- Email: [email protected]
Modelling the performance of a cross-flow grain drier
J. agric. Engng Res. (1987) 37, 43-57
Modelling the Performance of a Cross-flow Grain Drier M. E. NELLIST* A computer model of a cross-flow grain drier was developed in which the equations of heat and mass transfer were solved by Euler integration and the drying rate was defined as the product of a drying constant, k, and the excess of the instantaneous moisture, M, above an equilibrium value, !vf,. To validate the model, five different combinations of expressions for k and M, were used to predict the performance of three cross-flow driers for which test data were
available. Comparisons were made of the observed and predicted values of grain throughput, specific heat energy consumption, temperatures of output grain and grain germination. That combination of expressions for k and M,, which gave the best correlation of throughput of wet grain and specific heat consumption, was used in the computer model to correct the test results to standard conditions and thereby to separate the effect of design from that of operating conditions. A short study of the effect of bed depth, drying air temperature and ratio of drying area to cooling area was used to define a specification for an example cross-flow drier which, except for its having a larger cooling area, could be regarded as typical of contemporary models. The performance of this example drier was then calculated for initial moisture contents of from 16-26% w.b. and drying air temperatures from 40 to 100°C when drying wheat to 15% w.b. At the normal rating conditions of drying air temperature and initial moisture content of 65.5”C and 209/, w.b. respectively, the drier had a predicted throughput of 2.2 t/h and specific heat energy consumption of 4.99 MJjkg water evaporated. However, across the range of temperatures and moisture contents examined, throughput varied from 0.45 to 12.5 t/h and specific heat consumption from 8.74 to 3.74 MJ/kg water evaporated. Grain cooling was shown to depend upon the residence time determined by the interaction of drying air temperature and initial moisture content. In the example drier, total residence times greater than 0.6 h were necessary for output grain to be cooled to within 5°C of ambient temperature.
1. Introduction The main purpose of this paper is to present some of the results of a study by computer simulation of the performance of a simple cross-flow grain drier, i.e. a drier in which a moving stream of grain is successively dried and cooled by air flowing at right angles to the stream. For such a study, computer simulation has three main advantages over physical
experimentation.
These are
(a) A saving of several orders of magnitude in time and cost. (b) Important variables, which in practice are subject to uncontrollable variation, can be held constant. (c) Changes can be calculated in those quantities, grain temperature, for example, which are either not easy or impossible to measure by experiment. It follows that experimental data for validating drier models are at best imperfect’ and at worst unobtainable. Fortunately, in the present work it was possible to carry out a partial * Crop Drying and Ventilation Group, NIAE (now AFRC Institute of Engineering Research). Wrest Park, Silsoe, Bedford Received
MK45 4HS, UK
8 March
Paper presented
1985; accepted
in revised form 17 May 1986
at AG ENG 84, Cambridge,
UK. l-5 April
1984
43 002 I -8634:X7:050043
+ 15 $03.00/O
(‘8 1987 The British Society for Research
in Agricultural
Engineering
44
A CROSS-FLOW
GRAIN
DRIER
Notation
a, b empirical coefficients h heat transfer coefficient, W/m2 K k drying constant, s-l Y correlation coefficient r.h. relative humidity, ratio t time, s depth of bed, m specific heat capacity, kJ/kg K ; G mass rate of flow, kg/s m2 H absolute humidity of air, kg water/kg dry air L, latent heat of vaporization of water at O”C, kJ/kg LT latent heat of vaporization of water in grain, kJ/kg M grain moisture content, decimal d.b.
S T p
specific surface area of grain, mZ/m3 temperature, “C density, kg/m3
1
Subscripts
a abs e g 0 pa pg pl pw
air absolute equilibrium grain value of t = 0 air at constant pressure grain at constant pressure water liquid at constant pressure water vapour at constant pressure
validation using data from three NIAE User Tests. 2S3B4 These results were used not only to compare computed and observed performance but also to examine the choice of functions describing the drying properties of grain and to illustrate the use of simulation in adjusting test results to a common basis by which they may be compared. Only then does the paper proceed to consider the performance of an example cross-flow grain drier as affected by the temperature of the drying air and the moisture content of the input grain. 2. The computer model The heat and mass transfer equations used in the NIAE model of Nellist5 are essentially the same as those used by O’Callaghan et al.,’ and are solved in a similar manner by a simple Euler integration technique. The resulting program is very economical in computing time but provides no control of, or information about, integration error. A second program, which is based upon a more rigorous formulation and solution of the equations’ is available to check calculations but requires excessive computing time for everyday use. These two programs have given almost identical results when using the same functions describing the properties of the air and grain.’ There are a number of minor differences between the model of O’Callaghan et al.’ and Nellist’ of which, perhaps, the most important is the condensation routine.’ In addition, the NIAE model contains a procedure for calculating the loss of grain viability by the accumulation of “probit” viabilities’ and is able to search for a specified output moisture content by adjusting any one of the four input parameters, grain throughput, grain bed depth, air temperature and air flow. Although the computer program also contains facilities for modelling a number of possible variations on a simple cross-flow drier, these, apart from one exception discussed later, were not used in the present study. A greatly simplified flow chart is given in Fig. 1. In finite difference notation and in order of solution the equations solved are: AM = - k(M - M,) . At/( 1 + +k At),
(1)
M.
E.
45
NELLIST
t Initialize
t Calculate heat and moss transfer in ail layers for estimated residence time
I Calculate average within bed
,, 1, ,\
conditions
No
Yes
Set printing
Fig. 1. SimpIiJiedJlow-chart
q=
of a cross-j?ow drier
A+,~[~Y+~F]~[~+&(~B+~B+C,,AM)].(2) AT,=-
where
of the simulation
and
(AT,(B + C,, . AM) - AMF)
&
(3)
A= B= E= F= Y=
The most important of these is Eqn (1) which describes the rate of drying of the grain and contains two coefficients k and M, which, in their turn, are functions of drying air temperature and humidity. Values of k and M, are determined experimentally by fitting Eqn (l), in the integral form M-M -5
MO-M,
= exp(-kt)
46
A CROSS-FLOW
GRAIN
DRIER
by the method of least squares, to drying curves obtained in “thin” or exposed layer drying tests. Unfortunately, such curves are not well fitted by Eqn (5) because, in practice, they are not simple decaying exponentials and k is not a constant. Thus, the values of k obtained depend to a certain extent upon the circumstances of the experiment (cf. Sutherland and Ghaly” and Bailey and Smith”). Also it is conventional to use the Arrhenius equation: k = a exp (-b/T,,,)
(6)
to correlate k with drying air temperature. With the latter expressed on the Kelvin scale, small differences in the values of k cause large differences in the values of the coefficients a and b obtained by different workers for apparently compatible sets of data (see, for example, Watson and Bhargava’*). When the present work began the values of the coefficients in Eqn (6) in use at NIAE were those given by Boyce I3 for barley (a = 139.3, b = 4426). These had been found to give good agreement with results for drying wheat in mixed-flow driers14 when used in conjunction with the equation for equilibrium moisture content, M, given by Nellist and Dumont,” M, = 0.113-0.016
In T-0.079
In (1 -r.h.).
(7)
which was based on a correlation of results for wheat. Subsequently, this combination of coefficients was used in a study of the performance of mixed-flow driers.18 But doubt about the suitability of these expressions, at least for barley, was cast by Bowden et ~1.‘~ who suggested values of a = 3.485 x IO5 and b = 6942 in Eqn (6) and that, increasing the first coefficient in Eqn (7) from 0.113 to 0,143 gave better agreement with final moisture contents obtained under drying conditions. It may be significant also that the values of M, given by the modified equation are of the same order as those obtained for wheat by McEwen and O’Callaghan, ” but not adeq uately described by them for modelling purposes. Earlier, these authors’ k values were corrected by O’Callaghan er al.’ who found values of the coefficients a = 2000 and b = 5094 in Eqn (16). Although more, and possibly better, descriptions of the drying rate exist, these were the relationships which appeared to be more relevant at the time the present work was carried out. To determine which coefficients should be used in the study of an example drier, each of five combinations of the expressions were used to model the results of cross-flow drier tests. The coefficients selected for further use were those which gave the best correlation with grain throughput and specific heat energy consumption. 3. Validation 3.1. Experimental data Data for validation were obtained from tests on three driers, the Gascoigne 3/70 (Ref. 2), the Airwoods “40” (Ref. 3) and the Allmet 50 cwt, 400/63 (Ref. 4). Although the tests predated the standard ” BS 3986 : 1966, they conformed with it in all important respects. None of the drieis are now manufactured but their basic cross-flow design is still relevant to current practice (Fig. 2). The Gascoigne and Airwoods driers were simple cross-flow types with a single drying bed preceding, and continuous with, a cooling bed and without any recirculation of the air. The Allmet drier was slightly more complicated in that the grain passed along four separate beds, three for drying and one for cooling. On transfer between beds, the grain became mixed and this action was simulated in the computer program by re-initializing all the layers in the bed to the mean grain moisture content and temperature. In every case the length of the cooling section was about one-third the length of the drying section. However, in the Airwoods “40”, the reduced depth of the cooling bed meant that cooling time was reduced to approximately one-quarter of that in drying. The data from the tests converted to S.T. units are summarized in Table 1 and presented in
M.
