The use of mechanical refrigeration to improve the storage of pesticide treated grain G.R. Thorpe and W.B. Elder

Utilisation du froid m6canique pour am61iorer I'entreposage des grains trait6s par insec- ticide

Le refroidissement d'une masse de grains entrepos#s augmente beaucoup la persistance des insecticides chimiques qui leur sont appliques. Dans cette #tude on utilise un module math#matique du transport de chaleur et de masse et de la cin6tique chimique pour simuler /'influence de I'insufflation d'air refroidi m#caniquement ~ travers les grains sur la vitesse ult#rieure de d~gradation des insecticides. On montre que /e mat6riel convenant ~ cette t#che peut s'ap- puyer sur des conditionneurs d'air commerciaux nor- maux munis d'un syst#me de traitement d'air ap-

propri# susceptible de faire entrer /'air froid de force travers les grains.

Bien que la vitesse de d~gradation des deux insec- ticides, methacrifos et malathion, dans les masses de grains non refroidis soit tr~s sensible ~ la teneur en humidit~ et ~ la temperature initiales des grains, le refroidissement des grains rend leur d#gradation relativement insensible ~ ces variables. On montre que Iorsque les appareils de refroidissement des grains fonctionnent pendant un temps d~limit~ il existe un d~bit d'air optimal pour assurer la conservation op- timale des insecticides. Ce d~bit d'air se trouve dans le domaine de fonctionnement typique du condition- neur d'air commercial ~tudi& On recherche/'influence de la strat6gie de refroidissement et de/'emplacement g~ographique sur la conservation des insecticides dans les masses de grain refroidies. On examine en d~tail I'~tiologie physicochimique de ces ph~nom~nes.

Nomenclature

a i

c

c a

f l

f2

f3

f ,

h H k kl k2 n

P P

amplitude of diurnal temperature swing, o C concentration of pesticide, kg pesticide/m 3 moist grain specific heat of moist air at constant pressure, k J - 1 kg ° C - 1 air flow rate correction multiplier to sen- sible to total heat ratio ambient temperature correction multiplier to sensible to total heat ratio air f low rate correction multiplier to dry coil capacity air f low rate correction multiplier to wet coil capacity enthalpy of moist air, kJ/kg dry air enthalpy of moist grain, kJ/kg dry grain reaction rate constant, s -1 a constant, s a constant, ° 0 - 1 number of completed days of refrigera- tion unit operation pressure, Pa kernel density of grain, kg dry grain/m 3 moist grain

The authors are at the Commonwealth Scientific and Industrial Research Organization (CSIRO), Division of Mechanical Engineer- ing, PO Box 26, Highett, Victoria 3190, Australia. The authors wish to express their gratitude to the Wheat Industry Research Council, Australia, for financial support for this work. Paper received on 2 September 1979.

q

q, r

S T t t

t a d

t mi

t nw

t w

U

V

V

W

W

E

0 0'~ 0 Q

I

cooling capacity of refrigeration coil, kW compressor power consumption, kW fractional relative humidity of air sensible to total heat ratio temperature, °C ambient temperature, °C dry bulb temperature of air entering evaporator coil, °C mean daily temperature, °C notional wet bulb temperature, °C wet bulb temperature, °C flow ra:te of air, I s - 1 t - 1 interstitial air velocity, m -1 s volume flow rate of air through coil, m 3 S - - 1

moisture content of air in equilibrium with grain, kg water/kg dry air moisture content of grain, kg water/kg dry grain distance in direction of air flow, m

void fraction of grain overall fan-motor static efficiency time, s half-life of pesticide, s time function, s density of moist air, kg dry air/m 3 moist air frequency of daily temperature varia- tions, s - 1

0140-7007/80/020099-08 $02.00 Volume 3 Num&o 2 Mars 1980 © 1980 IPC Business Press Ltd. and I IR

