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Drying wheat in two foot beds IV: Residual moisture content
J. agric. Engng Res. (1969) 14 (I) 26-3 1
Drying
Wheat IV:
Residual
in Two Moisture
Foot
Beds
Content
R. G. CLARK*; W.J. LAMOND* The residual moisture content of 83 beds of freshly harvested wheat is reported. Each bed, 2 ft deep, was dried from either 35 or 45 % to 13.6 % m.c. (d.b.) by parallel through flow heated air. Typical bottom layer moisture content data are given. The effects of changes in rate of flow and temperature of the drying air on the relationships between the residual moisture content and the moisture status of the drying bed are given. The results are compared with those of previous investigators. 1.
Introduction
The extremes of moisture content in a bulk of grain may have a significant effect on the retention of its quality during post-drying processing. In farm-dried grain there is frequently a considerable residual variation in moisture content when drying from a high level of initial moisture and the normal farm practice of mixing grain by inter-bin transfer is not wholly effective in eliminating this residual variation despite the relatively ready interchange of moisture between intimately mixed heterogeneous grains.’ M’Ewen and O’Callaghan2 have reported the effect of drying air conditions on residual range of moisture content (wr) when drying in beds up to 10 in deep from 27.5”! initial m.c. Woodforde and Lawton and Warner and Browne4 have provided data for 6 and 24 in deep beds respectively, which permit the relationship between w, and drying conditions to be calculated when drying from 25 % initial m.c. There have also been numerous particular measurements of w, notably during the testing of commercial batch-type driers. However, no work on the effect of drying conditions on w, for grain dried from a high initial m.c. in relatively deep beds has been traced. It is these conditions, likely to be most critical in practice, which are the subject of the present investigation. 2.
Scope of work and metbod
The present work is part of an investigation of the drying of 83 batches of freshly harvested wheat.5 The apparatus used and some of the experimental procedure has been described previously.s-7 Each batch was dried in a stationary bed from either 35 or 45 e< m.c. by a parallel through-flow air stream. The drying air rate of flow and temperature were kept constant for each batch ranging from 0.99 to 6.27 lb min-* ft-2 and 22.2 to 75ao”C respectively. The moisture content of grain at the bottom of the bed during drying was found by withdrawing samples placed on trays in layers 2-3 kernels deep between the two floors of the drying unit. A sample was usually withdrawn every 30 min but the interval was decreased during the period of rapid change and increased during the period of slow change. Duplicate samples were taken periodically to check the consistency of moisture content determinations. 3.
Results and discussion
Typical changes of bed mean moisture content (w,) and top layer m.c. (w,) with time have been previously reported.5-7 The corresponding values of bottom layer m.c. (~2~) are shown in Fig. 1. The experimentally determined relationship of wr, to time was adjusted by extending the curve to the nominal m.c. and using this as the time scale origin. The mean of the differences between the standardized and experimentally determined values of wb was 0.05 (o. 0.08) at w,= 17.7 ; the *N.I.A.E. Scottish Station, Pemcuik, Midlothian 26
R.
G.
CLARK;
W.
J.
LAMOND
k
24 10 Time
1’2 from
lb 16 start
of drying,
I8
20
A
24 26 k3 3;
h
Fiir. I. Tvpic.a/ exanzples of drying progress of bottom luyer from rzbour 35 y0 (top) and 459:
curve
Test
Bin
I
9 12 16 II 6 8 18 3 18 17 1 17
3 5 4 2
2 3 4 5 6 7 8 9 IO II 12
1
5 2
I 4 2 5 5
Airflo+l., lb minm’ fi-’
___~__.._
1.78
I .66 6.21 2.70 4.25 4.05 1.72 3.21 4.52 1.70 3.02 4.35
_...
