US3714716A - Process for kiln drying lumber by means of a predetermined drying rate - Google Patents

Abstract

D R A W I N G
AN IMPROVED PROCESS FOR KILN DRYING WOOD WITH A CONDITIONED CIRCULATING FLUID, SUCH AS HEATED AIR, IS DESCRIBED. A DESIRED RATE OF MOISTURE LOSS IS INITIALLY CHOSEN. THIS RATE IS ACHIEVED AND MAINTAINED BY CONTROL OF THE RATE OF ENERGY TRANSFER FROM THE FLUID TO THE WOOD. IN CONTRAST TO PRIOR ART METHODS, THE RATE OF ENERGY LOSS FROM THE FLUID TO THE WOOD IS HERE HELD AS AN INDEPENDENT VARIABLE. THIS ENERGY LOSS IS SENSED AND THE ENTERING AIR IS ALLOWED TO ASSUME WHATEVER TEMPERATURE NECESSARY TO MAINTAIN THE SELECTED RATE OF MOISTURE LOSS. THE ENTERING AIR DRY BULB TEMPERATURE THUS BECOMES A DEPEDENT VARIABLE.

Description

United States Patent 3,714,716 PROCESS FOR KILN DRYING LUMBER BY MEANS OF A PREDETERMINED DRYING RATE Dallas S. Dedrick, Longview, Wash., assignor to Weyerhaeuser Company, Tacoma. Wash. Filed June 29, 1970, Ser. No. 50,561 Int. Cl. F26b 3/00 U.S. Cl. 3430 47 Claims ABSTRACT OF THE DISCLOSURE An improved process for kiln drying wood with a conditioned circulating fluid, such as heated air, is described. A desired rate of moisture loss is initially chosen. This rate is achieved and maintained by control of the rate of energy transfer from the fluid to the wood. In contrast to prior art methods, the rate of energy loss from the fluid to the wood is here held as an independent variable. This energy loss is sensed and the entering air is allowed to assume whatever temperature necessary to maintain the selected rate of moisture loss. The entering air dry bulb temperature thus becomes a dependent variable.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to kiln drying of wood products using a predetermined drying rate as an independent variable in the drying process.

(2) Prior art relating to the disclosure Traditionally, kiln drying of lumber has been carried out by imposing a set of kiln conditions (i.e., dry bulb temperature, wet bulb temperature, air velocity, etc.) and accepting whatever drying rate results from the interaction of the kiln atmosphere with the lumber. Drying conditions for a particular species of lumber in a certain configuration or charge have generally been arrived at through trial and error with no logical understanding of the results obtained. As is evidenced by the myriad process conditions advocated for drying lumber, it is clear that the application of a predetermined sequence of entering air conditions to kiln charges of similar lumber does not result in identical drying rates for the various charges. This is probably because of the many variables such as geographical differences in the wood, initial moisture content variations, board surface roughness, load geometry, etc. which affect the mechanism of energy transfer from circulating air to wood. It would be a distinct advantage to control the conditions in a kiln so that a predetermined rate of moisture removal could be obtained.

The instant invention wholly departs from the traditional practices of drying lumber in that the drying rate of the lumber in the kiln is selected as the independent variable and the kiln conditions then become the dependent variable.

SUMMARY OF THE INVENTION This invention provides a process for kiln drying wood by selecting a rate of moisture loss from the charge in the kiln and adjusting the rate of energy transfer from the drying fluid to maintain the selected rate. The removal of moisture from wood in a forced drying or kiln operation requires that energy in the form of heat be transferred to the wood being dried in order to vaporize and drive off the moisture contained in the wood. To attain a selected rate of drying a commensurate input of heat transferred from a drying fluid circulated across the surface of the wood must be attained to provide the selected rate. The method of this invention permits the pre-selection of a particular rate of drying or progression of rates of drying by ascertaining and controlling the energy transfer to the wood in order to maintain the pre-selected rate of progression of rates of moisture loss from the wood.