E.
47
NELLIST
Fig. 2. Schematic representation of the three test driers, the Airwoods “40” (top), Gascoigne 3170 (middle) and Allmet 50 cwt (bottom) to show their relative size, the grain paths and the disposition of drying and cooling beds
Table I Summary of test report data
T
Drier
Airwoods“40’ Gascoigne 3/7G No. of test runs Throughput of wet grain, t/h Moisture content, d.b. Initial Final Bulk density of dry matter, kg/m3 Temperature, “C Initial Final Germination, T/, Initial Final
0.983-3.38 0.266-0.269 0.159-0.220 650 [email protected] 18.1-33.9 83-87 64-85
4 1642.43 0.260-0.272 0.166-0.215 636 10.0 3.8-8.5 97 91
5 1.50-3.89 0.259-0.263 0.152-o-208 611 13.3-15.6 15.0-21.7 95 64-86
63.3-82.2 14.0-18.1
65.5-67.8 2.3-5.5
70&17.2 12.2-14.4
1
Air Temperature.
“C
Drying Cooling Drying Cooling
Flow, kg/s m2 Ambient
humidity,
g/kg
0.5
0.43-0.46
040
8.0
3.9-4.2
6.9-7.5
11.7 3.90 0,140 0,140
12.1 4.04 0.152 0.152
7.21-9.61
6.09-7.71
Drier
Bed area, mz
Drying Cooling Bed depth, m Drying Cooling Specific heat energy consumption, MJ/kg water evaporated
5.57 1.86 0.191-0.222 0.114 4.55-5.88
48
A CROSS-FLOW
GRAIN
DRIER
detail for each of the 16 runs in an Appendix. The derivation of some of these values requires comment. (a) Bed depth. All three driers used horizontal grain beds, in which the top surfaces were unconstrained so that bed depth tended to vary both across, and along, the length. Although some information on this variation was given in the test reports, it was considered adequate for the present work to use average values. (b) Bed urea. In the case of the Allmet drier, the areas of the drying and cooling beds were estimated from the capacity of the drier and the bulk density of the grain. (c) Air flow. Since in none of the tests was it possible to measure air flow, this vital quantity had to be estimated from the observed fuel consumption and mean rise in air temperature for each run. It was then assumed that the mass flow per unit area of cooling air would be the same as that of drying. (d) specific heat energy consumption. For valid comparison with values predicted by the computer program, the observed specific heat energy consumptions were calculated by the same algorithm relating grain throughput and moisture reduction to fuel use and assuming a net calorific value of 36 MJ/l. 3.2. Validation procedure Using these data, the 16 test runs were each simulated five times using different combinations of expressions for k and M,. Then each drier was simulated at standard conditions defined as (a) Grain moisture reduction Dry matter bulk density Temperature (b) Air ambient temperature Specific humidity Barometric pressure Drying air temperature
from 20 to 15% w.b. 642.6 kg/m3 15.O”C 15°C 0.00855 kg/kg 101.325 kPa 65.5°C. 3.3. Validation results
3.3.1. Results at observed inlet conditions No one single combination of the functions for k and M, predicted all the output variables better than other combinations, but the two most important parameters, the throughput of grain and the specific heat energy consumption, were best described by using a = 20Of1and . b = 5094 as given by O’Callaghan et al.’ in Eqn (6) for k and the coefficients given by Bowden et al.” for Eqn (7). Coefficients of the linear regression between observed and ‘predicted values of grain throughput, specific heat energy consumption, grain output temperature, and grain output germination are given in Table 2 and the data and fitted lines are plotted in Fig. 3. The excellent agreement between observed and predicted values of grain throughput and output grain temperature across the whole range of the data is clearly shown. For specific heat energy consumption, the correlation is not quite so good but, more importantly, confirms that the difference in heat consumption between-the driers is real and is a function of those quantities input to the simulation. But at this stage it is not possible to separate the effects of drier design from the effects of variation in the condition of the air and grain. The variation in germination of the output grain is not well predicted, but this is as might be expected for a biological property. The predictions will be improved by more accurate calculation of the rate of change in grain temperature and by better data on the rates of loss
49
M. E. NELLIST Table 2 Coefficients of linear regression between observed @) and predicted (x) parameters of performance
Quantity
Throughput, t/h Specific heat energy consumplion. Output grain temperature, “C Germination, yj0
MJjkg
Inlercept
Slope
-0.133
1.01 1.39 1.11 0.78
-2.14 -3.46 12.7
Correlation coefficient 0.98
0.91 0.98 0.52
“probit” viability under drying conditions. Nevertheless, since much of the variation in the observed data is due to experimental “noise”, predictions will always be less precise than those of physical properties. of
3.3.2. Results at standard conditions The results of simulating the three driers operating at identical conditions of the inlet air and grain, and identical moisture content of the output grain, provide a means of isolating the effect of differences in design from those of air and grain condition. In Table 3, the predicted values of throughput and specific heat energy consumption have been ordered
3 Predicted
throughput,
t/h
Predicted
output
grain temperature.
“C
@
//
I
1
I
0 6 Predicted specific consumption,
I
IO heat energy MJ/kg
/
I
40
,d;‘. / /. : /’
/I
.
.
I
60 80 Predicted germination.
I
I00 %
Fig. 3. Ptots of observed versus predicted values of throughput (top left), output grain temperature (top right), specific heat energy consumption (bottom left). a&germination (bottom right) for the Airwoods (m). Gascoigne (a) and Attmet (A) driers. 0. four coincident points
50
A CROSS-FLOW
GRAIN
DRIER
Table 3 Predicted throughputs and specific beat energy consumptions for tbe three test driers operating at standard air and grain conditions Drier
Drying bed depth, m Air mass flow, kg/m2 min Volumetric flow, kg/m3 min Throughput of wet grain: (a) t/h (b) kg/min m2 of drying bed Throughput of dry matter kg/min rn’ of drying bed Specific heat energy consumption, MJjkg Corrected specific heat energy consumption,
MJ/kg Mean of observed specific heat energy consumption, MJ/kg
Gascoigne
Allmet
Airwoods
0.140 21.1 194
0.152 24.3 159
0.197 30.0 152
2.47 211
2.44 201
1.42 255
169 6.74
161 6.34
204 6.20
7.22
6.67
6.48
7.54
6.13
5.42
across the page according to increasing depth of the drying bed. It was considered unnecessary to make the small correction to throughput indicated by the coefficients of Table 2, but the correction was applied to the specific heat energy consumptions to allow comparison with the observed values. Although differing considerably in physical appearance and mechanical design, the Gascoigne and Allmet driers had very similar drying and cooling areas and bed depths and this is reflected in their similar throughputs both overall (2.47 and 2.44 t/h), and per unit area of drying bed (211 and 201 kg/min m’). However, the predicted specific heat energy consumption of the Gascoigne (6.74 MJ/kg) is greater than the Allmet (6.34 MJ/kg). The corresponding values obtained by applying the regression relationship of Table 2 are 7.22 and 6.67 MJ/kg respectively. These values reduce the difference between the two driers given by the means of the observed values (7.54 and 6.13 MJ/kg) and indicate that the greater fuel use of the Gascoigne was not due solely to the low ambient temperature experienced during the test (see Table 1). Inspection suggests that a combination of slightly shallower bed with a higher air flow which results in an air flow per unit volume of grain some 22% higher in the Gascoigne than the Allmet, probably reduced partial saturation of the drying air. In physical appearance and mechanical design the Airwoods “40” was very similar to the Gascoigne, but it was smaller and the throughput reflects this. Also, the bed depth was over 40% deeper than the Gascoigne so that although the air mass velocity was greater, the flow per unit volume of grain was much lower. The result was that the throughput of wet grain per unit area was greater by >20% and the specific heat consumption was lower. Thus the Airwoods was the more efficient drier, although by only a relatively small margin. It can be concluded that differences in design of the three driers cause differences in performance, but these differences are smaller than the average results of the physical tests indicate because of unequal grain and air inlet conditions. 4. Performance
of an example cross-flow drier
4.1. Drier specification
The object of this part pf the study was to examine the effect of drying air temperature and moisture removal on the important parameters of drier performance. But first it was necessary to define a typical or example cross-flow drier.
M.