Temperuture, 32.3 66. I 27.2 53.9 43.3 64.4 43.3 30.0 27.2 64.4 75.0 65.6
“C
(hottom)
28
DRYING
WHEAT
IN
fW0
FOOT
BEDS
corresponding value of W, was 1.98 % d.b. (o, l-42). There was some uncertainty in defining graphically some of the relationships between W, and drying conditions, probably mainly due to imperfect control of grain initial moisture content, air humidity at intake and, in some instances, to the limited number of observations made. The mean lines shown are considered to give the best design values from the available information, and were fitted by eye. MS,and ~1~at a given w,, varied widely under different drying conditions (Table I). TABLE 1 Extreme values of w, and wy at three- levels of wm, y0 d.b.
Bed mean m.c.
Residual
range
_____
qf m.c.
-__
Top layer m.c.
23.5
6-39
27-46
17.7
3-36
17-42
13.6
5-25
14-36
3.1. Efsects qf’rate of airflow w, and w, fell with increase in rate of air flow under all conditions investigated when drying to a given w, (Figs 2 and 3). The fall was not as great when drying to a given w, (Fig. 2, top) and there was some evidence (bottom) that the trend may be reversed.
0
I
/
I *
/
I
i
4
a-
’
6 Rate
of air
flow,
lb rmnm’ ft
‘!
’
Fig. 2. The effect of air flow rate on w, at drying air temperatures of (left to right) 26.7, 43.3 and65.6” C (top); 43.3 and 65.6” C (bottom) and grain initial m.c. of 35% d.b. (top) and 45 ‘A (bottom) (I) Final bed mean m.c, 23.5%
d.6.: (2) 17.7; (3) 13.6; (4) Final top layer m.c. 17.7%
d.b.
The tendency for w, to fall with increase in rate of flow when drying to a given UJ, is similar to that reported by other workers. 2-4 As would be expected w, is much greater for given drying conditions than that reported by these workers who investigated drying from lower initial m.c. or bed thicknesses.
R.
G.
CLARK;
W.
UJ---2
J.
LAMONt)
I 4
I 6
Rate of air flow, lb min
Fig. J. The yfict
ft?
of air flow rate on w, al drying air temperatures of (left to right) 26.7, 43.3 and 6S@ C (/op): 43.3 and 65.6” C (bottom) and grain initial m.c. of 35 o/Od.6. (top) and 45% (bottom) (I) Final bed mean m.c. 23.5%
d.b.; (2) 17.7; (3) 13.6%
3.2. Eflects of drying air temperature M‘,and w, increase with drying air temperature (Figs 4 and 5). The relationship is usually approximately linear but under some conditions NJ, and w, become constant above 45-60’ C and at the lowest rate of flow there is little change in M’,when drying to w,=23.5 yO (Fig. 5, top). The increase of )v, with drying air temperature is a trend reported by other workers2,3 who investigated shallower bed depths and by Warner and Browne4 who dried from a lower initial M’,. However, the value of w, for similar air conditions and the rate of change of w, with change of drying air temperature were both greater for the drying bed conditions used in the present investigation. 3.3. Eflect of grain m.c. Values of w, and w, for similar drying air and drying bed conditions were usually greater when drying from 4.5 y(, than from 35 o/, (Figs Z-5). Under similar clrying air conditions w, and IV, both usually fell with terminal MI,,,within the range n’, : 13-6-23.5 % investigated. The above trends are similar to those reported by Warner and BrowneO when drying from approximately 25 “/;;,m.c. 3.4. Condensation It has frequently been asserted both by drier operators and in the literature that condensation of water on to the downstream layers of a bed of grain may occur during heated air drying. This condensation has been frequently associated with increased difficulty in drying. The precise conditions under which condensation occurs and the extent of the harmful effect have not however been defined, although HukilP suggests that condensation is likely to occur if the depth and moisture status of the bed and the drying conditions are so related that the “depth-unit” is greater than 4 to 5. Hall9 though less specific suggests that moisture will often condense at the
30
DRYING
30
IN TWO
~007‘
UEDS
r
IO
0
WHEAT
L
20
I 30
I 40
I 50
I
I
60
70
20
Temperature,
30
I 40
I 50
I 60
I 70
I 80
“C
Fig. 4. The effect of temperature on w, ut air flow rates (left to right) of I .65 and 3.30 lb min-’ f-” (top); 5.28 and 3.30 lb mix’ ft-” (bottom) and grain initial m.c. of 35% d.b. (45% bottom right) (I) Final bed mean m.c. 23.5% d.b.; (2) 17.7; (3) 13.6; (4) Final top layer m.c. 17.7% d.6.
