METHOD FOR THE CONTROLLING OF TEMPERATURE AND HUMIDITY

Abstract: A method for controlling the temperature and the humidity in storage spaces that tend to become warm and moist, in particular in vegetable and fruit warehouses. According to the method, the air in the storage space is circulated so that it passes across a cooling heat-exchange face, to which face the air delivers some of its heat. In order to keep the relative humidity of the air at its desired maximum value, in a way characteristic of the invention, the temperature of the cooling face is controlled substantially to a dewpoint temperature corresponding to the desired relative humidity, whereat any excess moisture is removed out of the air in the warehouse, but the air cannot become more dry than the desired value of humidity. The surface temperature of the heat exchanger can be brought to the said dewpoint temperature either by adjusting the surface temperature or, alternatively, by dimensioning the cooling apparatus, on the basis of empiric or experimental knowledge, in such a way relative the storage space that the heat exchanger assumes the said temperature in practice.

Description: Method for the controlling of temperature and humidity The present patent application is concerned with a method for the controlling of temperature and humidity which is in particular suitable for vegetable warehouses and in which method heat generated, and moisture delivered, by the products to be stored are removed out of the air in the storage space. This method is, in the first place, meant for the control of vegetable warehouses and in particular for shield-gas storage of vegetables and for research greenhouses, in which ventilation from the outside air is not desirable except in special cases. A shield-gas warehouse for vegetables means a warehouse in which the composition of the air is changed so that it differs from that of the normal atmosphere in order to obtain a better preservation of the vegetables. Thus, outside air cannot be used directly for cooling. Some of the most important factors to be adjusted in vegetable warehouses are the temperature and the relative humidity (RH) , both of which tend to be increased during the storage period as a result of the breathing activity of the vegetables. A typical storage temperature of vegetables is T = 0 ... 5°C, and the desired relative humidity RH = 85 ... 95 %. The sizes of the warehouses vary from the small warehouses of 10 to 50 tons at farms and retail stores to large warehouses of 100 to 1000 tons and up to 10,000 tons, which are kept by wholesale companies, food factories, and large specialized farms, etc. The storage space of a French-fries factory may be, e.g., 50,000 tons of potatoes per warehouse, divided into bulk potato heaps of 5,000 tons. High requirements are imposed on the system of controlling the temperature and the humidity in such a large warehouse. Owing to the large unified heap, the heat generated by the products has not time to OMPI escape through the surface of the heap, but an efficient current of air of precisely controlled temper¬ ature is required through the product heap, by means of which current of air the heat generated in the products can be removed. The air flow speeds used in the case of potato vary within the range of 40 to 120 cubic metres per potato ton and hour. A large quantity of air cools efficiently, but it also evaporates water out of the products efficiently, i.e. causes losses of weight. The extent of weight losses decisively depends on the relative humidity of the air used for cooling, the higher the humidity, the lower are the weight losses, but an excessively high relative humidity, or even fog contained in the cooling air, causes the risk of water being condensated on the pro¬ ducts, which promotes the development of plant diseases. Some of the control methods known in prior art for controlling the • temperature and the humidity in vegetable warehouses are as follows: 1. Cooling by means of a compressor-operated refrigeration machinery alone, so that the vaporizing radiator of the compressor is placed in the space to be cooled. When a compressor method is used, the air becomes dry, because, out of reasons of expenses, a high difference in temperature must be used between the air to be cooled and the vaporizing radiators (5 to 9 C) . Water must be added to the air in the warehouse arti¬ ficially, e.g., by means of evaporation humidifiers. Thus, the controlling of the relative humidity in the warehouse in this way involves waste of energy, because the same water that is first evaporated by means of electricity is soon condensed on the vaporizing radiators of the compressors either as water or as ice, and, in the latter case, the vaporizers must also be melted by means of electricity from time to time. On the other hand, the .operation of a compressor around the year OMPΓ " """ consumes an abundance of energy, and the equipment is soon worn out. The standard of technology of the refrigerated warehouses cooled by means of compressor-operated refrigeration machinery comes out, e.g., from Kylma- tekniikan oppikirja (Textbook of refrigeration techno¬ logy) , written by Vainδ Jaurola, published by Suomen kylmayhdistys r.y. (Finnish refrigeration association), printed by Oy ansi-Suomi, Rau a, 1979. 