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51
NELLIST
For simulation purposes the design of a simple cross-flow drier can be expressed in terms of bed depth; drying and cooling bed areas; and airflows in drying and cooling. In the present case the drying and cooling sections were taken to be of equal depth and the mass rate of airflow was fixed at 30 kg/min m* of drying or cooling area. The total bed area of the drier was fixed at 12 m* and, unless explicitly stated otherwise, the properties of the air and grain were as defined for standard conditions in Section 3.2. To select suitable values for bed depth and drying to cooling ratio, an initial series of simulations was carried out, with the moisture removal held constant at from 20 to 15% w.b. and the following were varied. 4.1.1. Bed depth Values of 0.2 and 0.3 m were chosen as being more representative of the range of depths on current (1984) designs than the 0.14 to 0.19 m of the older designs used in the validation exercise. 4.1.2. Drying area to cooling area ratio Conventionally, about one-quarter of the bed of a cross-flow drier is reserved for cooling. Although this may provide adequate cooling for the commonly accepted rating condition of a 5:/, w.b. moisture removal at a drying air temperature of 655°C there is good reason to believe that, in current practice, problems with cooling arise, either because moisture removal is less than 5% or because drying air temperatures are higher than 65.5”C or for both reasons. Thus the normal ratio of drying to cooling of 3 :l was compared with the increased cooling represented by a ratio of 1.4 : 1. In the example drier these ratios give equivalent bed areas of 9 and 3 m* and 7 and 5 m* respectively. 4.1.3. Drying air temperature The normal rating temperature of 65.5”C was compared with 90°C which is near the maximum temperature that would be used in a cross-flow drier. The effects of these variables upon grain throughput, output temperature and viability, and on specific heat energy consumption are shown in Table 4. This shows Table 4 Effect of drying air temperature, drying/cooling ratio and bed depth on five parameters of performance of an example cross-flow drier Drying air temperature, “C 65~5
90.0 Ratio drying: cooling area 1.4: 1
3:l
1.4: 1
3:I
Bed depth, m 0.2
0.3
0.2
0.3
0.2
0.3
0.2
0.3
Throughput of wet grain, t/h Output grain temperature, “C Specific heat energy
1.85 15.8
2.21 16.0
2.41 18.4
2.88 19.1
3.27 17.5
3.71 17.3
4.18 28.9
472 27.7
consumption, MJjkg Germination, 7;
5.98
4.99
5.89
4.93
5.03
4.43
5.06
4.48
(initial value 98.0) Probit loss of viability
97.7 0.056
97.8 0.049
97.7 0.055
97.8 0.049
96.0 0,307
95.9 0.314
96.0 0.310
95.9 0,319
A
52
CROSS-FLOW
GRAIN
DRIER
(a) Increasing the depth from 0.2 to 0.3 m was beneficial in that it increased throughput and reduced specific heat energy consumption without markedly affecting either output grain temperature or grain viability. (b) Reducing the drying area reduced the throughput and slightly increased specific heat energy consumption but was effective in improving cooling. Viability was unaffected. (c) Increasing the drying air temperature was also beneficial in increasing throughput and reducing specific heat energy consumption, but had the expected effect of increasing grain temperature, although, for the drier with the larger cooling area, the increase was < 2°C. Viability was also affected and although the predicted germinations do not appear significantly depressed, the equivalent changes in probit viability are large. On the basis of these conclusions it was decided to opt for a 0.3 m deep bed to give throughput and efficiency and a drying/cooling ratio of 1.4 : 1 to ensure reasonably adequate cooling at high temperatures and/or small moisture removals, i.e. the example drier has a
J
I
40
60 Drying
I
I
80
too
air temperature,
T
100 40
80
60 Drying
air
temperature,
100 “C
Fig. 4. The efSect of drying air temperature and initial moisture content (MO) upon the throughput (top Ieft), residence time (top right), spec$c heat energy consumption (bottom left) and output grain temperature (bottom right) of the example cross-flow drier. ??denotes normal rating condition
M.
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53
NELLIST
normal rating of 2.21 t/h with a specific heat energy consumption of 4.99 MJ/kg (= 8.6 litres oil/tonne) dried grain). 4.2. Drying air temperature and initial moisture content The performance of this chosen design was then simulated at temperatures from 40 to 100°C and for initial moisture contents from 16 to 26% w.b. which, with the final moisture content fixed at 15% w.b., represented moisture removals from 1 to 11% w.b. Fig. 3 shows, for each of six initial moisture contents, the variation with temperature of four measures of performance. 4.2.1. Throughput of wet grain (Fig. 4, top left) increases with increasing drying air temperature and decreases with initial moisture content. The total variation due to these two variables is from 0.45 t/h to 12.5 t/h, a factor of almost 28. This is why it can be misleading to describe the capacity of a drier by one single value-in this case, for example, 2.2 t/h. 4.2.2. Residence time (Fig. 4, top right) is the reciprocal of throughput and hence the relationship with temperature is hyperbolic. This means that residence times in the drier vary from 0.2 to 6.9 h depending upon grain moisture content and drying air temperature. The shorter the residence time, the easier it is to adjust the drier and subsequently to control it. Thus, in this respect it is advantageous to operate at higher temperatures. 4.2.3. Specific heat energy consumption (Fig. 4. bottom left) was greatest, 8.74 MJjkg water evaporated, at the lowest temperature, 40°C and initial moisture content, 16% w.b. Conversely, it was least, 3.74 MJ/kg at the highest temperature, 100°C and moisture content, 26% w.b. For initial moistures between 18 and 26x, the effect of increasing temperature was to reduce specific heat energy consumption, but at 16% a minimum of 7.13 MJ/kg was reached at about 70°C. This is because at higher temperatures too much sensible heat is carried away in the exhaust air and in the grain leaving the drier. 4.2.4. Grain temperatures at output (Fig. 4, bottom right) are within 1°C of the cooling air at all drying air temperatures when initial moisture contents exceed 22%. As they fall below 22x, cooling becomes less effective to an extent dependent upon the drying air temperature. In fact, the main effect of the higher drying air temperatures and lower moisture contents is not that the grain entering the cooling section is much hotter, but that the increased throughput reduces the residence time. A plot of grain temperature against total residence time in the drier (Fig, 5) shows that cooling is complete for all those conditions in which the residence time exceeds 1 h and that
y
.
35
.
2 ; 30 -1
. b
‘0
.
: 25z1 G 2 20: _ ; .s
.* . ?? e
,50.1
0.2
0.4
0.6
Residence
Fig, 5. The efSect
of residencetime
I.0 time,
on the grain temperature
2.0
4.0
7.1
h
at output of the example cross-yaw drier
54
A
CROSS-FLOW
GRAIN
DRIER
M,. % 26
d
2 16\o 0
24 22 20 18
16
40 Drying
60 air
80 temperature,
100 T
Drying
air temperature,
“C
Fig. 6. E#ect of drying air temperature and initial moisture content (IV,,) on the moisture range in output grain (top) and combinations of two variables giving a constant difference of 6.3% (bottom). ??denotes normal rating condition
a residence time of 0.6 h is sufficient to reduce the grain temperature to within 5°C of ambient. Unless cooling is done in the grain storage facility driers with reduced residence times will require larger cooling sections. It is inevitable that there will be a gradient of moisture content across the bed of grain leaving a simple cross-flow drier. Fig. 6 (left) shows that the difference between the inlet and exhaust sides increases with the drying air temperature and with initial moisture content. At 100°C the difference varies from 6% to 15% w.b. for initial moisture contents of 16 and 26% w.b. respectively. In most driers grain is mixed during its exit from the drier and the moisture difference will only be a problem if the moist grains mould before they equilibriate with the dry grains. The data of Kellermar? suggest that the limiting difference at which moulding occurs exceeds 16% w.b., so that the differences predicted in the present study need not be inhibiting in practice. ‘.
MO %
2624222~ .6 I8
I6 ,A
16 40 60 RO Drying air temperature.
“C
Fig. 7. EfSect of drying air temperature and initial moisture content (MO) on the germination (top) and probit loss (bottom left) in the example drier. Combinations of the two variables giving 0.05 units of probit loss are compared (bottom right) with safe combinations recommended by MAFF’ (dotted line). ??denotes normal rating condition
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55
NELLJST
the difference is considered limiting, it is possible to derive a curve, such as that in Fig. 6 of initial moisture content and drying air temperature giving the same moisture difference. In the example, the difference, 6.3% w.b., is that obtained at the normal rating condition of 655°C and 5% removal. The predicted effect on grain viability is illustrated in Fig. 7 which shows (left) the interaction between initial moisture content and drying air temperature assuming an input germination of 98%. At an initial moisture content of 16x, very little damage is predicted even at lOO”C, whereas damage to grain initially at 26% w.b. is predicted at temperatures below the rating temperatures (65.5”C). The same data are presented in “probit” form in Fig. 7 (middle). In Fig. 7 (right) the curve joins those combinations of input moisture content and drying air temperatures at which the predicted probit damage is the same (0.05 units) as at the rating temperature and initial moisture content. Also plotted are the maximum safe drying air temperatures for seed grain and quoted by MAFF” from the work of Cashmore.” This curve supports the indication from the validation exercise that the model underpredicts damage at temperatures in excess of 65°C and germination damage could be significantly understated. If
(right), joining those combinations
5. Conclusions 5.1. The best correlations between computed and observed throughputs (r = 0.98) and specific heat energy consumptions (r = 0.91) of three cross-flow driers were obtained when using the equation of the drying rate constant, k for wheat cited by O’Callaghan et ~1.~and the modified equation for equilibrium moisture content given by Bowden et all4 Good correlation (r = 0.98) was obtained with grain temperature at output but the prediction of germination was poor (I = 0.52).
