For both types of equipment, the goal of process heat management is to minimize the volume and temperature of the exhaust stream. Heat recovery is the process of utilizing the heat that is generated but not consumed by a process. This heat may be directed back to the process as primary heat recovery; utilized by a related or connected process as secondary recovery; or provided to an unrelated process as tertiary heat recovery.
Primary Heat Recovery in the OxidizerMost oxidizer designs incorporate a primary heat recovery system to preheat the incoming airstream. A heat exchanger is used to extract heat from the high temperature airstream exiting the combustion chamber and transfer it to the cooler airstream entering the combustion chamber. Depending on the type of heat exchanger employed in the design, an oxidizer is referred to as recuperative or regenerative.
A recuperative oxidizer utilizes an air-to-air heat exchanger to preheat the incoming airstream (figure 2). Typical thermal efficiency of a recuperative heat exchanger is 60 percent to 80 percent.
A regenerative oxidizer utilizes multiple beds of ceramic heat exchange media (figure 3). One bed absorbs heat from the outgoing airstream while another bed releases heat to the incoming airstream. At regular intervals, a switching valve reverses the airflow through the media beds. The beds cycle between absorbing heat from the process and releasing heat to the process. Typical thermal efficiency of a regenerative heat exchanger is 85 percent to 97 percent. High efficiency heat recovery combined with the exothermic combustion reaction creates the opportunity for the oxidizer to operate without the need for additional fuel. In general, the higher the solvent concentration in the airstream, the lower the heat exchanger efficiency required to maintain this “self-sustaining” operation. Regenerative oxidizers reach this operating condition at solvent concentrations as low as 5 percent LEL.
Secondary Heat RecoveryThe oxidizer design and process operating conditions will determine how much excess heat is available for secondary heat recovery. Heat recovery can be accomplished in two different ways: Extracting heat from the stack or extracting heat directly from the combustion chamber. Dryer exhaust air temperature, solvent concentration and heat exchanger efficiency determine the oxidizer stack air temperature. As oxidizer efficiency is increased, the stack temperature will decrease. Destruction efficiency requirements determine the combustion chamber temperature. When concentrations rise above the minimum energy required for self-sustaining operation, excess heat is generated in the combustion chamber. This excess heat presents a great opportunity for heat recovery. Whereas the oxidizer stack temperature will be 100 to 400oF (55 to 222oC) above the inlet temperature, air from the combustion chamber will be at 1,400 to 1,600oF. Depending on the need, various forms of heat recovery options are available.
Direct Air. Direct air heat recovery is an arrangement where heated air from the oxidizer is ducted directly to the process (figure 4). Coupled to a convection dryer, this air can be utilized as preheated makeup air. This system is simple and requires little auxiliary equipment: modulating dampers, fans and pressure control loops. One drawback is that direct air recovery requires a large amount of high temperature ductwork. This system often proves impractical when space is limited or the oxidizer is located a great distance from the dryer.
Air-to-Air. Similar to direct air heat recovery, heat recovery with an air-to-air heat exchanger (figure 5) should be considered if products of combustion from the oxidizer may contaminate the drying process. It is important to carefully analyze the dryer operating conditions whenever considering a blended air heating system. Dryer air temperature is directly related to the volume of makeup air. If air volume and temperature requirements cannot be balanced, an auxiliary heating system such as a gas fired burner may be required to supplement the heat recovery system.
Air-to-Steam. Air-to-steam heat recovery systems are similar in design and operation to the air-to-oil system (figure 6). Hot oil systems can operate at higher temperatures than steam, but steam offers higher heat transfer rates. Typically, steam is preferred in a facility already using steam. Otherwise, a hot oil circulation system will be used.
Absorption Chiller. A less common heat recovery alternative is using waste heat to provide chilled water. Usually, the cost and complexity of these systems are prohibitively high when compared to a conventional chiller system.
Heat Recovery ConsiderationsIt is important to understand that as the efficiency of the primary heat recovery systems is increased, the amount of heat available for secondary recovery decreases. A well-designed primary heat recovery system may eliminate the need for any secondary system. The design goal for any new system should be to first optimize the primary heat recovery system. Secondary heat recovery requires the interconnection of two separate operating systems. It is important to thoroughly analyze the operating cycles of both the dryer and oxidizer through their entire range of operation. The dryer provides fuel to the oxidizer; the oxidizer provides heat to the dryer. If supply and demand are not balanced for all operating conditions, an auxiliary heat source may be required at the dryer. Generally, it is not recommended to overfire the oxidizer to provide heat to the dryer.
This analysis gets even more complex when considering a tertiary heat recovery system. With primary and secondary recovery, the process requiring the heat operates on the same schedule as the process generating the heat. The tertiary recovery system may be providing plant heating or cooling, or energy to another process. Supply and demand are rarely balanced, so tertiary systems normally are considered supplemental.
Designing an efficient heat recovery system is not difficult. Waste heat sources are matched with potential users, and the appropriate heat transfer and control system is designed. However, determining the feasibility of a heat recovery system is difficult. Heat recovery feasibility is a financial decision. Energy savings must be balanced against capital, operating and maintenance costs over the life of the system. Accurately quantifying these variables can be extremely difficult. A thorough analysis of potential heat sources and users is necessary. The skill and experience of the system designer are critical. Fluctuating utility prices and changing regulations complicate payback calculations. The technology of heat recovery is well developed. The art lies in the application.
Sidebar: Projects for Reducing Energy ConsumptionSo where do you start an investigation to determine if your existing or proposed new system is energy efficient? The investigation on existing equipment will require a technical person skilled in measuring or obtaining the pertinent data related to the equipment evaluation. With new equipment, it should be provided in the proposal or obtained during technical reviews of the planned project. If you plan to perform an internal evaluation of the energy efficiency of your existing process, be sure to consider the following process characteristics:
Define the process lines heat load distribution. Determining where the energy is going will help identify where efficiency can be improved. How much heat is going to
- Substrate and coating?
- Solvent evaporation?
- Makeup air and exhaust?
Equally important, how much heat is available in exhaust solvents via recovery or combustion?
Define the solvent rate and range. How much variation exists in the solvent input rate for the product range produced? Factor in the amount of production run time on each product, e.g., a low solvent input rate may be a high percentage of your production. The line sizing and your process exhaust are likely based on the higher input rate product(s) produced.
Generate an accurate flow diagram for the process line being evaluated. Record the entire product input and output conditions that define your process. Obtain all the associated flow, temperature, pressure, installed and operating horsepower, etc., data vs. the product input data. This data gathering process can expend a great deal of time and energy, but it is essential to getting results.
Identify payback opportunities. If you have the necessary skills and manpower within your organization to evaluate and implement energy savings programs, congratulations, there are benefits available to your bottom line. Another option is concluding your process has opportunities to save energy, but you need help! Consulting engineering firms or capital equipment manufacturing companies that offer additional engineering services may be the best way to gather the data and analyze the savings opportunities.