GB2066651A - Process and apparatus for conditioning particulate foodstuffs - Google Patents

Abstract

A process is provided for conditioning (i.e. adjusting the moisture content and temperature of) particulate foodstuffs such as cereal grains which in by cooling the foodstuffs below ambient temperature allows them to be stored without deterioration although the moisture content remains higher than permissible for such storage at ambient temperature. In the process the foodstuff is heated in a first air current to reduce the moisture content, and is then refrigerated to below ambient temperature in a second air current, the heat abstracted in the latter step being recycled to preheat the first air current, thereby minimising the energy demand of the process. Apparatus for carrying out the process comprises a tower or silo (1) adapted to contain a slowly descending column of the foodstuff, means (3,4) for heating an upper region of the column with a first air current, means (9,5) for refrigerating a lower region of the column with a second air current, and heat exchange means (16) for recycling heat abstracted from the second air current to preheat the first air current. <IMAGE>

Description

SPECIFICATION Process and apparatus for conditioning particulate foodstuffs This invention relates to a process and apparatus fdrconditioning particulate foodstuffs, in particular cereal grain. In this specification the term "conditioning" means adjusting the moisture content and temperature to desired values thus adapting the foodstuff for storage with a reduced tendency to deteriorate. Causes of deterioration in particulate foodstuffs during storage include mould, yeast and bacterial growth, enzymic autolysis and fermentation however caused.

In commercial practice the drying of particulate foodstuffs by means of artificially applied heat energy is necessary, natural evaporation being too slow by far. This energy requirement for conditioning contributes significantly to the cost of the final product.

Conventional conditioning of particulate foodstuffs is often carried out as follows. A vertically disposed column of the foodstuff is maintained in a tower. Ambient air is collected and its temperature raised by heating means such as an oil burner or an internally steam-heated coil. The resulting hot air is drawn by means of reduced pressure through the foodstuff through a plurality of ducts which penetrate the tower wall and traverse the column of foodstuff. These ducts discharge the hot air downwardly into the foodstuff through a longitudinal opening in each duct. The resulting cooled and moisture-laden air exits from the tower through a second plurality of similar ducts to a common exhaust vent in which an exhaust fan creates the necessary air pressure differential.Untreated foodstuff is constantly added to the top of the column, and dried but warm foodstuff is constantly withdrawn at the same rate from the bottom of the column so that the process is continuous. The dried, warm foodstuff is collected from the tower, cooled to ambient temperature, and stored.

The cooling may be a continuous or a batch process.

It is known to carry out both the drying process and the cooling process in a single tower, an upper portion of which constitutes at least one drying zone and a lower portion, at least one cooling zone. This procedure simplifies the construction of the apparatus, thereby reducing initial cost. The inlet and outlet ducts for drying are located in the drying zone(s), and those for cooling in the cooling zone(s), and the particulate foodstuff itself acts as a mechanical barrier to prevent mixing of the air stream in the or the lowermost drying zone with the airstream in the or the uppermost cooling zone.

It is also known to collect the exhaust airfrom the cooling process and to use it, together with ambient air, to feed the drying process. This effects a small economy in the nett required heat input to the heating element(s). It also simplifies the ducting of the exhaust from the cooling zone(s) to the drying zone(s).

For each of a large number of particulate foodstuffs (including animal fodders) a threshold or "safe" value of the moisture content is known, below which the foodstuff when stored at room temperature will remain in usable condition. "Room temperature" in practice can vary from 15 to 250C.

Wheat grain, for example, if dried to a moisture content of 14-15% by weight, can be stored in silos at room temperature without deterioration, and this is conventional.

It is an object of the invention to provide a process for conditioning particulate foodstuffs which is less energy intensive than the known processes. The invention seeks to exploit the fact that a given particulate foodstuff, if brought to a temperature substantially below the customary room temperature, can be satisfactorily stored thereat notwithstanding that the moisture content of the foodstuff is greater than the threshold value aforesaid. Wheat grain, for example, having a moisture content as high at 18% by weight, can be satisfactorily stored if the temperature of storage is not allowed to exceed about 7"C.

The invention seeks to make such low temperature storage economical despite the additional energy requirement involved in refrigerating the foodstuff.

