A custom-engineered industrial heat recovery system utilizes thermal recycling, reusing process energy that would otherwise be wasted. One method for recovering process heat is to integrate the process heating equipment with the air pollution control unit. This creates a closed-loop system that yields a net-zero or even a net-positive operation. In addition, heat exchangers, which transfer heat from one fluid such as air or liquid at a higher temperature to another fluid at a lower temperature, can be added into the process at strategic stages.
This approach incorporates primary, secondary and even tertiary heat recovery methods into the overall operations. Based on the unique process stream and requirements, different types of heat exchangers can be selected and sized for optimal heat recovery. A comprehensive multi-stage heat recovery solution can provide enhanced system performance and improved energy efficiency as well as operating cost savings.
Multi-Stage Heat Recovery Methods Explained
Primary heat exchange systems are categorized as either recuperative or regenerative. They use hot air to preheat the process stream entering the air pollution control system or thermal oxidizer unit.
A recuperative heat exchange system typically employs shell-and-tube heat exchangers. In these heat exchangers, a stream of cold process gas passes through a series of tubes and is heated by a stream of hot process gas as it moves over the tubes’ outer surfaces (all within the shell of the heat exchanger). Because the surface area of the heat exchanger tubes is where the heat transfer takes place, for higher effectiveness, the number of tubes and length of the heat exchanger can be increased and sized for optimal heat recovery. Generally, these types of systems are used with low to medium process flow rates and can recover up to 80 percent of the thermal energy.
Regenerative heat exchange systems can operate with process flow rates in the low or high range; however, they can yield a much higher thermal recovery efficiency — up to 95 percent or greater. Regenerative systems use combustion chambers lined with ceramic media to absorb and transfer heat. The operation of a regenerative heat exchanger is cyclic, whereby the process gas is brought into the preheated ceramic bed and is combusted or oxidized. Two or three ceramic beds alternate the duty of heating, combusting and purging while the valves simultaneously open and close. The gas leaving the regenerative or recuperative thermal oxidizer, acting as the primary heat exchanger, can continue to the secondary exchanger.
The secondary heat exchange systems use the hot gas from the oxidation process to run other equipment within the manufacturing facility. Many applications are possible. A few examples are supplying the process heating equipment, where the hot air can be reused for systems like ovens or other curing devices, or for operating equipment, where the hot gas can be used to heat water for washers.
When applicable, a third phase of heat recovery can be integrated into the system to further enhance thermal efficiency. Tertiary heat exchangers recover heat that would be released after the secondary heat exchanger. The tertiary loop is used to capture and repurpose that thermal energy to propel other mechanical forces of the facility operations. Examples include being used to compress vapors, pump liquids or move turbines that generate electricity, or for ambient comfort heating throughout a facility.
Typically, secondary and tertiary heat exchange systems are stand-alone shell-and-tube types like those in recuperative thermal oxidizers. Additionally, these heat exchangers can utilize thermal fluid heaters such as waste-heat hot water heaters and waste-heat boilers. Thermal fluid heaters are industrial heating systems that recirculate a special heat transfer liquid, often a form of oil, through a fired heat exchanger. Many types of thermal fluid heaters are available.
4 Examples: Heat Recovery in Practice
The following four case studies demonstrate how the incorporation of various types of heat exchangers — as well as the integration of process heating operations with air pollution control systems — can create a multi-stage heat recovery solution.
Custom-engineered closed-loop systems can maximize process efficiencies across the operation. By utilizing thermal recycling and advanced airflow management throughout the process, heat recovery often results in burnerless or indirectly fired systems, which may lead to reduced fuel consumption and lower operating energy costs.
1. Integrated System with Prime and Finish Ovens, Thermal Oxidizer and Heat Recovery for a Coil-Coating Application. A metal processing facility needed to replace a coil-coating line comprised of multiple ovens and a stand-alone thermal oxidizer system. The main objective was to meet stringent environmental regulations while improving operational efficiency and not compromising on fuel consumption.
After evaluation and consideration of the existing system operation, a combination system layout coupling multi-zone ovens with the air pollution control unit was implemented. The new system integrated the three-zone prime oven, a four-zone finish oven with two heat exchangers, and a single recuperative thermal oxidizer.
As a result, the new system maintained the facility’s compliance with air emissions regulations. At the same time, it increased operational efficiency by as much as 50 percent, resulting in savings of $420,000 per year on fuel costs.
2. Thermal Cleaning and Air Pollution Control with Heat Recovery. An HVAC equipment supplier needed an efficient way to remove the residual oil present in the coils it manufactured. The company also needed an air pollution control system to destroy the volatile organic compounds (VOCs) evaporated during the thermal cleaning process.
After an evaluation, the final design installed was an integrated system with a burnerless oven configuration. It recovered the waste heat generated in a recuperative-style thermal oxidizer that had primary and secondary heat recovery arrangements. The primary heat exchanger preheated the oven exhaust air to minimize the energy demand in the thermal oxidizer system. The secondary exchanger served as an alternate heat source to heat ambient air to 650°F (343°C) before it was supplied to the oven heating chamber.
The integrated oven-oxidizer system configuration resulted in fuel savings of 5.5 million BTU/hr, which equated to approximately $325,000 per year.
3. Recuperative Thermal Oxidizer and Heat Recovery System for Aluminum Recycling Operation. An aluminum recycling plant was replacing an inefficient gas, direct-fired thermal oxidizer (DFTO) and rotary kiln. The existing systems were consuming more than 32 million BTUs per hour, and the company wanted an integrated heat recovery system that was able to reduce fuel costs.
For this application, an 18,000 scfm recuperative thermal oxidizer with multi-stage heat recovery was installed. Both primary and secondary heat exchangers were incorporated into the system design, eliminating the need for the natural gas burner in the rotary kiln. The hot air leaving the oxidizer went to the primary or preheat exchanger, then continued to the secondary heat exchanger. The secondary exchanger recycled the waste heat from the combustion process to run other parallel production processes throughout the manufacturing facility such as ovens and furnaces as well as provide general comfort heating for the facility.
The newly installed system consumes 10 million BTU/ hr due to the heat recovery generated by the primary and secondary heat exchangers. This resulted in savings of more than 22 million BTU/hr and significant operating capital.
4. Direct Fired Thermal Oxidizer and Waste Heat Boiler for Petrochemical Manufacturer. A large petrochemical manufacturer needed new equipment to comply with increasingly stringent air pollution emission regulations. Additional demands were to achieve a higher destructive rate efficiency (DRE) and minimize overall fuel consumption.
The engineering team decided on a 15,000 scfm direct-fired thermal oxidizer to work in conjunction with a waste heat boiler (WHB) equipped with a bypass to atmosphere to control the steam pressure and volume.
After the installation, the new system achieved more than 97 percent heat recovery efficiency while simultaneously destroying nearly all process VOCs. Ultimately, the system allowed the facility to remain compliant and operational with significantly reduced fuel demands.
In conclusion, as these project case studies show, the inclusion of multi-stage heat recovery can enhance thermal efficiency and the overall system performance. In many cases, such systems can provide a sustainable or even a net-zero operation.
When designing a new facility or retrofitting an existing operation, consult a heat recovery expert that can integrate the process heating equipment with the air pollution control system and evaluate the use of multiple heat exchangers. Regardless of the application, recycling thermal energy is one of the best ways to preserve natural resources and save on operating costs within your production facility.