BRIEF DESCRIPTION OF THE DRAWING The figure is a graph of percent moisture content versus kiln resident time (hours) for a load of 2" x 8" western hemlock using an initial drying rate of 0.018 lb./sq. ft./hr. achieved by a temperature drop between the entering and leaving air of 25 F. with air circulating over the charge at a velocity of 200 ft./min.

DETAILED DESCRIPTION OF THE INVENTION Optimization of the kiln drying of lumber can be attained by providing heat transfer to the wood at a rate which gives the optimum mass transfer of moisture from the wood which minimizes the kiln residence time for the Wood being dried, while minimizing degradation of the wood surface. The goal of minimum kiln residence time, surprisingl enough, can be accomplished by the utilization of rather mild initial kiln conditions which are formulated to provide a rate of heat transfer to the wood surface commensurate with the desired rate of moisture loss from the wood.

An energy balance equation can be formulated for a drying system assuming that the system behaves adiabatically. Such an equation is:

where:

dQw/d0=Rate of water loss from wood (lb./hr.)

A=Rate of dry air flow over lumber (lb./hr.)

a=Specific heat of dry air (B.t.u./lb.-degree) V=Rate of water vapor flow over lumber (lb/hr.)

v=Specific heat of water vapor (B.t.u./lb.-degree) dT=Change in temperature of air-vapor mixture during contact with lumber surfaces (degrees) W=Oven dry weight of wood (1b.)

w=Specific heat of dry wood (B.t.u./lb.-degree) F=Weight of free water in lumber (1b.)

f=Specific heat of free water (B.t.u./lb.-degree) B=Weight of bound water in lumber (1b.)

b=Specific heat of bound water (B.t.u./lb.-degree) dT*/d0=Rate of temperature change of lumber (degrees/ L*=-Apparent heat of vaporization of water-air system (B.t.u./lb.).

Reference to the above equation shows that, other factors remaining constant, the temperature drop of the drying fluid across the load is directly related tothe rate of moisture loss (dQw/dB). By maintaining a predetermined temperature drop across the charge in the kiln the kiln conditions can be so controlled that a predetermined amount of energy is transferred from the circulating air to the wood per unit time. The rate of water loss by wood (dQw/dfl) is also dependent upon the following independent or semi-independent variables, (1) the air velocity through the charge, (2) the composition of the air flowing over the charge (moisture content of the air), (3) the partition of energy between latent and sensible heat (dT/do), and (4) the apparent heat of vaporization of water (L*).

Referring again to the equation it can be seen that the rate of moisture loss from lumber in a dry kiln is high when (1) the air velocity is high, (2) the rate of temperature rise of wood is low, (3) the effective heat of vaporization is low, and (4) the temperature drop across water is left to be removed. To remove the bound water the bonding energy must be overcome as Well as energy required to remove the vapor from the wood. It has been found that when the average moisture content approaches 30% the energy associated with the loss of a unit quantity of water increases rapidly and at the 23% level the value becomes approximately ten times that found at moisture contents above 30%, indicating that the moisture removed at these levels is predominantly bound water and not free moisture.

By the instant process, the rate of water loss can be controlled at the desired level by programming the temperature drop across the load or by programming the air velocity or both. Referring again to the figure, if it is desired to maintain the rate of moisture loss below 43% approximately the same as above 43% it is necessary only to increase the temperature drop across the load gradually or to increase the air velocity gradually or both. In either case more heat energy per unit time will be transferred to the lumber to provide the extra energy necessary to compensate for the larger amount of energy required for freeing a unit mass of water at moisture levels below 43% than the energy required for freeing moisture above 43%.