The invention accordingly provides a process for conditioning a particulate foodstuff which has a given actual moisture content, and which is known to be satisfactorily storable at room temperature if its moisture content is reduced below a known threshold value, which process comprises initially heating the foodstuff in a warm air current to reduce the moisture content thereof to a value below said given actual value but substantially above said known threshold value, thereafter refrigerating the foodstuff in a cold air current to a temperature substantially below room temperature for subsequent storage, and recycling a substantial proportion ofthe heat transferred in the refrigeration step to assist in warming the airforthewarm air current.

Preferably, the cold air current is prepared by means of refrigeration apparatus having a refrigerant condenser which dissipates heat, said condenser being located so as to preheat said warm air current.

Preferably the cold air is caused to travel upwards through a depth of 0.5 to 3 meters of the foodstuff to refrigerate the same, more preferably 1.5 to 2.5 meters.

It will be clear that the heat energy needed in the invented process for drying the foodstuff is less than that conventionally expended, since the moisture content of the product remains substantially greater than the customary threshold value. This saving is made all the greater by the use of the heat pump principle whereby heat generated in refrigerating air to produce the cold air current is employed in preheating airto produce (after further heating) the warm air current.

The invention also provides apparatus for performing the process described above, which apparatus comprises a tower adapted to contain a column of the foodstuff, means for adding to the The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

column at the top and withdrawing from the column at the bottom thereof so as continuously to maintain the column, means for heating a first current of air and for passing said first current through the foodstuff in an upper zone of said tower, and means for refrigerating a second current of air and for passing said second current through the foodstuff in a lower zone of said tower, wherein the refrigerating means comprises a coolant condenser located in the path of the first current of air to constitute preheating means for said first current.

The invention will be appreciated in greater detail from the following description of a particular embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which Fig. is a schematic longitudinal section of a grain conditioning silo; Fig. 2 is a schematic longitudinal section of the silo of Fig. 1, in a plane perpendicular to that of Fig.1, and further simplified; and Fig. 3 is an isometric exploded view of part of the silo of Figs. 1 and 2.

Referring now to the drawings, a grain conditioning silo comprises a grain tower 1, air preheaters 2, air heaters 3, warm air inlet ducts 4, a cold air inlet duct 5, an air outlet duct 6, a compressor fan 7, an extractorfan 8, and a refrigerator 9.

The grain tower 1 comprises a grain inlet hood 10, a grain outlet hopper 11, a vestibule section 12, three grain drying sections 13 and a grain cooling section 14.

The refrigerator 9 comprises an air cooling unit 15 and three refrigerant condensers 16 connected in parallel across the cooling unit by insulated pipes 17.

Each condenser 16 is located in a respective air preheater 2, and constitutes the heating element therefor.

The grain inlet hood 10 comprises two vertical spaced apart grain-level-sensitive switches 18 and the grain outlet hopper 11 comprises two vertically spaced apart grain-level-sensitive switches 19. A grain conveyor (not shown) is controlled by the switches 18 to introduce increments of raw grain to the tower 1, and a grain outlet valve 20 is controlled by the switches 19 to discharge increments of conditioned grain.

Referring particularly to Fig. 3, each grain drying section 13 comprises a drying box 21, two detachable insulated side panels 22, the inlet duct 4, the air heater 3, the air preheater 2 and a transition piece 23. Each drying box 21 comprises two parallel sheet metal panels 24,25 connected and spaced apart by alternate horizontal rows of warm air distributor ducts 26 and spent air collector ducts 27. An ambient air inlet manifold 29 communicates with each transition piece 23 (Fig. 1).

Each duct 26,27 is of triangular cross-section with an apex uppermost, and comprises two sloping sides of sheet metal, the base being open, although in use the grain occupies all the space between the ducts 26,27 and a surface of bulk grain takes the place of a base for each said duct. Each distributor duct 26 terminates at a triangular orifice 28 in the panel 24, its other end being closed. Each collector duct 27 terminates at a similar triangular orifice in the panel 25, its other end being closed. The grain inlet hood 10 caps the uppermost grain drying section 13.