2. Cooling by means of outdoor air alone. In this method, the warmed-up interior air of the warehouse is replaced by colder outdoor air by means of gates and blowers placed in the wall. Figure 1 shows the principle of cooling by means of outdoor air. For the cooling of the products, a mixed air 12 is used, which has been produced by mixing together outdoor air 16 and indoor air 15 by means of the gate 9 so that the temperature of the mixed air is within the limits of the ability of toleration of plants, i.e. 2 to 3 C colder than the plants. The mixed air 12 is passed to the products 11 typically via underfloor ducts 20. The relative humidity of the mixed air is determined in accordance with the relative humidities and temperatures of the available outdoor air and indoor air, and in practice it is almost always excessively low, causing weight losses of up to 10...15 % during a storage season. Out of the above reason, the control of humi¬ dity in such a warehouse almost always means increasing of the humidity, and this increasing is performed by vaporizing or atomizing water into the mixed air. The use of humidifying devices has, however, not become common, because their use is difficult or uneconomical, out of the following reasons: a. Successful controlling of the humidity of the cooling air is excessively dependent on the relia¬ bility of the devices of measurement of the relative OMPI humidity and on adjustability of the devices for increasing the humidity. The cooling air flows in the duct typically at a rate of 5 m/s, whereat the time available for vaporizing the water and for measuring the humidity remains very short. Owing to an error in the apparatus of measurement of relative humidity or owing to poor adjustability of the humidifier, water may be added momentarily too much, in which case the face of the vegetables becomes moist and diseases that cause storage losses can spread easily. b. Humidity detectors and humidifiers require abundant calibration and servicing, and thereby increase the maintenance cost. 3. Combination of compressor cooling and cooling by means of outdoor air. By means of combination of the methods men¬ tioned above, the warehouse can be obtained for use all the year round and the energy consumption can be reduced significantly as compared with a warehouse cooled by means of a compressor alone. However, what was stated about the poor economy of the present moistening methods is still applicable. This method cannot be applied to shield-gas warehouses. It is characteristic of all of the prior-art methods suitable for large warehouses that the control of temperature and humidity is performed by means of separate apparatuses and as separately controlled. Cooling as well as heating by means of outdoor air cause intensive drying, and attempts are made to compensate for this drying by adding water to the warehouse air by means of separate humidifying devices. In all of the prior-art methods in which it is really attempted to control the humidity of the air, the controlling is, at some stage, based on addition of water, e.g. Patent publications US 3,877,512 and US 4,118,945. In prior art, a method suitable for little packaging and cold warehouses is also known, described OMPI in the German Patent No. 944,495 and in the U.S. Patent .. No. 3,633,375, in which method the relative humidity of the space to be controlled becomes high, but in those cases it is, however, not possible to speak of controlling of the relative humidity, but the humidity by itself assumes a level determined by the conditions. In the warehouses in accordance with the patents mentioned above, the vaporizing radiator of the compressor is not placed in the cold storage space, but the coldness generated by it is passed from an adjoining freeze warehouse into the cold warehouse through a twofold structure or "mantle", provided in one or all of the walls, by convection. Such a "mantle refrigeration method" is suitable for combination warehouses only, which have both a freeze warehouse and a cold warehouse, and even then for very small warehouses only, owing to internal ventilation problems in the warehouse, which problems will be examined in more detail later on. The warehouse must be designed and built in accordance with the requirements of this refrigeration method right from the beginning, i.e. it is poorly applicable to existing buildings. In addition to the drawbacks mentioned above, the methods in accordance