Cooling a bulk of stored grain greatly increases the persistence of chemical pesticides applied to it. In this work, an established mathematical model of heat and mass transfer and chemical kinetics is used to simulate the effects of blow- ing mechanically refrigerated air through grain on the subsequent rate of decay of pesticides. It is demonstrated that the hardware suitable for such a task may be based on standard, commer- cial, air conditioners fitted with an appropriate air handling system capable of forcing cold air through the grain. Although the rates of decay of the two pesticides, methacrifos and malathion, in un-

cooled grain bulks are very sensitive to initial, grain moisture content and temperature, grain cooling renders their degradation relatively in- sensitive to these variables. It is shown that when the grain cooling units are operated for a fixed time there is an optimum air f low rate for achieving the maximum preservation of pesticide. This air f low rate is in the typical operating range of the commercial air condi- tioner studied. The effects of cooling strategy and geographical location on pesticide preserva- tion in cooled grain bulks are investigated. The physico-chemical aetiology of these phenomena is discussed in some detail,

Grain storage insects are an economic liability to grain exporting countries such as Australia which are con- tractually bound to supply the international grain market with insect-free grain. Left uncontrolled, grain storage insects can devastate stored grains within months and in India, for example, they have been identified 1 as the principal cause of damage. A widely accepted method of keeping grain free from insects is to spray it with chemical pesticides when it enters a grain store. Dramatic reductions in the numbers of grain storage insects were observed in the early 1960's when malathion, an organo-phosphorus insec- ticide was first introduced for this purpose. However, it soon became apparent that stored product insects were becoming resistant to malathion and by 19732 resistance had been reported in 70 countries. Insec- ticide resistance is of considerable commercial impor- tance as may be gauged by the fact that one strain of the Indian meal moth, Plodia interpunctella, requires an 800-fold increase in application of malathion to achieve the same mortality as susceptible strains. 2 Such an application would exceed the rates laid down by regulatory authorities. New, and generally more expensive chemicals are being introduced but they are expected to have a finite useful life.

The population dynamics of grain storage, insect pests respond to the micro-climate of the grain bulk in which they are found. For example, Birch 3 observed that the rice weevil, Sitophilus oryzae, can more than double its population in a week in 14% moisture con- tent grain at about 30°C whereas at 20°C the weekly rate of population growth is only 1.25. At low temperatures the life processes of the insects are slow and the life-cycle period from egg to adult is long thus few insect generations are able to develop during a given storage period. For example, it may be calculated from Hardman's 4 data that a newly laid egg of S. oryzae takes 27 days to develop into an adult at 28°C as against over 120 days at 16°C. Fur- thermore, the reproduction of most grain storage, in- sect pests ceases below about 20°C 5. Thus, we can anticipate that the cooling of stored grain will delay the onset of insect resistance to novel pesticides.

The rate of decay of chemical pesticides is also a function of grain moisture content and temperature. For example, Desmarchelier and Bengston e give the half-life of methacrifos, a newly developed organo-

phosphorus pesticide, as only eight weeks on 11% moisture content grain at 30°C but about 30 weeks at 20°C. That grain cooling extends the useful life of methacrifos has been demonstrated by Bengston et al. ? who observed that over a 41 week storage period, methacrifos applied to initially 10.2% moisture content wheat at 30°C degraded much more slowly in a grain bulk cooled by blowing cold night air through it compared with pesticide applied to uncooled grain. The methacrifos became partly ineffective against stored product insect pests after six months in the un- cooled grain but remained fully effective after eight months in the aerated silo. The field trials showed that grain cooling allows a lower quantity of pesticide to be applied to grain if it is cooled, yielding the possibility of overall cost savings.

It is therefore apparent that cooling grain enables a reduction in pesticide useage, and offers the possibili- ty of delaying the onset of insect resistance to new chemical pesticides.

Object ives of this w o r k

A mathematical model of heat and moisture transfer in grain and the chemical reaction kinetics of pesticide decay has been used to predict the amount of insec- ticide degradation in unaerated and naturally aerated grain bulks 8.

The model has been shown to predict accurately the rate of cooling of grain in naturally aerated silos and after six months or so grain temperatures are calculated to within 2°C. This error in temperature prediction has negligible effect on the calculation of the rate of pesticide decay which is low at low temperatures. Thorpe s has also shown that the chemical kinetics model is accurate in both unaerated and naturally aerated grain bulks. In this work the model is used to investigate the cooling of grain, and the resulting pesticide preservation when the condi- tions of the aeration air are modified by a commercial air conditioning unit in which the air handling system has been suitably adapted so that it is capable of forc- ing air through a grain bulk.

