AU741613B2 - Automatic fan controller using the differential principle - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT

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AUTOMATIC FAN CONTROLLER USING THE DIFFERENTIAL PRINCIPLE The following statement is a full description of this invention, including the best methods of performing it known to me: AUTOMATIC FAN CONTROLLER USING THE DIFFERENTIAL PRINCIPLE Technical Field This invention concerns the automatic control of fans or pumps used to pass fluid (gases or liquid), air to which future references will be made, from one space to another. In particular it describes an automatic fan controller that may be used to lower the moisture content, to cool or warm durable commodities in storage containers such as silos or used to alter the relative humidity or temperature in one space by conveying suitable air from another space. Examples of the field include but are not restricted to:ooooo 1. A controller for the fans used in the ambient drying of durable commodities in silos such as nuts, grain, seed etc; S2. A controller for the fans used in Silos to lower the temperature of the stored commodities such as grain, seed, nuts etc; 3. A controller for a fan or fans used to transfer warm air from a passively heated ceiling space in houses to living areas below.

It relates to the Patent Classification AI01F 25/00, B65 3/08, A23N 12/08, F26B 21/06, 15/00, G05B 11/00, G05B 19/00, G05B 21/00 with F26B 21/06 being most relevant to this particular application.

Background It is common practice for durable commodities such as grain and nuts to be placed in storage facilities such as silos soon after harvest while they are awaiting shipment to processors or awaiting export to other markets. When these commodities are freshly harvested they are frequently at moisture contents or temperatures that are higher than those considered safe for storage in confined spaces such as silos. It is therefore common practice to place these commodities in ventilated storages through which air may be forced to either cool the commodity or to reduce excess moisture from the atmosphere within the commodity. For this purpose, fans are used which are sized to provide what is considered to be an appropriate flow rate through the commodity. In some cases warm, ambient air may also be forced through cold grain to warm the grain slightly and thus prevent moisture condensation on the grain when it is outloaded from silos in warm moist conditions.

It is recognised by the operators of such facilities that the air available from outside the storage is sometimes unsuitable for passing through the commodity because the ambient temperature may be too high or the relative humidity in the ambient air may be higher than the equilibrium relative humidity of the commodity.

It is also known that virtually all durable commodities absorb and desorb moisture and will reach an equilibrium with the air within the commodity when it is confined in spaces such as •silos. The air within the commodity is known as the interstitial air or intergranular air. This equilibrium is attained slowly and is affected by temperature. For a given temperature, if the :.--relative humidity of the air within the commodity is lower than the equilibrium relative 15 humidity, moisture from the commodity will desorb into the intergranular air and the moisture content of the commodity will thus be decreased. Therefore by actively passing air of a lower humidity through a commodity a lowering of the moisture content can be achieved. This is the principle of grain driers although they achieve the lowering of intergranular relative humidity by raising the temperature of the air introduced into the S" 20 commodity and progressively raising the temperature of the commodity. If unheated air from outside a silo is used, drying will mainly occur if the air from outside the silo is at a lower relative humidity than the intergranular air of the commodity. Under normal atmospheric conditions this condition will vary diurnally and with changes in weather patterns. It should be noted that because of the thermal mass of most durable commodities in silos the relative humidity is of greater significance than the absolute amount of water vapour in the atmosphere. However, under some conditions moisture content of the air must be taken into account to achieve efficient ambient-air drying.

Unless suitable conditions of temperature and relative humidity (or moisture content of the commodity) are achieved within a commodity stored in structures such as silos spoilage can occur and significantly affect the marketability of the commodity. At temperatures greater than about 15' C, moisture contents in excess of 13% in wheat for example, rapid growth of moulds can occur which can have serious consequences for the marketability of such products and their use for either animal or human consumption. Mould growth in such commodities may lead to the production of mycotoxins many of which have serious health implications, the production of aflatoxin in peanuts. Effective storage practices to alter the conditions leading to the production of mycotoxins are therefore essential and as indicated above, forced ventilation plays a key role in achieving this.