with the above German Patent No. 944,495 and the U.S. Patent No. 3,633,375 also involve the drawback that, when they are used, it is difficult to arrange the internal ventilation of the warehouse. In warehouses in which such vegetables or fruits are stored as generate an abundance of heat, it is not sufficient, for keeping the products at the correct temperature, that the walls and the ceiling of the warehouse are at the correct temperature, but it is necessary to blow a current of air through the products, by means of which current of air the heat generated by the products is removed from the interior of the heap of products. By means of these methods, which are intended for little warehouses and mainly for storage o ?r of foods that do not generate heat, it is difficult to - control the temperature and the relative humidity in a large warehouse, because the current of air that is blown through the vegetables should also be made to flush the walls of the warehouse, through which the heat generated in the warehouse is conducted away. When the volume of a warehouse increases, the area of its walls increases considerebly more slowly than the mass that is stored, whereat the use of the walls as the cooling face becomes difficult. In large warehouses (100 to 1000 tons) the situation is made even more difficult by the fact that at least one fourth of the wall area must be used for large doors and drive passages for fork trucks. All of the prior-art methods, the U.S. Patents Nos. 3,633,375 and 4,118,945 and the German Patent No. 944,495 further involve the drawback that therein it has not been possible to prevent the de¬ position of ice onto the vaporizing radiator of the compressor. By means of the method of controlling of temperature and humidity in accordance with the present patent application, it is possible to eliminate the said drawbacks. The objective is to provide a method for controlling the temperature and the humidity in vegetable and fruit warehouses, by means of which method it is possible, in cooling, to take advantage both of the use of the outdoor air and of mechanized cooling and, at the same time, to control the temperature and the relative humidity in the warehouse precisely and in such a way that it is not necessary to use apparatuses intended for addition of water. Since carbon dioxide cannot escape out of the warehouse along with the cooling air when this method is used, the outdoor air can be used for cooling even in the case of shield-gas ware¬ houses, which has not been possible in prior art. The method can be applied equally well to small and large, O PI new and old, box warehouses and bulk-goods warehouses, as well as to shield-gas warehouses, because, by means of the method, it is possible to perform internal ventilation of the goods stored in an efficient way, among other things. Cooling equipment operating in accordance with the method can be accomplished so that it requires little space, and, owing to its variability, it can also be installed easily in existing warehouses even if the ware- house had been planned originally so that it operates by some other cooling method. In view of obtaining the advantages mentioned above and in view of avoiding the drawbacks of the prior-art methods, the method in accordance with the present patent application for controlling the temper¬ ature and the humidity in vegetable and fruit ware¬ houses is mainly characterized in what is stated in the patent claims. The method and devices and constructions in accordance with the method will be described in the following in detail with the aid of the attached figures, without, however, in any way confining the method in accordance with the application to the said figures. Figure 1 shows a prior-art temperature con- trol system. Figure 2 is a plan view of a large 10,000 ton potato warehouse, which is divided into ten compartments of equal size. The air conditioning of the warehouse has been designed in accordance with the present invention. Figure 3 illustrates the principle of the method in accordance with the invention and the devices required by the method when the mechanized refrigeration is not in operation. Figure 4 illustrates the principle of the method in accordance with the invention and the devices required by the method when the mechanized refrigeration is in operation. OMPI Figure 5 is an axonometric view of the method -- in accordance with the invention and of the devices required in the said method. The controlling of the temperature and humidity in the warehouse is performed by cooling the interior air in the warehouse by means of a heat exchanger and by adjusting the temperature of the heat exchange face of the heat exchanger to the dewpoint temperature cor¬ responding to the desired relative humidity of the air in the warehouse. Thereby, out of the air to be cooled, only any excess moisture is removed