Referring once again particularly to Fig. 3, the grain cooling section 14 comprises a cooling box 30, two detachable insulated side panels 31, and a detachable insulated front panel 32, the lower portion of which is a transition piece 33 for connection with the cold air inlet duct 5of Fig. 1. The cooling box 30 comprises two parallel sheet metal panels 34; 35 connected and spaced apart by a horizontal row of cold air distributor ducts 36 and a horizontal row of spent air collector ducts 37. The ducts 37 are similar to the ducts 27; each terminates at a triangular orifice in the panel 35, its other end being closed. The ducts 36 however have metal bases, and are provided with perforations in their sloping sides to admit the cold air to the grain.The ducts 36 terminate at triangular orifices 38 in the panel 34, their other ends being closed. The ducts 36 thus communicate with the transition piece 33. The vertical separation of the ducts 36 from the ducts 37 is 2 meters.

The air outlet duct 6 (Fig. 1) embraces all the panels 25 and the upper part of the panel 35, thus communicating with all the spent air collector ducts 27,37. The grain outlet hopper 11 underlies the grain cooling section 14. However, as shown only in Fig. 2, the grain cooling section 14 comprises a plurality of bottom outlet slots 39, each guarded by a powered grain feed control roller 40. Each slot 39 runs along the bottom of a respective one of a plurality of parallel grain feed troughs 41 which troughs together constitute a floorforthe grain cooling section 14.

In use ofthe above apparatus to perform the process of the invention, a column of grain occupies the tower 1 from the troughs 41 up to a level intermediate the switches 18, and additional grain occupies the outlet hopper 11 from the outlet valve up to a level intermediate the switches 19. The rollers 40 feed conditioned grain continuously into the outlet hopper 11, and the outlet valve 20, opened by the upper one and closed by the lower one of the grainlevel-sensitive switches 19, discharges conditioned grain intermittently for subsequent storage.The grain in the column descends slowly and continuously, butthe column is maintained by addition of raw grain from a conveyor (not shown) above the grain inlet hood 10 in a feedlinethereto. Said feed valve is opened by the lower one and closed by'the upper one ofthe grain-level-sensitive switches 18, so that the grain feed to the tower, like the grain discharge therefrom, is intermittent.

The descending grain is diverted, baffled and to some extent agitated by the obstacles provided by the ducts 26, 27, 37. Partly for this purpose the ducts 26 are positioned in horizontally staggered relationship to the ducts 27 above and below them. The grain, having flow praperties similar to those of a fluid, flows beneath each duct 26, 27,37 closing off the bottom outlet thereof with an air-grain interface.

The extractor fan 8 generates a reduced air pres sure causing a current of ambient air to enter the manifold 29 and the transition pieces 23. The current traverses the preheaters 2, the heaters 3 and the entry ducts 4, continues warm into the several dis tribute ducts 26, and, passing through the interposed grain, enters the several collector ducts 27 from where it exits to atmosphere by way of the outlet duct 6 and the extractor fan 8. The grain is thus partially dehydrated while descending through the several drying sections 13, and arrives warm into the grain cooling section 14.

The compressor fan 7 creates a forced current of ambient air and passes it through the refrigerator 9.

The refrigerated air passes by way of the inlet duct 5 and the transition piece 33 to the distributor ducts 36, from the perforations in the sloping sides of which said air enters the grain. On upward passage through the grain a distance of 2 meters the air emerges from the grain into the collector ducts 37 and is voided to atmosphere through the (common) air outlet duct 6 and the extractor fan 8. The partially dehydrated grain aforesaid is thus further dehydrated as well as being refrigerated to a sub-ambient storage temperature, at which temperature it is discharged by the rollers 40 into the outlet hopper 11, and shortly thereafter discharged therefrom through the outlet valve 20.The further dehydration occurs because the equilibrium moisture content ofthe partly dehydrated grain in contact with the cold air current is lower than the actual moisture content thereof at the moment of entry to the cooling section 14. In these drying conditions the evaporation of further moisture from the grain assists the grain cooling process due to the latent heat requirement involved.

The air cooling unit 15 of the refrigerator 9 produces a constant supply of refrigerant at an elevated temperature, and this is circulated through the several condensers which comprise the preheaters 16.

The heat evolved from these condensers is thus utilised to warm the incoming air from the manifold 29 to an intermediate temperature above ambient. The heaters 3 are adapted to burn fuel oil in the air current thereby raising the latter to a working temperature.

The following Example illustrates the energy saving attained in using the process of the invention Example.

In an apparatus similar to that described above with reference to the drawings, except that it contained 5 drying sections, Irish malting barley grain of moisture content 21% by weight was processed at a rate of 20 tonnes per hour. The ambient temperature, both of the air and of the raw grain, was 15 C.