At temperatures greater than about 15'C insect infestations can develop in grain which can lead to considerable losses both in terms of quality and quantity. Grain temperatures at harvest are frequently higher than 30'C creating conditions that will favour rapid development of most species of insects. To minimise losses due to insects a preferred .ooooi method is to reduce the temperature of the grain as soon as possible after harvest and to eventually cool it to temperatures around or less than 15'C This cooling of the grain is achieved by forcing cool air through the grain, i.e. air at a lower temperature than that of the 15 grain. The air suitable for this purpose must also be at a suitable relative humidity that will not lead to any significant increase in the moisture content of the grain which can lead to caking of the grain around aeration ducts and possible mould development.

0: In some countries grain is cooled to temperatures at or near freezing point. When this grain 20 is later outloaded from such storages during warm weather moisture from the atmosphere condenses on the cold grain which can lead to an increase in the grain's moisture content 6 a.

S* ,and create conditions that will favour mould growth.

To ensure that fans only pass air of suitable conditions through a commodity the current practice is for operators of such silos to switch the fan ON and OFF when, in their judgement, conditions are appropriate. Time clocks are sometimes used for this purpose. A further method that has been employed to achieve the required conditions is to use a simple thermostat or humidistat or a combination of both and to select specific set points at which the fan will switch ON or OFF. In addition, in aeration systems used to cool grain, the interaction of temperature and relative humidity, the use of wet-bulb temperature of the ambient air and the intergranular dry-bulb temperature has also been used. Disclosure relating to this method can be found Patent AU-A 1-48,856/79. However, a major defect in the method described in this patent is that it fails to measure the relative humidity of the intergranular air of the grain and hence its moisture content and wet-bulb temperature. Although Claim 2 of that patent refers to the "moisture content of the air surrounding the grain" it is in fact referring to the equilibrium relative humidity of the grain associated with the moisture content of the grain and that this relative humidity is selected as the upper limit for ambient air that is suitable for cooling the grain according to the selected wet bulb temperature limit. A further defect of the method described in this patent is that it relies on a wet-bulb temperature sensing means which usually employs a water-fed wick the evaporation from which produces a lowering of the temperature response of the sensing device which is correlated with the relative humidity of the surrounding air. Waterfed wick devices for measuring wet-bulb temperatures are inherently unreliable as practical devices because of the failure to maintain the wick and water supply with the result that control signals generated by such devices are also unreliable and could lead to damage of the commodity being aerated.

A further aeration controller is that described in Australian Patent No. 438732 "Means for conditioning a conditionable means". This controller is essentially a moving set-point controller that uses ambient dry-bulb temperatures only. There are no sensors in the grain and the device changes the set-point on ambient air according to a selected rate of change 20 designed to take into account seasonal changes. This controller does not have provision to set an upper limit on the relative humidity of suitable air and it does not have means for sensing the temperature or relative humidity of the grain being aerated.

These approaches frequently fail to take into account the fact that as air is passed from one space to another the conditions of temperature or equilibrium humidity will change and therefore affect the efficiency of the operation. For example, in the case of cooling a commodity in a storage using ambient air, the temperature of which changes diurnally, the fan will switch ON when the air temperature drops to the selected set point and continue to operate until, during the rising phase of the ambient temperature, the set temperature is again reached and exceeded. If during the period when the fan is ON the commodity temperature is reduced to below the level of the set point of the fan's thermostat some of the benefits of this will be lost during the rising phase of the ambient temperature and heat will be added back to the commodity before the fan switches OFF. To cope with this the set point of the thermostat would need to be adjusted manually from time to time. A further disadvantage of this approach is that it will use more energy than is necessary for the purpose.

It is the purpose of the present invention to achieve maximum efficiency when transferring air from one space to another and in preferred applications of the principles to achieve maximum efficiency of either reduction of temperature or moisture content of a commodity using ambient air, cooling or drying without temperature or moisture regain.

It is further the purpose of this invention to allow cooling, drying or warming of commodities, stored in silos, using ambient air with the help of a controlling device.

