Process Energy Efficiency Improvement in Wisconsin Cheese Plants

Abstract

Wisconsin continues to lead the nation in the production of cheese, providing about 30% of the national supply. The industry is less secure than it once was, however, due to narrowing profit margins and competition from large plants in California and elsewhere. In 1994, the production of cheese consumed 87.7% of the milk produced in Wisconsin. Thus the success of the Wisconsin cheese industry is closely linked to the success of the Wisconsin dairy industry at large, which provides about three billion dollars per year in gross income.

In this study, the utility use of two representative cheese plants is examined. Utility costs generally represent about 11% of the total manufacturing cost of cheese. Utility use is one of the few variables that a plant manager can influence to improve plant profitability.

The largest fraction of the energy used at a typical plant is devoted to processing whey, a byproduct of significant food value. Two types of equipment are commonly used to process or remove moisture from whey: evaporation systems and spray dryers. Through the course of this research project, models for both evaporation and spray drying equipment were developed. Pinch analysis has been applied to investigate heat recovery options such as open cycle heat pumps and heat exchange units.

Significant opportunities for reduction of utility use have been identified for both the evaporation system and the spray drying system examined. Making use of the low temperature vapor rejected from the evaporation system by preheating raw milk provides two simultaneous benefits. Steam use by the

ii pasteurizer is decreased, and the cooling rejection load on the cooling tower (equipped with a 60 hp motor) is eliminated. Cost savings associated with these benefits are estimated to exceed $70,000 per year.

In the spray dryer system studied, outdoor air is heated to 240�F before being introduced into the drying chamber. Exhaust air leaves the chamber at 155�F. The use of either direct or indirect heat exchange between the supply air before it reaches the burner and the exhaust air is explored. Energy cost savings in the range of $40,000 to $70,000 are predicted for this opportunity.

In addition to the heat recovery analysis just described, this study explores an alternative control strategy to reduce electricity costs related to maintaining a cold storage warehouse through demand shifting. By sub-cooling the stored cheese during the off-peak period, it is possible to meet all or most of the cooling load in the warehouse as the sub-cooled cheese returns to its normal storage temperature. In this way, operation of the cooling equipment during peak-time can be avoided or reduced significantly. This control strategy has been examined using a finite difference model of the warehouse. The model has demonstrated the approach to be feasible. It has been estimated to result in reduced annual electric costs of about $15,000.