top of a bin of cold grain. Woodforde and Osborne, lo observed condensation in the upper layers of wheat artificially dampened to 20% m.c. w.b. and dried in layers 2 ft deep. M’Ewen and 0’Callaghan2 and Woodforde and Lawton refer to the possibility of condensation occurring. In the present investigation the initial grain temperature was 6-52” C below that of the incoming drying air and O-1 1”C above the atmospheric temperature at the start of drying; “depth-units” were from 3 to 27, 90% being greater than 5. Under these conditions the change in top layer moisture content was usually less than 2 % and never greater than 4 “A, d.b. No adverse effect on the drying process was observed. 4.
Conclusions
(a) Drying conditions can considerably affect the range of moisture content (w,) and the top layer moisture content (u’,) of a bed at given terminal value of mean moisture content (w,). (b) Both w, and w, at a given MI,,,usually increase in drying air temperature.
increased
(c) Both w, and w, at a given M’, increased when drying to higher grain final m.c. values. (d) No harmful
condensation
with either reduction
with grain initial
of water in the downstream
(e) Further and more precise data are required m.c. on the residual moisture content.
in rate of air flow or
m.c. and were usually
greater
layers of the grain bed was detected.
on the effect of rate of air flow and grain initial
R.
G.
CLARK;
IO----J-20
W.
30
J.
31
LAMOND
’
40
I
I
50
60
I 70
20
Temperature,
I
I
I
I
I
I
30
40
50
60
70
80
“C
on w, at air flow rates of (left to right) 1.65 and 3.30 lb min-’ ft-” (top): 5.28 and 3.30 r’b min- ’ft-" (bottom) and grain initial m.c. of 35% d.b. (45% bottom right)
Fig. 5. The effkcr of temperature
(I) Final bed mean m.c. 23.5% d.6.; (2) 17.7: (3) 13.60/
REFERENCES
Williamson, W. F’. Transjkr of moisture from damp to dry wheat in storage. Tech. Memo. 71 nat. Inst. agric. Engng, 1952 M’Ewen, E.; O’Callaghan, J. R. Through drying of deep beds of wheat grain. Engineer (Land), 1954, 198 (5159) 817 Woodforde, J.; Lawton, P. J. Drying cereal grain in beds six inches deep. J. agric. Engng Res., 1965, 10 (2) 146 Warner, M. G. R.; Browne, D. A. Drying wheat 2 ft deep in a 50ft2 tray. J. agric. Engng Res., 1962, 7 (2) 112 Clark, R. G.; Lamond, W. J. Drying wheat in 2 ft beds: I. Rate oj’drying. J. agric. Engng Res. 1968, 13 (2) 141 Clark, R. G. An installation for crop drying research. J. agric. Engng Res. 1966, 11 (1) 58 Clark, R. G.; Lamond, W. J. Drying wheat in 2 ft beds. III: Exhaust air humidity. J. agric. Engng Res., 1968, 13 (4) 33.5 Anderson, J. A.; Alcock, A. W., Ed. Storage of cereals and their products. Pub]. Am. Assoc. of Cereal Chem., 1954, 422 Hall, C. W. Drying farm crops. Publ. Agric. Consulting Ass. Inc., Reynoldsburg, 1957, 268 ” Woodforde, J.; Osborne, L. E. The drying of wheat in beds one and two feet deep. J. agric. Engng Res. 1961, 6 (4) 300
Drying
Wheat IV:
Residual
in Two Moisture
Foot
Beds
Content
R. G. CLARK*; W.J. LAMOND* The residual moisture content of 83 beds of freshly harvested wheat is reported. Each bed, 2 ft deep, was dried from either 35 or 45 % to 13.6 % m.c. (d.b.) by parallel through flow heated air. Typical bottom layer moisture content data are given. The effects of changes in rate of flow and temperature of the drying air on the relationships between the residual moisture content and the moisture status of the drying bed are given. The results are compared with those of previous investigators. 1.
