Effect of Wet Bulb 

Depression on Heat 

Sterilization Time of 

Slash Pine Lumber 

William T. Simpson 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Paper 

FPL−RP−604 
Abstract 

For international trade, heat sterilization of wood products is

often required to prevent the spread of insects and pathogens. Application of heat sterilization requires estimates of 

the time necessary to heat the center of the wood configuration to the temperature required to kill the insect or other 

pest. The nature of the heating medium was found to have a 

significant effect on heating time of slash pine lumber. The 

heating time increased exponentially with wet bulb depression. When the wet bulb temperature in the heating chamber 

was below the desired center temperature, heating times 

were extended far beyond the times when target center temperature was greater than the wet bulb temperature. This 

effect was less in air-dried lumber than in green lumber. 

Keywords: heat sterilization, international trade, kiln drying

June 2002 

Simpson, William T. 2002. Effect of wet bulb depression on heat sterilization time of slash pine lumber. Res. Pap. FPL-RP-604. Madison, WI: U.S.

Department of Agriculture, Forest Service, Forest Products Laboratory. 6 p.

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an equal opportunity provider and employer.
Effect of Wet Bulb Depression on Heat 

Sterilization Time of Slash Pine Lumber 

William T. Simpson, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin 

Introduction 

Heat sterilization of wood in various forms is currently

receiving attention as a means of killing insects and pathogens to prevent their transfer from one region of the world to

another as a side effect of international trade. One issue of

concern is the time required to heat wood of various species 

and sizes to a temperature that will kill the insect or pathogen. Recent research (Simpson 2001) has shown that the 

heating medium has a large effect on heating time. In the 

nondrying medium of saturated steam, heating proceeds 

rapidly. But when the drying air is not saturated, the wood 

also dries during heat treatment. This drying causes the 

surface to cool below the air temperature, which in turn 

reduces the center-to-surface temperature gradient. The 

temperature gradient is the driving force for heat conduction;

when it is reduced, heat conduction is slowed. We would 

expect that as drying conditions become more severe, the 

surface cooling will increase and the time required for the 

center to reach some temperature will also increase. Previous 

work showed heating times of green lumber in severe drying 

conditions were in some cases extended to more than five 

times that in a saturated steam environment. Heat sterilization therefore should ideally be done in saturated steam, 

which can sometimes be accomplished in a tight dry kiln by 

heating with just the steam spray. However, some dry kilns 

are not capable of maintaining saturated conditions, and 

furthermore, some specially constructed heat sterilization 

chambers may have no provision for introducing steam. 

The purpose of this study was to further define the effect of 

simultaneous drying on heating times. The previous study

(Simpson 2001) looked only at the two extremes—nondrying saturated steam and a very severe drying condition of 

55°F (30.6°C) wet bulb depression (the difference between 

dry bulb temperature, that is, the air temperature, and wet 

bulb temperature)—but did not examine drying conditions 

of intermediate severity. This study includes the effect of

several levels of wet bulb depression on heating times of 

slash pine lumber 1.0 and 1.8 in. thick (25 and 46 mm thick) 

heated at green moisture content and 1.0 in. (25 mm) thick 

heated at air-dried moisture content. 

Experimental Methods 

The slash pine lumber was obtained from a plantation in

northern Florida and was received in the form of 8-ft- 

(2.4-m-) long nominal 2- by 4-in. (nominal 51- by 102-mm)

lumber freshly sawn and undried. Some lumber for the study

was used in the thickness as received (1.8 in. (46 mm)), and 

the rest was surfaced to 1.0 in. (25 mm) thick for the study. 

Board width averaged about 3.85 in. (98 mm). 

The experimental heating runs were conducted in a 

1,500 board foot (3.5 m3

) laboratory dry kiln. Most of them

were run at a dry bulb temperature of 160°F (71°C). The 

target levels of wet bulb depression for both 1.0- and 1.8-in.- 

(25- and 46-mm-) thick lumber were 0, 5, 10, 15, 20, 30, and 

50°F (0, 2.8, 5.6, 8.3, 11.1, 16.7, and 27.8°C), with corresponding equilibrium moisture content levels of +20%, 

15.2%, 11.5%, 9.4%, 7.9%, 5.8%, and 3.2%. Because there 

was an excess of experimental material, several additional

heating runs were conducted. With the 1.8-in.- (46-mm-) 

thick lumber, we missed some target wet bulb depressions 

by an undesirably wide margin; so two additional runs were 

conducted to fill in the large gaps between depression levels. 