.oo.oi Other features and advantages of the present invention will be apparent from the following :....detailed summary.

Summary of the present invention.

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The primary object of the present invention in its broadest concept is to have fans or transfer pumps (hereinafter referred to as fan switched ON only when the conditions of one 20 space on the upstream side of the fan also referred to as the ambient side of the fan) will be of benefit to the downstream space including but not restricted to a silo or storage structure containing a durable commodity including but not limited to grain). This is illustrated in FIG. 1 in which Zone A may be on the upstream side of a fan 1 the operation of which is controlled by a controller 2 which puts the fan 1 into the ON state only when the air from Zone A will benefit Zone B as determined by the sensors 3 and 4, when the difference between the output of the sensors is either positive or negative depending on the objective of the fan operation.. If the objective is to cool Zone B by passing the air of Zone A through Zone B the fan will only be set in the ON state when the temperature of Zone A as measured by sensor 3 is less than the temperature of Zone B as measured by sensor 4. If the temperature of Zone A is changing regularly, diurnally it is to be expected that the period of operation of the fan will decrease progressively as the temperature of Zone B decreases and eventually the temperature of Zone B would approximate the minimum temperature of Zone A. By contrast, if a simple thermostat were used to monitor the temperature in Zone A and control the switching of the fan, the period of fan operation would remain constant and a lower rate of decrease would occur in Zone B and the final or equilibrium temperature of Zone B would approximate the average of the difference between the set point and the minimum temperatures that occurred in Zone A, not as low as that achieved with the continuously variable differential controller.

A similar situation occurs if the primary objective of the fan operation is to lower the relative humidity in Zone B using gas such as air from Zone A only when the relative humidity of Zone A is less than that of Zone B. In this case, using the continuously variable differential fan controller, the minimum moisture content of a commodity in Zone B will approximate that determined by the equilibrium moisture content of the lowest relative humidity that occurs in Zone A The objective of the aeration fan controller of this invention is achieved by using sensors in each of the spaces and an electronic-control circuit that selects one state or another ON or OFF based on whether the difference between the outputs from the two or more sensors is either positive or negative.

When the primary objective is to lower the moisture content of a commodity, the logic of 20 the switching circuit will be such that. the fan will only switch ON when the relative humidity or moisture content of the ambient air is less than the relative humidity or moisture o content of the intergranular air within the commodity. It is an additional objective of the Sinvention to use the fan in the ON mode when RHB is greater than or equal to RHA (1 C) where RHA is an ambient air humidity and RHB is a relative humidity of the intergranular :"25 air, whereas C a rate-determining constant 0 to Cmax, 0 to 1. Likewise, if the primary objective is to cool the commodity the logic of the fan switching circuit will only switch the fan ON when the air outside the storage is at a lower temperature (wet-bulb or dry-bulb temperature) than the air within the commodity. However, in this case, in particular, it is necessary to include logic governing the relative humidity of the ambient air and the intergranular air so that in the process of cooling, the moisture content of the commodity is not raised to unacceptable levels. In this case sensors measuring both temperature and relative humidity are used. It is a further objective of the invention to ,Ncalculate a rate of lowering of the intergranular humidity. If the rate of lowering is substantially low the fan will be set in the OFF mode in which case a further minimisation of power consumption can be achieved.

It is also a component of the present invention that air from one space can be transferred to another space using a fan that is activated by the difference in the selected conditions between the two spaces. An example of this invention is the transfer of warm ceiling air (heated by solar radiation) in an insulated house to the cooler living spaces below. If a simple thermostat is used the fan will switch ON when the temperature of the air in the ceiling exceeds the set point of the thermostat and continue to transfer air from the ceiling until, as the afternoon progresses and the sun begins to set, the temperature falls below the set point. As the temperature in the ceiling begins to fall it will then be transferring air to the space below that is cooler than that now in the space below so that the benefits gained during the warmer part of the day are subsequently lost. By contrast, the present invention *.ooo V is designed to switch the fan OFF as soon as the temperature of the ceiling air drops below 15 the temperature of the air in the space below thus achieving maximum efficiency in the operation of such a fan.