5.2. Simulation of the performance of the three test driers at standard ambient conditions and for the same drying air temperatures and moisture removal showed that the differences in their specific heat energy consumption were due more to design than to test conditions. 5.3. A specification for an example cross-flow drier was derived from a short study of bed depth, drying air temperature and ratio of cooling to drying. It was concluded that a bed depth of 0.3 m and a drying/cooling area ratio of 1.4 : 1 would give good efficiency and adequate cooling. 5.4. Simulation of the example drier at the range of initial moisture contents and drying air temperatures typically encountered in cross-flow driers, showed that behaviour at one single operating condition is inadequate as a description of the performance of a drier. Figs 4 to 7 demonstrate that simulation can be used to map the wide variation to be expected in practice and to derive combinations of input factors at which some measure of performance is constant. Although the example drier had a larger than normal cooling area, cooling was not adequate for those combinations of initial moisture content and drying air temperature giving total residence times of less than 0.6 h.
References ’ Netlist, M. E. Crop drying and mathematical models. In: Proceedings of Operational Research Workshop, Report No. 32, National Institute of Agricultural Engineering, Silsoe, 1979 2 Report on Test of Gascoigne 3/70 Grain Drier. Test Report No. 314, National Institute of Agricultural Engineering, Silsoe, 1962 3 Report on Test of Airwoods “40” (Gascoigne 4140) Grain Drier. Test Report No. 360, National Institute of Agricultural Engineering, Silsoe, 1963
56
A CROSS-FLOW
GRAIN
DRIER
4 Report on Test of Allmet 50 cwt Grain Drier D400/63 Series. Test Report No. 414, National Institute of Agricultural Engineering, Silsoe, 1964 J Nellist, M. E. The drying of ryegrass seeds in deep layers, Ph.D. Thesis, University of Newcastleupon-Tyne, 1974 (umpubl.) 6 O’Callaghan, J. R.; Menzies, D. J.; Bailey, P. H. Digital simulation of agricultural drier performance. Journal of Agricultural Engineering Research 197 1, 16(3): 223-244 7 Laws, N.; Parry, J. L. Mathematical modelling of heat and mass transfer in agricultural grain drying. Proceedings of Royal Society, London A, 1983, 385: 169-187 a Parry, J. L. Mathematical modelling and computer simulation of heat and mass transfer in agricultural grain drying. Ph.D. Thesis, Cranfield Institute of Technology, November 1983 9 Nellist, M. E. Predicting the viability of seeds dried with heated air. Seed Science and Technology 1981, 9: 439-455 lo Sutherland, J. W.; Ghaly, T. F. Heated-air drying of oilseeds. Journal of Stored Products Research 1982, 18: 43-54 using ” Bailey, P. H.; Smith, E. A. Strategies for control of near-ambient grain driers-simulation 1968 Turnhouse (Edinburgh) weather. Departmental Note SIN/330, Scottish Institute of Agricultural Engineering, Penicuik, 1982 l2 Watson, E. L.; Bhargava, V. K. Thin-layer drying studies on wheat. Canadian Agricultural Engineering 1974, 16( 1): 18-22 l3 Boyce, D. S. Grain moisture and temperature changes with position and time during through drying. Journal of Agricultural Engineering Research 1965, lO(4): 333-341 l4 Bruce, D. M. Simulation of multiple-bed concurrent-, counter, and mixed-flow grain driers. Journal of Agricultural Engineering Research 1984, 30(4): 361-372 isotherms for wheat. Departmental Note l6 Nellist, M. E.; Dumont, S. D. Desorption DN/CDV/983/06010, National Institute of Agricultural Engineering, Silsoe, 1979 l6 Miller, P. C. H.; Whiffield, R. D. The predicted performance of a mixed-flow grain drier. Journal of Agricultural Engineering Research 1984, 30(4): 373-380 l7 Bowden, P. J.; Lamond, W. J.; Smith, E. A. Simulation of near-ambient grain drying. 1. Comparison of simulations with experimental results. Journal of Agricultural Engineering Research 1983, 28(4): 279-300 l* McEwen, E.; O’Callaghan, J. R. The effect of air humidity on through drying of wheat grain. Transactions of the Institution of Chemical Engineers 1955, 33: 135-154 l9 Methods of Test for Agricultural Driers. British Standard 3986 : 1966. British Standards Institution, London, 1966 za Kellermann, C. uber die bedeutung von ungleichmbsigkeiten des getreidefeuchtegehaltes beim ftillen und entleeren von beluftungs- und warmluftsatztrocknern. (Uneven moisture content of grain when filling and emptying batch driers employing natural or heated air). Grundlagen der Landtechnik 1966, 16(4): 187-191 21 Farm Grain Drying and Storage (3rd Edition), Ministry of Agriculture, Fisheries and Food, Her Majesty’s Stationery Office, London, 1966 ** Cashmore, W. H. The effect of heat on the germination of grain. Technical Notes on Mechanised Farming, No. 2, Institute of Research in Agricultural Engineering, University of Oxford, 1932
bed Area, m2 Depth, m Cooling bed Area, mz Depth, m Specific heat consumption, MJ/kg water evaporated
Drier Drying
Throughput of wet grain, t/h Throughput of dry matter, kg/m2 h Moisture content, d.b. Initial Final Bulk density of d.m., kg/m Temperature, “C Initial Final Germination, 7; Initial Final Air Temperature, “C Drying Cooling Flow, kg/s m* Drying Cooling Ambient humidity, g/kg 5.57 0.191 1.86 0.114 5.88
5.55
65.3 16.9 0.5 0.5 8.0
65.6 14.5 0.5 0.5 8.0 5.51 I.222 1.86 I.114
0.98 139 0.267 0.159 650 16.9 20.3 83.0 83.0
2
1.33 189 0.267 0.178 650 14.5 18.1 84.0 85.0
1
F
4.87
5.57 0.192 1.86 0.114
6.5.3 18.1 0.5 0.5 8.0
1.58 223 0.267 0.159 650 IS.1 22.5 84.0 84.0
3
4.55
5.57 0.191 1.86 0.114
63.3 16.7 0.5 0.5 8.0
2.35 332 0.266 0.209 650 16.7 24.7 87.0 83.0
4
Airwoods
5.40
5.57 0.191 1.86 0.114
72.8 14.0 0.5 0.5 8.0
1.56 220 0.267 0.177 650 14.0 18.3 84.0 80.0
5
“40”
5.29
5.57 0,110 1.86 0,114
82.5 17.0 0.5 0.5 8.0
1.39 197 0.267 0.152 650 17.0 21.7 83.0 64.0
6
7
5.06
5.57 0.197 1.86 0.114
82.2 17.0 0.5 0.5 8.0
3.38 478 0.269 0,220 650 17.0 33.9 84.0 67.0
Appendix
8.30
11.7 0.140 3.90 0.140
66.7 2.3 0.451 0,451 3.9
2.42 164 0,263 0.185 636 10.0 3.8 97.0 97.0
I
9.61
11.7 0.140 3.90 0.140
67.8 4.0 0.431 0,431 4.0
1.64 111 0,260 0.166 636 10.0 5.1 97.0 97.0
2
Run number
7.21
11.7 0,140 3.90 0,140
65.5 5.5 0.46 0.460 4.2
3.13 251 0.271 0.199 636 10.0 7.2 97.0 97.0
3
Gascoigne 317’0
Drier
7.50
11.7 0.140 3.90 0.140
66.7 2.3 0.451 0,451 3.9
2.43 163 0.272 0.125 636 10.0 8.5 97.0 97.0
4
6.67
12.1 0.152 4.04 0.152
70.6 12.2 0405 0405 7.2
2.17 142 0.263 0.171 611 13.3 15.0 95.0 77.0
I
7.71
12.1 0.152 4.04 0,152
70.6 14.2 0405 0405 7.3
1.50 97.9 0.263 0.152 611 14.2 15.6 95.0 74.0
2
6.35
12.1 0.152 4.04 0,152
70.6 13.9 0405 0405 6.9
2.61 171 0.259 0.182 611 14.4 17.8 95.0 81.0
3
4
6.09
12.1 0.152 4.04 0,152
70.6 13.3 0405 0405 6.9
3.89 254 0.263 0.208 611 13.6 21.7 95.0 86.0
Allmet 50 cwr
6.56
12.1 0.152 4.04 0,152
71.2 14.4 0405 0405 7.5
2.45 161 0,259 0.171 611 15.6 16.1 95.0 64.0
5
n
Modelling the Performance of a Cross-flow Grain Drier M. E. NELLIST* A computer model of a cross-flow grain drier was developed in which the equations of heat and mass transfer were solved by Euler integration and the drying rate was defined as the product of a drying constant, k, and the excess of the instantaneous moisture, M, above an equilibrium value, !vf,. To validate the model, five different combinations of expressions for k and M, were used to predict the performance of three cross-flow driers for which test data were
available. Comparisons were made of the observed and predicted values of grain throughput, specific heat energy consumption, temperatures of output grain and grain germination. That combination of expressions for k and M,, which gave the best correlation of throughput of wet grain and specific heat consumption, was used in the computer model to correct the test results to standard conditions and thereby to separate the effect of design from that of operating conditions. A short study of the effect of bed depth, drying air temperature and ratio of drying area to cooling area was used to define a specification for an example cross-flow drier which, except for its having a larger cooling area, could be regarded as typical of contemporary models. The performance of this example drier was then calculated for initial moisture contents of from 16-26% w.b. and drying air temperatures from 40 to 100°C when drying wheat to 15% w.b. At the normal rating conditions of drying air temperature and initial moisture content of 65.5”C and 209/, w.b. respectively, the drier had a predicted throughput of 2.2 t/h and specific heat energy consumption of 4.99 MJjkg water evaporated. However, across the range of temperatures and moisture contents examined, throughput varied from 0.45 to 12.5 t/h and specific heat consumption from 8.74 to 3.74 MJ/kg water evaporated. Grain cooling was shown to depend upon the residence time determined by the interaction of drying air temperature and initial moisture content. In the example drier, total residence times greater than 0.6 h were necessary for output grain to be cooled to within 5°C of ambient temperature.
