Kiln heating

[MichaelM]

What can you do if a kiln isn't heating fast enough?
For the answer, let’s assume the kiln is mechanically working – it is getting steam of sufficient pressure, the valves are opening, and the traps are operating.  Let’s also assume that the kiln is wired correctly and the heat valves are fully open.  In other words, to heat faster requires either a capital improvement or some change in the way the kiln is operated.

I would not suggest using steam spray to speed things up.  This is an expensive way to heat as the water is not returned to the boiler.  Any water that is condensed on the wood or in the kiln needs to be evaporated later in the cycle.  It can be hard on the kiln in terms of corrosion.

I would suggest keeping the vents closed until the kiln is at or near the desired dry-bulb temperature.  The amount of water held by the air increases with temperature.  When venting at a dry/wet-bulb condition of 140/110F, 20 pounds of makeup air are required to remove one pound of water.  At 180/150F, this reduces to 5 pounds of makeup air to remove one pound of water.  This does not make the whole process 4x more efficient, but it probably does increase the efficiency by a few percent.  The kiln should be capable of coming up to temperature with a maximum 10-15F wet-bulb depression if the vents are closed (assuming the kiln is well insulted, the kiln doesn’t leak, and the lumber is not frozen).  This may not be good for stain-prone wood.

Other suggestions would be staggering the start of the kilns, which most operators try to do.  A quick change over from the previous charge can prevent the kiln structure from cooling too much. 

If the kiln is tight, increasing the fan speed (if not maxed out already) might help by increasing the heat transfer from the steam coils.   There’s no clear answer because the increase in heat transfer has to be great enough to offset any leakage (the pressure difference from plenum to plenum will increase and any leaks will get worse at high fan speed).

What has worked for you?  I realize there are many mechanical factors involved such as pipe and valve sizes, steam pressure, and coils area that go beyond the way the kiln is operated.

[MichaelM]

I do not have experience with heat recovery in terms of using a system.  There are, of course, several on the market and they heat the incoming air using the exhaust air.  Heating incoming air represents 5 to 15% of the thermal energy to dry.  There may be some other indirect benefits (reducing cold spots near vents, reduce condensation, better control, less heat loss because vents are sealed, better airflow through the load because the vent blower(s) is separate from the kiln fans) that lead to a somewhat higher energy recovery.

I would suggest asking mills in your area how it's worked for them - both that currently operate with heat recovery or have in the past and for whatever reason do not now.  The devices have been around for kilns for more than 20 years so there are some of both.

Maybe someone can comment who has done a side-by-side or before-after comparison?

[fbushaw]

I have run across this same problem, especially if I have WF in all 5 of my kilns. I have tried not letting the vents open untill the kiln is up to temp. This did help, but it also lengthened out my schedule too much. I try to stagger my kilns a little when we are drying WF, but I dont have the luxury of being able to leave any kilns down very long. The planer will run out of lumber, and the kilns will fall behind on the sawmill..

[river guy]

operating on demand venting helps, keeping the the vents closed maybe not until you are totally up to temp but close. with this though you have to have tight control of your venting. slow and easy when you start or you will lose most of what you gained in the first place

[HencoV]

What can you do if a kiln isn't heating fast enough? This can be done either 1. adding more energy/boiler capacity or by 2. decreasing energy loss or 3. optimizing the drying schedule for minimum values

If nr1  is not an option, nr 2 has been checked and rechecked, nr 3 could become an option for advanced kiln managers and kiln analysers

Kiln control systems are designed to control on maximum values, eg, maximum dry bulb, vent when maximum WB value (vent set point) is exceeded. To control on minimum energy is a completely different ball game.

It is about balancing energy input, and getting rid of moisture, while not compromising the highest temp achievable to maintain maximum moisture movement. ie. managing the venting strategy in such a way that temperature is not lost, while still getting rid of moisture to a point where the available energy is not exceeded by the energy take up(TDAL) as timber dries.

[GeorgeCulp]

I think one is kinda stuck if their kiln isn't heating fast enough. It's normally too costly to add boiler capacity and too difficult to juggle kiln startups to compensate.

If one has recently done something to increase airflow and now the kiln takes longer to heat up, then the airflow increase is the answer. Normally, when airflow increases there is an increase in heat transfer from the fin pipe, however there is also an increase in heat transfer to the wood. If there is not enough steam available to replace the increased amount transferred due to the increase in airflow then the kiln will take longer to heat up because the heat is being transferred to the wood and not staying in the air.

But what do you want? Hot wood or hot air?

[Craig Jensen]

I imagine you have to be careful with what I am about to suggest if you are drying dimension, as we are a stud mill and my experience only applies to 2x6 and smaller.

I am assuming that we are not getting to our setpoint, which means we are not really controlling our kiln - it's wide open trying to hit a high setpoint.

I am also assuming that the driving force in drying is depression.  Early in the schedule there is a lot more mass in the kiln to heat, because we have not removed the water yet, but free moisture comes out easy.

I have found that if we can't reach set point temperature, we can achieve drying as quickly by lowering the wet bulb set point a little.  This results in venting,.  However, if you have wood that is over 2/3 water by weight, sometimes releasing the water through the vents a little earlier will reduce the mass sooner in the schedule and allow you to get what you are after.  So if I have a schedule that had a tight depression early but I can't reach the dry bulb setpoint, sometimes lowering the dry bulb setpoint 5 degrees, but the wet bulb 10 degrees will allow us to hit that set point, remove the water that comes out easy, and then tighten up a little later and hit higher temperatures. 

Like I said, I know you need to be careful, as this can increase the drying defect if done too quickly.  But the driving force of drying is depression as much as temperature, so if you can't have temp, maybe you can have depression?

Just something we have played around with and found a little more success.  You have to play with it a little to get that sweet spot where you are heating up as quickly as possible.