Modified commercial air conditioners have been used successfully to refrigerate a thermally insulated,

100 International Journal of Refrigeration

15 000 t capacity, grain store fitted with cold air re- circulation, in order to eradicate insects by cold s without the use of chemicals. However, the present application envisages mobile refrigeration units that they will force mechanically cooled ambient air through chemically treated grain via perforated aera- tion ducts installed in the grain store. Since grain in a 2000 t capacity silo can be cooled in four or five days, one refrigeration unit will serve several silos and great- ly reduce the capital cost per unit of grain cooled.

Analysis The heat and mass transfer model of a bulk of aerated grain is based upon the following assumptions: the air and the grain are in thermal and sorption equilibrium at all locations in the grain bulk; the sorp- tion process is reversible; the air flow through the bulk is uniform; thermal and moisture diffusion in the direction of air flow are zero; and the system is adiabatic.

Where symbols have the meaning ascribed to them in the nomenclature the moisture and energy conserva- tion equations may be written:

v l+ ,1 =0 (1)

I %1+ @1+ =0

where/~ = P(1 - ~ ) /~ . Analytical and numerical methods of solving these equations have been treated in detail elsewhere 8' lo, 11

Chemical reaction kinetics The rates of decomposition of the chemical pesticides considered here are propor- tional to insecticide concentration and water activity, expressed as fractional relative humidity, r, of the in- terstitial air. Desmarchelier and Bengston e present the following relationship for the half-life of pesticides on grain with an equilibrium relative humidity of 50%:

tions, namely up to two I s - 1 t - l , forced convec- tion has no effect on pesticide concentration. Ex- amination of experimental data presented by Storey 12 on the persistence of malathion on aerated grain justifies this assumption.

Refrigeration unit performance The performanc eof nominally 50 kW cooling capacity commercial air con- ditioner was determined from manufacturer's data 13. The relationship between the sensible to total heat ratio, ST, and the dry-bulb temperature, t d, and wet- bulb temperature, t of the air entering the evaporator coil is represented t~y

S T = 0.05556 t d - - 0.08333 t + 0.87 where S T ~ 0.9

ST = 0.1176 t d - - 0.1765 t + 0.84 where 0.5 < ST < 1

(6)

(7)

The correction multiplier, f l , to the sensible to total heat ratio for the air flow rate through the evaporator coil is

fl = 0.665 + 0.124 V + 0.004 V 2 (8)

where Vis the volume flow rate of air. The effect of ambient temperature, t a, on ST, is calculated from the multiplier:

f= = 0.924 - 0.0013 t + 0.0001 t 2 (9)

If the sensible to total heat component is unity, ie the evaporator coil is dry, the cooling capacity is given by

q = 0.833 te - 0.345 t a + 30.63 (10)

and the dry coil capacity correction multiplier for air flow rate is

f3 = 0.525 + 0.275 V - 0.034 V 2 (11)

When ST < 1, the wet coil case, the cooling capacity relationship is

e, h = kl 1 0 - k , C r - 301 (3) q = 1.36 t -- 0.44 t a + 34.5 (12)

and the rate of decay is

dc d--e = - kc (4)

where the rate constant, k, is given by

k = Hn (0.5) (5) 0.5 e,/2

The values of kl and k2 for methacrifos are given by Desmarchelier and Bengston 6 as 4.84 x 10es and 0.055oc -1 . In this work it is assumed that at aeration flow rates normally encountered in industrial applica-

and the correction multiplier for air flow rate is

f4 = 0.755 + 0.138 V - 0.016 V 2 (13)

The power consumption of the compressor when the evaporator is wet may be found from

q i = 0.3 t w + 0.16 t a + 6.7 (14)

If the coil is dry it is necessary to calculate a notional wet-bulb temperature, t- w from the equation

tnw = 0.6667 t d - - 0.7333 (15)

Compression temperature rise In commercial, air con- ditioners adapted to meet the requirements of grain

Volume 3 Number 2 March 1980 101

cooling, it is customary to place the aeration fan downstream of the evaporator coil. Thus the hear of compression in the fan reduces the relative humidity of the air before it enters the grain and prevents the formation of mould in the vicinity of the aeration ducts. The expression used for the pressure drop, &o, through a 15 m high bed of aerated wheat is taken from the graphical data of Holman~4 and is written

&o = 218.37 + 710.2 u +65.3 u 2 where 0.25 < u < 2.0 (16)

where the unit of pressure is Pa and the flow rate of air, u, is expressed in I s - 1 t - 1 of moist grain. The overall static efficiency, n, of the fan-motor assembly which is installed in the aeration air stream is taken to be 50% and the temperature rise due to compression is calculated from &t = & o / ( Q c a rl).