by condensing onto the heat exchange face, and, when the operation takes place in accordance with the method of the inven¬ tion, the air need not be humidified by addition of water ever, because excessive drying cannot take place. In this connection, the notion "control" is to be interpreted in a wide sense, for the desired effect can, besides by active controlling, also be produced by dimensioning the equipment empirically or by experi- menting,in such a way in relation to the space to be cooled that the face temperature of the heat exchanger assumes the dewpoint temperature substantially corres¬ ponding to the desired relative humidity. The heat exchanger used in the method in accordance with the invention may have a construction of a cell-type plate heat exchanger, wherein the inside air and the outdoor air are passed so that they flow at opposite sides of thin plates, made, e.g., of alu¬ minium, so that the flow of heat between the currents of air passes through the plate. Fig. 3 illustrates the principle of operation of the method in accordance with the invention. The heat exchanger 6 is here illustrated as a principle, being provided with one thin sheet only, and in a corresponding way only the wings 7 and 8 are shown of the blowers, and the opening and closing mechanisms of the mixing gates 9 are not shown. OMPΓ A microprocessor-based control device is used --_ for measuring the temperatures and for controlling the actuating devices, the said control device being capable of making the necessary logical conclusions and cal- culations. The control device is not shown herein, but the detectors operated by it are as follows: T1 is the detector for the temperature of the vegetables 11, T2 is the detector for the temperature of the cooling air 12 coming out of the exchanger 6 and going to the vegetables 11 , T3 is the detector for the temperature of the mixing result consisting of outdoor air 16 and recir¬ culated secondary air 17 and flowing at the secondary side of the exchanger, which detector measures the temperature of the air right before it enters into the exchanger 6. T4 is the detector for the temperature of the " outdoor air 16. The temperature in the warehouse is kept at the desired value with a precision of, e.g., +/- 0.4 . More than 90 % of the controlling of the temperature in vegetable warehouses involves cooling, and the cooling is advantageously performed periodically. The processor measures the temperature of the vegetables by means of the detector T1 , and if the temperature has become higher than the permitted range and if it is sufficiently cold outdoors, the cooling is started. The cooling takes place so that the blower 7 that circulates the indoor air 15 of the warehouse through the primary side 13 of the heat exchanger and the blower 8 that circulates the outdoor air 16 through the secondary side 14 are started. The primary air 13 and the secondary air 14 pass through the heat exchanger 6 by means of the upstream principle or downstream principle, the upstream principle being the better mode. The indoor air 15 is cooled when it flows through the heat exchanger 6, and is heated again in the vegetables 11. The cooling blowing is continued until the temperature of the vegetables has gone down to its set -£O £-C OMPI value. The cooling output, i.e. the extent of cooling of the inside air in the heat exchanger 6, is deter¬ mined in accordance with the desired relative humidity of the warehouse. The lowering of the temperature of the indoor air 15 taking place in the heat exchanger 6 is con¬ trolled by controlling the flow of heat transferred from the indoor air 15 to the outdoor air 16. This flow of heat can be controlled by changing the ratios of the flow quantities of the outdoor air 16 and indoor air 15, by adjusting the efficiency of the heat exchanger 6, or by passing a result of mixing together* of outdoor air 16 and recirculated secondary air 17 into the secondary circulation 14. Fig. 3 illustrates the latter mode, by means of which the best control result is obtained. Any excess moisture contained in the air in the warehouse is removed in connection with the cooling by condensation. If it is desired to adjust the rela- tive humidity .to 90 %, the indoor air 15 is allowed to meet a surface temperature, in the heat exchanger, which is the dewpoint temperature corresponding to a relative humidity of 90 % of the indoor air 15. If the relative humidity of the indoor air 15 before the exchanger 6 is higher than 90 %, water is condensed from it in the exchanger 6, and when the air leaves the exchanger 6, its relative humidity is 100 %. When this air 12 becomes warm again (when it flows through the vegetables 11) being warmed up to the temperature of the vegetables 11, its relative humidity is, in theory, 90 %. In practice, the cooling air 12 flowing through the vegetables 11, however, binds new