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Another Long Cold Winter Of Discontent Or Will You Use Your Wood-Waste To Keep Warm

Clare Hollister

Chris Franklin, Managing Director of Ranheat Engineering Ltd. One of the UK’s leading manufacturers of wood-waste burning equipment, continues his series of articles exclusively for Panel and Joinery Production.

Many potential customers are unaware as to what is needed to install a wood fired heating system. They are also unaware as to how little wood-waste is needed to effectively heat a factory.

In general, within the woodworking industry, the heat load is divided into three main factors. The obvious one is transmission losses through the fabric of the building, this is heat lost through the roof, walls and floor of the factory unit itself. Modern buildings have low transmission losses, older buildings were not so well insulated. The second form of heat load is infiltration losses. In simple terms how many doors are left open all day for loading finished goods for delivery or receiving materials from suppliers. This is normally referred to as “air changes” The third and very relevant heat loss in the wood-working industry is extraction losses, these are from the dust extraction system and spray booths.

Some dust extraction companies supply dust extraction filters which return air to the building. Extraction losses from spray facilities cannot be returned.

So how much energy does a factory use? As a “rule of thumb” a 20,000 ft2 factory will need 300kW 10,000 ft2 factory 150 kW. It is generally predicted by OFGEM that the energy used per heating season is the power of the heater in kW multiplied by 1314. So, 150 kW heater 150 x 1314 = 197,100 kW hours or around 61 tonnes per heating season. If we take the heating season to be 32 weeks, this needs 1.9 tonnes per week. Equivalent gas usage at 25p per kW hour would be £49,275 per year.

If we do the same calculation with a 300kW heater and a 20,000ft2 factory 122 tonnes per year would replace a gas bill of almost £100,000. and during the heating season needs 3.8 tonnes per week.

If we take an example of a site with dust extraction and off-cuts available filling a 12 m3 skip every month with extracted material and producing a 12m3 skip of off-cuts per week.

If we assume that the density of the extracted material is 500kgs per m3 and the offcuts is 250kgs per m3. Then this site would have 1.5 tonnes per week of extracted material and 1.5 tonnes of off-cuts. If the off cuts are granulated, then they can be mixed with the extracted material and the site has 3 tonnes per week available as fuel all of the year round.

With a small amount of storage of the off cuts in the summer the site could be self-sufficient in heating. During the summer months the waste produced can be efficiently burned and the heat produced released to atmosphere.

If we consider a woodworking factory with a spray facility, heating to the spray room is still required even in summer but at a much lower rate. In winter the extraction losses are often more than the transmission losses.

The design of the heating system can effectively reduce the energy needed to heat a factory if we can cut down on heat lost from the building.

An example of this would be an air curtain over each loading door.

The use of high-level unit heaters, these take the hottest air and heat it and blow it down to floor level so as well as acting as heaters they act as heat recovery units.

Unit heaters and door curtains can be thermostatically controlled, or the door curtains can switch on when the door is raised, automatically.

High level unit heaters can be controlled in zones allowing storage areas to be held at lower temperatures than work areas.

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