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=1list 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 kilnsNow, 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 ManagementUsing 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