Comparison of heat insulation effects of several kinds of grain surface gland materials

Low temperature storage is one of the development directions of grain storage. Perennial low-temperature grain storage is mainly achieved by reducing grain temperature and heat insulation. Reducing grain temperature can be achieved by mechanical refrigeration and ventilation. In order to achieve low-temperature grain storage for many years, thermal insulation and cold preservation measures are the key, especially during the summer period, if the grain reactor is not properly insulated, the grain temperature will rise too fast. The number of artificial refrigeration increases, the cost of grain storage is too high, or the low temperature requirements are not met, resulting in accelerated food deterioration and frequent occurrence of pests.

The heat of the grain pile mainly comes from the roof and the surrounding walls, doors and windows, and the roof is the most, accounting for more than 70%. In order to alleviate the external heat entering the grain pile, it is an important means to extend the low temperature state of the grain pile by using the heat insulation material to cover the grain surface, blocking the heat insulation amount to the grain pile, and delaying the grain temperature rise.

Our library is located in the temperate zone. The summer climate is hot and the low temperature season is short. We use the cold and dry climate in winter to reduce the average temperature of stored grain to about 7 °C through natural ventilation and mechanical ventilation. Before the advent of summer, PEF polymer insulation board, polystyrene foam board and heat insulation inflatable cushion were used as the grain cover gland insulation materials to test the grain temperature increase, fatty acid value change trend, pest occurrence, The unit storage grain cost and other indicators are evaluation criteria. The insulation performance of the three materials is analyzed and compared, and a grain surface gland insulation material with better heat insulation effect, convenient operation and economy is selected.

1 Materials and methods 1.1 Materials Three kinds of grain surface gland insulation materials are: PEF polymer insulation board, each piece size 2mX1m, thickness 2cm; polystyrene foam board, each piece size 2mX1m, thickness 8cm , PVC plastic film outsourcing; inflatable cushion, each inflatable cushion size is 1.5m, each of which adopts a seamless connection, and is inflated with a pneumatic pump to form a plurality of small airbags with a thickness of about 16cm.

1.2 Test warehouse warehouse, select the 5th warehouse as the control warehouse. The specific storage conditions of each warehouse are shown in Table 1. Table 1 Test warehouse and control warehouse grain situation Warehouse number Variety storage mode Warehouse type Rice bulk arch room room warehouse rice bulk arch house room warehouse rice bulk arch room warehouse warehouse rice bulk High-level flat warehouse 1.3 test method 1.3.1 Preparations From the end of 2004 to the beginning of 2005, the centrifugal fan was used to grasp the low temperature time in winter, and the warehouses were ventilated and cooled to make the grain pile reach a low temperature state, and the average grain temperature was around 7 °C. The temperature of each layer was balanced. In April 2005, the windows and vents of each bin were insulated with polystyrene foam board.

1.3.2 Operation method Before the temperature rises at the end of April 2005, the heat-insulating gland of each test warehouse is separately carried out. The specific methods are shown in Table 2. Table 2 The use of each cover gland insulation material How to use the thermal insulation material Conventional storage inflatable cushion grain cover gland polystyrene foam film film envelope after grain surface gland polymer insulation board grain surface gland after film sealing 1.3.3 temperature detection in the warehouse outside the ground height about 1m and no sunlight Select a direct spot, measure the temperature with a mercury thermometer; set the space on the grain surface in the warehouse evenly by 3 points, measure with a thermometer, take the average value as the warehouse temperature; obtain the grain temperature from the pre-embedded temperature measurement cable in the warehouse, and seek each Layer average. Checked regularly at 9 am every Monday. Combined with the climatic characteristics of the high temperature period in the summer where the library is located, the data collection time is from April to October 2005. 1.3.4 Quality Inspection In April and September of 2005, a comprehensive sample of each warehouse was taken for 2 results and analysis. 2.1 Inflatable pad grain surface gland insulation effect No. 6 warehouse adopts inflatable cushion grain surface insulation gland, temperature during test And the quality changes are shown in Table 3. The temperature reached a maximum of 29.5 °C in July, and the lowest 20.0 grain temperature in April reached a maximum of 20.1 ° in July. The minimum of 10.7 in April shows that the grain temperature is more susceptible to temperature and warehouse temperature. In particular, the surface grain temperature fluctuated up and down with the temperature and warehouse temperature in the sixth, seventh, eighth and third months, and the amplitude was around 7 °C. The fatty acid value has risen. Application evaluation: Although this material has a certain heat insulation effect, it is not ideal. First, the heat insulation effect is not strong, and the grain temperature is easily affected by the outside world. Second, the price is 36兀/m2, which is the highest among the three kinds of heat insulation materials. Third, it is inconvenient to use, and the time spent in inflation and deflation is higher. Long; fourth, because the material is inflated and formed into a balloon shape, it brings inconvenience to cleaning, checking food, and pest control.