Introduction
The extremes of moisture content in a bulk of grain may have a significant effect on the retention of its quality during post-drying processing. In farm-dried grain there is frequently a considerable residual variation in moisture content when drying from a high level of initial moisture and the normal farm practice of mixing grain by inter-bin transfer is not wholly effective in eliminating this residual variation despite the relatively ready interchange of moisture between intimately mixed heterogeneous grains.’ M’Ewen and O’Callaghan2 have reported the effect of drying air conditions on residual range of moisture content (wr) when drying in beds up to 10 in deep from 27.5”! initial m.c. Woodforde and Lawton and Warner and Browne4 have provided data for 6 and 24 in deep beds respectively, which permit the relationship between w, and drying conditions to be calculated when drying from 25 % initial m.c. There have also been numerous particular measurements of w, notably during the testing of commercial batch-type driers. However, no work on the effect of drying conditions on w, for grain dried from a high initial m.c. in relatively deep beds has been traced. It is these conditions, likely to be most critical in practice, which are the subject of the present investigation. 2.
Scope of work and metbod
The present work is part of an investigation of the drying of 83 batches of freshly harvested wheat.5 The apparatus used and some of the experimental procedure has been described previously.s-7 Each batch was dried in a stationary bed from either 35 or 45 e< m.c. by a parallel through-flow air stream. The drying air rate of flow and temperature were kept constant for each batch ranging from 0.99 to 6.27 lb min-* ft-2 and 22.2 to 75ao”C respectively. The moisture content of grain at the bottom of the bed during drying was found by withdrawing samples placed on trays in layers 2-3 kernels deep between the two floors of the drying unit. A sample was usually withdrawn every 30 min but the interval was decreased during the period of rapid change and increased during the period of slow change. Duplicate samples were taken periodically to check the consistency of moisture content determinations. 3.
Results and discussion
Typical changes of bed mean moisture content (w,) and top layer m.c. (w,) with time have been previously reported.5-7 The corresponding values of bottom layer m.c. (~2~) are shown in Fig. 1. The experimentally determined relationship of wr, to time was adjusted by extending the curve to the nominal m.c. and using this as the time scale origin. The mean of the differences between the standardized and experimentally determined values of wb was 0.05 (o. 0.08) at w,= 17.7 ; the *N.I.A.E. Scottish Station, Pemcuik, Midlothian 26
R.
G.
CLARK;
W.
J.
LAMOND
k
24 10 Time
1’2 from
lb 16 start
of drying,
I8
20
A
24 26 k3 3;
h
Fiir. I. Tvpic.a/ exanzples of drying progress of bottom luyer from rzbour 35 y0 (top) and 459:
curve
Test
Bin
I
9 12 16 II 6 8 18 3 18 17 1 17
3 5 4 2
2 3 4 5 6 7 8 9 IO II 12
1
5 2
I 4 2 5 5
Airflo+l., lb minm’ fi-’
___~__.._
1.78
I .66 6.21 2.70 4.25 4.05 1.72 3.21 4.52 1.70 3.02 4.35
_...