This controller can be used in a variety of conditions to transfer warm air to a cooler space or vice versa. For example, it may also be used to control fans to warm grain that has been cooled to very low temperatures. Such grain, without warming, could attract moisture condensation if later outloaded in warm, moist ambient conditions, a situation known to occur in some countries.

Description of the preferred embodiments of this invention The following description refers to the use of the automatic fan controller in a commodity storage situation where the primary objective is to lower the moisture content of the commodity which is one mode of operation of the proposed controller. A diagram showing the installation of such a controller is shown in FIG. 2. The logic associated with the operation of this controller in Drying Mode is given in FIG. 4 which is a flowchart of typical logic to program such a controller.

The automatic fan controller 2 consists of electronic circuitry which measures the difference in, output voltage from sensors in the air outside the silo 3 and the air within the commodity 6 inside the silo 4. The sensors measure both temperature and relative humidity.

In Drying Mode the operation of the Controller may be set in either Air Moisture Content (Air MC) Mode or Relative Humidity (RH) Mode FIG. 4 In Air MC Mode if the ambient temperature is less than the intergranular temperature differences in RH will be used to switch the fan whereas if the ambient temperature is greater than the intergranular temperature Air MC will be used to switch the fan. Because of the stable thermal mass of commodities in storage air of higher temperature will be rapidly cooled as it begins to pass through the commodity and the RH of the air will rise. Thus it is important that under such conditions the increased RH is less than the equilibrium RH of the commodity so that drying can proceed.

ooooo 15 When the selected condition is suitable a signal will be generated that will activate the fan through a suitable relay and commence to pass the drier air from outside the silo through a perforated plate or other diffuser 5 and then through the commodity 6. By achieving a lower relative humidity in the intergranular air of the silo than the equilibrium relative humidity of the commodity, moisture from the commodity will desorb in accordance with the hygroscopic equilibrium involved and hence the moisture content of the commodity will be lowered. The automatic controller will continue to compare the signals from the two sensors and when the air outside the silo is no longer suitable the fan will switch OFF. Thus because the fan operation has lowered the moisture content of the commodity and hence the relative humidity within the intergranular air, the fan will switch OFF at a lower ambient relative humidity or moisture content than when it switched ON. It follows from this that the sensor in the commodity is placed at the opposite end of the flow path of the air through the commodity.

The relative humidity in the ambient air changes diurnally and with changes in atmospheric conditions such as during periods of rain. With this fan controller which represents one embodiment of the present invention, the maximum rate of decrease of moisture content of the commodity will be achieved with minimum fan operation and the moisture content will lowered to that at or near the equilibrium moisture content of the average minimum relative humidity of the ambient air during the storage period.

It should be noted that although voltage has been referred to in the foregoing example other applications of the controller may use different sensors with different outputs including but not restricted to resistance, capacitance, current or inductance.

A particular example of a commodity for which the primary objective is to lower the moisture content is the storage of Macadamia nuts. These are stored as nut-in-shell and when freshly harvested from the field may have moisture contents in excess of 20%. In some sections of the industry it is also a common practice to pass the nut through water and to discard those nuts that float (immature nuts). The nuts that sink (mature nuts) are then transferred to the storage silos and the intergranular relative humidity is likely to be close to saturation At these high moisture contents and intergranular relative humidity 15 there is a high probability that moulds will develop that will lead to spoilage, downgrading and possible total loss of the nuts.

For further clarification reference will be made to accompanying drawings FIGS. 3 and 4.

FIG. 3 is an example of the controller's functional diagram while FIG. 4 is a flow chart 20 of typical logic to program the controller.