Table 36 warehouse temperature and grain quality change month temperature warehouse temperature surface grain temperature (C) average grain temperature (C) 2.2 polystyrene foam board insulation effect No. 10 warehouse using polystyrene foam board for grain surface gland, pressure Before the cover, we made plastic film coating treatment to prevent the foam board from polluting the grain surface and facilitating the sanitary cleaning of the warehouse. The temperature and quality changes during the test are shown in Table 4, Table 4, No. 10 warehouse temperature and grain quality change month temperature. Warehouse temperature, grain temperature (C), average grain temperature, overall grain temperature trend is stable, grain temperature is low in April, the highest average grain temperature in the year is 20.4 °C and the highest surface grain temperature is 26.8 °C in September, instead of In July, when the temperature and warehouse temperature are the highest, the delay and the lag of temperature rise are obvious for about 2 months. The fluctuation of grain temperature is small, and the rise and fall are stable. The increase in fatty acid value has a significant thermal insulation effect. However, due to the untreated roof, a large amount of heat is transferred into the warehouse, resulting in a warehouse temperature that is too high. After a long period of heat conduction, heat can enter the grain pile, and there is a gap between each foam board. If the heat in the tank is removed in time The effect is necessarily better; the price of the foam board is low (13 yuan/m2), the operation is convenient, and it is durable, and after the foam board is pressed to cover the grain surface, the grain surface is neat and beautiful.

2.3 Insulation effect of polymer thermal insulation board No. 33 warehouse firstly use PEF polymer thermal insulation board to cover the grain surface, and then seal it with 0.18mm plastic film. The temperature and quality changes during the test are shown in Table 5. Table 5 33 No. of warehouse temperature and grain quality change month temperature warehouse temperature surface grain temperature average grain temperature fatty acid value 33 warehouse grain temperature stable, the highest average grain temperature in the year 20. The highest surface grain temperature 30.3 °C appeared in August, the highest ratio The temperature and warehouse temperature were delayed for about 1 month in July, and the temperature rise and fall lag was more obvious. The fatty acid value increased by 1.2 mg KOH / 100 g. We believe that this insulation material has a certain thermal insulation effect, but it is not ideal. First, the thickness of the insulation material is relatively thin, similar to the insulation effect of the No. 10 warehouse foam board, but the price (25 yuan / m2) is 12 yuan / m2 more expensive than the foam board; second, passed to the grain pile The heat is not easy to dissipate, the natural ventilation effect is poor in the low temperature season, and the ventilation energy is increased. Third, the fumigation of the insect body is difficult to clean between the film and the heat insulation board, which affects the appearance.

2.4 General storage temperature and quality changes The temperature and quality changes during the routine storage test in the No. 5 warehouse are shown in Table 6. The temperature reaches the highest in July 29.5 The lowest in April 20.0 The warehouse temperature reaches the highest in July 31.3 The lowest in April is 19.8 °Q The grain temperature reached the highest in 19.5° in July and the lowest in 10.4°C in April. It can be seen that the change of grain temperature is basically consistent with the change of temperature and warehouse temperature, and there is no obvious hysteresis in the rise and fall of temperature, especially the surface grain temperature with the temperature. Changes in July quickly rose nearly 6.0 fatty acid values ​​increased Table 6 No. 5 warehouse temperature and grain quality change month temperature warehouse temperature surface grain temperature average grain temperature fatty acid value (down to page 22) Table 1 T09 air tightness test table determination Point pressure (Pa) time (s) in the middle of the north wall test in the middle of the east wall test table 4 T12 air tightness test table test point pressure (Pa) time (s) in the middle of the north wall test 3 conclusion 2.2 brick wall The airtightness test results of the two layers of the inner and outer canopies of the earth core warehouse type are combined with the bead and the bottom bedding. See Table 2. From the beginning of the pumping, after reaching the negative pressure of 1235Pa in 480 seconds, it will not rise. The lack of 1000Pa to 500Pa in the table is due to the fact that when the fan is completely removed from the gap, the pressure in the chamber has recovered to less than 1000 Pa, so no time is recorded.

