The application of energy conservation techniques in an industrial plant

Abstract

An energy conservation program was developed in the fall of 1975 at General Extrusions, Inc. to maintain the operation of the plant at full production while staying within the natural gas allocation of 35,600,000 cubic feet per year. This program when fully implemented will minimize the use of propane as an alternate fuel. Before the energy conservation program, 8,000 gallons of liquid propane (720,000 cubic feet equivalent of natural gas) were used monthly to supplement the energy needs of the plant operation.

Group meetings were held with the plant personnel, stressing the importance of energy conservation in maintaining their jobs and seeking their help in this program. The energy conservation program then became a group effort with management and labor working jointly. A complete and detailed survey and study of all energy usage in the plant was done with the help of the maintenance staff and the operating supervisors. The survey and study was completed in two months and all the information and recommendations by the group were evaluated by a management committee.

The following problems were found during the energy usage study: the operation of the plant consisted of six days per week and twenty-four hours per day which left no flexibility in operating hours for cycling of energy loads; the anodizing building required thirteen air changes per hour (consists of 157,000 CFM exhaust and 150,000 CFM fresh outside supply air) to purge the toxic fumes from the building continuously twenty-four hours per day, seven days per week to prevent corrosive attack on the equipment and finished product stored in the building; the 157,000 CFM exhaust consisted of low temperature though high humidity exhaust (80°F and 80% relative humidity); the make-up air heaters and the exhaust fans were positioned on the roof in a way that would make it difficulty for the addition of heat recovery coils or wheels; even though the exhaust air went through wet scrubbers, most of it was too toxic or too humid to return to the plant; and finally, the first cost of installing heat recovery equipment was relatively high.

The following recommendations were implemented immediately: 1. The plant thermostat temperatures were set at 65°F and the employees agreed to dress accordingly, 2. Exhaust air from three tank immersion heaters was directed to another tank to supply 250°F temperature air for drying of finished extrusions (thereby eliminating the need for 200 CFH burner), 3. Inlet duct thermostats were installed in each of the three 50,000 CFM make-up air heaters to automatically turn off the burners when outdoor temperatures rose to 65°F (during unseasonable warm weather, even though the burners were on maximum turn down, 80°F air would be supplied to the plant, wasting as much as 500 CFH.)

Two additional recommendations of energy conservation systems were evaluated based on the following design constraints: first cost, operating and maintenance cost, quantity and cost of energy conserved, effect on in-plant air quality, and effect on employee environment. Based on these evaluations, both systems were economically justified and it was determined that they would not degrade the in-plant air quality, therefore they were implemented. The following is a description of the two systems and a tabulation of the calculated design energy savings. The systems were fully operational on March 1, 1976.

The first energy conservation system was implemented to take advantage of the effectiveness of the exhaust fan-scrubber (20,800 CFM) that is handling air from the sulfuric acid tanks. This scrubber removes the contaminants from the exhaust air well below the allowable level for in-plant air designated by OSHA. Therefore, this heated cleaned air (65°F) was recycled back into the plant with the addition of a return duct system. This system reduces the make-up air requirements by 20,800CFM and therefore saves the quantity of energy normally required to heat this amount of air from outside temperatures to 65°F during the winter heating season. For the Youngstown area which as 6400 degree days per year and a plant operation of twenty-four hours per day six days per week, the quantity of energy conserved is estimated to be 3x10 to the 9 BTU's annually.

The second energy conservation system that was implemented uses a heat pipe non-regenerative air-to-air heat recovery unity (Q-Dot)to recover energy from a 35,25 CFM exhaust fan (80°F and 80% relative humidity). This recovered energy is used to preheat 50,000CFM of fresh outside supply air that is to be reheated to the 65°F required building temperature by a gas direct fired make-up air heater. The design conditions for this heat recovery system are : temperature of exhaust air is 80°F, temperature of supply air entering the coil is 37°F, temperature of supply air leaving the coil is 66°F and the recovery factor is .667. As can be seen from the design conditions, when the outside temperature is 37°F or greater, no gas will be burned in the make-up air heater and 50,000 CFM will be supplied to the building at 65°F or greater. If the outside temperature is greater than 37°F a greatly reduced quantity of gas will be burned to maintain the supply air at 65°F. Computer modeling of this system was done to determine the optimum design and to calculate the estimated energy savings of 7 X10 to the 9 BTU's annually. The energy required for the space heating of the anodizing building before the energy conservation program was implemented was 20x10 to the 9 BTU's annually. Therefore, these two system will save 10x 10 to the 9 BTU's annually or 50% of the space heating requirement of the building.

The two energy conservation systems have a first cost of $48,000.00 total. At current costs of $1.26 per MCF for natural gas and $.35 per gallon for propane, the energy savings of 10 billion BTU's is equivalent to approximately $36,000.00 (first 8.6 billion BTU's at propane costs and balance of savings as gas costs). Based on the above figures the cost of money at 10%, the payback period for the total installation is approximately 1.5 years. This thesis presents the detailed investigation, design and calculations of the two energy conservation systems that have been implemented. The present heating and ventilating system is described and alternate energy conservation and heat recovery systems are presented in detail. Computer modeling and economic analysis of the alternate systems is included.