Dehumidifier basics and energy for drying

GeneWengert-WoodDoc · January 22, 2015, 09:08:31 AM

When dry air (under 100% RH) is blown across of piece of undried lumber, water will be evaporated from the wood. This increases the air's RH. A dehumidifier removes water vapor from the air, making the air drier and increasing the ability to remove more water from the lumber.

In round numbers, it takes 1000 BTUs of energy to evaporate one pound of water from wood. That is the same as 0.293 kWH.

With 1000 board feet of oak lumber, a green load will have around 2500 pounds of water to evaporate to achieve 7% MC. If we used electric strip heat, (or light bulbs in a simple kiln), this would mean we would need about 732 kWh of electricity just for evaporation. Building heat losses and vent losses would double this number to 1464 kW. At 10 cents per kWh, that is $146 total or $2.40 per 1% MC removed per 1000 BF.

A dehumidifier is a heat pump, and so it is very efficient. A 60 pint per day unit will condense 60 pounds of water per day and use about 630 watts per hour. This is true for any kiln even a solar kiln with DH assist, or even in a basement. (Any 60 pint DH that runs continually for one hour will draw about 6 amps of current or 0.630 kWh or $.63 worth of electricity at 10 cents per kWh, which is a low rate for many people.) So, to evaporate 2500 pounds of water will require roughly 630 kWh or $63. Stated another way, with 1 MBF of lumber, each 1% moisture removed by a DH unit takes 10 kW. That is $1 per 1% moisture removed from 1000 BF of oak.

Another comparison is that with a fairly efficient wood heating system, it will take about 1/5 of a cord of air dried oak fireood to dry 1000 BF of oak lumber in a kiln. Perhaps that is $20 worth of wood or $0.33 per 1% per MBF.

Using natural gas in a furnace would require about $24 of gas at today's low prices, or $0.40 per 1% per MBF.

Of course air drying is free energy. So is a 100% solar heated kiln.

scsmith42 · January 22, 2015, 12:55:36 PM

Gene, thank you for the writeup. It's been my experience that - depending upon the time of year - in the second half of the drying cycle the heat strips are frequently operating in order to keep the kiln temp at 120F (or higher for the sterilization cycle). Heat strip usage is higher in the winter when the ambient temps are lower too.

Have you looked at the KW consumption for the sterilization cycle, as well as the 120F cycles?

GeneWengert-WoodDoc · January 23, 2015, 01:26:15 AM

I have not looked at them.

GeneWengert-WoodDoc · January 23, 2015, 01:32:17 AM

Your experience reinforces the idea that a DH kiln building should be extremely well insulated, unless the auxiliary heat comes from wood. Oftentimes the power company or university extension can do an audit of your kiln to determine building losses and suggest energy saving ideas.

FarmingSawyer · January 23, 2015, 05:36:22 AM

A very helpful writeup! Thank you!

rjwoelk · January 29, 2015, 09:04:42 PM

What would be the best insulation, I was thinking spray foam, closed cell type.

GeneWengert-WoodDoc · January 29, 2015, 10:00:02 PM

Closed cell, due to the high moisture environment is essential. A good poly sheet on the inside as a vapor barrier is also encouraged, patching any holes for the vents, etc. Closed cell rigid boards will work as well as spray foam. Some foams will not withstand temperatures much over 130 F, so these low temperature foams need to be avoided

johnnyllama · February 06, 2015, 01:29:04 PM

Gene,
Would rigid polyisocyanurate foam with foil facing be an acceptable insulation for the kiln if it is going to be heated hot enough for sterilization? And if dehumidifiers are used will electric strip heating provide enough heat to reach sterilization temps ( with high enough r-values and sealed for air infiltration)? I'm building a 15' x 6' x 8' kiln for our own use, not commercial use, and trying to keep cost down. Considered solar but decided to go dehumidification. It's located near our outdoor boiler too so possability of running a loop from that. We would air dry most woods other than maple first.