In carrying out the invention the following steps are taken:

(1) A quantity of drying medium (air and water vapor) to be passed through and over the load of lumber in a unit time is selected. Low air velocities are preferred in this invention since laminar rather than turbulent flow is conducive to low degradation. The present invention is not limited to any particular air velocity range although, as mentioned previously, air velocities ranging from 50- 1200 ft. per min. and preferably from 150-800 ft. per min. may be used. The quantity of air chosen, expressed in lbs, of air per unit time, is multiplied by the heat capacity of the drying medium to obtain the number of heat units leaving the air and entering the wood per unit time when the air is cooled a specified amount. The heat energy which enters the wood-water system comprises sensible heat which acts to raise the temperature of the system including the lumber being dried, and latent or evaporative heat which acts to liberate free vapor within the wood and transport the vapor to the wood-air interface where it enters the air stream. In drying lumber the effective energy for causing the mass transfer of moisture from the wood into the drying fluid is the latent heat. For softwood dimension lumber evidence has shown that the latent heat represents 80-85% of the total heat which enters the wood, at least during the initial stages of drying green lumber. Thus, in calculating the drying potential of the circulating air, it is considered to be 85% of the amount of heat loss per unit time due to the drop in temperature across the kiln load.

(2) A schedule of rates of drying is selected for the particular species and moisture content of the wood in the kiln. The rate of drying is expressed in units of lbs. of water lost per sq. ft. of major surface of lumber per hour. This drying rate which is selected from experience based upon the drying characteristics, permeability, and shrinkage characteristics of the particular species of lumber being dried, when multiplied by the total area of major board surface in the charge yields the number of lbs. of water lost each hour from the total charge. This quantity in turn multiplied by the heat of vaporization of water yields the quantity of latent heat which must be supplied to the kiln each hour.

(3) The temperature drop which must be experienced by the drying medium to provide the quantity of latent heat required is then calculated using the parameters set forth in steps 1 and 2 above. In step 1, the amount of heat energy resulting from a given temperature drop across the kiln was shown. In step 2 the required amount of heat energy per hour to provide the desired rate of drying was calculated. If the total latent heat requirement from step 2 is divided by the latent heat potential from step 1 the number of degrees Fahrenheit through which the drying medium must cool in order to supply the required amount of heat.v to the lumber charge per unit time is found. This calculated result is the required temperature drop across the load necessary to achieve the desired rate of drying.

The apparatus used to dry lumber according to the process described is a conventional kiln having dry bulb temperature sensing heads on both the entering and leaving air sides of the charge. The two temperatures are compared by a suitable sensing control device which signals the need for higher entering air temperatures when the measured temperature dilference between the entering and leaving air is less than the set point of the instrument as a result of the calculations of step 3 above. These sensing control devices are known and commercially available.

The drying process is normally started at ambient temperature. A temperature gradient is quickly established across the entire charge at relatively low temperatures. As a consequence, a major portion of the moisture removal takes place at considerably lower temperatures than conventionally used in most kiln drying processes.

In order to establish the desired drying across the entire load it is necessary that the wet bulb temperature be maintained lower than the leaving air dry bulb temperature. To accomplish this it has been found effective to sense the leaving air dry bulb temperature and the wet bulb temperature and control the vents and spray steam in terms of a predetermined wet bulb depression on the leaving air side. The wet bulb depression at the leaving air side may range from 0.5" F. to 10 F. with a preferable wet bulb depression at the leaving air side of about 5 F. at dry bulb temperatures below 220 F. As the end of the drying cycle is approached, greater wet bulb depression will be observed. It should be noted that since essentially adiabatic conditions are maintained during passage of air through the load there will be little or no change in wet bulb temperature across the load.

Load width also dictates the wet bulb depression. The wider the load the greater the resulting temperature drop across the load for a given air velocity, slot thickness and drying rate. For wide loads the entering air humidity may be too low to be controllable or two low to preserve product quality on the entering air side. There is an optimum relationship of slot thickness, air velocity, humidity, temperature drop, and drying rate which must be established empirically for the various species of wood.

The initial optimum drying rate of step 2 may range from 0.001 to 0.5 lb./sq. ft./hr. Typical drying rates for western hemlock and Douglas fir are about 0.02 lb./sq. ft. /hr.; for western red cedar 0.005 lb./sq. ft./hr.; and for birch, 0.01 lb./ sq. ft./hr.