C l i m a t o l o g i c a l s i m u l a t i o n Ambient dry and wet bulb temperatures are simulated by an equation of the form

t = tin, + a sin co 0 (17) I I where o.) = 6.23 x 1 0 - e and 0 = e + 2.52 x 104 - 8.64n x 104 if 0 < e - 8.64n x 104 ~< 5.04 x 104 and co = 8 . 7 3 x 10 - e and 0 = 0 - 6.84 x 10 - 4 - 8./64n x 10 4 if 5.04 x 01 4 < 8.64n x 10 4 < 8.64 x 104

I00

8O o o c

60

o

~ 4 0

20

R

/ / L°west c°ncentr°ti°..~..~.......~ No oerotion n

I I J 0 0.5 1.0 1.5 2.0

Air flow rate, ts-lt -I

Fig. 1 Highest, lowest and mean methacrifos concentrations after 180 days in a bulk of wheat initially of 11% m.c. and at 30°C cool- ed for four days at Warracknabeal, Victoria as a function of air f low rate

Fig. 1 Concentrations max/kna/es, rein/males et moyennes de methacrifos au bout de 180 jours dons une masse de grains/nitia/e- ment ~ une teneur en eau de 11% b 30°C, refroidis quatre jours b Warracknabea/, Victoria, en fonction du d#bit d'air

8 0

60

'E "6

g "6 40

8

~, 2o .c:

Mean (Warracknobeal) j

Unaeroted

I 1 I 2 4 6

Grain cooling unit aperation, d

Fig. 2 The response to the number of days of cooling of mean and lowest methacrifos concentrations after 180 days storage in initially 11% m.c. wheat at 30°C cooled with 1.51 s -~ t -~ of air t Warracknabeal, Victoria and Moree, New South Wales

Fig. 2 R~action b un certain nombre de jours de refroidlssement de concentrations moyennes et minima/es de methacrifos au bout de 180 jours d'entreposage dans du bl# ~ one teneur en eau in/t/ale de 11% ~ 30°C, refroldi avec 1.5 / s - 1 t - 1 d'air a Warracknabea/, Victoria e t a Moree, Nouve/le-Gal/es du Sud

and n is the number of completed days of grain cooler operation.

The amplitudes, a,, of the diurnal swing in dry bulb and wet bulb temperatures were calculated from reference 15 to be 8.3ac and 2.5°C respectively at Warracknabeal in Victoria in January, and the cor- responding values during December at Moree where the wheat harvest is earlier are 7.55°C and 2.0°C respectively. The mean wet and dry bulb temperatures, t are 14.7°C and 22.1ac respectivley m,, at Warracknabeal and 18.0°C and 25.15°C at Moree.

S c o p e o f t h e inves t iga t ion

Australia's wheat belt is encompassed by a range of climatic conditions varying from subtropical in northern New South Wales and southern Queensland to temperate in Victoria. In this study we shall consider the viability of grain cooling for pesticide preservation at Warracknabeal, Victoria and Moree, in northern New South Wales.

The impact of the two independent variables within an air conditioning system, air flow rate and duration of cooling, on the persistence of the pesticides methacrifos and malathion will be investigated. The effects of initial grain moisture content and of temperature on pesticide decay will also be studied but the investigation will be confined to grain stores of 2000 t capacity, a common unit in the Australian grain handling system.