Table 2 T09 air tightness test table measuring point pressure (Pa) time (s) in the middle of the north wall test in the middle of the east wall detection 2.3 surrounding the scattered earth embankment type soil dike two layers of canopy first with sandbags Into the sandbag ditch, and then bury the tarpaulin in the soil (buried in February 2002), the air tightness test results are shown in Table 3. From the beginning of pumping, after 600 seconds to reach a negative pressure of 1400Pa (full pressure of the fan) ).

Table 3 T12 airtightness test table test point pressure (Pa) time (s) in the middle of the north wall detection 2.4 surrounding bag loose soil embankment type earth embankment two layers of canopy first pressed into the sandbag ditch with sandbags, and then The tarpaulins are stacked neatly, and the tarpaulin is pressed with sandbags. The spacing of the sandbags is 50cm. The airtightness test results are shown in Table 4. From the beginning of the pumping, after reaching the negative pressure of 882Pa in 600 seconds, it will not rise.

3.1 Whether it is the brick surrounding earth core type soil embankment or the surrounding package earth embankment type earth embankment, under the two kinds of glanding methods, the airtightness meets the national airtightness requirements of the warehouse ( The tall flat warehouse is no less than 40 seconds, and the shallow round warehouse is not less than 1 minute).

3.2 Whether it is the brick surrounding earth core type soil embankment or the surrounding earth embankment type earth embankment, the four sides of the canopy are buried in the soil, and the airtightness is obviously better than not embedding the canopy in the soil. Inside. From the half-life of 500Pa~250Pa, the former increased by 255%, and the latter increased the airtightness of the 3.3-environmental embankment-type soil embankment, which was better than the airtightness of the brick-and-soil-soil-type soil embankment.

3.4 Earth embankment has better air tightness. When fumigation and insecticide, it can be considered to control the dosage below the standard of shallow warehouse and shallow round warehouse.

3.5 According to the practice of our unit in the past two years, fumigation pipes are arranged at a depth of 20cm in the surface layer of the earthen bank, and low-dose (1.5g/m3) fumigation kills pests, which can achieve one year of fumigation. If you bury the canopy around the 20cm deep trench, you only need to fumigate once a year, or even fumigation throughout the year.

3.6 It is conceivable to carry out the atmosphere storage of the soil storage in the earth bank.

(Continued from page 15) 3 Conclusions 3.1 Through the test we can see that the polymer insulation board, polystyrene foam board, thermal insulation inflatable pad, these three insulation materials have a certain insulation effect, To some extent, delay the rise of grain temperature.

3.2 Compared with the control warehouse, the surface temperature of the test warehouse is 3.0~5.0°C. Due to the low heat conduction of the insulation material, the natural heat dissipation performance of the grain pile is reduced, so the rise and fall of the grain temperature have obvious hysteresis.

3.3 The polystyrene insulation effect of the three materials is the most ideal, which obviously delays the rising speed of the surface grain temperature and the deterioration of the grain quality; the polymer thermal insulation board also has a certain heat insulation effect; although the heat insulation inflatable cushion also has a certain heat insulation effect But it is worse than the first two insulation materials.

3.4 In summary, the polystyrene foam board has good heat insulation performance, low price, convenient operation, durability, and the use of foam board to cover the grain surface, the grain surface is neat and beautiful, and it is a cheap and good quality. Thermal insulation material.

4 Discussion Through the experiment we can see that although each warehouse adopts thermal insulation materials, in the four high temperature months of 6, 7, 8, and 9, the temperature and warehouse temperature still have a serious impact on the surface temperature of the grain pile. It is because the accumulated heat in the warehouse was not eliminated in time. Even with the insulation gland, it still has a great impact on the grain temperature. Therefore, we believe that: to strengthen the thermal insulation performance of the warehouse, especially the thermal insulation of the warehouse roof, and then take the grain surface polystyrene foam board insulation gland, the high temperature season to do ventilation in the warehouse, reduce the warehouse temperature, will It is a scientific and effective method of heat insulation and cold storage, which can keep the grain pile in a quasi-low temperature state for a long time, reduce or eliminate mechanical refrigeration, and achieve the purpose of low-cost green grain storage.

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