China is the world’s largest grain producer. The total of 1992 Chinese grain crop production was 443 million tons (Mt), including 95 Mt of maize, 196 Mt of rice, and 102 Mt of wheat. Maize in China is grown mainly in the Northern part of the country. Most maize is harvested manually as ear maize in October and November, and is shelled before being transported in bags during December-February to a local grain depot for drying.
When the maize reaches the depot, it is usually segregated into stacks by moisture content, e.g. 22%-26%,18%-21%,<18%, its temperature is between -10℃ and -15℃. Some of the wet maize is stored as long as 3 to 4 months but does not deteriorate because of the low grain temperature. When a mechanical dryer is available, drying commences in November and continues through April. However, At many Chinese grain depots sun-drying is still the prevalent method for moisture extraction, it is a very slow process during the extremely cold months of January, February and March.
The need for mechanical maize dryers in Northeast China is great. Dryers manufactured overseas can be afforded by few grain depots. Therefore, the Chinese grain dryer manufacturing industry is developing rapidly. A recent survey ( by the senior author ) of 23 major Chinese dryer companies indicated that in the 1992-1993 period over 1500 dryers in the 5-25t/h capacity range were sold in Northeast China.
It appears that dryer designers in China are primarily concerned with the capacity and the energy efficiency their of designs. No data has been published on the stress-crack and breakage-susceptibility increase in maize dried in Chinese dryers. Since kernel breakage is a major complaint of Chinese maize, it is essential that such information become available to depot managers. This paper fulfills this need partially. The data has been obtained in Northeast China during the 1993-1994 and 1994-1995 drying seasons, and covers four main types of dryer used in China, i.e. crossflow, mixed-flow, concurrent-/counter-flow, and steam dryers. For comparison, maize quality data was collected at several sun-drying installations.
2 Experiment
2.1 Dryer Description
The specifications of the dryers are tabulated in Table 1.
2.1.1 Crossflow dryer
Three crossflow (No.1-No.3) dryers were tested. Dyer No.1 contains two crossflow columns, each column consists of two drying sections, one turn-flow device and a cooling section, and with a grain column thickness in the upper drying section of 0.277m and 0.304m in the lower section. The dimensions of dryer No.1 are 6.517m×3.400m×13.482m. No.2 dryer is similar to No.1, but is smaller and has a lower capacity. No.3 is a cylindrical crossflow dryer with two drying sections, one turn-flow device and a cooling section.
2.1.2 Mixed-flow dryer
The mixed-flow dryer (No.4) is composed of eight modular sections. Each module has dimensions of 1.8m×1.0m×0.8m. The tow bottom modules serve as cooling stage. Dryer No.5 consists of two mixed-flow drying towers in series, similar in design as dryer No.4.
Table 1 The Specifications of the Dryers Investigated
2.1.3 Concurrent-/Counter-flow dryer
Dryers No.6-No.10 are concurrent-/counter-flow dryers, consisting of two or more towers. The number in front of “-CCF” indicates the number of towers. Each tower has two counter-/concurrent-flow drying sections. The last tower, or part of the last tower, is used for cooling. The depth of the counter-/concurrent-flow beds is 0.6m, the cross-sectional area of each tower is 2.2m×2.2m. It should be noted that overseas CCF dryers do not have counter-flow drying stages, only a counter-flow cooling stage.
2.1.4 Steam Dryer
The steam dryer is a unique dryer widely used in grain depots in China. It consists of 3-6 indoor drying and cooling towers, and a boiler. Each tower contains in its upper section a series of steam pipes, and in its lower section a number of inlet/outlet air ducts. The grain is heated by conduction as it flow over the steam-heated pipes, and is subsequently treated with ambient or slightly-heated recycled drying-air. The steam pressure in the pipes is 2×105Pa--4×105Pa (130-140℃), the diameter of the pipes is 42-48mm, and the distance between pipes is 110-120mm. The grain retention time is long (4-6 hours for 10-point moisture removal) due to the relatively low grain temperature.
2.1.5 Sun-Drying
Sun drying is still the main method of grain drying in China. Almost every village or depot has a special area for the sun drying of grains. The grain is spread out in 5-10cm thin layer and is stirred occasionally. In Northeast China where the ambient temperature is very low when maize is harvested, sun drying is a very slow process.
2.2 Experiment
The stress crack conditions of each kernel are checked on a candling table. There samples, each of 100 kernels, are checked. The stress-crack percentage (SC) and index (SCI) as follows[2],
SC=% single crack + % multiple crack + % checked
And
SCI=1×(% single crack ) + (% multiple ) + 5×(% checked ).
3 Result and Discussion
The experimental results are give in Table 2.
Table 2 Results of Stress-Crack Percentage(SC) and Stress-Crack Index(SCI)
of Maize Dried by Different Drying Methods
The best maize quality, i.e. the maize with the lowest stress-crack percentage and lowest SCI, is produced by sun-drying and by steam drying. The sun-drying depots can produce maize with few stress cracks and low SCI values. The samples dried in the steam dryers also show few stress cracks and low SCI values. In comparing the sun-dried and steam-dried kernels, it should be understood that the quality of sun-dried maize greatly depends on the weather conditions and the grain-mixing strategy. In contrast, the quality of the maize dried in steam dryers is not affected by weather or the operating conditions, and thus can be expected to be similar to that of the steam-dried samples analyzed in this paper.
Of the mechanical dryers, steam drying shows the lowest increase in stress-cracks. Multi-stage concurrent-/counter-flow and crossflow drying exhibit highest increase, the more stages in a dryer, the more stress-cracks. Mixed-flow drying show a moderate increase in kernel stress-cracks. The average increased in the percentage of stress-cracks is 11 for steam drying, 30 for mixed-flow drying, 45 for crossflow drying and 60 for concurrent-/counter-flow drying.
The stress-crack figures of maize reported in this study Chinese dryers are similar to those report for mechanical dryers in the USA by Montross et al[3]. According to Montross research stress-crack increase for concurrent-flow drying (remember : US concurrent-flow dryers do not dry grain by counter-flow ) is 50%-74%, for mixed-flow drying 66% (only one dryer was tested), and for cross-flow drying 81%-89%. The USA dryers show a higher percentage of stress-cracked kernels than Chinese dryers of the same type, especially the cross-flow dryers. This could be due to variety differences but is likely caused by the higher drying temperatures used in the USA than in China[1].
The main conclusions to be drawn from this study of maize drying in China are:
1) Sun drying is able to produce maize with a minimum number of stress cracks if properly implemented.
2) Steam-dried maize usually has a small percentage of stress cracks.
3) Of the three major high-temperature types of dryer, mixed-flow dryers produce maize of higher quality, i.e. with fewer stress cracks than crossflow dryers and concurrent-/counter-flow dryers.作者: yuandian 时间: 2007-5-20 14:39
你从哪搞来的.我翻译的不是太好不敢发给你作者: li5jun1 时间: 2007-5-25 15:09
翻译软件翻的，看看如何？