Because high temperature dry kilns are common in the 

Southern Pine lumber industry, we also included several

heating runs at a target dry bulb temperature of 240°F 

(116°C). For the 1.0-in- (25-mm-) thick lumber, we included 

target wet bulb depressions of 60°F (15.6°C) and 110°F 

(43.3°C), resulting in target wet bulb temperatures of 180°F 

(82.2°C) and 130°F (54.4°C). For the 1.8-in.- (46-mm-) 

thick lumber, the target wet bulb depression was 60°F 

(15.6°C). Air velocity was about 600 ft/min (3 m/s). All

these tests were conducted at green moisture contents. Because of the anticipated effect that surface cooling during 

drying would extend heating times, the question arose of 

whether or not heating times would also be extended if the 

drying during heating were reduced. With that question in

mind, we air-dried enough of the 1.0-in.- (25-mm-) thick 

boards before heat treatment for six levels of wet bulb depression. Table 1 summarizes the experimental conditions. 
2 

Table 1—Experimental heating conditions used in study. 

Temperatures listed are target temperatures and differ slightly 

from actual temperatures attained

1.0 in. (25 mm) thick 1.8 in. (46 mm) thick 

Heating Target wet bulb Heating Target wet bulb 

temperature depression temperature depression

°F (°C) °F (°C) °F (°C) °F (°C) 

Heated at green moisture content

160 (71) 0 160 (71) 0 

5 (2.8) 5 (2.8) 

10 (5.6) 10 (5.6) 

15 (8.3) 15 (8.3) 

20 (11.1) 20 (11.1) 

30 (16.7) 30 (16.7) 

50 (27.8) 50 (27.8) 

240 (116) 60 (33.3) —a

110 (61.1) 240 (116) 60 (33.3) 

Heated at air-dried moisture content

160 (71) 0 

10 (5.6)

15 (8.3)

20 (11.1)

30 (16.7)

50 (27.8) 

a

Two additional runs at 160°F (70°C) were included with excess 

 experimental material. Target wet bulb depressions were 15°F 

 (8.3°C) and 20°F (11.1°C). See Table 2 for actual temperatures. 

The center temperature of interest was 133°F (56°C) because 

at the time of this study, import–export regulations required 

that temperature to be reached and held for 30 min. 

The choice of experimental wet bulb temperatures brackets 

this required center temperature, and the results will show

that the relationship between the wet bulb temperature and 

the required center temperature has a pronounced effect on 

heating time. 

Each heating run included 33 boards arranged in three layers 

of 11 boards each. Temperature measurements were made on 

the center nine boards in the center layer. The layer above 

and the layer below were present so that the study boards 

were flanked by wet boards and thus more closely simulated 

boards in a large kiln filled with lumber. 

The lumber was stacked on a kiln truck outside the kiln, 

which was already running at the target conditions, and 

thermocouples were inserted into the nine study boards. 

Each of these nine boards was fitted with two thermocouples, one inserted to the center and one placed on the surface 

(Fig. 1a and b). The center thermocouple was inserted in a 

slightly oversized hole drilled into the edge of the board, and

the hole was plugged with a round toothpick to minimize the 

influence of the outside kiln temperature on the measured 

center temperature. The surface thermocouples were held 

tightly to the board surfaces with plastic-headed (nonconducting) push pins and were brought into even closer contact

Figure 1—Thermocouples placed (a) on the surface and (b) in the center of board. 
with the surface by pressing down gently on the thermocouple junction. The nine boards were stacked edge-to-edge 

with the surface thermocouple on the underside of the board. 

With all thermocouples in place, the kiln door was opened, 

the kiln truck wheeled in, and the door closed as quickly as 

possible to minimize recovery time of the desired kiln conditions. Because of this required recovery time and the inherent difficulty of attaining precise control of kiln conditions 

(especially in a kiln loaded to only a small fraction of its 

capacity), the exact target wet and dry bulb temperatures 

were not always attained to a high degree of precision. However, they were attained to a level fully sufficient to satisfy 

the objectives of the study. Both center and surface temperatures were read to a computer file at intervals ranging from

0.5 to 2 min, depending on the expected rate of temperature 

rise. Runs were terminated after the last of the nine center 

temperatures reached 133°F (56°C). Each of the nine boards 

was weighed before and after heat treatment and then 

ovendried for moisture content determination. 