182 Revue Internationale du Froid

Resul ts

Air f low rate Fig. 1 shows the influence of the rate of air flow on the predicted highest, lowest and mean methacrifos concentrations remaining in the grain bulk expressed as % of initial concentration. The wheat was initially assumed to be at 30°C and with a 11% (w.b.) moisture content, cooled for four days and then from January stored for 180 days at War- racknabeal. The predicted mean concentration of methacrifos has a maximum value when the air flow rate is 1.5 I s - ~ t - ~ the flow rate across the evaporator coil is 30001 s -~ t -~, the lowest concen- tration of methacrifos in the wheat is the same as that of unaerated grain. Increasing the air flow rate reduc- ed the spread of concentration so that both the lowest and highest concentrations approach the mean. With malathion, the maximum mean concen- tration is 70% also when the air flow is 1.5 I s - t - ~ . It should be pointed out that for air flow rates below about 1 I s - ~ t - ~ the curves in Fig. 1 are in- dicative of trends only because their derivation in- volves extrapolation of manufacturer's data.

Duration of cooling Fig. 2 shows the predicted effect of changing duration of cooling on the final methacrifos concentration at both Warracknabeal and Moree using an air flow rate of 1.5 I s - ~ t - ~. At both locations the mean concentrations become almost constant after about six days of cooling and the lowest values approach a constant value below the asymptote of the mean. At least two days of cooling are needed to ensure that the lowest concen- tration of methacrifos in the grain exceeds that which would occur in an unaerated grain bulk.

Location It is also clear from Fig. 2 that more pesticide was retained by the better cooling achieved at Warracknabeal than at Moree. For example, if grain initially at 11% m.c. and 30°C was immediately cool- ed for four days with an air flow rate of 1.5 I s - t - ~ and then stored for 180 days at Warracknabeal or Moree the mean concentrations of methacrifos would be 63% or 47% of the initial value respectively. If the grain were left warm and unaerated the concen- tration would be only 11% of this value. Hence refrigerated cooling in either location would result in substantial pesticide preservation.

Initial moisture content The rate of decay for both malathion and methacrifos is sensitive to grain moisture content 8 and as can be seen from Fig. 3 this is reflected by a decrease in their concentration at high moisture content in uncooled grain at 30°C. In contrast, cooling renders the rate of decay of pesticide relatively insensitive to moisture content.

Initial temperature Increasing the initial temperature also speeds pesticide decay in uncooled grain bulks (Fig. 4). For instance, after 180 days of storage the methacrifos concentration on uncooled 11% moisture content wheat at 40°C decays to only 0.01% of its in- itial value. Prompt cooling of the grain, however, limits the degradation and the final concentration of pesticide is not greatly affected by brief exposure to the initial grain temperature (Fig. 4).

Power consumption The power consumption of the compressor averaged over 24 h operation is about 15 kW, whilst that of the fan delivering 3000 I s -1 is about 9 kW. The energy consumed per t of grain after being cooled for four days is therefore 1.15 kWh.

Discuss ion

A bulk of grain is a porous hygroscopic medium. The thermal behaviour of an aerated grain bulk is therefore dictated by the initial state of the grain and that of the aeration air. The heat and mass transfer phenomena which occur in aerated grain bulks have been elucidated elsewhere ~ but for the sake of clarity we shall describe briefly the case of grain cooling. If air, at a temperature lower than that of a bulk of grain and with a relative humidity greater than that of the initial interstitial humidity, is forced through the grain, a cooling and drying wave passes through the grain. The velocity of this wave depends principally on the

I00

80

_== o

"6 60

§ "6 40

20

\

~\ . . . . ~ ~ ~ Malathion (aerated)

\ \

\ \

\ \ \

Methacrifos (aerated)

\ \ \

\ \

\ \

\ \

~Malathion (unaerated)

~ Methacrifas (unaerated)

8 9 10 II 12 13 14

Initial grain moisture content,% WB

Fig. 3 The relationship between moisture content at 30°C wheat and pesticide concentrat ion after 180 days of storage after no cool- ing and four days of cooling wi th an air f low rate of 1.5 I s - 1 t - 1 at Warracknabeal, Victoria

Fig. 3 Re/arian entre la teneur en eau ~ 30°C de b/e et /a concentra- tion d'insecticide au bout de 180 jours d'entreposage sans refroidissement et apr#s quatre /ours de refroldissement avec un d#bit d'air de 1.5 1 s- ' t -~ ~ Warracknabeal, Victoria

Volume 3 Num~ro 2 Mars 1980 103

grain temperature and it is approximately 250 times slower than the face velocity of air when the grain is 30°C. Behind the cooling and drying wave a much more slowly moving cooling and wett ing wave is pro- pagated; between the passage of the two waves the grain is at the so called dwell state which is, principal- ly, a function of the wet bulb temperature of the aera- tion air and the initial grain moisture content. The in- itial grain temperature has a secondary effect on the dwell state.

