The Cost of Over-designing Belt Conveyors
For projects that include troughed belt conveyors for the transportation of bulk materials, a detailed engineering analysis is frequently omitted. Many companies seek to standardize their material handling equipment by specifying components and designs that are similar or identical to existing equipment. This can benefit the end user by reducing maintenance inventory. Other times, conveyors are built "like the last one". More times than not the individual pieces of equipment for such projects tend to be over-designed. This is not necessarily bad because the end result can be equipment that is quite robust and "bomb-proof". But, before we send a man to do a boy's job, let us examine the effects on operating cost by such over-designing.
Consider an actual example for a simple troughed belt conveyor. The given design parameters for this conveyor were to convey material with a density of 90 pounds per cubic foot over a horizontal distance of 50 feet at a nominal capacity of 100 tons per hour. In actuality, the conveyor would only be loaded approximately 20 percent of the time and would be running empty for the remaining 80 percent of the time. The conveyor will operate 24 hours per day, 365 days per year.
Using the design recommendations specified in "Belt Conveyors for Bulk Materials" published by the Conveyor Equipment Manufacturer's Association; the mechanical components for the conveyor were determined. The recommended components were as follows: a belt width of 24 inches, an idler troughing angle of 35 degrees, carrying and return idler roll diameters of 4 inches, a belt speed of 150 feet per minute, and a 3 horsepower drive motor and gear reducer. These components were selected after considering the operating, maintenance, and environmental conditions.
However, to conform to the desires of the end user, the belt width was arbitrarily increased from 24 to 30 inches wide, the idler roll diameter was increased from 4 to 5 inches, the belt speed was increased from 150 to 240 feet per minute, and the drive motor was increased from 3 to 5 horsepower. Undoubtedly, the larger conveyor desired by the end user would perform the intended task, but an economic evaluation reveals that these arbitrary requirements have costly short and long-term effects.
The immediate effect of these changes is obvious. The fabrication of the conveyor will be more expensive due to the increased costs of idlers, belting, drive motor, gear reducer, belt cleaners, etc. For larger systems with larger drive motors there can also be an increased cost for the electrical equipment. In addition to these immediate effects on the initial cost, the energy usage to operate the two different designs is significant.
Most of the work done by small horizontal belt conveyors is moving the belt and turning the idlers and pulleys. This is the empty belt friction force. For this conveyor the empty belt friction force was increased slightly by the greater weights of the larger pulleys, idlers and belting. The actual power required, however, is not just a function of the force required to move the belt and other components, but is defined as the rate of work. This rate of work is simply force (pounds) multiplied by velocity (feet per minute). In US units the equation for horsepower (neglecting the effects of drive system inefficiencies) is:
where F is the force required at the drive pulley to pull the belt (in pounds) and V is the speed of the belt (in feet per minute). By examining the equation above it is clear that increasing the empty belt friction force (F) will increase the horsepower requirements of the drive system. The greatest effect on power consumption, however, is caused by the increase in belt speed (V) from 150 to 240 feet per minute.
A detailed engineering analysis of component weights, belt tensions, mechanical system efficiencies, and operating parameters will allow the engineer to calculate the amount and cost of the electricity required to operate the conveyor over a given time period. For the two conveyor designs previously described we can perform these calculations.
For the proposed conveyor design the empty belt friction, or belt pull, is equal to 438 pounds. The belt pull for the loaded conveyor is 490 pounds. Since the conveyor will be empty 80 percent of the time and loaded 20 percent of the time we can calculate an average belt pull with the formula:
For the conveyor design specified by the end user the empty belt friction, or belt pull, is equal to 446 pounds. The belt pull for the loaded conveyor is 493 pounds. The average belt pull for this design is:
In order to calculate the operating energy costs of the two different conveyor designs, two assumptions will be made. The cost of electricity is 5 cents per kilowatt-hour and the total efficiency of the motor/reducer drive system is 85 percent. Also note here that 1 horsepower is equal to 0.746 kilowatts.
For the proposed design the power consumption is equal to:
For the design specified by the end user the power consumption is equal to:
Examining these results shows that the design specified by the end user will consume 62 percent more electricity than the proposed design. Using the assumed cost of electricity the annual operating costs for the proposed design is:
For the design specified by the end user the annual operating costs is:
This equals and annual difference of $486 and over a ten-year period will accumulate to $4,860 of unnecessary expense.
It is important to bear in mind that this analysis applies to a single conveyor in a facility with dozens of various sized belt conveyors. The cumulative effects of many properly designed conveyors can have a significant impact on energy cost reduction in a large facility. It is also important to note that this conveyor is a short, low capacity unit. The effects of wasted energy due to over-designing will be more significant as the conveyor size is increased.
A detailed engineering analysis of a belt conveyor will take into account variables such as idler sizes and spacing, pulley sizes, product loading conditions, belt tension and speed, skirting length, capacity and lump size, equipment maintenance, temperature, and many others. Accurately examining the effects of altering these variables can be a tedious process. Because of this, it is quite common to use conservative calculations and rules of thumb when determining component specifications. This short cutting of the engineering process can be another cause for over-designed belt conveyors. An experienced engineer, however, will recognize the long-term benefits of designing a conveyor not only to perform the task required but for cost efficient fabrication and long term operating costs.
Ó 2000, Gary R. Sharlow. All rights reserved.
Also published in Powder & Bulk Magazine.