Results and Discussion 

As the wet bulb depression increased during heating, the rate

of drying was expected to increase. One consequence of the 

increased drying rate was cooling of the surface below the 

heating temperature of 160°F (71°C). This cooling is shown 

in Figure 2 for 1.0-in.- (25-mm-) thick boards heated at

green moisture content, in Figure 3 for 1.8-in.- (46-mm-) 

thick boards heated at green moisture content, and in Figure 4 for 1.0-in.- (25-mm-) thick boards heated at air-dried 

moisture content. In Figures 2 to 4, the average surface 

temperature of the nine boards in each heating run is plotted 

against wet bulb depression at several times during heating. 

All figures clearly show that as wet bulb depression increased, surface temperature decreased. For example, with 

1.0-in.- (25-mm-) thick boards heated at green moisture 

content (Fig. 2), surface temperature was about 157°F 

(69.4°C) after 20 min when the average wet bulb depression 

during the run was 3.5°F (1.9°C). When wet bulb depression 

was 49°F (27.2°C), the surface temperature after 20 min was 

only about 112°F (44.4°C), indicating surface cooling due to 

the evaporation of water from the surface during drying. 

Figure 4 shows that even after 1.0-in.- (25-mm-) thick 

boards were air-dried to approximately 15% moisture content, surface cooling was still taking place, although as expected, not as much as when the boards were heated at green 

moisture contents. Even though the air-dried boards were at

15% moisture content, the equilibrium moisture content in

the kiln was below that level (except for the minimum wet 

bulb depression level; Table 2) and evaporation from the 

surface could still occur. 

The results of the heating time experiments are summarized 

in Table 2, and the effect of wet bulb depression on the time 

required to heat the center of the boards to 133°F (56°C) is 

shown in Figure 5 for boards heated at green moisture 

Figure 2—Dependence of surface temperature on wet 

bulb depression at several times during the heating of

green 1.0-in.- (25-mm-) thick slash pine at 160°F (71°C) 

dry bulb temperature (°C = (°F – 32)/1.8). 

Figure 3—Dependence of surface temperature on wet 

bulb depression at several times during the heating of

green 1.8-in.- (46-mm-) thick slash pine at 160°F (71°C) 

dry bulb temperature (°C = (°F – 32)/1.8). 

0 10 20 30 40 50

110

120

130

140

150

160

170

10 minutes

30 minutes

60 minutes

60 m

Wet bulb depression (°F) 

Surface temperature (◦F) 

30 m

10 m

170

160

0 10 20 30 40 50

100

110

120

130

140

150

10 minutes

20 minutes

30 minutes 

45 minutes

Surface temperature (◦F) 

Wet bulb depression (◦F) 

3 
Figure 4—Dependence of surface temperature on wet 

bulb depression at several times during the heating of

air-dried 1.0-in.- (25-mm-) thick slash pine at 160°F (71°C) 

dry bulb temperature (°C = (°F – 32)/1.8). 

160 Table 2—Results of heating time experiments with 1.0- and 

1.8-in.- (25- and 46-mm-) thick slash pine heated at target 

temperatures of 160°F (71°C) and 240°F (116°C)a

Wet

bulb 

depression 

(°F) 

Average 

dry bulb 

temp. 

(°F) 

Average 

wet bulb 

temp. 

(°F) 

Average 

green 

moisture 

content 

(%) 

Average 

moisture 

content 

after 

heating 

(%) 

Average 

time for 

center to 

reach 

133°F 

(min) 