1. Introduction The main purpose of this paper is to present some of the results of a study by computer simulation of the performance of a simple cross-flow grain drier, i.e. a drier in which a moving stream of grain is successively dried and cooled by air flowing at right angles to the stream. For such a study, computer simulation has three main advantages over physical
experimentation.
These are
(a) A saving of several orders of magnitude in time and cost. (b) Important variables, which in practice are subject to uncontrollable variation, can be held constant. (c) Changes can be calculated in those quantities, grain temperature, for example, which are either not easy or impossible to measure by experiment. It follows that experimental data for validating drier models are at best imperfect’ and at worst unobtainable. Fortunately, in the present work it was possible to carry out a partial * Crop Drying and Ventilation Group, NIAE (now AFRC Institute of Engineering Research). Wrest Park, Silsoe, Bedford Received
MK45 4HS, UK
8 March
Paper presented
1985; accepted
in revised form 17 May 1986
at AG ENG 84, Cambridge,
UK. l-5 April
1984
43 002 I -8634:X7:050043
+ 15 $03.00/O
(‘8 1987 The British Society for Research
in Agricultural
Engineering
44
A CROSS-FLOW
GRAIN
DRIER
Notation
a, b empirical coefficients h heat transfer coefficient, W/m2 K k drying constant, s-l Y correlation coefficient r.h. relative humidity, ratio t time, s depth of bed, m specific heat capacity, kJ/kg K ; G mass rate of flow, kg/s m2 H absolute humidity of air, kg water/kg dry air L, latent heat of vaporization of water at O”C, kJ/kg LT latent heat of vaporization of water in grain, kJ/kg M grain moisture content, decimal d.b.
S T p
specific surface area of grain, mZ/m3 temperature, “C density, kg/m3
1
Subscripts
a abs e g 0 pa pg pl pw
air absolute equilibrium grain value of t = 0 air at constant pressure grain at constant pressure water liquid at constant pressure water vapour at constant pressure
validation using data from three NIAE User Tests. 2S3B4 These results were used not only to compare computed and observed performance but also to examine the choice of functions describing the drying properties of grain and to illustrate the use of simulation in adjusting test results to a common basis by which they may be compared. Only then does the paper proceed to consider the performance of an example cross-flow grain drier as affected by the temperature of the drying air and the moisture content of the input grain. 2. The computer model The heat and mass transfer equations used in the NIAE model of Nellist5 are essentially the same as those used by O’Callaghan et al.,’ and are solved in a similar manner by a simple Euler integration technique. The resulting program is very economical in computing time but provides no control of, or information about, integration error. A second program, which is based upon a more rigorous formulation and solution of the equations’ is available to check calculations but requires excessive computing time for everyday use. These two programs have given almost identical results when using the same functions describing the properties of the air and grain.’ There are a number of minor differences between the model of O’Callaghan et al.’ and Nellist’ of which, perhaps, the most important is the condensation routine.’ In addition, the NIAE model contains a procedure for calculating the loss of grain viability by the accumulation of “probit” viabilities’ and is able to search for a specified output moisture content by adjusting any one of the four input parameters, grain throughput, grain bed depth, air temperature and air flow. Although the computer program also contains facilities for modelling a number of possible variations on a simple cross-flow drier, these, apart from one exception discussed later, were not used in the present study. A greatly simplified flow chart is given in Fig. 1. In finite difference notation and in order of solution the equations solved are: AM = - k(M - M,) . At/( 1 + +k At),
(1)
M.
E.
45
NELLIST
t Initialize
t Calculate heat and moss transfer in ail layers for estimated residence time
I Calculate average within bed
,, 1, ,\
conditions
No
Yes
Set printing
Fig. 1. SimpIiJiedJlow-chart
q=
of a cross-j?ow drier
A+,~[~Y+~F]~[~+&(~B+~B+C,,AM)].(2) AT,=-
where
of the simulation
and
(AT,(B + C,, . AM) - AMF)
&
(3)
A= B= E= F= Y=
The most important of these is Eqn (1) which describes the rate of drying of the grain and contains two coefficients k and M, which, in their turn, are functions of drying air temperature and humidity. Values of k and M, are determined experimentally by fitting Eqn (l), in the integral form M-M -5
MO-M,
= exp(-kt)
46
A CROSS-FLOW
GRAIN
DRIER
by the method of least squares, to drying curves obtained in “thin” or exposed layer drying tests. Unfortunately, such curves are not well fitted by Eqn (5) because, in practice, they are not simple decaying exponentials and k is not a constant. Thus, the values of k obtained depend to a certain extent upon the circumstances of the experiment (cf. Sutherland and Ghaly” and Bailey and Smith”). Also it is conventional to use the Arrhenius equation: k = a exp (-b/T,,,)
(6)
to correlate k with drying air temperature. With the latter expressed on the Kelvin scale, small differences in the values of k cause large differences in the values of the coefficients a and b obtained by different workers for apparently compatible sets of data (see, for example, Watson and Bhargava’*). When the present work began the values of the coefficients in Eqn (6) in use at NIAE were those given by Boyce I3 for barley (a = 139.3, b = 4426). These had been found to give good agreement with results for drying wheat in mixed-flow driers14 when used in conjunction with the equation for equilibrium moisture content, M, given by Nellist and Dumont,” M, = 0.113-0.016
In T-0.079
In (1 -r.h.).
(7)
which was based on a correlation of results for wheat. Subsequently, this combination of coefficients was used in a study of the performance of mixed-flow driers.18 But doubt about the suitability of these expressions, at least for barley, was cast by Bowden et ~1.‘~ who suggested values of a = 3.485 x IO5 and b = 6942 in Eqn (6) and that, increasing the first coefficient in Eqn (7) from 0.113 to 0,143 gave better agreement with final moisture contents obtained under drying conditions. It may be significant also that the values of M, given by the modified equation are of the same order as those obtained for wheat by McEwen and O’Callaghan, ” but not adeq uately described by them for modelling purposes. Earlier, these authors’ k values were corrected by O’Callaghan er al.’ who found values of the coefficients a = 2000 and b = 5094 in Eqn (16). Although more, and possibly better, descriptions of the drying rate exist, these were the relationships which appeared to be more relevant at the time the present work was carried out. To determine which coefficients should be used in the study of an example drier, each of five combinations of the expressions were used to model the results of cross-flow drier tests. The coefficients selected for further use were those which gave the best correlation with grain throughput and specific heat energy consumption. 3. Validation 3.1. Experimental data Data for validation were obtained from tests on three driers, the Gascoigne 3/70 (Ref. 2), the Airwoods “40” (Ref. 3) and the Allmet 50 cwt, 400/63 (Ref. 4). Although the tests predated the standard ” BS 3986 : 1966, they conformed with it in all important respects. None of the drieis are now manufactured but their basic cross-flow design is still relevant to current practice (Fig. 2). The Gascoigne and Airwoods driers were simple cross-flow types with a single drying bed preceding, and continuous with, a cooling bed and without any recirculation of the air. The Allmet drier was slightly more complicated in that the grain passed along four separate beds, three for drying and one for cooling. On transfer between beds, the grain became mixed and this action was simulated in the computer program by re-initializing all the layers in the bed to the mean grain moisture content and temperature. In every case the length of the cooling section was about one-third the length of the drying section. However, in the Airwoods “40”, the reduced depth of the cooling bed meant that cooling time was reduced to approximately one-quarter of that in drying. The data from the tests converted to S.T. units are summarized in Table 1 and presented in
M.
