Airflow and Energy -- Opportunities for Your Kiln
Our experts say more powerful airflow systems can make a difference in uniformity and MC.
by Peter Garrahan, Vincent Lavoie, and Dany Normand - FPInnovations
Kiln manufacturers and lumber producers have realized the benefits associated with drying with faster air velocity. As a result, kilns these days are generally equipped with more powerful airflow systems. Increased air velocity through the lumber stack improves heat transfer and moisture pick up and therefore shortens drying time. That's the good news, but with every gain there is a little pain and in this case that is the extra cost of creating the faster air flow. Electrical energy consumed by the air circulation system may only represent 5 to 10% of the total energy used to dry SPF lumber but, due to the higher cost of electricity, it can represent up to 20% of your energy costs.
What the Science Has Shown
As mentioned above, increased air velocity can be beneficial in terms of both a faster drying rate and achieving better uniformity in final moisture content. There are several rules of thumb that can be followed:
High MC, more permeable (faster drying) woods will benefit more from higher airflow.
For Canadian SPF, faster airflow has been shown to be beneficial for black spruce and balsam fir. There is approximately a 2% reduction in total drying time when air velocity is increased by 100 fpm. This applies to kilns operating with an air velocity of less than 1000 fpm.
Dry kilns with wider loads will benefit more from a faster air velocity due to a reduction in the TDAL (temperature drop across the load).
Benefits from faster airflow are only seen when the wood is above the fibre saturation point (FSP) which is approximately 25-30%. An air velocity of approximately 300 fpm is sufficient once the wood has been reduced below the FSP
Airflow Objectives
Based on the above, your airflow objectives should be as follows:
- As fast an air velocity as possible from green to FSP.
- Reduce air velocity to about 300 fpm for the final stages of drying (below FSP).
- As uniform an airflow as possible at all stages of drying.
Opportunities
Since electrical energy is your most expensive form of energy, the key is to achieve the above in the most economical fashion possible. Here are the opportunities for achieving greater efficiency:
Switch to higher efficiency motors. When building a new kiln or replacing motors in an existing kiln consider switching to high efficiency motors. Relatively speaking, the improvement in efficiency is greater for small motors than for large. Even a 1 or 2% improvement can pay for itself several times over on the life of a motor. An added benefit is that higher efficiency motors tend to be more reliable and have a longer service life than standard motors (source Natural Resources Canada, www.nrcan.gc.ca).
Variable frequency drives (VFDs) can be used to reduce air velocity in kilns operating with more than 300 fpm. The savings here are considerable. Reducing the airflow by 50% can reduce energy consumption at the fan motor by more than 80%. Therefore, even reducing airflow by small amounts for a portion of the drying cycle can result in a rapid return on investment. Our analyses have shown that payback periods of 1 year are common for kilns operating with high air velocity. For kilns with lower air velocity (i.e. 500 to 600 fpm) the payback may be more in the 2 to 4 year range but some of the energy incentive initiatives from government or utilities can help to reduce this. The key to doing this without affecting productivity or quality is to link the operation of the VFD with a system that provides feedback on the MC of the wood.
Optimize the operating parameters (pitch angle and rpm) of your variable pitch fans to achieve greater operating efficiency. FPInnovations and Hydro Quebec have worked together to develop a methodology to identify the optimum operating parameters for dry kiln fan systems. By applying this process on industrial dry kilns we have identified opportunities for energy savings of up to 30%. We are now working with other agencies in Nova Scotia, New Brunswick, Ontario, and B.C. to offer a similar service to the industry in those regions. Contact Vincent Lavoie at our Quebec lab for more information (vincent. lavoie@fpinnovations.ca).
Taking care of the basics is the final but perhaps the most important opportunity. Gains from any of the above mentioned opportunities will only be realized and/or maximized if you are taking care of all of the things around a kiln that affect airflow. This is a long list of items but the major ones are:
- Good piling.
- Proper kiln loading with good placement of packages, maintaining a proper chimney between packages, and filling the kiln from end to end and top to bottom.
- Maintain and use baffles to block any and all openings around the load.
- Keep all mechanical and electrical equipment well maintained and in good operating condition.
- Regardless of the level of airflow, uniformity is a key factor. Our recommendation is to have a variation of no more than +/- 100 fpm from the average.
Auditing Your Airflow System
All kiln operations can benefit from applying the above principles. The key to assessing if the opportunities apply to you or not is information. The first step in the information process is to know the airflow properties of your kiln and some basic information on electrical energy consumption. If you don't already own one, buy an air velocity meter similar to the one shown in the accompanying photo and conduct an extensive airflow test on your kiln. Once you know the level and uniformity of air velocity in your kiln and the power consumption of your system you can start to assess which opportunities apply to you.