Fig. 5 illustrates the calculated temperature profiles in initially 11% m.c. grain at 30°C cooled for four days at Warracknabeal with air f low rates of 1.0, 1.5 and 2.0 I s -1 t - 1 . As the air f low rate increases, the wet bulb temperature of the air leaving the evaporator coil in the refrigeration unit increases because the air is less well cooled. The compression temperature rise of the air in the aeration fan also increases with f low rate and we observe that the dwell temperature of the grain between the cooling and wett ing, and cooling and drying waves increases as a consequence of these phenomena. Fig. 5 also shows that the speeds of the cooling waves increase with air f low rate and when the air f low rate is 1.0 I s - 1 t -~ most of the faster moving cooling wave is still in the bed after four days of cooling unit operation. At this time the

80

6 0

o

c

i o

E 4O

g o

~ 20

0 20

~ ~ ~ ~ ~ Ma la th ion ( a e r a t e d )

M e t h a c r i f o s ( a e r a t e d ) X - \ x\

2',,, _ ~ \ X \ \

2 5 3 0 3 5 4 0

In i t ia l Qrain t e m p e r a t u r e , ° C

Fig. 4 The relationship between initial grain temperature of 11% m.c. wheat and pesticide concentration after 180 days of storage after no cooling and four days of cooling with an air flow rate of 1.5 I s -1 t -1 at Warracknabeal, Victoria

Fig. 4 Re/ation entre la temperature initia/e des grains b une teneur en eau de 11% du b/# et /a concentration d'insecticide au bout de 180 jours d'entreposage sans refroidissement et apr#s quatre jours de refro/dissement, avec un d~bit d'air de 1.5 / s -~ t -~ ~ War- racknabea/, Victoria

30

? . 2 5

20

E

6

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . q

1.0 ts -I t - I ]

•

O 5 I0 15

Dis tance from air inlet , m

Fig. 5 Temperature distribution in a bulk of grain initially of 11% inc. and 30°C cooled at Warracknabeal for four days with air flow rates of 1.0 I s -1 t -~, 1.5 I s -1 t -1 and 2.0. s - ' t - t

Fig. 5 R#partition des temperatures dans une masse de grains ~ une teneur en eau initia/e de 11%, ~ 30°C, refroidis ~ Warracknabeal pendant quatre jours avec des d#bits d'air de 1.0 / s -1 t -1, 1.5 / s -~ t - et 2.0 / s-1 t-~

average grain temperature is 19.8°C. When the air f low rate is 2.0 I s - 1 t - 1 it is observed that the dwell temperature of the grain is higher but most of the faster cooling wave is expelled from the grain which has a mean temperature of 19.6°C. The op- t imum air f low rate for pesticide preservation occurs when the air f low rate is 1.5 I s - 1 t - 1 and the mean grain temperature has the minimum value of 18.9°C. It is clear that an optimum mean pesticide concentra- tion arises because as the air f low rate increases there is a trade-off between rapid expulsion of heat initially in the bed and an increase in grain dwell temperature. The lowest concentration of pesticide increases with air f low rate because that grain furthest downstream of the air inlet is subject to the initial grain state for a decreasing period. The highest concentration of methacrifos is in the vicinity of the aeration duct and as the air f low rate increases this grain is subject to higher dew point temperatures hence the pesticide concentration in this region decreases.