E.
47
NELLIST
Fig. 2. Schematic representation of the three test driers, the Airwoods “40” (top), Gascoigne 3170 (middle) and Allmet 50 cwt (bottom) to show their relative size, the grain paths and the disposition of drying and cooling beds
Table I Summary of test report data
T
Drier
Airwoods“40’ Gascoigne 3/7G No. of test runs Throughput of wet grain, t/h Moisture content, d.b. Initial Final Bulk density of dry matter, kg/m3 Temperature, “C Initial Final Germination, T/, Initial Final
0.983-3.38 0.266-0.269 0.159-0.220 650 [email protected] 18.1-33.9 83-87 64-85
4 1642.43 0.260-0.272 0.166-0.215 636 10.0 3.8-8.5 97 91
5 1.50-3.89 0.259-0.263 0.152-o-208 611 13.3-15.6 15.0-21.7 95 64-86
63.3-82.2 14.0-18.1
65.5-67.8 2.3-5.5
70&17.2 12.2-14.4
1
Air Temperature.
“C
Drying Cooling Drying Cooling
Flow, kg/s m2 Ambient
humidity,
g/kg
0.5
0.43-0.46
040
8.0
3.9-4.2
6.9-7.5
11.7 3.90 0,140 0,140
12.1 4.04 0.152 0.152
7.21-9.61
6.09-7.71
Drier
Bed area, mz
Drying Cooling Bed depth, m Drying Cooling Specific heat energy consumption, MJ/kg water evaporated
5.57 1.86 0.191-0.222 0.114 4.55-5.88
48
A CROSS-FLOW
GRAIN
DRIER
detail for each of the 16 runs in an Appendix. The derivation of some of these values requires comment. (a) Bed depth. All three driers used horizontal grain beds, in which the top surfaces were unconstrained so that bed depth tended to vary both across, and along, the length. Although some information on this variation was given in the test reports, it was considered adequate for the present work to use average values. (b) Bed urea. In the case of the Allmet drier, the areas of the drying and cooling beds were estimated from the capacity of the drier and the bulk density of the grain. (c) Air flow. Since in none of the tests was it possible to measure air flow, this vital quantity had to be estimated from the observed fuel consumption and mean rise in air temperature for each run. It was then assumed that the mass flow per unit area of cooling air would be the same as that of drying. (d) specific heat energy consumption. For valid comparison with values predicted by the computer program, the observed specific heat energy consumptions were calculated by the same algorithm relating grain throughput and moisture reduction to fuel use and assuming a net calorific value of 36 MJ/l. 3.2. Validation procedure Using these data, the 16 test runs were each simulated five times using different combinations of expressions for k and M,. Then each drier was simulated at standard conditions defined as (a) Grain moisture reduction Dry matter bulk density Temperature (b) Air ambient temperature Specific humidity Barometric pressure Drying air temperature
from 20 to 15% w.b. 642.6 kg/m3 15.O”C 15°C 0.00855 kg/kg 101.325 kPa 65.5°C. 3.3. Validation results
3.3.1. Results at observed inlet conditions No one single combination of the functions for k and M, predicted all the output variables better than other combinations, but the two most important parameters, the throughput of grain and the specific heat energy consumption, were best described by using a = 20Of1and . b = 5094 as given by O’Callaghan et al.’ in Eqn (6) for k and the coefficients given by Bowden et al.” for Eqn (7). Coefficients of the linear regression between observed and ‘predicted values of grain throughput, specific heat energy consumption, grain output temperature, and grain output germination are given in Table 2 and the data and fitted lines are plotted in Fig. 3. The excellent agreement between observed and predicted values of grain throughput and output grain temperature across the whole range of the data is clearly shown. For specific heat energy consumption, the correlation is not quite so good but, more importantly, confirms that the difference in heat consumption between-the driers is real and is a function of those quantities input to the simulation. But at this stage it is not possible to separate the effects of drier design from the effects of variation in the condition of the air and grain. The variation in germination of the output grain is not well predicted, but this is as might be expected for a biological property. The predictions will be improved by more accurate calculation of the rate of change in grain temperature and by better data on the rates of loss
49
M. E. NELLIST Table 2 Coefficients of linear regression between observed @) and predicted (x) parameters of performance
Quantity
Throughput, t/h Specific heat energy consumplion. Output grain temperature, “C Germination, yj0
MJjkg
Inlercept
Slope
-0.133
1.01 1.39 1.11 0.78
-2.14 -3.46 12.7
Correlation coefficient 0.98
0.91 0.98 0.52
“probit” viability under drying conditions. Nevertheless, since much of the variation in the observed data is due to experimental “noise”, predictions will always be less precise than those of physical properties. of
3.3.2. Results at standard conditions The results of simulating the three driers operating at identical conditions of the inlet air and grain, and identical moisture content of the output grain, provide a means of isolating the effect of differences in design from those of air and grain condition. In Table 3, the predicted values of throughput and specific heat energy consumption have been ordered
3 Predicted
throughput,
t/h
Predicted
output
grain temperature.
“C
@
//
I
1
I
0 6 Predicted specific consumption,
I
IO heat energy MJ/kg
/
I
40
,d;‘. / /. : /’
/I
.
.
I
60 80 Predicted germination.
I
I00 %
Fig. 3. Ptots of observed versus predicted values of throughput (top left), output grain temperature (top right), specific heat energy consumption (bottom left). a&germination (bottom right) for the Airwoods (m). Gascoigne (a) and Attmet (A) driers. 0. four coincident points
50
A CROSS-FLOW
GRAIN
DRIER
Table 3 Predicted throughputs and specific beat energy consumptions for tbe three test driers operating at standard air and grain conditions Drier
Drying bed depth, m Air mass flow, kg/m2 min Volumetric flow, kg/m3 min Throughput of wet grain: (a) t/h (b) kg/min m2 of drying bed Throughput of dry matter kg/min rn’ of drying bed Specific heat energy consumption, MJjkg Corrected specific heat energy consumption,
MJ/kg Mean of observed specific heat energy consumption, MJ/kg
Gascoigne
Allmet
Airwoods
0.140 21.1 194
0.152 24.3 159
0.197 30.0 152
2.47 211
2.44 201
1.42 255
169 6.74
161 6.34
204 6.20
7.22
6.67
6.48
7.54
6.13
5.42
across the page according to increasing depth of the drying bed. It was considered unnecessary to make the small correction to throughput indicated by the coefficients of Table 2, but the correction was applied to the specific heat energy consumptions to allow comparison with the observed values. Although differing considerably in physical appearance and mechanical design, the Gascoigne and Allmet driers had very similar drying and cooling areas and bed depths and this is reflected in their similar throughputs both overall (2.47 and 2.44 t/h), and per unit area of drying bed (211 and 201 kg/min m’). However, the predicted specific heat energy consumption of the Gascoigne (6.74 MJ/kg) is greater than the Allmet (6.34 MJ/kg). The corresponding values obtained by applying the regression relationship of Table 2 are 7.22 and 6.67 MJ/kg respectively. These values reduce the difference between the two driers given by the means of the observed values (7.54 and 6.13 MJ/kg) and indicate that the greater fuel use of the Gascoigne was not due solely to the low ambient temperature experienced during the test (see Table 1). Inspection suggests that a combination of slightly shallower bed with a higher air flow which results in an air flow per unit volume of grain some 22% higher in the Gascoigne than the Allmet, probably reduced partial saturation of the drying air. In physical appearance and mechanical design the Airwoods “40” was very similar to the Gascoigne, but it was smaller and the throughput reflects this. Also, the bed depth was over 40% deeper than the Gascoigne so that although the air mass velocity was greater, the flow per unit volume of grain was much lower. The result was that the throughput of wet grain per unit area was greater by >20% and the specific heat consumption was lower. Thus the Airwoods was the more efficient drier, although by only a relatively small margin. It can be concluded that differences in design of the three driers cause differences in performance, but these differences are smaller than the average results of the physical tests indicate because of unequal grain and air inlet conditions. 4. Performance
of an example cross-flow drier
4.1. Drier specification
The object of this part pf the study was to examine the effect of drying air temperature and moisture removal on the important parameters of drier performance. But first it was necessary to define a typical or example cross-flow drier.
M.
E.