Temperuture, 32.3 66. I 27.2 53.9 43.3 64.4 43.3 30.0 27.2 64.4 75.0 65.6
“C
(hottom)
28
DRYING
WHEAT
IN
fW0
FOOT
BEDS
corresponding value of W, was 1.98 % d.b. (o, l-42). There was some uncertainty in defining graphically some of the relationships between W, and drying conditions, probably mainly due to imperfect control of grain initial moisture content, air humidity at intake and, in some instances, to the limited number of observations made. The mean lines shown are considered to give the best design values from the available information, and were fitted by eye. MS,and ~1~at a given w,, varied widely under different drying conditions (Table I). TABLE 1 Extreme values of w, and wy at three- levels of wm, y0 d.b.
Bed mean m.c.
Residual
range
_____
qf m.c.
-__
Top layer m.c.
23.5
6-39
27-46
17.7
3-36
17-42
13.6
5-25
14-36
3.1. Efsects qf’rate of airflow w, and w, fell with increase in rate of air flow under all conditions investigated when drying to a given w, (Figs 2 and 3). The fall was not as great when drying to a given w, (Fig. 2, top) and there was some evidence (bottom) that the trend may be reversed.
0
I
/
I *
/
I
i
4
a-
’
6 Rate
of air
flow,
lb rmnm’ ft
‘!
’
Fig. 2. The effect of air flow rate on w, at drying air temperatures of (left to right) 26.7, 43.3 and65.6” C (top); 43.3 and 65.6” C (bottom) and grain initial m.c. of 35% d.b. (top) and 45 ‘A (bottom) (I) Final bed mean m.c, 23.5%
d.6.: (2) 17.7; (3) 13.6; (4) Final top layer m.c. 17.7%
d.b.
The tendency for w, to fall with increase in rate of flow when drying to a given UJ, is similar to that reported by other workers. 2-4 As would be expected w, is much greater for given drying conditions than that reported by these workers who investigated drying from lower initial m.c. or bed thicknesses.
R.
G.
CLARK;
W.
UJ---2
J.
LAMONt)
I 4
I 6
Rate of air flow, lb min
Fig. J. The yfict
ft?
of air flow rate on w, al drying air temperatures of (left to right) 26.7, 43.3 and 6S@ C (/op): 43.3 and 65.6” C (bottom) and grain initial m.c. of 35 o/Od.6. (top) and 45% (bottom) (I) Final bed mean m.c. 23.5%
d.b.; (2) 17.7; (3) 13.6%
3.2. Eflects of drying air temperature M‘,and w, increase with drying air temperature (Figs 4 and 5). The relationship is usually approximately linear but under some conditions NJ, and w, become constant above 45-60’ C and at the lowest rate of flow there is little change in M’,when drying to w,=23.5 yO (Fig. 5, top). The increase of )v, with drying air temperature is a trend reported by other workers2,3 who investigated shallower bed depths and by Warner and Browne4 who dried from a lower initial M’,. However, the value of w, for similar air conditions and the rate of change of w, with change of drying air temperature were both greater for the drying bed conditions used in the present investigation. 3.3. Eflect of grain m.c. Values of w, and w, for similar drying air and drying bed conditions were usually greater when drying from 4.5 y(, than from 35 o/, (Figs Z-5). Under similar clrying air conditions w, and IV, both usually fell with terminal MI,,,within the range n’, : 13-6-23.5 % investigated. The above trends are similar to those reported by Warner and BrowneO when drying from approximately 25 “/;;,m.c. 3.4. Condensation It has frequently been asserted both by drier operators and in the literature that condensation of water on to the downstream layers of a bed of grain may occur during heated air drying. This condensation has been frequently associated with increased difficulty in drying. The precise conditions under which condensation occurs and the extent of the harmful effect have not however been defined, although HukilP suggests that condensation is likely to occur if the depth and moisture status of the bed and the drying conditions are so related that the “depth-unit” is greater than 4 to 5. Hall9 though less specific suggests that moisture will often condense at the
30
DRYING
30
IN TWO
~007‘
UEDS
r
IO
0
WHEAT
L
20
I 30
I 40
I 50
I
I
60
70
20
Temperature,
30
I 40
I 50
I 60
I 70
I 80
“C
Fig. 4. The effect of temperature on w, ut air flow rates (left to right) of I .65 and 3.30 lb min-’ f-” (top); 5.28 and 3.30 lb mix’ ft-” (bottom) and grain initial m.c. of 35% d.b. (45% bottom right) (I) Final bed mean m.c. 23.5% d.b.; (2) 17.7; (3) 13.6; (4) Final top layer m.c. 17.7% d.6.