It is the finite time for the passage of the cooling waves which causes the average and lowest concen- trations to increase with aeration time (Fig. 2). When the refrigeration units operate for only a short time, the cooling wave does not exit the bed hence some grain remains warm and pesticide decay is rapid. However, after about six days cooling, most of the grain is at the dwell state and further operation of the refrigeration unit does not result in any improvement in the preservation of pesticide, although the slower cooling wave advances through the bed and the mean grain temperature continues to decrease. The reason for this is that the grain state in the cooling and wet- ting wave closely fol lows a wet bulb line on a psychrometric chart, and in the range of temperatures and moisture contents considered, the rates of decay of the pesticides examined are almost constant for a given wet-bulb temperature. It is also seen that the lowest pesticide concentration in the bed approaches a value, a fixed amount lower than the mean value, as the number of days of cooling increases. The two values cannot converge because some pesticide is ir- revocably lost before the cooling wave has been ex- pelled from the bed. In the range of cooling times

104 International Journal of Refrigeration

Table 1. The fractional concentration of methacrifos initially at a concentration of unity and then subject to a total of 90 and 180 days of storage following alternative cooling strategies

Tableau I. Fraction de concentration de methacrifos ~ une concentration intiale de 1, puis soumis autota/ ~ 90 et 180 jours d'entreposage apr#s divers types de refroidissement

Location Air flow Duration of unit Initial grain Initial r a t e o p e r a t i o n , moisture grain

I s-1 t -1 d content, % tempera- w.b. ture °C

Fractional methacrifos concentration

90 days 180 days

Lowest Highest Mean Lowest Highest Mean

Warracknabeal 0 0 11.0 30 1.5 2 11.0 30 1.5 4 11.0 30 1.5 6 11.0 30 1.5 8 11.0 30

1.0 4 11.0 30 1.25 4 11.0 30 1.75 4 11.0 30

0 0 9.0 30 1.5 4 9.0 30

0 0 9.0 40 1.5 4 9.0 40

Moree 1.5 2 11.0 30 1.5 4 11.0 30 1.5 5 11.0 30 1.5 6 11.0 30 1.5 8 11.0 30

1.5 5 9.0 30

0.33 0.33 0.33 0.11 0.11 0.11 0.36 0.84 0.63 0.13 0.66 0.43 0.68 0.84 0.78 0.47 0.70 0.63 0.78 0.84 0.81 0.63 0.70 0.66 0.79 0.84 0.81 0.65 0.71 0.66

0.47 0.88 0.74 0.22 0.78 0.57 0.59 0.86 0.77 0.36 0.74 0.61 0.73 0.82 0.78 0.54 0.68 0.62

0.61 0.61 0.61 0.38 0.38 0.38 0.74 0.83 0.79 0.55 0.70 0.63 0.11 0.11 0.11 0.012 0.012 0.012 9.67 0.83 0.77 0.47 0.69 0.60

0.35 0.73 0.56 0.12 0.53 0.34 0.62 0.73 0.68 0.39 0.54 0.47 0.67 0.73 0.69 0.46 0.54 0.48 0.68 0.73 0.69 0.48 0.54 0.49 0.68 0.74 0.70 0.48 0.54 0.49

0.63 0.71 0.65 0.40 0.51 0.43

considered the constancy of the difference between lowest and mean methacrifos concentrations is seen after six days cooling at Moree where the higher dwell temperature results in a more rapidly travelling trailing edge of the cooling and drying wave resulting in its expulsion from the bed in this time. The higher grain dwell temperature at Moree is a reflection of the 5°C higher average wet-bulb temperature immediately after the grain harvest compared with Warracknabeal.

It has been pointed out that the dwell temperature in an aerated grain bulk is principally a function of initial grain moisture content and air inlet wet bulb temperature. Hence for given air inlet conditions, as occurs for the results presented in Fig. 3, we may observe that after four days of cooling the mean grain temperature for grain bulks initially of 8%, 11% and 14% m.c. are 30.3, 18.9 and 15.6°C respectively. The resulting grain states lie very close to a line of cons- tant wet bulb and again, as reported above, the rates of decay of the pesticides considered are approx- imately constant along such a line, ie the effect on decay rate of increasing moisture content is offset by the decreasing temperature. This explains the obser- vation in Fig. 3 that aerated grain cooling renders the rate of pesticide decay a weak function of initial grain moisture content.

After the passage of the cooling and drying wave all of the grain is in equilibrium with air at a constant wet bulb temperature. For this reason, grain bulks which have been aerated for six days or more with an air flow rate of 1.5 I s - 1 t - 1 have fairly uniformly distributed pesticide concentrations, as seen in Table 1.