Heated at green moisture contents

1.0 in. (25 mm) thick 

3.5 160.3 156.8 111 109 15.1

5.0 158.9 153.9 109 111 15.4

9.5 158.4 148.9 116 114 18.7

14.7 159.6 144.9 120 117 20.9

19.3 160.0 140.7 109 106 26.2

28.6 159.8 131.2 106 75 109

49.2 160.0 110.8 106 34 438

68.5 241.2 172.7 113 110 9.9

105.5 238.7 132.6 118 95 48.0

1.8 in. (46 mm) thick 

4.0 160.6 156.6 100 98 38

6.1 159.7 153.6 112 110 43

11.5 159.0 147.5 90 90 47

13.5 159.2 145.7 93 93 52

15.7 159.7 144.0 81 79 57

18.8 159.3 140.5 88 84 60

21.0 161.4 140.4 80 79 64

29.5 161.0 131.5 68 62 102

48.6 160.1 111.5 76 57 198

57.4 232.7 175.3 109 104 25

Heated at air-dried moisture contents

1.0 in. (25 mm) thick 

5.6 157.4 151.8 15.3 15.3b 8.7

12.3 158.6 146.3 14.4 10.9b 10.2

16.6 159.1 142.5 15.4 9.3b 11.3

19.0 157.3 138.2 15.2 8.6b 14.2

28.5 158.4 129.8 15.6 6.6b 18.1

48.8 160.5 111.6 15.5 3.6b 23.0

a

°C = (°F – 32)/1.8. b

Equilibrium moisture content conditions in kiln. 

5 minutes 

10 minutes

15 minutes 155

Surface temperature (◦F) 

150

145

140

5

135

10 15 20 25 30

Wet bulb depression (°F) 

content and in Figure 6 for boards heated at air-dried moisture content. It is clear that as wet bulb depression increased, 

heating time also increased. For 1-in.- (25-mm-) thick boards 

heated at green moisture content, with a dry bulb temperature of 160°F (71°C), heating time increased from about

15 min at a wet bulb depression of 3.5°F (1.9°C) to 438 min 

at a wet bulb depression of 49.2°F (27.3°C), a factor of 29. 

For 1.8-in.- (46-mm-) thick boards heated at green moisture 

content, heating time increased from 38 min at a wet bulb 

depression of 4.0°F (2.2°C) to 198 min at a wet bulb depression of 48.6°F (27.0°C), a factor of 5.2. For 1.0-in.- 

(25-mm-) thick boards heated at 160°F (71°C) at air-dried 

moisture content, heating time increased from 8.7 to 23 min, 

a factor of only 2.6. 

As Figure 5 shows, the increase in heating time with increase in wet bulb depression is relatively small at the lower 

values of wet bulb depression. But at the larger values of wet

bulb depression, the increase is quite large and follows an 

exponential relationship. The point at which the heating time 

begins to increase sharply with wet bulb depression is when 

the wet bulb temperature in the kiln is about equal to the 

desired center temperature. In the heating runs listed in

Table 2, the runs where the wet bulb temperature was below 

133°F (56°C) (wet bulb temperatures targeted for either 

110°F or 130°F) (43.4°C or 54.5°C) showed sharply longer 

heating times than runs where the wet bulb temperature was 

greater than 133°F (56°C). 

Figure 7 illustrates the progression of surface and center 

temperatures when the wet bulb temperature is less than the 

desired center temperature (heated at green moisture content). In this example, dry bulb temperature was 160°F 

(71°C), wet bulb depression was 49.2°F (27.3°C), wet bulb 

temperature was 110.8°F (43.8°C), and 1.0-in.- (25-mm-) 

thick boards were used. The center temperature rose to the 

wet bulb temperature relatively quickly (about 50 min), then 

gradually increased, but even after 800 min, it was only

about 140°F (60°C). The surface temperature rose relatively 

quickly to about 125°F (51.7°C) and then gradually rose to

4 
5 

about 150°F (65.6°C) after 800 min. In contrast, when the 

wet bulb depression was only 3.5°F (1.9°C), the center 

reached 110.8°F (43.8°C) in about 11 min and 133°F (56°C) 

in about 15 min. In addition, the surface temperature reached 

160°F (71°C) in about 12 min. 

The results suggest that there are two causes that extend 

heating times beyond those for heating in a saturated steam

environment. The first is the surface cooling effect that 

results from the increased drying when wet bulb depression 

is increased. This reduces the center-to-surface temperature 

gradient, and because this temperature gradient is the driving 

force for heat conduction, heating time increases. Secondly,

if the wet bulb temperature is less than the desired center 

temperature, the heating time may be increased because of 

evaporative cooling in the center during drying. Heating 

time to 133°F (56°C) (at 160°F (71°C) dry bulb temperature) 

increased by a factor of 29 from a wet bulb depression of 

3.5°F (1.9°C) to a wet bulb depression of 49.2°F (27.3°C) 

for green 1.0-in- (25-mm-) thick boards. But the increase 

between those two extremes was only a factor of 5.2 for the 

1.8-in.- (46-mm-) thick boards. The reason for this is that as 

thickness increases, internal transfer mechanisms play a 

more important role compared with surface heat and mass 

transfer mechanisms (Fleischer 1953). 