Any form of Wood having a surface over which a drying fluid can be circulated may be dried by the process of this invention. Typically the wood is in the form of boards, dimension lumber, veneer or shingles, but other equivalent forms of wood and lumber may be advantageously dried.

The process has been predominantly used for drying the species of woods used for structural applications. These species include Douglas fir, yellow pines, white pines, hemlocks, cedars, spruces and true firs. Many other species as well have been advantageously dried including oaks, birch, maples, walnut, cherry, hickories, gums, ashes, poplars and alder. Several of the more exotic woods such as tropical woods including luan and the like may also be dried by the process of this invention into wood products of superior quality in a minimum of time.

Reference to the examples shows that, even at very low drying rates, the kiln residence time is much less than required with conventional kiln drying processes. Also the drying is done primarily at low kiln temperatures, reducing degradation of the wood being dried.

7 EXAMPLE I Ninety-six 2" x x 8" western hemlock boards, stacked eight boards per layer in twelve layers, and separated by narrow /2 thick stickers, providing 4 sq. ft. of open cross section through the charge, were placed in a conventional kiln. The average green moisture content of the lumber was 77% by weight.

For this particular example an air velocity through the load of 300 it. per min. was chosen and a drying rate of 0.020 lb./sq. ft./ hr. To calculate what the setting of the temperature drop across the load should be for this predetermined drying rate, the following calculations were made in a manner described previously.

The velocity of 300 ft. per min. through 4 sq. ft. of cross section is equivalent to 1200 cu. ft. per min. or 72,000 cu. ft. per hour. The density of air is approximately 0.065 lb. per cu. ft. Multiplying 72,000 cu. ft. per hr. by. the density of air gives 4680 lbs. per hour of air passed through the kiln. The heat capacity of air is approximately 0.24 B.t.u.s per pound, per degree; therefore, 1120 B.t.u.s is lost by this quantity of air on cooling 1 F. Experience with the kiln being used showed that approximately 80% of the energy lost by the circulating air medium was used as latent heat for freeing and transporting moisture in the wood to the wood surfaces; thus, the drying potential of the circulating air is approximately 900 B.t.u.s per degree.

The major surfaces of a 2" x 10 x 8' board total 15.3 sq. ft. or 1470 sq. ft. for the total charge. Using the predetermined drying rate of 0.020 lb./sq. ft./hr., the rate of moisture loss would be 29.4 lbs. per hour. Experience has shown that western hemlock has an effective heat of vaporization of moisture from the wood of about 1050 B.t.u.s per lb. Considering these figures, a total evaporative or latent heat load of 30,900 B.t.u.s per hr. would be required to yield the desired drying rate of 0.020 lb./sq. ft./hr. If the circulating medium has a drying potential of 900 B.t.u.s per degree, a temperature drop of greater than 34 F. is required to furnish the required amount of evaporative energy, i.e., 30,900 B.t.u.s per hr.

Using these calculations the fan speed of the kiln was adjusted to a velocity of about 300 ft. per min. and the entering and leaving dry bulb temperature sensor-controller set to maintain a temperature drop across the load of 35 F. as calculated above. The leaving air dry bulb-wet bulb sensor-controller was set to maintain a wet bulb depression of 5 F. on the leaving air side. No wet bulb temperature control is needed or desired for the entering air side. The initial temperature of both lumber and the kiln atmosphere was approximately 80 'F. The drying process was started and after about 6 hrs. a steady drying rate of 28 lbs. per hr. had been achieved. This rate is very close to the calculated rate considering the difliculty of obtaining a precise rate of air flow and the uncertainty of the precise distribution coefficient between latent and sensible heat.

