Design of an Exhaust Gas Heat Recovery System to Heat Domestic Water for Use at the Muncy Kellogg Facility

Executive Summary

      The Kellogg's company has requested that a team of Penn State engineers design a heat recovery system to recover waste heat from the exhaust of natural gas fired ovens to heat domestic process water. The company would like to see a temperature increase from 40 °F to 140 °F in the water.

      The sponsor has not been able to provide the Penn State team with the daily usage of hot water at the facility, which is necessary to design and size the specific system. In the absence of this critical data, the team has performed a parametric analysis of a cross flow continuous fin heat exchanger to demonstrate relationships between exchanger characteristics (both geometrical and fluid) and the amount of thermal energy that can be recovered. To do this efficiently, the team developed a custom computer code to perform multiple heat transfer calculations rapidly and obtain trends.

      The team used the custom computer code to determine the best dimensions of the heat exchanger and storage tank. A 220 gallon tank was found to be optimal. It was determined that it is possible to extract 50 kW from the exhaust stack (out of 246 kW available), producing a maximum heat exchanger outlet temperature of 209 °F and a storage tank steady state temperature of 135 °F, when the optimal 220 gallon storage tank is used.

      The system will reduce the plant's use of natural gas fired boilers to heat domestic process water. Based on the amount of energy saved and current natural gas prices, the team expects to see an operational cost savings up to $17,000 per year. The initial investment cost in system hardware is estimated to be approximately $8300. The payback period on the investment is then just under 6 months.

**Note: The heat recovery system specified here may not be the optimum choice for the Kellogg's plant due to insufficient data. The results here are parametric and provide a basis for future designs should proper data become available.

Objectives

• Heat incoming city water at 40°F (12.7°C) to as close to 140°F (60°C) as possible
• Include redundant systems to limit system failures
• Tie proposed solution into current boiler water heating system
• Provide cost analysis of new design verses old boiler system

Design Specifications

      In the design of the heat recovery system, many different parameters need to be considered. Due to the large number of variables and the complexities of the system, the team decided that a parametric analysis was the best approach. A custom computer code was designed to perform a parametric analysis of a cross flow continuous fin heat exchanger. The code simulates the heat recovery system, taking into account the properties of the two working fluids, city water and exhaust gases, the flow rates of the working fluids, the dimensions of the heat exchanger and storage tank, and the water demand. The code outputs the temperature of the storage tank versus time, the temperatures of the gas and water at the outlets of the heat exchanger. It also calculates the inner, outer, and overall heat transfer coefficients for the heat exchanger.

Conclusions

• Successfully designed and proposed a waste heat recovery system to Kellogg’s corporation.
• Developed computer code to evaluate system effectiveness for wide variety of inputs.
• Estimated heat recovery of ~50 kW which raises water to ~135° F.
• Designed as standalone system with no user input necessary.
• Estimated initial cost of equipment is approx. $8,300.
• Potential savings of $17,000/yr which leads to a payback period of less than 6 months.

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