The air humidity, expressed as kg water per kg dry air, was 0.008. Ambient air entering the manifold 29 was raised to 35 C by the preheaters 16 and then to 70"C by the heaters 3. The drying air current had a value of 54,050 kg per hour. Its temperature at exhaust was 40"C. The cooling air current generated by the compressor fan 7 was brought down to a temperaure of 5"C by the refrigerator 9. It had a value of 29,000 kg per hour, and an exhaust temperature of 30"C.

The conditioned grain issuing from the outlet valve 20 had a moisture content of 17.5% by weight and a temperature of 7"C. It was stored for 2 months without significant deterioration, after which period its temperature was found to be 8"C. This was a test run.

A comparison run was made in a similar apparatus (having, however, 17 drying sections) with malting barley grain of the same moisture content and other properties. The refrigerator 9 was inoperative during the comparison run, ambient air alone being used to cool the grain to ambient temperature, which was 15"C. However, the extractor fan 8 was run at a higher speed of revolution to generate a heavier warm air current, the value of which was 220,000 kg per hour.A greater quantity of fuel oil per unit time was burned by the heaters 3 in order to raise this heavier air current to the working temperature of 70"C. The heavier warm air current was necessary in orderto dry the grain from 21% to 15% moisture content by weight, this lower moisture content of the dried grain being necessary to permit subsequent storage of the conditioned grain at ambient temperature.

The energy balance of the two runs was as fol lows:

Test Comparison Heat required for drying, in Kcal/hr. 723,900 (a) 2,414,000 (c) Heat recovered by recycling, Kcal/hr. 125,400 (b) Nett Heat consumption, Kcal/hr. 598,500 (a-b) 2,414,000 (c) Heat saved, Kcallhr 1,815,500 (c-(a-b)) equivalent to kg heating oil/tonne grain 8.4 Fan horse power 70 225 Refrigeration horse power 50 NIL Total horse power 120 (e) 225 (f) Power saved (H.P.) 105 (f-e) equivalent to KWh/tonne grain 5 The energy saving thus attributable to the employment of the process of the invention amounted to 8.4 kg of fuel oil plus 5 kilowatt-hours, pertonne of raw grain, a considerable improvement over the conventional process used in the comparison run.

Claims (11)

1. A process for conditioning a particulate foodstuffwhich has a given actual moisture content, and which is known to be satisfactorily storable at room temperature if its moisture content is reduced below a known threshold value, which process comprises initially heating the foodstuff in a warm air current to reduce the moisture content thereof to a value below said given actual value but substantially above said known threshold value, thereafter refrigerating the foodstuff in a cold air current to a temperature substantially below room temperature for subsequent storage, and recycling a substantial proportion of the heat transferred in the refrigeration step to assist in warming the air for the warm air current.

2. A process as claimed in claim 1 wherein the cold air current is prepared by means of refrigeration apparatus having a refrigerant condenser which dissipates heat, said condenser being located so as to preheat said warm air current.

3. A process as claimed in claim 1 or2, wherein the cold air is caused to travel upwards through a depth of 0.5 to 3 meters of the foodstuff to refrigerate the same.

4. A process as claimed in claim 3, wherein the depth is 1.5 to 2.5 meters.

5. A process as claimed in any of claims 1 to 4 wherein the particulate foodstuff is a cereal grain.

6. A process as claimed in claim 5 wherein the cereal is selected from malting barley, feeding barley, and wheat.

7. A process as claimed in claim 5 or 6 wherein the final temperature of the cereal grain does not exceed 10 C.

8. A process as claimed in claim 6 or 7 wherein the final moisture content of the cereal grain exceeds 16% by weight.

9. A process as claimed in claim 1, substantially as herein described.

10. Apparatus for performing the process claimed in any of claims 1 to 9, which apparatus comprises a tower adapted to contain a column of the foodstuff, means for adding to the column at the top and withdrawing from the column at the bottom thereof so as continuously to maintain the column, means for heating a first current of air and for passing said first current through the foodstuff in an upper zone of said tower, and means for refrigerating a second current of air and for passing said second current through the foodstuff in a lower zone of said tower, wherein the refrigerating means com prises a coolant condenser located in the path of the first current of air to constitute preheating means for said first current.

11. Apparatus as claimed in claim 10, substan tially as herein described with reference to or as illustrated in the accompanying drawings.