After the passage of the cooling and drying wave the dwell state is almost independent of initial grain temperature. Since the time of passage of the cooling wave is much shorter than the pesticide half-life, even when the initial grain temperature is high, only a small amount of pesticide decay takes place during the cooling of the grain. This concatenation of phenomena results in the overall rate of pesticide decay being insensitive to initial grain temperature in rapidly cooled graih bulks.

C o n c l u s i o n s

From the predictions made in this work, we conclude that a standard air conditioner fitted with an ap- propriate air handling system should be able to cool grain bulks in the Australian environment sufficiently to restrict the rate of pesticide decay to acceptable proportions. In any event the cooling will inhibit the build up of insect populations and hence inhibit the development of resistance to insecticides. At War- racknabeal in Victoria the prompt cooling of wheat, initially at 30°C and of 11% moisture content, can be expected to reduce the loss of methacrifos during six months of storage from about 90 to about 30%, and the energy requirement to do this is 1.15 kWh t - 1 of grain. In the warmer climate of Moree the loss would reduce to about 50%.

The analysis has also shown the existence of an op- timum air flow rate for pesticide preservation, and the effects of duration of grain cooling unit operation have been demonstrated. The rates of decay of the pesticides malathion and methacrifos in aerated bulks

Volume 3 Number 2 March 1980 LJ.R. 3.2 D

are i nsens i t i ve to in i t ia l g ra in m o i s t u r e c o n t e n t and t e m p e r a t u r e b e c a u s e t he bu l ks are c o o l e d q u i c k l y t o t e m p e r a t u r e s w h e r e b r e a k d o w n is s l o w .

The above results have been discussed in the light of t he p h y s i c s o f hea t a n d s ing le a d s o r b a t e t r a n s f e r be t - w e e n a f lu id and a h y g r o s c o p i c p o r o u s m e d i u m .

References 1 Krishnamurthy, K. Post harvest losses in food grains Bu//Grain

Tech 13 1 (1975) 33-49 2 Dyte, C.E. Problems arising from insecticide resistance in

storage pests EPPO Bu//4 3 (1974) 275-289 3 Birch, L.C. Experimental background to the study and abun-

dance of species Eco/ogy 34 (1953) 698-711 4 Hardman, J.M. A logistic model simulating enwronmental

changes associated with the growth of populations of rice weevils, Sitophilus, oryzae, reared in small cells of wheat J App/ Eco/ 15 (1978) 65-87

5 Howe, R.W. A summary of optimal and minimal conditions for population increase in some stored products insects J stored Prod Res I (1965) 177-184

6 Desmarchelier, J.M., Bengston, M. Chemical residues of newer grain protectants, 2nd International Conference on Stored Pro- ducts Entomology, tbadan, Nigeria (1978)

7 Bengston, M., Connel, M., Davies, R.A.H., Desmarchelier, J.M., Elder, W.B., Hart, R.J., Phillips, M.P., Ridley, E.G., Ripp, R.E., Snelson, J.T., Sticka, R, Chlorpyrifos methyl plus bioresmethnn, methacrifos, pirimphos methyl plus bioresmethrin and synergised bioresmethrin as grain protectants for wheat (submitted for publication)

8 Thorpe, G.R. The persistence of chemical pesticides in aerated grain bulks, Part I a mathematical analysis (submitted for publication)

9 Thorpe, G.R., Elder, W.B. Refrigerating a horizoutal ,grain storage Bu/k Wheat 12 (1978) 47-49

10 Banks, P.J. Coupled equilibrium heat and single adsorbate transfer in fluid flow through a porous medium, 1 characteristic potentials and specific capacity ratios Chem Eng Sci27 (1972) 1143-t 155

11 Sutherland, J.W., Banks, P.J., Griffiths, H.J. Equilibrium heat and moisture transfer in air f low through grain J Agr/c Engng Res 16 (1971) 368-386

12 Storey, C.L. The effects of air movement on the biological ef- fectiveness and persistence of malathion in stored wheat, Pro- ceedings of North Central Branch, Entomological Society of America 27 (1972) 57-62

13 Hall, A. Private communication 14 Holman, L.E. Aeration of grain in commercial storages,

Marketing Research Report No 178 (1966) United States Department of Agriculture

15 Department of Science and Consumer Affairs, Bureau of Meteorology. Climatic averages, Australia. Australian Govern ment Publishing Service, Canberra, (1975)

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