3.RESULTS
3.1. optimum. belt. Width in terms of engineering-economics Figure. 2 shows the relationship between the total cost of the conveyor and the belt width. The vertical axis shows the conveyor total cost (baht), while the horizontal axis shows the belt width (mm). The convey capacity shown in the Fig.2. is 50-500 ton h^-1. Figure 2a-2b show the calculation for the conveyor with the inclination angle of 0*, whose length is 10 and 100 m respectively, while Fig. 2c-2d show the calculation for the conveyor with the inclination of 20* It was found from Fig. 2a-2b that for every belt width and every conveyor inclination angle, with the convey capacity of 50 ton h^-1, the belt width contributing to the lowest total cost of the conveyor was 400 mm. It was also found that when the belt width increased, the total cost of the conveyor increased. When the conveyor capacity was higher than 50 ton h^-1 the belt width contributing to the lowest total cost was 500mm. The total cost of conveyor tented to decrease when the belt width was increase from 400 to 500 mm, while the total cost tented to increase when the belt width was larger than 500 mm. This cost characteristics could be found for every conveyor length and inclination angle of 50 ton h^-1 conveyor
3.2. The effect of the Conveyor Length on the Lowest Total Cost
Figure. 4 shows the relationship between the conveyor length and the conveyor’s lowest total cost for the convey capacity of 100 ton h^-1. The horizontal axis shows the belt length (m), while the vertical axis shows the conveyor’s lowest total cost (baht).
3.3. The Effect of the Overdesign of Convey Capacity on the Change of the Conveyor’s Lowest Total cost
Figure 6a-6b show the relationship between the lowest total cost increasing and the normal convey capacity-the convey capacity before an overdesigned value of 50 ton h^-1 was added. The vertical axis shows the increasing rate in percentage of the lowest total cost in case of the overdesigned capacity compared to the lowest total cost in case in the normal convey capacity. The horizontal axis shows the normal convey capacity.
4.DISCUSSION
4.1.Optimum Belt Width in terms of Engineering-Economics
If the lowest total cost of conveyor was defined as the optimum point for engineering-economics aspect of belt width design, it can be seen from Fig. 2 that the optimum belt width could be divided into 2 categories. The first one was the optimum belt width without the turning point that occurred when the convey capacity was 50 ton h^-1 and the other one was the optimum belt width with the turning point that occurred when the convey capacity was higher than 50 ton h^-1. This was because of the effect from cost of the energy used to run the conveyor. The evidence of explanation above could be seen in Fig.3. The figure shows the various costs of the conveyor with the convey capacity of 50 and 100 ton h^-1 respectively. Both conveyors were 10 m long with the inclination angle of 0 degree.
Fig.2. The relationship between the total cost of conveyor and the belt width (a) belt width: 10 m; inclination angle: 0 degree, (b) belt length: 100m; inclination angle: 0 degree, (c) belt length: 10m;
It could be seen that when the convey capacity was 50 ton h^-1, the decreasing rate of the energy cost of the conveyor with belt width range from 400 to 500 mm was lower than the increasing rate of other costs. As a result, the total cost of the conveyor increased with the increasing width of the belt and the total cost was lowest at the belt width of 400 mm that was the lowest width used for calculation. When the convey capacity was higher than 50 ton h^-1, the decreasing rate of the energy cost of the conveyor with belt width range from 400 to 500 mm was higher than the increasing rate of other costs, while the decreasing rate of the energy cost was lower than the increasing rate of other costs when the belt width was higher than 500 mm. As a result, the turning point of the total cost was found at the belt width of 500 mm, causing the belt width of 500 mm to become the lowest total cost of the conveyor.
It could be concluded that the optimum belt width of conveyor could be classified into 2 cases. The first one was the optimum belt width without the turning point of the total cost that occurred when the convey capacity was 50 ton h^-1. For this case, the optimum belt width that made the lowest total cost was 400 mm. The other one was the optimum belt width with the turning point of the total cost that occurred when the convey capacity was higher than 50 ton h^-1. The lowest total cost of the conveyor occurred at the turning point of the total cost and it was the belt width of 500 mm for every length and inclination angle. This was because of the effect of the energy cost.