Stainless steel coils deliver the performance needed to allow effective, efficient lumber dehumidification.

Lewis shows a dehumidification cabinet ready for shipment and installed in a dry kiln.

Some milling operations still believe the best way to cure or kiln-dry raw lumber used for furniture, construction and thousands of other applications is to simply turn up the gas burners -- or burn more wood in a steam boiler in a conventional kiln. But Don Lewis discovered his better way more than 30 years ago: dehumidification dry kilns that use electricity and recycle heat.

Along the way, an unusual, long-lasting association developed between his company, the Nyle Corp., and Minneapolis-based Super Radiator Coils (SRC) -- a relationship that played a significant role in the growth of both companies. SRC developed reliable, thin-tube stainless steel condenser coils that withstand highly corrosive vapors released from wood during the drying process. As a result, Nyle continues to provide dehumidification dry kilns that are dependable and efficient.

It took several years for SRC to perfect the coils for Nyle's application. The design maximized heat transfer by using the coils with stainless steel tubes, aluminum fins and a stainless steel distributor for routing refrigerant through the tubes. It is a classic success story about both companies overcoming major challenges, with a twist.

In this case, it was Lewis (the customer) who pursued SRC (the vendor) to develop the coils that Nyle needed for its kilns, rather than the other way around. "We started having SRC do them, and basically they've done all of our stainless coils ever since," he says.

A close-up of the condenser coils in the dehumidification cabinet is shown.

Stainless Steel Stands Up

"Early on in our development of dehumidification dry kilns, we were having problems with copper/aluminum coils from other manufacturers," Lewis says. "Another company built coils out of galvanized steel, which just don't work in lumber kilns. We also got coils that looked sloppy."

After trying coils made from a range of metals, Lewis concluded that stainless steel was best for withstanding corrosion from the acetic acids released by raw lumber during the curing process. Copper, which is the best metal for transferring heat, cannot be used because it corrodes too easily when the coils are wet.

Other problems also emerged with the early stainless steel coil designs, including gaps between the tubes and fins that reduced heat transfer. Also, the welds that connected the tubes tended to corrode and leak. "A lot of people who tried early dehumidification systems had a lot of trouble with coil leaks and gave up on the idea," Lewis says.

Lewis contacted Super Radiator Coils' Richmond, Va., plant in the mid-1980s to ask the company to develop a coil that addressed the problems. However, at the time, the company did not have the technology necessary to produce stainless steel coils.

So, Lewis used coils from another manufacturer. Before long, customers began to call with complaints about rust on the coils -- rust that proved the coils Lewis had bought were not made of stainless steel. At one customer installation, Nyle spent more than $10,000 on repairs. "That was my last experience with that vendor," Lewis said. "I tried a couple of others, but they could never seem to get parts.

The condenser coil and blower assembly are integral parts of the dehumidification system.

Getting in the Game

Late in 1987, SRC contacted Lewis to say the company had acquired the expertise needed to develop a better stainless steel coil. They learned how to properly weld stainless steel tubes and how to force a metal ball, or "bullet," through them to expand the hard metal and contact aluminum fins for good heat transfer. It was not long before SRC built several models of coils and worked with Nyle to test them under field conditions. Ultimately, they developed the configurations needed and SRC began shipping production units to Nyle in 1989.

"We had bought some coils from Super Radiator and tried different ones early on," Lewis says. "We liked their products better because they came through right. They were welded right and not patched together. We could count on them, and gradually they began to get most of our coil business, including the copper/aluminum coils we use for other applications."

A few years later, SRC developed a more effective method to expand stainless steel coils by using hydraulic pressure at 10,000 to 20,000 psi. The process also can produce very large coils that are completely "square," meaning they have the same diagonal measurement from one corner to another in both directions, which is desirable for the best performance.

According to the company, this was the first time hydraulic pressure was used to expand stainless steel coils. SRC was also the first to develop a stainless steel mechanical distributor for coolant fluids, and the company used it on coils manufactured for Nyle.

SRC's business with Nyle grew steadily over the years. In addition to stainless steel coils for drying lumber, the company provides copper, carbon steel and other coils for Nyle kilns used for other applications such as drying bread into croutons, apples for cereals and baking mixes, vegetables, fish and pet food products.

Stainless steel condenser coils from are key to the performance of this dehumidification cabinet.

The Dehumidification Difference

Besides the obvious benefit of curing wood, dehumidification offers a number of advantages to sawmills and lumber processors, and to end-users. First and foremost, the process uses less energy and recycles heat.

Unlike conventional gas-fired equipment, dehumidification dry kilns operate at a constant temperature, Lewis says, which prevents operators from drying wood too rapidly and produces cured wood with more uniform moisture content. Wood that is heated too rapidly may become brittle, split or dry unevenly, which may cause warping. In addition, burning wood chips to produce steam for drying lumber has become less attractive because mills have developed more lucrative markets for their wood byproducts.

Drying times for these types of processing are about the same, Lewis explains, because technology developed by the company reportedly allows dehumidifiers to operate at temperatures as high as 220°F (104°C). Until then, heat pump dehumidification systems were limited to lower temperatures that slowed drying.

Comparing a conventional kiln for drying wood to a clothes dryer, Lewis says, "They both suck in large volumes of air, heat them up, absorb water and exhaust vapor back into the air. For every trailer load of lumber, you're looking at about 20,000 pounds of water that has to be removed, which equates to huge amounts of air."

"The other thing that's lost is the heat that warmed up all that air," he continues. "With dehumidification, we recover the heat and route the moisture-laden air over cold stainless steel coils to condense the vapor into water and drain it."

Lewis said a typical gas-fired or wood-fired steam conventional kiln takes about 5 million BTUs to dry about 1,000 board feet of raw lumber. A dehumidification dry kiln takes about 400 kWh for the same amount of wood. Given the increased cost of fossil fuels in recent years and comparable drying times with both processes, dehumidification dry kilns are an option that warrants consideration.