The entering air dry bulb temperature at the onset of steady state drying was 140 F. and the leaving air dry bulb temperature was 105 F. The wet bulb temperature on the leaving air side was 100 'F. The constant rate drying continued until the average moisture content of the lumber in the kiln reached approximately 48% by weight (about 28 hours). At this point the drying rate became less until a second constant rate was achieved at 22 lbs. per hr. (36 hours drying time). This second constant rate continued until the moisture content of the wood reached about 27% by weight, at which time a second change in drying rate to 16 lbs. per hr. took place. This last drying rate was continued until drying was stopped at 14% average moisture content after about 76 hours. The entering air dry bulb temperature at the first transition was 155 F. and at the second transition, 170 F. A final temperature of 210 F. was recorded on the entering air side and 175 F. dry bulb and 170 F. wet bulb on the leaving air side.

In the above experiment the air direction was not reversed as is conventionally done in most kiln drying processes. Although this was done there was no detectable moisture gradient across the load, proving that a constant rate of heat transfer from air to wood took place across the entire 8 ft. of load width. Also the rate of drying on the leaving air side was the same as on the entering air side even though the air temperature on the leaving air side was always 35 F. cooler than on the entering air id? and even though the wet bulb depression was 35 ess.

The kiln residence time, even at a very low drying rate, was 40 to 50 hrs. less than commonly used for drying western hemlock boards of this type. The moisture in the wood was being lost at the rate of 16 lbs. per hr. when drying was stopped. The major portion of the drying took place at low temperatures preventing any degradation of the wood.

EXAMPLE II A charge of 2" x 10 x 8' Douglas fir boards, 12 boards high and 8 boards wide, separated by /2" thick stickers, was placed in the same kiln used in Example I. An initial drying rate of 0.014 lb./sq. ft./hr. was chosen for this example. The air velocity was set at 400 ft. per

, min. and the entering and leaving dry bulb temperature sensor-controlled set to maintain a temperature drop across the load of 25 F. The initial moisture content of the wood was 36% by weight. After initiating drying at first equilibrium stage at a steady drying weight of 20 lbs. per hr. down to an average moisture content of 29% by weight was achieved. At this point the drying rate became less until a constant rate of 11 lbs. per hr. was achieved. This second constant rate continued until the moisture content reached approximately 21% by weight at which time a third change in rate took place. This latter rate was continued until the drying was stopped at 10% average moisture content after 51 hrs. Normal drying time for Douglas fir to 15% moisture content is hrs. The lumber was removed from the kiln and examined and found to be of excellent quality.

EXAMPLES III-VI Hemlock lumber two inches thick was dried in several runs of the kiln to demonstrate the effect of various temperature drops across the load as a basis for operating the kiln. Charges of 2" x 10" x 8' hemlock lumber, 12 boards high and 12 boards wide separated by stickers were placed in the same kiln used in Example I. The lumber was dried under the conditions shown in the table below and lumber of excellent quality resulted in Examples III through V.

Kiln conditions Drying Dryin' Velocity, time, rate Example ATL, F. t.p.n1. hours rams m! III 25 300 46 0 024 V 40 300 an 0. 03s 19 0.069

1 Prior art drying schedule.

2 Less than 0.01 average.

EXAMPLE VII Planks of western red cedar 10" wide and 1%" thick were dried from 72.7% moisture to 5.9% moisture in 52.5 hours using a AT of 25 F. with a drying fluid flow rate of 200 feet per minute. A conventional process for where:

N =Reynolds number w=mass of fluid/ time L =wetted permiter (here 2 widths and 2 lengths) =fluid viscosity It can then be shown that 2 X y1 p y u where 7=velocity and =density.

This equation has been solved for several operating conditions with the results presented below:

Velocity, ftJmin.

Slot

NEE dimension 104 F. 212 F. 300 F.

The process of this invention lends itself to many modifications. For example, if one desires to maintain the drying rate constant at 0.02 lb./sq. ft./hr. throughout the entire drying period it would be necessary only to increase the rate of heat flow from the air to the Wood. Referring to Example I the first transition represents a reduction in drying rate from 28 lbs. per hr. to 22 lbs. per hr. By increasing the rate of heat transfer by a factor of 28/22 a constant drying rate can be maintained. This can be done by increasing the air velocity from 300 ft. per min. to 382 ft. per min., by increasing the temperature drop across the load from 35 F. to 446 F., or by a combination of air velocity and temperature drop changes. It is obvious that the drying schedules may be programmed to follow any predetermined pattern which may be dictated by the particular item being dried.

