Heat-pipe energy-recovery technology gained significant market acceptance in the 1960s. Performance and durability in those early years was not questioned, but the cost of heat pipes for industrial applications was extreme compared to other technologies. Since those early days, however, several advancements have been made in the manufacturing process for heat pipes that have enabled them to become more efficient and cost-effective for use in industrial applications.
One of these advancements has been the introduction of thermosyphon heat pipes into the market. Since then, developments in manufacturing techniques have both improved the performance of the pipes as well as reduced the cost of manufacturing them. These technology improvements have made thermosyphon heat pipes an economically viable alternative to more traditional heat-recovery technologies.
Why, then, is this technology rarely discussed? If government estimations point toward a 20-50% loss in industrial energy input through waste heat in the forms of hot exhaust gases, cooling water and heat lost through hot equipment surfaces and heated products, then why aren’t industries scrambling to see this new option in heat recovery?
The short, and rather blunt, answer is that many are stuck in the past when the topic of heat recovery is brought up. Traditionally, heat-exchange technologies were largely limited to plate and frame, shell and tube, heat wheels and radiant systems, each of which have their place but can experience a variety of problems that reduce the overall effectiveness of the technology in certain applications. Often, these problems include thermal stress cracking due to differential temperature expansion, cold-spot condensation leading to corrosion, and pitting of the thin metal surfaces used to optimize heat transfer. All of these create a vulnerability of the metals within the exchangers to fatigue, erosion and corrosion.
On top of this, many see new heat-recovery projects as a financial stretch. If an energy-recovery system was not specified when a production unit was installed, it is often considered to be an expensive exercise with poor return on investment. And should a heat exchanger fail, the downtime could cost a company an exorbitant amount of money. These fears are understandable considering the problems we have already addressed regarding more traditional heat-recovery systems. Even the U.S. Department of Energy, in an article on recuperators for aluminum melting furnaces from 2007, stated that, “recuperators have been successfully used to preheat the air, however, in many cases, the metallic recuperator tubes have a relatively limited lifetime – 6 to 9 months.”
But with energy being lost the way it is, the market is still there for a viable method of recovering and reusing that energy in some way. At their best, heat exchangers are an essential component for industrial processes that consume large amounts of energy. When these systems run well, with minimal downtime, they provide the user with significant savings on energy costs. And these are the benefits that newer thermosyphon heat-pipe technologies can provide the industry with – an energy-recovery process that is simple, effective and safe.
In a gas-to-air heat-pipe heat exchanger, the thermosyphon heat pipes are free to expand and contract during operation without applying any stress to the housing. This minimizes the potential for thermal stress cracking due to the differential expansion between the heat-exchange surfaces and the unit casing.
Within a heat-pipe heat-exchange system, the pipes operate as individual heat exchangers (Fig. 1) to provide a multiple redundancy capacity and minimize the potential for a single catastrophic failure point within the unit. Should a single heat pipe fail, the system will continue to operate with a minimal loss in performance, measured as 1/n where n represents the total number of pipes within the system.
Thermosyphon heat pipes operate isothermally. For example, when one end of the heat pipe is placed in a hot exhaust stream and the other end is exposed to another transfer media (air, water or oil), the working temperature of the heat pipe will remain constant along the full length of the pipe. The temperature along the entire heat pipe remains uniform. Therefore, within the heat exchanger there are no cold spots to provide points for condensation and possible corrosion.
The heat-transfer capacity of thermosyphon heat pipes is not affected by wall thickness, thus allowing thicker walls – typically 2.5 mm or 3.5 mm – that offer a higher resistance to pitting in high-particulate atmospheres. Thermosyphon heat pipes manufactured by Econotherm LLC and distributed in North America by Mantra Innovative Systems can effectively operate with waste streams between 100-1000°C (212-1832°F).
As a result of the new manufacturing processes and the improved performance of the pipes, more energy can be recovered from an exhaust stream using the same number of thermosyphon heat pipes. That means, given the same operating parameters (exhaust temperature and mass flow), thermosyphon heat pipes perform more efficiently than older heat-pipe technology.
Thermosyphon heat-pipe heat exchangers offer flexibility within the design process, allowing the footprint of the exchanger housing to be determined by the space available around the production unit, a luxury that many other technologies used for energy recovery cannot offer. The size and functionality of the system are normally determined by the operating parameters of the production unit and the efficiency level necessary to recover the amount of energy required for a task. Calculations are then made to determine the exposed surface area of the heat pipes required to recover the energy necessary to accomplish the desired task.
