Kiln Drying Forum
Wood Drying Topics => Dry Kiln Control => Topic started by: TILLAMOOK on September 27, 2017, 10:29:18 AM

Robbing Peter to pay Paul? Peter is starting to get upset.

Great Topic TILLAMOOK!
I am probably by far the least experienced regarding this topic than all our other more qualified forum experts. Having said that, I would like to make a few mentions which I hope will be notable or at least entice further discourse. I would like to speak from a reference I recently read which seemed to help clarify some basic main physical processes relating to temperature and pressure. Page 20 of this resource: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/5057/Optimizing_Use_ocr.pdf;jsessionid=13C7402FAC6580CE00E4B727BD10E0C7?sequence=1
list a portion of the steam tables which provide bases for analyzing work and efficiencies.
Concerning systems/piping,The steam tables demonstrate that it takes little to no extra heat to generate
steam at 250 psig as it does to generate steam at 100 psig. At higher pressures the
steam occupies a smaller space because the specific volume of the steam is decreased.
This means that if we generate steam at high pressure we can use smaller diameter
piping to transmit the steam to the dry kilns
Now, the reason that we use steam in a dry kiln instead of hot water or hot oil is that
the steam gives up all of its useful heat at constant temperature. From the steam tables we learn that it takes
270
Btu's of heat
to the raise the temperature of one pound of water from 70F
to 338F at a pressure of 100 psig.
If we add an additional 880 Btu's of heat to the pound of water
we will convert the hot water to one pound of saturated steam
at a temperature of 338F and a pressure of 100 psig.
From the steam tables we learn that the total heat in steam at
a pressure of 100 psig is 1189 Btu's
The steam tables demonstrate that it takes little to no extra heat to generate
steam at 250 psig as it does to generate steam at 100 psig. At higher pressures the
steam occupies a smaller space because the specific volume of the steam is decreased.
This means that if we generate steam at high pressure we can use smaller diameter
piping to transmit the steam to the dry kilns.
Using Steam
When steam is used in a dry kiln, the generating process is reversed. If 100 psig
steam is used in the dry kiln coil, 880 Btu's of heat are removed from the steam to dry the
lumber at a constant temperature of 338F. The pound of steam is converted to a pound
of hot water at 338F and the water must be removed from the kiln coil to keep from
flooding the coils. If the condensate is left in the coil and additional heat is removed the
water temperature will drop.
The reason that we use steam in a dry kiln instead of hot water or hot oil is that
the steam gives up all of its useful heat at constant temperature.
In this example the hot water exits the kiln coil at 338F and it is discharged into
a vented condensate receiver. The maximum temperature in the vented condensate
receiver is 212F. Some of the hot condensate is converted to flash steam and the
energy and water is vented to atmosphere. This results in energy loss or efficiencies less than 1.
This is a picture of the vent on a condensate receiver with dry kilns operating at
100 psig. This is a significant amount of energy and water loss. Tabulated below is the
potential energy loss from dry kilns operating at various pressures.
Dry Kiln Pressure % Energy Loss
20 psig 4.7%
50 psig 8.6%
100 psig 12.6%
150 psig 15.4%
If we use low pressure steam in the dry kilns we will convert more of the total heat in the
steam to useful heat, the steam in kiln coils will be dryer, and the loss from the
condensate receiver will be reduced.
Steam Engineering's Axioms for Steam Use
From the steam table, it can be seen that the specific volume decreases as the
steam pressure increases. This suggests our first axiom.
1.
Always generate and transmit steam at the highest practical pressure. At higher
pressures, equal volumes of steam occupy smaller spaces. The physical size of
the boiler, valves, and piping can be reduced. In this axiom, the word "practical"
is important, because as pressure exceeds 150 psig and 300 psig, the cost of
piping and valves will increase, and analysis must be done to determine the most
practical (economic) pressure for generation.
This table also indicates that the latent heat of vaporization in the steam
decreases with increased pressure, which provides reasons for our second and
third axioms.
2.
Always utilize steam for process work at the lowest practical pressure. Process
work is accomplished by condensing the latent heat contained in the steam. The
lower the pressure, the greater the available latent heat in the steam and the
more efficient is the steam conversion to useful process work. Therefore, if the
process does not require an extremely high temperature, the steam should be
used at a low pressure.
3.
Once energy has been sent to the process plant as steam, extract every useful
Btu possible before it is returned to the boiler for reheat. If a high pressure
process is utilized, the steam table shows that much of the total heat in the steam
remains as useless sensible heat in the condensate. If the high pressure
condensate can be flashed to a lower pressure, then the sensible heat will be
converted to latent heat at the lower pressure and will be available to do more
useful work
Steam Load Management
Using what we have learned in the previous sections
of this paper many dry kiln operations could be improved by
operating the boiler at its highest practical pressure,
transmitting the steam to the dry kilns at high pressure and
then installing a pressure reducing station at the dry kilns to
operate the dry kilns at the lowest practical pressure. We have
used a simple electronic single loop controller with an output
rate feature to operate the recommended pressure reducing
station. The effect is that we are able to dampen the spikes and oscillations often
associated with dry kiln operation
This resource from which this information has been garnered provides a bit more insights with conclusions. I encourage everyone to have a look; it is only a few pages but stoked with billowing puffs of delightful insights.
resource: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/5057/Optimizing_Use_ocr.pdf;jsessionid=13C7402FAC6580CE00E4B727BD10E0C7?sequence=1