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36 • ROCK products • March 2018 www.rockproducts.com Charleston Stone FIGURE 3: Van Tarble founded Charleston Stone in 1958. This experience demonstrated that electric rock crushers required less maintenance and less down time. The Tarbles estimated the cost to replace their primary crusher at $1.2 million, which is a larger expense than they incurred in 1999 when the entire plant was replaced. This was a major invest- ment for the company. This case study explores the benefits of converting to elec- tric rock crushing. A benefit that is often uppermost in the minds of co-ops and other utilities on industrial projects is the energy savings. The primary rationale for this project is improvements in production, which makes it similar to many industrial projects. Energy savings was not an important driver and was not analyzed. This case study includes esti- mates of energy savings, but these estimates are not based on a detailed energy analysis, which typically includes on-site testing and measurement. Description of the Technology Application John Hutchins, the CEO of Aggregate Processing, which sup- plied the new primary crusher to Charleston Stone, said that quarry operators prefer electric motors to diesel for a number of reasons, including operating cost. Even so, there are many diesel engines in use in the quarries Hutchins sup- plies, so Charleston Stone's use of a diesel powered crusher is not unusual. Hutchins said that many quarries still have diesel engines because it can be easier to start up a new quarry with diesel. Comparing the cost of operating a diesel crusher and an elec- tric crusher can begin by looking at the price for each fuel and the efficiency of each motor. The amount of electricity needed to provide the equivalent amount of energy as a gallon of diesel depends on the effi- ciency of the motors. The old diesel engine likely operated at 35 to 45 percent efficiency. The new electric crusher motor is 95 percent efficient. Figure 4 shows how much less the cost of an equivalent amount of electricity would cost than the price of a gallon of diesel. It only costs $1.04 for the electricity needed to run a 95 percent efficient electric motor that will provide the same output that a 35 percent efficient diesel engine will produce with a $2.50 gallon of diesel. 9 An important factor is not addressed on this graph. The price of the electricity does not include demand charges. Some co-ops do not charge for demand, others do, and this can have a dramatic effect on this comparison. Charleston Stone's aging portable diesel-fired crusher was powered by a 300- to 350-hp diesel engine. This engine also powered a 100-kW generator which, in turn, powered several smaller electric motors on the crushing station. 10 FIGURE 4: Price of kWh's equivalent to 1 gal. of diesel. The replacement crushing station was designed, fabricated, and installed by Aggregate Processing. 11 (See Figure 5.) It can process 400 to 500 tph of material. The station is made up of a number of components: • Portable chassis mainframe assembly. • Collection transfer conveyor with 40-hp drive motor. • 300,000-lb. hydraulic leveling package kit with 20-hp drive motor. • 100,000-lb. quad axle running gear package. • Hazemag 1515 HD horizontal impact crusher with 400-hp drive motor. • Deister 5220 vibrating grizzly feeder with 60-hp drive motor. The crusher station is mobile. To move it, power is discon- nected at the pole and the connecting wire and plugs travel on the trailer to the new location (see Figure 6). The trans- former is even mobile. Charleston Stone purchased and installed the transformer on the trailer, which they expect will help reduce demand charges. 12 How Does the Customer Benefit? The primary benefits to Charleston Stone are reductions in maintenance costs and down time. Diesel engines require