FIG. 3 is a schematic diagram of an example of the implementation of the automatic

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aeration controller which is the embodiment of this invention. Integrated solid stage relative humidity (RH) and temperature sensors, e.g. Vaisala Oy, Finland, are connected to microprocessor (uP) 14 via a multiplexer 13 which interfaces to the multisensor array. The sensor 11 measures relative humidity (RHa) and temperature (Ta) of the ambient air whereas the sensors 12 measure the parameters RHi and Ti of the intergranular air of the commodities stored in silos, e.g. in the silo i where 1 i m and m being a maximum number of sensors or silos. The parameters of the silo's sensors 12 will be compared with the ambient sensor 11 in the microprocessor 14 according to the differential principle of measurement.

The input of the microprocessor 14 has an analogue to digital converter to transfer the sensor's analogue signals into digital numbers. From the input data for Ta, T1, Tm and RHa, RH1, RH2 RHm from sensors 11 and 12 the microprocessor computes values of wet bulb temperature (Twa) as a function of RH and T for an ambient air and for each silo Twl, Tw2 Tm using for example, an iterative mathematical algorithm. The parameters such as air moisture content as a function of RH and T for ambient conditions (AMCa) and silos atmosphere (AMC1, AMC2 AMCm) are also computed by the microprocessor 14 in addition to wet bulb temperature. The sensor's data for Ta, T1, T2 Tm, RHa, RH1, RH2 RHm, and computed values of Twa, Twl, Tw2 Twm, AMCa, AMC1, AMC2 AMCm are compared between themselves according to the controller logic FIG. 4. The types of equations used in the logic (FIG. 4) and computed by the microprocessor 14 are: Ta (Ti ±DT) 2 0; Twa-(Twi±DT) 0; RHa (RHi DRH) 2 0 and AMCa (AMCi ±DAMC) 0 (1) 15 OR (Ta±DT)-Ti 0; (Twa±DT)-Twi 0; (RHa±DRH) RHi 0 and (AMCa DAMC) AMCi 2 0 (2)

OR

Ta Ti(1 C) 2 0; Twa Twi(1 C) 0; RHa RHi(1 C) 2 0 and 20 AMCa AMCi(1 C) 0 (3) 9o, OR Ta(1 C)-Ti 0; Twa(l ±C)-Twi 0; RHa(l C) RHi 2 0 and AMCa(1 C) AMCi 0 (4)

OR

Any combinations of equations described in the sets of equations to Where 1 5 i m and m is the number of silo sensors; Ta, Twa are the ambient temperature and the ambient wet bulb temperature. Ti, Twi are the intergranular temperature and the intergranular wet bulb temperature of the commodity in the i silo; RHa, AMCa are the relative humidity and air moisture content of the ambient air; RHi, AMCi are the relative humidity and air moisture content of the intergranular air in the i silo; DT, DRH and DAMC are parameters which by adding or subtracting can be used for example to improve the efficiency of cooling or can be used to achieve warming of a commodity by offsetting T, Tw, RH or AMC.

The parameters Ta, Ti, RHa, RHi are derived from the RH/T sensors 11 and 12 and Twa, Twi, AMCa, AMCi have been computed by the microprocessor 14. The sets of equations and will determine the state ON or OFF of the fans supplying ambient air to the silos in the selected cooling, drying or warming modes. Thus the logic to control fans ON or OFF is based on the difference between parameters of the ambient air and those of the intergranular air of each silo.

The use of difference parameters for fan control has an additional advantage to produce more stable results over long periods of time considering that similar sensors tend to change *006 their measured parameters (T and RH and corresponding output voltages of the sensors in doses: so* this instance) in the same direction. The error will have a tendency to be cancelled once the 15 difference between the parameters is calculated according to equations and In practical terms it acts to prolong time between recalibration of the sensors. Aeration controllers that do not use this differential principle such as set point controllers or timeeg proportioning controllers and use sensors only in the ambient air that slowly change their o characteristics over time will eventually lead to unsatisfactory ambient-air cooling, drying or 20 warming.