Drying rate may be readily monitored by such conventional methods as observation of the rate of weight loss from the kiln charge or by measurement of lumber moisture content as drying progresses. A preferred apparatus for in process measurement of lumber moisture is described in application Ser. No. 639,134, now US. 3,430,357, filed May 17, 1967, and assigned to the as signee of the present invention.

Although the drying mechanism for lumber is quite complex, the process herein described allows the rate of moisture loss of a specific species of lumber to determine the optimum kiln conditions under which it is to be dried. The rate may be held as a linear function of time if desired and may be increased, decreased, held constant or controlled in a programmed combination to optimize the factors of drying time and product quality. Alternatively, the initial rate selected may be used to determine kiln parameters of air velocity and dry bulb temperature drop across the load and these may then be held constant during the remainder of the drying cycle. This practice is distinctly different than past methods wherein conditions arrived at by trial and error were imposed on a given species of lumber in order to attain a particular result.

What is claimed is:

1. A process for removal of a portion of the moisture content of wood wherein said wood is subjected to a flow 10 of drying fluid across the surface thereof, said process comprising:

selecting a rate of moisture removal from said wood;

and

providing a rate of energy transfer to said wood from said drying fluid by providing a constant temperature drop in the drying fluid across the wood in a drying zone during at least a portion of the drying cycle to establish said rate of moisture removal.

2. The process of claim 1 wherein said rate of moisture removal is substantially constant.

3. The process of claim 1 wherein said rate of moisture removal varies substantially linearly as a function of time.

4. The process of claim 1 wherein said rate of moisture removal ranges from 0.001 to 0.5 pound of water per square foor per hour.

5. The process of claim 1 wherein said flow of drying fluid is laminar across said surface.

6. The process of claim 1 wherein said flow of drying fluid is varied during the course of drying to correspond to the various stages of drying.

7. The process of claim 1 wherein the drying fluid leaving said drying zone has a wet bulb temperature depression of at least 0.5 F.

8. The process of claim 1 wherein said rate of moisture removal changes during the course of drying to correspond to the various stages of drying.

9. The process of claim 1 wherein said wood is in the form of boards, dimension lumber, veneer or shingles.

10. A process for removal of a portion of the moisture content of wood placed in a drying zone of a kiln, said kiln constructed to permit circulation of a drying fluid into contact with said wood, said process comprising:

selecting a constant temperature drop for the drying fluid across said drying zone;

controlling the temperature of the entering drying fluid to obtain and maintain said constant temperature drop for at least a portion of the drying cycle; and, passing said drying fluid across the surface of said wood.

11. The process of claim 10 wherein the rate of moisture removal from said Wood ranges between 0.001 and 0.5 pound of water per square foot per hour.

12. The process of claim 11 wherein said rate of moisture removal from said wood is substantially constant with respect to time.

13. The process of claim 11 wherein the rate of moisture removal varies substantially linearly with time.

14. The process of claim 10 wherein the drying fluid leaving said drying zone has a wet bulb temperature depression of at least 0.5 F.

15. The process of claim 10 wherein said wood is in the form of boards, dimension lumber, veneer or shingles.

16. The process of claim 10 wherein the velocity of the drying fluid circulating across the surface of said wood is altered during the course of drying to correspond to different stages of drying.

17. The process of claim 10 wherein the temperature drop is altered during the course of drying to correspond to different stages of drying.

18. The process of claim 11 wherein the rate of moisture removal is altered during the course of drying to correspond to diiferent stages of drying of the wood.

