Companies that perform vacuum brazing know that the two most significant operating costs are the electricity used to run the furnace and the water used to cool it. By adding a chiller and several heat exchangers, Lytron was able to eliminate the furnace's water expense and provide building heat, at no additional ongoing cost.
While annual costs will vary with the furnace size, usage and local utility rates, at Lytron, electricity accounted for the greatest portion of the high operating costs. At its plant, the furnace has a moderately large (41 x 36 x 54") hot zone surrounded by resistive heaters. At peak load, it draws 205 kW, and the furnace's roughing and diffusion pumps, which create the vacuum, and various electronics, which control the furnace, draw another 27 kW. Lytron estimates that the typical usage in an 8-hr shift is approximately 750 kW-hr. At roughly $0.10 per kW-hr (typical rates for the Boston metropolitan area), this translates into an annual cost of $45,000 for a three-shift, 200 days/year operation.
The source of the other significant operating cost -- water -- is used in several locations in the operation. A water-cooled circuit around the furnace's jacket is designed to keep the outside walls from getting too hot. It requires a 10 gal/min flow rate. In addition, chilled cooling water is needed for the roughing pump (2.0 gal/min), diffusion pump (1.5 gal/min) and transformers (1.5 gal/min), for a total cooling water requirement of 15 gal/min.
Where municipalities do not prohibit it, some processors use city water to provide cooling. When Lytron first received its brazing furnace, it used a hybrid cooling arrangement: The 10 gal/min water used to cool the jacket was a part of a closed-loop system that utilized a heat exchanger exhausted outdoors. The remaining 5 gal/min chilled water required for the pumps and transformers was supplied by city water and dumped into the sewer.
The cost of water varies widely across North America. In Woburn, MA, where Lytron is located, water costs $3.70 per 1,000 gal. Sewer fees are an additional $11.26 per 1,000 gal, for a combined rate of almost $15 per 1,000 gal. (Water rates are high in the Boston metropolitan area due to costs associated with the Boston harbor cleanup.) At these rates, the company spent $21,600 per year to provide the 5 gal/min chilled water needed for a three-shift operation.
Obviously, cost reductions were in order. Just as it would for a customer, Lytron analyzed the operating costs and found a solution.
Reducing the Electric BillThe company realized that dumping the heat from the 10 gal/min water jacket loop outdoors was wasteful during the winter months -- the factory needed to be heated. The water jacket loop was redesigned with two heat exchangers mounted inside the building (figure 1). Now, the company uses the outside cooling loop in the summer, but during the winter months, the new auxiliary cooling loop is used to dump the process's excess heat into the plant. This arrangement has eliminated the winter heating bill. The company pays the same amount for electricity to run the furnace, so in essence, it heats its 60,000 ft2facility for free.
Reducing the Water BillThe $21,600 water bill for cooling the roughing pump, diffusion pump and transformers was reduced using a recirculating chiller. Instead of dumping city water down a drain, one of the company's recirculating chillers is used to cool the heated process water with a refrigeration circuit. The water is recycled back to the equipment, and no water is consumed. Lytron found that the additional electricity cost (approximately $5,000 to run the chiller year round) has been more than offset by the $21,600 savings from not using city water.
Every user will have different process variables to consider when developing design solutions. Even so, if you operate an oven that requires a cooling loop, consider whether a chiller can save your company money.