oo Values of additional parameters such as maxima and minima of relative humidity and egos temperature, some coefficients and setpoints, etc. are introduced into the microcomputer 14 via the keypad 15 by an operator. Selected parameters will be computed in the microprocessor using RHa, RHI RHm, Ta, T1 Tm, etc in accordance with the control algorithms and modes of operation FIG. 4 to switch silo fans 20 ON or OFF. Some of the programmable parameters like the minimum time of operation the fan (t oN m and t OFF min FIG. have been included in the control logic to prevent rapid cycling of fans when differences are small and fluctuating. The modes of the operation of the proposed controller can be chosen by an operator from the menu and introduced into the microprocessor 14 using the keypad The m control outputs 16 from microprocessor 14 are connected to the m electrical switching means 18 with the help of the amplifiers 17, e.g. an array of transistors. The electrical switches 18, e.g. electromechanical relays, contactors, or electronic solid state relays activate the fans electric motor 20 by connecting them to single or 3-phase electricity supply mains 19.

The values of temperature and relative humidity measured by the sensors 11 and 12 for ambient air and silo intergranular air are displayed on the controller display 21 in connection with the microprocessor 14. The Controller 10 has external communications including remote control via a modem, datalogging, etc provided by means of an interface, an RS232 port 22 controlled by microprocessor 14. The output 24 of the microprocessor 14 controls an alarm device 23. Alarm conditions identified by the microprocessor include if the sensors have not been connected to the controller or if they fail to produce an electrical signal to the controller. One of the examples of such an algorithm for an alarm can be 15 Vhmin Vhi Vhmax; Vhmin Vha Vhmax and VTmin VT i VTmax; VTmin VTa VTmax, where i 1 to m; Vhi is relative humidity signal from silo i sensor, Vha is RH signal from the ambient sensor; Vhmin and Vhmax are minimum and maximum voltage range from the sensor. Likewise, VTi is the voltage corresponding to the temperature in i silo and VTa is for an ambient 20 temperature, whereas VTmin, VTmax is the voltage range for temperature. The minimum voltages identify short circuits and maximum voltages are used to identify open circuit. The fault is an alarm situation displayed by means 21 and inputs of audio or visual alarm devices 23 connected to line 24. In case of alarm the affected silo's fan will be switched OFF and in case of the ambient's sensor failure all fans will be disabled. As shown in FIG. 4 one example of the proposed Automatic Aeration Controller has 5 modes of operation of which 3 are cooling (Cl, C2, C3) which are used mostly for grains and 2 are drying (Dl and D2) designated primarily for nuts, beans, etc.

Although exemplary implementation of the present invention has been described above it should be mentioned that variations in those implementations may be effected without departing from the present inventive concept.

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Claims (5)

1. An automatic aeration controller for cooling, drying or warming commodities stored in silos or other storages by controlling an ambient air flow through said commodities, comprising an array of integral sensors for measurements of relative humidity (RH) and temperature wherein one sensor of the array is designated for ambient air and the other sensors being placed into said commodities of m silos to measure RH and T of intergranular air; a microprocessor or a similar computational means connected to the said sensor array and programmed to calculate wet bulb temperatures from RH and T values as for the ambient air (Twa) as well as for the air in each of the m silos (Twl to Twm), and further programmed to compute air moisture contents of the said ambient air (AMCa) and the intergranular atmospheres (AMC1 to AMCm) in m silos and thereafter programmed to calculate differences between parameters from the said sensors and the values derived by the computer using said parameters for the intergranular air compared to the ambient air, for example, TI -Ta 0, to Tm Ta 0 RH1- RHa 0, to RHm RHa 2 0 9 Twl Twa 0, to Twm Twa 2 0 o