For example, a heat exchanger requiring a calculated number of lineal inches of exposed surface area may use nearly any combination of “pipe length x # of pipes” equaling the number of lineal inches required to recover the energy necessary for the task. As energy prices climb, this innovative design aspect becomes very useful when contemplating the installation of new energy-recovery systems for existing production units.
A recently completed case study of a thermosyphon heat-pipe heat exchanger in an aluminum foundry (Fig. 2) shows remarkable longevity and an excellent return on investment.
In 2005, the longest continuously operating Mantra-installed thermosyphon heat-pipe heat exchanger commenced operations at an aluminum foundry in the Midwest (Fig. 3). In July 2013, after eight years of continuous operation, Mantra conducted an inspection of this cross-flow heat-pipe heat exchanger to monitor the performance of the system compared to the original specification and to evaluate the condition of the metal used in the collection system, the heat-exchange housing and the actual heat pipes (Fig. 4).
The aluminum furnace was operating at ≈2000°F with 5% excess air. After the introduction of dilution air to the exhaust stream, the heat-pipe heat exchanger received the exhaust at 896°F (480°C). The incoming ambient combustion air supply was reaching the heat exchanger at 80°F and was exiting the heat exchanger at 700°F. With a process temperature of 2000°F (1093°C) at 5% excess air and a preheated combustion air temperature of 700°F (371°C), the furnace energy demand was reduced by 23.37%.
Once the heat-pipe energy-recovery system has been installed and production parameters established, there is minimal operator involvement with the system, as was the case in this study. Hot exhaust gas is pulled from the furnace stack and delivered to the heat-pipe heat exchanger. During production, the exhaust contains corrosive gases, resulting from the use of fluxing materials.
The heat pipes in this unit use a gravity-fit locator, and the differential pressure between the exhaust stream and the incoming combustion air ensure that there is no cross contamination between the two gas flows. This form of fitting allows for easy extraction of the heat pipes if necessary (Fig. 5).
When the heat pipes were extracted from the heat exchanger eight years after installation, they were examined for physical deterioration. They were also performance tested against the original performance specification. There was a particulate buildup on the pipes that was easily removed with a wire brush, but there was zero pitting and minimal corrosion visible. The outside diameter of the pipes was measured, and there was zero loss after eight years of operation (Fig. 6).
Performance tests were then carried out on the heat pipes extracted from the units. The heat pipes performed ≥97% when compared to the performance levels measured when the pipes were first manufactured.
After eight years of continuous operation, each thermosyphon heat pipe within the heat exchanger was still functioning at near optimal capacity.
As we can see from this study, many of the fears and concerns regarding the more traditional methods of energy recovery within the foundry industry do not apply to the newer thermosyphon heat-pipe technology, especially the one about having too short of a life span. These new heat exchangers provide the manufacturing industry with an efficient method to recover and reuse energy that would otherwise be discharged from the facility. Energy from clean, dirty, particulate-laden, corrosive, cool or very hot exhaust streams can now be collected more efficiently at higher levels and with increased confidence that the system will continue to operate.
For more information: Erin Yates, marketing and communications, Mantra Innovative Systems, LLC, 200 Wingo Way, Suite 100, Mt. Pleasant, S.C. 29464; tel: 843-724-3404; e-mail: Erin.Yates@MantraInnovative.com; web: www.MantraInnovative.com
- Econotherm UK Ltd, R&D department.
- FINAL TECHNICAL REPORT MATERIALS FOR INDUSTRIAL HEAT RECOVERY SYSTEMS Task 1 IMPROVED MATERIALS AND OPERATION OF RECUPERATORS FOR ALUMINUM MELTING FURNACES DOE Award Number DE-FC36-04GO14035 Project Period 04/01/2004 to 03/30/2007
A recent case study conducted by Econotherm shows improved energy return with the introduction of new thermosyphon heat pipes.
In 2013, changes required to an existing heat-pipe heat exchanger at a steel mill in the Czech Republic provided the opportunity to directly compare the performance of traditional heat pipes to the newly produced thermosyphon heat pipes. This particular heat exchanger was returning 6 MW of energy from a steel-mill blast furnace. The recovered energy was being used to preheat combustion air for the mill. After replacing just one-third of the original heat pipes with new thermosyphon heat pipes, the heat exchanger was able to dramatically improve the level of recovered energy with temperatures reaching 8% higher than previously recorded.