Ugly Tree · February 06, 2015, 10:00:38 PM

Gene, I have access to a good mess of structurally insulated panels. Metal both sides. I wonder if the glues used in SIPS would hold up to the high temps of solar kilns we can get them up to here in the South. Any experience with SIPS for kilns?

GeneWengert-WoodDoc · February 06, 2015, 11:57:36 PM

Johnnyllama: The isocyanate has a variety of formulations, so I cannot comment about the specific product you will be using. The manufacturer would be the best source. A panel with foil is a good idea. However, the key to insulation for any kiln is to have thermal stability, closed cell construction, plus adequate thickness. As far as thickness, I would suggest 6" minimum for walls and floor and ceiling, with 8" being better (reduced energy usage). In a DH kiln, you will use over 1/3 of the energy input through the walls, floor and ceiling, so extra insulation will save energy and reduce the size of the power plant. Your comment about infiltration is also very important...leaks can result in huge energy losses indeed.

Depending on fuel cost for your boiler, oftentimes, energy is too cheap to save, so the extra insulation may not be economically justified; but it is from an energy conservation viewpoint. (Insulation does indeed cost money today, but the benefit is lower monthly costs for years to come.)

Your comment about air drying is also excellent; you save more energy by air drying first, plus the kiln can dry more lumber per year. The profit in a kiln is really based on throughput and quality, so air drying does indeed greatly help throughput. Poor air drying practices can cost plenty if there are quality losses however, so watch that. Again, air drying hard maple is not a good idea, from a white color standpoint; slow air drying color can be poor.

Incidentally, to get the core of wood up to 133 F for sterilization, it will be necessary to maintain the kiln air temperature at 150 F for as long as 24 hours, depending on lumber thickness.

UGLY TREE: Regarding SIPs, I have seen them used. Obviously, a thin panel is not as good as a thick panel. The overall insulation value (R rating) would be important. The joint connection is also very important, as a lot of heat can be lost through a metal joint. (Analogy: A bucket can hold lots of water, but even a small hole will soon empty the bucket. Insulated walls, floor and roof can hold in a lot of heat, but a few small holes (metal joints for example) can quickly drain away a lot of heat.) I think that an R=30 would be the minimum for the panel and that is with an insulated joint of some sort. The temperature sensitivity of the glue would indeed be important, as well as of the insulation itself.

xlogger · February 07, 2015, 07:04:54 AM

I slab up a couple red maple logs about 6 months ago into 2" and put them under a shed close to the end so they could get some air flow. Are you saying this was a bad idea?

GeneWengert-WoodDoc · February 07, 2015, 07:19:43 AM

Shed air drying with good air flow is almost always a great idea. For most species, reasonably fast drying, meaning good air flow and modest humidity, gives excellent quality in a short time. Keeping sun and rain off the lumber is important.

WDH · February 07, 2015, 07:31:01 AM

Unless there is very good airflow, sticker stain is a big problem with air drying maple in the South. Supplemental fans are required to keep the air velocity up for the first few weeks. Forum member @Yellowhammer has conducted some experiments, and has posted results here on the Forum.

johnnyllama · February 07, 2015, 08:39:39 AM

Thanks Gene. I'm also looking into the logosol kiln system, probably the 800 model. Cost is more and electric is also more but we are thinking about a solar grid tied system for our home and shop area and the sizing of the system for tax incentives and grants is based on overall usage so we may just go that route and increase the solar sizing to include those costs. Got to look into that some more.

GeneWengert-WoodDoc · February 07, 2015, 08:50:08 AM

Solar can be very attractive, but remember that drying of air dried lumber requires about 3 million BTU per MBF. For a ten day cycle, that would be 300,000 BTU per day. Solar input is about 1000 BTU per sq. ft. per day, so you would need 300 sq ft of collector per MBF. Of course, wintertime operation would be slow and summertime would be faster.