top of a bin of cold grain. Woodforde and Osborne, lo observed condensation in the upper layers of wheat artificially dampened to 20% m.c. w.b. and dried in layers 2 ft deep. M’Ewen and 0’Callaghan2 and Woodforde and Lawton refer to the possibility of condensation occurring. In the present investigation the initial grain temperature was 6-52” C below that of the incoming drying air and O-1 1”C above the atmospheric temperature at the start of drying; “depth-units” were from 3 to 27, 90% being greater than 5. Under these conditions the change in top layer moisture content was usually less than 2 % and never greater than 4 “A, d.b. No adverse effect on the drying process was observed. 4.
Conclusions
(a) Drying conditions can considerably affect the range of moisture content (w,) and the top layer moisture content (u’,) of a bed at given terminal value of mean moisture content (w,). (b) Both w, and w, at a given MI,,,usually increase in drying air temperature.
increased
(c) Both w, and w, at a given M’, increased when drying to higher grain final m.c. values. (d) No harmful
condensation
with either reduction
with grain initial
of water in the downstream
(e) Further and more precise data are required m.c. on the residual moisture content.
in rate of air flow or
m.c. and were usually
greater
layers of the grain bed was detected.
on the effect of rate of air flow and grain initial
R.
G.
CLARK;
IO----J-20
W.
30
J.
31
LAMOND
’
40
I
I
50
60
I 70
20
Temperature,
I
I
I
I
I
I
30
40
50
60
70
80
“C
on w, at air flow rates of (left to right) 1.65 and 3.30 lb min-’ ft-” (top): 5.28 and 3.30 r’b min- ’ft-" (bottom) and grain initial m.c. of 35% d.b. (45% bottom right)
Fig. 5. The effkcr of temperature
(I) Final bed mean m.c. 23.5% d.6.; (2) 17.7: (3) 13.60/
REFERENCES
Williamson, W. F’. Transjkr of moisture from damp to dry wheat in storage. Tech. Memo. 71 nat. Inst. agric. Engng, 1952 M’Ewen, E.; O’Callaghan, J. R. Through drying of deep beds of wheat grain. Engineer (Land), 1954, 198 (5159) 817 Woodforde, J.; Lawton, P. J. Drying cereal grain in beds six inches deep. J. agric. Engng Res., 1965, 10 (2) 146 Warner, M. G. R.; Browne, D. A. Drying wheat 2 ft deep in a 50ft2 tray. J. agric. Engng Res., 1962, 7 (2) 112 Clark, R. G.; Lamond, W. J. Drying wheat in 2 ft beds: I. Rate oj’drying. J. agric. Engng Res. 1968, 13 (2) 141 Clark, R. G. An installation for crop drying research. J. agric. Engng Res. 1966, 11 (1) 58 Clark, R. G.; Lamond, W. J. Drying wheat in 2 ft beds. III: Exhaust air humidity. J. agric. Engng Res., 1968, 13 (4) 33.5 Anderson, J. A.; Alcock, A. W., Ed. Storage of cereals and their products. Pub]. Am. Assoc. of Cereal Chem., 1954, 422 Hall, C. W. Drying farm crops. Publ. Agric. Consulting Ass. Inc., Reynoldsburg, 1957, 268 ” Woodforde, J.; Osborne, L. E. The drying of wheat in beds one and two feet deep. J. agric. Engng Res. 1961, 6 (4) 300