51
NELLIST
For simulation purposes the design of a simple cross-flow drier can be expressed in terms of bed depth; drying and cooling bed areas; and airflows in drying and cooling. In the present case the drying and cooling sections were taken to be of equal depth and the mass rate of airflow was fixed at 30 kg/min m* of drying or cooling area. The total bed area of the drier was fixed at 12 m* and, unless explicitly stated otherwise, the properties of the air and grain were as defined for standard conditions in Section 3.2. To select suitable values for bed depth and drying to cooling ratio, an initial series of simulations was carried out, with the moisture removal held constant at from 20 to 15% w.b. and the following were varied. 4.1.1. Bed depth Values of 0.2 and 0.3 m were chosen as being more representative of the range of depths on current (1984) designs than the 0.14 to 0.19 m of the older designs used in the validation exercise. 4.1.2. Drying area to cooling area ratio Conventionally, about one-quarter of the bed of a cross-flow drier is reserved for cooling. Although this may provide adequate cooling for the commonly accepted rating condition of a 5:/, w.b. moisture removal at a drying air temperature of 655°C there is good reason to believe that, in current practice, problems with cooling arise, either because moisture removal is less than 5% or because drying air temperatures are higher than 65.5”C or for both reasons. Thus the normal ratio of drying to cooling of 3 :l was compared with the increased cooling represented by a ratio of 1.4 : 1. In the example drier these ratios give equivalent bed areas of 9 and 3 m* and 7 and 5 m* respectively. 4.1.3. Drying air temperature The normal rating temperature of 65.5”C was compared with 90°C which is near the maximum temperature that would be used in a cross-flow drier. The effects of these variables upon grain throughput, output temperature and viability, and on specific heat energy consumption are shown in Table 4. This shows Table 4 Effect of drying air temperature, drying/cooling ratio and bed depth on five parameters of performance of an example cross-flow drier Drying air temperature, “C 65~5
90.0 Ratio drying: cooling area 1.4: 1
3:l
1.4: 1
3:I
Bed depth, m 0.2
0.3
0.2
0.3
0.2
0.3
0.2
0.3
Throughput of wet grain, t/h Output grain temperature, “C Specific heat energy
1.85 15.8
2.21 16.0
2.41 18.4
2.88 19.1
3.27 17.5
3.71 17.3
4.18 28.9
472 27.7
consumption, MJjkg Germination, 7;
5.98
4.99
5.89
4.93
5.03
4.43
5.06
4.48
(initial value 98.0) Probit loss of viability
97.7 0.056
97.8 0.049
97.7 0.055
97.8 0.049
96.0 0,307
95.9 0.314
96.0 0.310
95.9 0,319
A
52
CROSS-FLOW
GRAIN
DRIER
(a) Increasing the depth from 0.2 to 0.3 m was beneficial in that it increased throughput and reduced specific heat energy consumption without markedly affecting either output grain temperature or grain viability. (b) Reducing the drying area reduced the throughput and slightly increased specific heat energy consumption but was effective in improving cooling. Viability was unaffected. (c) Increasing the drying air temperature was also beneficial in increasing throughput and reducing specific heat energy consumption, but had the expected effect of increasing grain temperature, although, for the drier with the larger cooling area, the increase was < 2°C. Viability was also affected and although the predicted germinations do not appear significantly depressed, the equivalent changes in probit viability are large. On the basis of these conclusions it was decided to opt for a 0.3 m deep bed to give throughput and efficiency and a drying/cooling ratio of 1.4 : 1 to ensure reasonably adequate cooling at high temperatures and/or small moisture removals, i.e. the example drier has a
J
I
40
60 Drying
I
I
80
too
air temperature,
T
100 40
80
60 Drying
air
temperature,
100 “C
Fig. 4. The efSect of drying air temperature and initial moisture content (MO) upon the throughput (top Ieft), residence time (top right), spec$c heat energy consumption (bottom left) and output grain temperature (bottom right) of the example cross-flow drier. ??denotes normal rating condition
M.
E.
53
NELLIST
normal rating of 2.21 t/h with a specific heat energy consumption of 4.99 MJ/kg (= 8.6 litres oil/tonne) dried grain). 4.2. Drying air temperature and initial moisture content The performance of this chosen design was then simulated at temperatures from 40 to 100°C and for initial moisture contents from 16 to 26% w.b. which, with the final moisture content fixed at 15% w.b., represented moisture removals from 1 to 11% w.b. Fig. 3 shows, for each of six initial moisture contents, the variation with temperature of four measures of performance. 4.2.1. Throughput of wet grain (Fig. 4, top left) increases with increasing drying air temperature and decreases with initial moisture content. The total variation due to these two variables is from 0.45 t/h to 12.5 t/h, a factor of almost 28. This is why it can be misleading to describe the capacity of a drier by one single value-in this case, for example, 2.2 t/h. 4.2.2. Residence time (Fig. 4, top right) is the reciprocal of throughput and hence the relationship with temperature is hyperbolic. This means that residence times in the drier vary from 0.2 to 6.9 h depending upon grain moisture content and drying air temperature. The shorter the residence time, the easier it is to adjust the drier and subsequently to control it. Thus, in this respect it is advantageous to operate at higher temperatures. 4.2.3. Specific heat energy consumption (Fig. 4. bottom left) was greatest, 8.74 MJjkg water evaporated, at the lowest temperature, 40°C and initial moisture content, 16% w.b. Conversely, it was least, 3.74 MJ/kg at the highest temperature, 100°C and moisture content, 26% w.b. For initial moistures between 18 and 26x, the effect of increasing temperature was to reduce specific heat energy consumption, but at 16% a minimum of 7.13 MJ/kg was reached at about 70°C. This is because at higher temperatures too much sensible heat is carried away in the exhaust air and in the grain leaving the drier. 4.2.4. Grain temperatures at output (Fig. 4, bottom right) are within 1°C of the cooling air at all drying air temperatures when initial moisture contents exceed 22%. As they fall below 22x, cooling becomes less effective to an extent dependent upon the drying air temperature. In fact, the main effect of the higher drying air temperatures and lower moisture contents is not that the grain entering the cooling section is much hotter, but that the increased throughput reduces the residence time. A plot of grain temperature against total residence time in the drier (Fig, 5) shows that cooling is complete for all those conditions in which the residence time exceeds 1 h and that
y
.
35
.
2 ; 30 -1
. b
‘0
.
: 25z1 G 2 20: _ ; .s
.* . ?? e
,50.1
0.2
0.4
0.6
Residence
Fig, 5. The efSect
of residencetime
I.0 time,
on the grain temperature
2.0
4.0
7.1
h
at output of the example cross-yaw drier
54
A
CROSS-FLOW
GRAIN
DRIER
M,. % 26
d
2 16\o 0
24 22 20 18
16
40 Drying
60 air
80 temperature,
100 T
Drying
air temperature,
“C
Fig. 6. E#ect of drying air temperature and initial moisture content (IV,,) on the moisture range in output grain (top) and combinations of two variables giving a constant difference of 6.3% (bottom). ??denotes normal rating condition
a residence time of 0.6 h is sufficient to reduce the grain temperature to within 5°C of ambient. Unless cooling is done in the grain storage facility driers with reduced residence times will require larger cooling sections. It is inevitable that there will be a gradient of moisture content across the bed of grain leaving a simple cross-flow drier. Fig. 6 (left) shows that the difference between the inlet and exhaust sides increases with the drying air temperature and with initial moisture content. At 100°C the difference varies from 6% to 15% w.b. for initial moisture contents of 16 and 26% w.b. respectively. In most driers grain is mixed during its exit from the drier and the moisture difference will only be a problem if the moist grains mould before they equilibriate with the dry grains. The data of Kellermar? suggest that the limiting difference at which moulding occurs exceeds 16% w.b., so that the differences predicted in the present study need not be inhibiting in practice. ‘.
MO %
2624222~ .6 I8
I6 ,A
16 40 60 RO Drying air temperature.
“C
Fig. 7. EfSect of drying air temperature and initial moisture content (MO) on the germination (top) and probit loss (bottom left) in the example drier. Combinations of the two variables giving 0.05 units of probit loss are compared (bottom right) with safe combinations recommended by MAFF’ (dotted line). ??denotes normal rating condition
M.
E.
55
NELLJST
the difference is considered limiting, it is possible to derive a curve, such as that in Fig. 6 of initial moisture content and drying air temperature giving the same moisture difference. In the example, the difference, 6.3% w.b., is that obtained at the normal rating condition of 655°C and 5% removal. The predicted effect on grain viability is illustrated in Fig. 7 which shows (left) the interaction between initial moisture content and drying air temperature assuming an input germination of 98%. At an initial moisture content of 16x, very little damage is predicted even at lOO”C, whereas damage to grain initially at 26% w.b. is predicted at temperatures below the rating temperatures (65.5”C). The same data are presented in “probit” form in Fig. 7 (middle). In Fig. 7 (right) the curve joins those combinations of input moisture content and drying air temperatures at which the predicted probit damage is the same (0.05 units) as at the rating temperature and initial moisture content. Also plotted are the maximum safe drying air temperatures for seed grain and quoted by MAFF” from the work of Cashmore.” This curve supports the indication from the validation exercise that the model underpredicts damage at temperatures in excess of 65°C and germination damage could be significantly understated. If
(right), joining those combinations
5. Conclusions 5.1. The best correlations between computed and observed throughputs (r = 0.98) and specific heat energy consumptions (r = 0.91) of three cross-flow driers were obtained when using the equation of the drying rate constant, k for wheat cited by O’Callaghan et ~1.~and the modified equation for equilibrium moisture content given by Bowden et all4 Good correlation (r = 0.98) was obtained with grain temperature at output but the prediction of germination was poor (I = 0.52).