19. The process of claim 10 wherein said drying fluid flows in laminar flow across the surface of said wood.

20. A process for removal of a portion of the moisture content of wood wherein said wood is placed in a drying zone having means for passing a drying fluid across the surface of said wood from an inlet position to an outlet position, comprising:

establishing a first constant temperature drop in the dry ing fluid across the drying zone which provides sufficient energy transfer from said fluid to said wood to give a first rate of moisture removal in the range of 0.001 to 0.5 pound of water per square foot per hour;

1 1 controlling the inlet temperature to maintain said first constant temperature drop for that portion of the drying cycle until a predetermined moisture content in the wood is attained;

changing the temperature of said fluid at said inlet position to establish a second temperature drop across the drying zone which provides suflicient energy from said drying fluid to said wood to give a second rate of moisture removal from said wood in the range of 0.001 to 0.5 pound of water per square foot per hour; and,

controlling the inlet temperature to maintain said second temperature drop for that portion of the drying cycle until a second predetermined moisture content in the wood is attained.

21. The process of claim 20 wherein the drying fluid in said outlet position has a wet bulb temperature depression of at least 0.5 F.

22. The process of claim 20 wherein said second rate of moisture removal is constant with respect to time.

23. The process of claim 20 wherein said second rate of moisture removal varies substantially linearly with respect to time.

24. The process of claim 20 wherein said second rate of moisture removal has various values corresponding to the various stages of drying.

25. The process of claim 20 wherein said wood is in the form of boards, dimension lumber, veneer or shingles.

26. The process of claim 20 wherein said second temperature drop is altered during the course of drying to correspond to different stages of drying.

27. The process of claim 20 wherein the rate of flow of said drying fluid across the surface of said wood is altered during the course of drying.

28. The process of claim 20 wherein the rate of flow of said drying fluid across the surface of said wood produces laminar fluid flow.

29. The process of claim 20 wherein said first temperature drop is in the range of 25 F. to 75 F.

30. The process of claim 20 wherein said second temperature drop is in the range of 25 F. to 75 F.

31. The process of claim 20 wherein said drying fluid is circulated across the surface of said wood at a velocity in the range of 50 to 1200 [Eeet per minute.

32. The process of claim 20 wherein said drying fluid is circulated across the surface of said Wood at a velocity in the range of 150 to 800 feet per minute.

3 3. The process of claim 10 wherein said drying fluid is circulated across the surface of said wood at a velocity in the range of S to 1200 feet per minute.

34. The process of claim wherein said drying fluid is circulated across the surface of said wood at a velocity in the range of 150 to 800 feet per minute.

35. The process of claim 1 including controlling the rate of flow of drying fluid across the Wood to maintain the required rate of energy transfer to said wood to achieve the selected rate of moisture removal from the wood.

36. The process of claim 20 including controlling both said second temperature drop in the drying fluid across the Wood and the velocity of flow of drying fluid across the wood to establish the required rate of energy transfer of said wood to provide said second rate of moisture removal from the wood.

37. A method of drying wood including:

placing the wood to be dried in a drying zone,

providing a flow of drying fluid across a surface of said wood within said zone,

selecting a desired rate of water loss trom said surface,

determining a required constant temperature drop in the drying fluid flowing across said load that will establish said desired rate of water loss, and

establishing and maintaining said determined constant temperature drop by varying the temperature of said drying fluid on the upstream side of said load in 12 response to changes in the temperature of said drying medium on the downstream side of said load.

38. The method of claim 37 wherein the rate of flow of the drying fluid through said zone during a drying period results in laminar flow of said fluid across said wood surface.

39. The method of claim 68 wherein the drying fluid leaving said drying zone has a wet bulb temperature depression.

40. The method of claim 37 wherein the temperature drop in the drying fluid across said zone, the rate of flow of said drying fluid through said zone and the humidity of said drying fluid are controlled so as to provide a rate of moisture removal from said wood surface within a range of from 0.001 to 0.5 pound of water per square foot of wood surface per hour.