9. AMC1 AMCa 0, to AMCm AMCa 0 20 to generate control signals to switch fans of each of the m silos ON or OFF in order to produce conditions of aeration beneficial for cooling, drying or warming of grains, nuts or other commodities; means to switch ON and OFF said fans controlled from said microcomputer; an alarm circuit to prevent operation of one or all fans if one of the S said sensors is at fault. 2. An automatic aeration controller as claimed in Claim 1 where ambient wet-bulb *999 temperatures (Twa) are compared with m intergranular wet-bulb temperatures (Twl to STwm) to control cooling or warming of the commodity by the switching ON for cooling or OFF for warming said m fans using the following algorithm: (Twl+ DTI) (Twa DT 2 0, to (Twm DTI) (Twa DT 2 0 where DTI and DT 2 are rate-determining constants with values DT 0 to DTmax or the algorithm: K P 1 T Twl(l+Ci) Twa(l+C 2 0, to Twm(l+Ci) Twa(l+C 2 2 0 where C 1 and C 2 are rate-determining constants with values C 0 to Cmax 3. An automatic aeration controller as claimed in Claim 1 where ambient temperatures (Ta) are compared with m intergranular temperatures (T1 to Tm) to control cooling or warming of the commodity by the switching ON for cooling or OFF for warming the said m fans using the following algorithms: (Tl+ DTI)-(Ta DT 2 0, to (Tm DTI) (Ta DT 2 0 where DTI and DT 2 are constants with values DT 0 to DTmax or: T1(1+C 1 Ta(l+C 2 2 Tm(l+C 1 Ta(l+C 2 2 0 where CI and C 2 are constants with values C 0 to Cmax 4. An automatic aeration controller as claimed in Claim 1 in which the value of a set wet- bulb temperature Twset is selected at a value at or above maximum prevailing ambient temperatures and a value of a selected constant Tmin is set at or above the commodity temperature so as to achieve heating of the said commodity by switching fans ON when ambient temperature Ta Tmin. 20 5. An automatic aeration controller as claimed in Claim 1 in which said wet bulb temperatures calculated by the said computer are used to control cooling of the commodity and in which the control algorithm to switch ON or OFF said m fan will be as follows: TI Twa(l+C)> 0, Tm Twa(l+C) 0 where a rate-determining constant C equals 0 to Cmax. 6. An automatic aeration controller as claimed in Claim 1 in which relative humidities, air moisture contents and temperature of the ambient and intergranular airs are compared by said microcomputer to achieve drying of a commodity in such order that if the ambient air temperature is greater than the temperature of the commodity the silo fans will be switched ON if: AMC1 AMCa 2 0, to AMCm AMCa 2 0 and if the ambient air temperature is less than the temperature of the intergranular air the fans will be switched ON if: RH1 RHa RHm RHa(1+C) 0, where a constant C 0 to Cmax. 7. An automatic aeration controller as claimed in Claims 1, 2 and 3 in which the value of DT 1 and C 1 are selected at a value so that Ti +DT 1 or Ti(l+C 1 will be at or above maximum prevailing ambient temperatures (wet-bulb or dry-bulb), values of DT 2 and C 2 are set at zero and Tmin is set at or above the commodity temperature and with values of DT or C and Tmin set this way the fans will be ON when conditions are suitable to warm grain that may have been cooled to very low temperatures. 8. An automatic aeration controller as claimed in Claims 1, 2, 3, 4, and 5 in which the said microcomputer controls the said fans to be switched OFF if: Tl to Tm Tmin where Tmin is set to prevent overcooling of the said commodity. .2 9. An automatic aeration controller as claimed in Claims 1, 2, 3, 4, and 5 in which the said 20 microcomputer controls the said fans to be switched OFF if: RHa RHmax where RHmax is set to prevent spoilage of the said commodity.

10. An automatic aeration controller as claimed in Claims 1 and 6 in which the said 9 microcomputer is programmed to switch OFF the said fans if: RH1 to RHm RHmin where RHmin is a set constant, to prevent the said commodity from overdrying or to select a preferred value for density sorting or for other purposes.

11. An automatic aeration controller as claimed in Claims 1 and 6 in which the said microcomputer is programmed to switch one of the drying modes to one of the cooling modes if the temperature Ti of the commodity exceeds some maximum temperature which can overheat said commodity.

12. A storage silo equipped with an automatic aeration controller as claimed in any one of the claims. I 0th September 1998 Robert Gordon Winks for WRC Technology U5321215