Figure 5—Dependence of the time required for the center 

of green slash pine boards to reach 133°F (56°C) on wet 

bulb depression (wbd) when heated at 160°F (71°C) dry

bulb temperature (°C = (°F – 32)/1.8). 

Figure 6—Dependence of the time required for the center 

of air-dried slash pine boards to reach 133°F (56°C) on 

wet bulb depression (wbd) when heated at 160°F (71°C) 

dry bulb temperature (°C = (°F – 32)/1.8). 

Figure 7—Increase in surface and center temperatures 

when green 1.0-in.- (25-mm-) thick slash pine boards 

were heated at 160°F (71°C) dry bulb temperature with a

49.2°F (27.3°C) wet bulb depression (°C = (°F – 32)/1.8). 

0 200 400 600 800

160

40

60

80

100

120

140

Center temperature

Surface temperature

Temperature (◦F) 

Time (min) 

500

0 10 20 30 40 50

0

100

200

300

400

1 in. thick: 

Time = 9.01 exp(0.0776 wbd) (R2

 = 0.955) 

1.8 in. thick: 

Time = 31.6 exp(0.0374 wbd) (R2 = 0.989)

Wet bulb depression (°F) 

Time for center to reach 133◦F (min) 

24

0 10 20 30 40 50

8

12

16

20

Time = 8.12 exp(0.0232 wbd) (R 2 = 0.923)

Time for center to reach 133◦F (min) 

Wet bulb depression (°F) 
The moisture content data in Table 2 show the extent of 

drying when the wet bulb depression is large. When the 

green 1.0-in.- (25-mm-) thick boards were heated with wet

bulb temperatures above 133°F (56°C), there was little 

decrease in moisture content during heating. Moisture contents were reduced by only a few percent during drying. In 

contrast, with the two wet bulb temperatures below 133°F 

(56°C), moisture contents were reduced from slightly more 

than 100% to 75% and 34% for wet bulb temperatures of 

131.2°F (55.1°C) and 110.8°F (43.8°C), respectively. 

The results in Table 2 also show that heating times were 

extended at a wet bulb temperature below 133°F (56°C) 

even when the heating temperature was 240°F (116°C). With

green 1.0-in.- (25-mm-) thick boards and a wet bulb temperature of 172.7°F (78.2°C), heating time was 9.9 min. But

when the wet bulb temperature was 132.6°F (56°C), the 

heating time was 48 min. 

Concluding Remarks 

This study investigated the effect of wet bulb depression at

constant dry bulb temperature on the time required to heat

the center of green 1.0- and 1.8-in.- (25- and 46-mm-) thick 

slash pine boards to 133°F (56°C). The expectation was that 

as wet bulb depression increased, drying rate would increase 

and result in a cooling effect that would slow heating relative 

to conditions where little or no drying occurred. Measurements showed that surface cooling increased as wet bulb 

depression increased. The time required for the center to 

reach 133°F (56°C) increased exponentially with wet bulb 

depression at a constant dry bulb temperature of 160°F 

(71°C). The increase was greater for the 1.0-in.- (25-mm-) 

thick boards than for the 1.8-in.- (46-mm-) thick boards. 

When the wet bulb temperature was less than the desired 

center temperature, the time required for the center to reach 

the desired temperature was greatly extended far beyond the 

time required when the wet bulb temperature was greater 

than the desired center temperature. Boards that had been 

air-dried to 15% moisture content before heating also 

showed these effects but to a lesser degree than boards 

heated at green moisture content. 

Literature Cited 

Fleischer, H. 1953. Veneer drying rates and factors 

affecting them. Forest Products Journal 3(3): 27–32. 

Simpson, W.T. 2001. Heating times for round and rectangular cross sections of wood in steam. Gen. Tech. Rep. FPL–

GTR–130. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 103 p. 

6