5.2. Simulation of the performance of the three test driers at standard ambient conditions and for the same drying air temperatures and moisture removal showed that the differences in their specific heat energy consumption were due more to design than to test conditions. 5.3. A specification for an example cross-flow drier was derived from a short study of bed depth, drying air temperature and ratio of cooling to drying. It was concluded that a bed depth of 0.3 m and a drying/cooling area ratio of 1.4 : 1 would give good efficiency and adequate cooling. 5.4. Simulation of the example drier at the range of initial moisture contents and drying air temperatures typically encountered in cross-flow driers, showed that behaviour at one single operating condition is inadequate as a description of the performance of a drier. Figs 4 to 7 demonstrate that simulation can be used to map the wide variation to be expected in practice and to derive combinations of input factors at which some measure of performance is constant. Although the example drier had a larger than normal cooling area, cooling was not adequate for those combinations of initial moisture content and drying air temperature giving total residence times of less than 0.6 h.
References ’ Netlist, M. E. Crop drying and mathematical models. In: Proceedings of Operational Research Workshop, Report No. 32, National Institute of Agricultural Engineering, Silsoe, 1979 2 Report on Test of Gascoigne 3/70 Grain Drier. Test Report No. 314, National Institute of Agricultural Engineering, Silsoe, 1962 3 Report on Test of Airwoods “40” (Gascoigne 4140) Grain Drier. Test Report No. 360, National Institute of Agricultural Engineering, Silsoe, 1963
56
A CROSS-FLOW
GRAIN
DRIER
4 Report on Test of Allmet 50 cwt Grain Drier D400/63 Series. Test Report No. 414, National Institute of Agricultural Engineering, Silsoe, 1964 J Nellist, M. E. The drying of ryegrass seeds in deep layers, Ph.D. Thesis, University of Newcastleupon-Tyne, 1974 (umpubl.) 6 O’Callaghan, J. R.; Menzies, D. J.; Bailey, P. H. Digital simulation of agricultural drier performance. Journal of Agricultural Engineering Research 197 1, 16(3): 223-244 7 Laws, N.; Parry, J. L. Mathematical modelling of heat and mass transfer in agricultural grain drying. Proceedings of Royal Society, London A, 1983, 385: 169-187 a Parry, J. L. Mathematical modelling and computer simulation of heat and mass transfer in agricultural grain drying. Ph.D. Thesis, Cranfield Institute of Technology, November 1983 9 Nellist, M. E. Predicting the viability of seeds dried with heated air. Seed Science and Technology 1981, 9: 439-455 lo Sutherland, J. W.; Ghaly, T. F. Heated-air drying of oilseeds. Journal of Stored Products Research 1982, 18: 43-54 using ” Bailey, P. H.; Smith, E. A. Strategies for control of near-ambient grain driers-simulation 1968 Turnhouse (Edinburgh) weather. Departmental Note SIN/330, Scottish Institute of Agricultural Engineering, Penicuik, 1982 l2 Watson, E. L.; Bhargava, V. K. Thin-layer drying studies on wheat. Canadian Agricultural Engineering 1974, 16( 1): 18-22 l3 Boyce, D. S. Grain moisture and temperature changes with position and time during through drying. Journal of Agricultural Engineering Research 1965, lO(4): 333-341 l4 Bruce, D. M. Simulation of multiple-bed concurrent-, counter, and mixed-flow grain driers. Journal of Agricultural Engineering Research 1984, 30(4): 361-372 isotherms for wheat. Departmental Note l6 Nellist, M. E.; Dumont, S. D. Desorption DN/CDV/983/06010, National Institute of Agricultural Engineering, Silsoe, 1979 l6 Miller, P. C. H.; Whiffield, R. D. The predicted performance of a mixed-flow grain drier. Journal of Agricultural Engineering Research 1984, 30(4): 373-380 l7 Bowden, P. J.; Lamond, W. J.; Smith, E. A. Simulation of near-ambient grain drying. 1. Comparison of simulations with experimental results. Journal of Agricultural Engineering Research 1983, 28(4): 279-300 l* McEwen, E.; O’Callaghan, J. R. The effect of air humidity on through drying of wheat grain. Transactions of the Institution of Chemical Engineers 1955, 33: 135-154 l9 Methods of Test for Agricultural Driers. British Standard 3986 : 1966. British Standards Institution, London, 1966 za Kellermann, C. uber die bedeutung von ungleichmbsigkeiten des getreidefeuchtegehaltes beim ftillen und entleeren von beluftungs- und warmluftsatztrocknern. (Uneven moisture content of grain when filling and emptying batch driers employing natural or heated air). Grundlagen der Landtechnik 1966, 16(4): 187-191 21 Farm Grain Drying and Storage (3rd Edition), Ministry of Agriculture, Fisheries and Food, Her Majesty’s Stationery Office, London, 1966 ** Cashmore, W. H. The effect of heat on the germination of grain. Technical Notes on Mechanised Farming, No. 2, Institute of Research in Agricultural Engineering, University of Oxford, 1932
bed Area, m2 Depth, m Cooling bed Area, mz Depth, m Specific heat consumption, MJ/kg water evaporated
Drier Drying
Throughput of wet grain, t/h Throughput of dry matter, kg/m2 h Moisture content, d.b. Initial Final Bulk density of d.m., kg/m Temperature, “C Initial Final Germination, 7; Initial Final Air Temperature, “C Drying Cooling Flow, kg/s m* Drying Cooling Ambient humidity, g/kg 5.57 0.191 1.86 0.114 5.88
5.55
65.3 16.9 0.5 0.5 8.0
65.6 14.5 0.5 0.5 8.0 5.51 I.222 1.86 I.114
0.98 139 0.267 0.159 650 16.9 20.3 83.0 83.0
2
1.33 189 0.267 0.178 650 14.5 18.1 84.0 85.0
1
F
4.87
5.57 0.192 1.86 0.114
6.5.3 18.1 0.5 0.5 8.0
1.58 223 0.267 0.159 650 IS.1 22.5 84.0 84.0
3
4.55
5.57 0.191 1.86 0.114
63.3 16.7 0.5 0.5 8.0
2.35 332 0.266 0.209 650 16.7 24.7 87.0 83.0
4
Airwoods
5.40
5.57 0.191 1.86 0.114
72.8 14.0 0.5 0.5 8.0
1.56 220 0.267 0.177 650 14.0 18.3 84.0 80.0
5
“40”
5.29
5.57 0,110 1.86 0,114
82.5 17.0 0.5 0.5 8.0
1.39 197 0.267 0.152 650 17.0 21.7 83.0 64.0
6
7
5.06
5.57 0.197 1.86 0.114
82.2 17.0 0.5 0.5 8.0
3.38 478 0.269 0,220 650 17.0 33.9 84.0 67.0
Appendix
8.30
11.7 0.140 3.90 0.140
66.7 2.3 0.451 0,451 3.9
2.42 164 0,263 0.185 636 10.0 3.8 97.0 97.0
I
9.61
11.7 0.140 3.90 0.140
67.8 4.0 0.431 0,431 4.0
1.64 111 0,260 0.166 636 10.0 5.1 97.0 97.0
2
Run number
7.21
11.7 0,140 3.90 0,140
65.5 5.5 0.46 0.460 4.2
3.13 251 0.271 0.199 636 10.0 7.2 97.0 97.0
3
Gascoigne 317’0
Drier
7.50
11.7 0.140 3.90 0.140
66.7 2.3 0.451 0,451 3.9
2.43 163 0.272 0.125 636 10.0 8.5 97.0 97.0
4
6.67
12.1 0.152 4.04 0.152
70.6 12.2 0405 0405 7.2
2.17 142 0.263 0.171 611 13.3 15.0 95.0 77.0
I
7.71
12.1 0.152 4.04 0,152
70.6 14.2 0405 0405 7.3
1.50 97.9 0.263 0.152 611 14.2 15.6 95.0 74.0
2
6.35
12.1 0.152 4.04 0,152
70.6 13.9 0405 0405 6.9
2.61 171 0.259 0.182 611 14.4 17.8 95.0 81.0
3
4
6.09
12.1 0.152 4.04 0,152
70.6 13.3 0405 0405 6.9
3.89 254 0.263 0.208 611 13.6 21.7 95.0 86.0
Allmet 50 cwr
6.56
12.1 0.152 4.04 0,152
71.2 14.4 0405 0405 7.5
2.45 161 0,259 0.171 611 15.6 16.1 95.0 64.0
5
n