41. The method of drying a load of wood comprising:

selecting a desired rate of drying for said load, determining a drying medium flow rate by selecting a quantity of drying medium to be passed over the major surfaces of said load in a given unit of time,

determining the required temperature drop in the drying medium across the load which must be experienced to provide said desired rate of drying in accordance with the expression:

where dQw/d6=Rate of water loss from wood (lb./hr.)

A=Rate of dry air flow over lumber (lb./hr.)

a=Specific heat of dry air (B.t.u./lb.-degree) V=Rate of water vapor flow over lumber (lb/hr.)

v=Specific heat of water vapor (B.t.u./lb.-degree) dT=Change in temperature of air-vapor mixture during contact with lumber surfaces (degrees) W=Oven dry weight of food (1b.)

w=Specific heat of dry Wood (B.t.u./lb.-degree) F=Weight of free water in lumber (1b.)

f=Specific heat of free water (B.t.u./lb.-degree) B=Weight of bound water in lumber (1b.)

b=Specific heat of bound Water" (B.t.u./lb .-degree) d1 */d9=Rate of temperature change of lumber (degrees/hr.)

L*=Apparent heat of vaporization of water-air system then passing said drying medium over said load surfaces at the selected flow rate While monitoring the dry bulb temperature of the drying meduim at the upstream and downstream sides of said load and adjusting the temperature of the drying medium at said upstream side to maintain the required temperature drop in the drying medium across said load.

42. The method of claim 41 including maintaining a wet bulb temperature depression in the drying medium at the downstream side of said load.

43. The method of claim 41 including maintaining the rate of flow of drying medium across said load within the laminar flow range.

44. The method of claim 41 including varying the flow rate of the drying medium across the load to control the drying rate.

45. The method according to claim 37 wherein said temperature drop remains constant through at least a predetermined portion of a drying cycle.

46. The method according to claim 37 wherein said temperature drop is maintained constant throughout substantially the entire drying cycle.

47. The method according to claim 37 wherein different desired rates of water loss from said load are selected for different stages of the drying cycle and the required temperature drop in the drying fluid across said load 13 14 is changed for each difierent stage to establish said desired 3,259,994 7/ 1966 Klinkmueller et a1 3426 rates of water loss. 3,404,464 10/ 1968 Dedrick 34-246 3,434,222 3/ 1969 Malmquist 34-165 References 61M 3,131,034 4/1964 Marsh 34 30 UNITED STATES PATENTS 5 2,763,069 9/1956 Vaughan 34 26 CARROLL B-DORITY,JR-,Pr1mary r United States Patent [191 Beard, Jr. et al.

[541 KNIT CYCLE FOR CLOTHES DRYER [75] lnventors: William L. Beard, Jr., Benton Harbor; Ronald E. Hahn, 'St. Joseph, both of Mich.

[73] Assignee: WEE- 15661 Corp Benton Harm;

Mich.

221 Filed: use. 11, 1960 21 Appl. No.: 97,242

[52] US. Cl. ..34/45, 34/48, 318/483 [51] Int. Cl ..F26b 13/10 [58] Field of Search ..34/45, 48; 318/483 [56] References Cited UNITED STATES PATENTS Smith ..34/45 Elders et al..... Miller et al. ..34/45 1 Feb. 6, 1973 7 1970 Niewyk etal ..34 45 9/1966 Guentheretal ..34/4s Primary ExaminerCarroll B. Dority, Jr. Attorney-James S. Nettleton, Thomas E. Turcotte, Donald W. Thomas, Gene A. Heth, Franklin C. Harter and Robert L. Judd [5 7 ABSTRACT A knit cycle is provided for clothes dryers to prevent shrinkage caused by excessive drying due to inadertent presetting of the dryer programmer to an excessively dry setting. During an automatic drying cycle, moisture sensing apparatus is employed to stall or interrupt operation of a cycle terminating timer motor. Selection of a knit cycle overrides an incorrect setting by limiting the ability of the moisture sensing apparatus to affect the operation of the timer motor.

5 Claims, 6 Drawing Figures

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