heat treatment kilns each holding 1400 euro pallets.

Rail through pallet drying kilns for high volume production.

Two direct forklift loaded pallet drying & heat treatment kilns. Each of the kilns can hold 600 standard or 700 Euro pallets. The kilns are heated using a Biomass boiler and are registered under the Renewable Heat Incentive scheme. The drying control system is fully automatic and has specially designed heat treatment and drying programs.

A single module pallet & heat treatment kiln that holds 150 pallets in stacks which are 25 pallets high. Each batch load of pallets is heat treated in approximately 4 hours. The kiln is extremely efficient and can heat treat 450 pallets per day. This kiln can be extended by adding additional modules, increasing their capacity to heat treat 900 pallets per day. The kiln control system is fully automatic and can be used to dry pallets down to a specific moisture contents thus eliminating the possibility of Blue Stain and Fungal growth.

Weighing Up The Options

For some time, there has been a discussion within the industry regarding the problems associated with higher moisture contents in pallets, facilitating blue stain and fungal growth.

These unsightly problems along with the very real danger of moisture condensating onto goods have focused many within the industry to ensure moisture content is kept to a level which minimises the risks. It has been generally accepted that once the moisture content is below 20%, the above problems are reduced, however many companies now dry to between 18 and 15% to provide a higher safety margin.

The question is how do we obtain a moisture content of say 18% in every component of a pallet, of course there is only one answer to this which is not practical and generally uneconomical, that is to dry each thickness separately to 18% prior to manufacture. Discounting this idea does mean that we have to accept that the drying rate in different positions and thicknesses within the pallet will vary. For example, the open area of a top deck board with good air flow over the surface will dry much faster than the area covered by the stringer board and block, this on a stack of pallets is a solid column of timber components with very little surface area other than the end grain, from which moisture can evaporate.

Of course, composite blocks can be used to eliminate the moisture of solid timber blocks, however drying rates will still be considerably different. Another idea that would improve the drying would be to place spacing sticks between each pallet, but again this is considered uneconomical and labour intensive, so again is not a sensible solution.

Therefore, when stating the average moisture content of a pallet, are we referring to the open area of a top deck board, the top deck or stringer board in the area of the block, or the moisture content of the block.

It would not be unusual after drying to have moisture content variations of several percent within the same pallet, although some equalizing will take place after drying, customers should be aware that the average moisture content of say 18% does not mean the maximum. You may find examples of 21% around the blocks, whilst in the open area a top deck board could be 15%. The variation in moisture will depend upon many factors, pallet design but most importantly, the initial moisture content of components prior to drying.

Most kiln control systems designed for pallet drying will use simple time-based programs or a number of measuring electrodes which measure the electrical resistance between each pair. This is then displayed as the average moisture of the batch and determines the end of each drying cycle.

When drying a kiln batch of sawn timber packs with boards of the same thickness and sticks between each layer, a number of electrodes will provide a good indication of the average moisture. Having said this, there are still some important points that must be adhered to. Firstly, the kiln operator should assess the batch to be dried, if for example the initial moisture content range varies from say 55-45%, the operator should try to find sample boards of around 50-51% to insert the electrodes.

Electrodes should be positioned in the centre of the board and to a depth of to ½ of the board thickness. Electrodes should all be installed at 35mm centres, not close to knots or other imperfections. It is important to understand that the electrodes are only measuring an area of 35mm of wood between the pins, as with a moisture meter you could move 500mm along the board and get a considerably different reading, especially if the pins are not at the same depth.

So getting back to the point, where does this leave us with controlling the drying program for pallets? Firstly, where do we insert the electrodes - if in the open area of the top deck boards, drying will be fast but we will still have areas of high moisture around the blocks and stringer boards. Inserting electrodes into the overlapping areas, perhaps into the stringer board will greatly extend drying and could result in an over-dry top deck board. How should the kiln operator assess each load to determine where to insert the electrodes? Unfortunately there are no easy answers, which is why some companies revert to simple time-based programs, sometimes leading to the over or under-drying of batches, returned loads and loss of customer confidence.

Around 2 years ago, we decided to investigate the possibility of drying by weight, after all, the moisture content of wood is expressed as a percentage of its dry weight density, so to control the process by weight made complete sense. Initially, it was difficult to find suitable load cells that could put up with the harsh conditions and high moisture within the kiln, also the temperature increase would affect the accuracy of the cells. After working closely with a manufacturer, we succeeded in developing a load cell which is very robust to IP68, and fully temperature compensated up to 80° centigrade. We were quite excited about this, as it opened up the possibility of fully automated pallet-drying, controlled by a target weight.

Integrated Pallets in Northern Ireland have just installed their second kiln with the drying by weight system. Michael Carson stated that he has been delighted with its simplicity of use and accuracy, with ½kg coming off the weight every 5 minutes. That's around 140 litres per hour from each kiln batch.

Although there are many advantages for those companies drying large quantities of the same types of pallets, some companies drying a moisture of pallets and timber packs may prefer a dust option with both probes and the drying by weight system.

When considering your pallet drying requirements, it is well worth weighing up the options.

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