moisture out of the vegetables 11, and the theoretical value of relative humidity determined by the surface temperature of the exchanger 6 is not reached precisely. In each process, it remains a duty of the control operator to adjust the surface temperature in the exchanger 6 to such a value at which the correct relative humidity of the warehouse is reached. As a basic setting, it is, however, possible to use the values given in the following Table 1 for the relationship between dewpoint temperature and relative humidity, e.g., for a warehouse in which the indoor temperature is kept at +4 C. If a compressor cooling is added to the system for summer storage (Fig. 4) , only a vaporizer 19 and closing gates 10 have to be added to the construction shown in Fig. 3. The vaporizer 19 is placed in the second¬ ary circulation 14 of the heat exchanger 6, in which case it is in connection neither with the indoor air 15 nor with the outdoor air 16. It is separated from the indoor air 15 by the heat exchanger 6 and from the outdoor air 16 by the gates 10 closed in summer. From the indoor air 15 flowing in the primary circuit 13 the heat is trans¬ ferred with a little difference in temperature by means of the large-area cell-type plate heat exchanger 6 into the air flowing in the secondary circuit 14. The air flowing in the secondary circulation 14 consists partly of the air 18 that passed through the vaporizer 19 of the compressor and partly of the air 17 coming again to the secondary circuit 14 from the front side of the gates 9, the said air 17 having not been cooled in the vaporizer 19. Thereat, the vaporizer 19 of the com¬ pressor can be designed so that it operates with a high difference in temperature between the vaporizer 19 face and the air 18 flowing through the vaporizer, for the vaporizer 19 can never be deposited with ice in the closed air circulation of the secondary circuit 14, because after the quantity of water, in itself harmless, contained in the secondary circuit 14 has condensed, no new water has access into it, and the condensing is discontinued. Around the vaporizer 19, no free space should be allowed to remain except what is necessary, i.e. the space for air ducts and gates. The relative humidity of the cooling air 12 ,_. flowing out of the primary circuit 13 of the heat ex- _ changer 6 is controlled further by adjusting the surface temperature of the heat exchanger 6 by means of the mixing gates 9. It is not necessary to measure the said surface temperature of the heat exchanger 6 and the relative humidity of the air 12 directly, but the data required for the control are obtained by measuring the tempera- tures of the currents of air flowing through the ex¬ changer 6. For example, the surface temperature of the heat exchanger 6 is obtained by calculating the halfway value between the measurement results of the temperature detectors T2 and T3. For the implementation of the method, other kinds of embodiments of equipment may also be used, consisting of apparatuses known for a person skilled in the art. Table 1 RH % 100 99 98 97 96 95 94 93 T °C 4 3.86 3.71 3.57 3.43 3.29 3.14 3.0 RH % 92 91 90 89 88 87 96 85 T °C 2.84 2.68 2.52 2.37 2.21 2.05 1.89 1.73 RH % 80 75 70 T °C 1 0 -0.9 - υRE OMPI

Claims: WHAT IS CLAIMED IS: 1. Method for the controlling of temperature and humidity, e.g., for vegetable and fruit warehouses, in which method heat generated, and moisture delivered, by the products to be stored are removed out of the air in the warehouse by circulating air so that it by-passes a cooled heat-exchange face, c h a r a c t e r i z e d in that the temperature of the heat exchange face is controlled to a dewpoint temperature substantially cor¬ responding to the desired relative humidity of the air in the warehouse either by adjusting the temperature of the heat exchange face or by dimensioning the heat exchange equipment in the storage space in such a way that the temperature of the heat exchange face assumes the- said temperature in practice. 2. Method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that the temperature of the heat exchange face is adjusted by passing air cooled by means of outdoor air or by mechanized refrigeration across the opposite side of the said face. 3. Method as claimed in claim 2, c h a r ¬ a c t e r i z e d in that the temperature of the cooling air is controlled by mixing together circulated air and cooling air. 4. Method as claimed in claim 2, c h a r ¬ a c t e r i z e d in that a closed circuit is used for the mechanically refrigerated air. 5. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d in that the cooled air in the storage space is passed into close contact with the products to be stored. -£TJRE_C O PI

Inventors: RUOKOLAINEN ANTTI, RUOKOLAINEN, ANTTI
Applicants: ITUMIC OY
Application year: 1984
Main classification: F25D13
IPC codes: A23B7, A23L3, F24F3, F24F5, F25D23