Advancing Waste Heat Recovery Solutions in Industrial Operations

Case study offers insight into air pre-heater replacement at U.S. power station

With increasing emphasis on corporate environmental responsibility, industrial operators across various industries are re-exploring opportunities to repurpose otherwise wasted heat. When implemented effectively, the benefits of Waste Heat Recovery have shown to be significant, ranging from a reduction in primary energy inputs to fewer greenhouse gas emissions.

Many operators have long started down this path in the form of heat pipe air pre-heaters (APHs). The technology, which traces its origins back more than a century, is touted by its ability to successfully navigate the complexities of recovering energy from often-overlooked streams such as combustion flue gases that come with heavy particulate loads, high activity and extreme temperature fluctuations.

In the case of Shuykill Energy Resources (SER), the U.S.-based independent power producer had been operating a Q-Dot heat pipe APH at its 80-MW St. Nicholas Electric Power Generation Station in Shenandoah, Pennsylvania, U.S. since 1987.

Q-Dot built and installed dozens of large heat pipe APHs in power stations, refineries and chemical plants around the world during the 1980s and ’90s. Shuykill’s power station in Pennsylvania utilises waste coal culm and silt to generate steam and electricity.

Over decades, the performance of the heat pipe APH at SER’s power station had degraded due to several known issues, including pipe corrosion, leakage, degradation of the organic working fluids. SER decided to replace the faulty heat pipes and address the leakage issues affecting the unit. The following case study details critical issues facing SER’s heat pipe APH, as well as the solution, design considerations and results.

But first, what is an APH?

A heat pipe APH is constructed as a cased unit comprised of an array of heat pipes, each acting as an individual heat exchanger.  The heat pipe itself is a sealed tube filled with a small amount of working fluid at saturation condition. The latent heat of that working fluid is used for the energy exchange.

Heat pipe APHs can receive two process streams, which are isolated using a separation plate that contains affixed heat pipes that contact each process stream. On the primary side, also known as the evaporator section, the pipes contact the hot process stream, causing the liquid working fluid within each heat pipe to boil.  On the secondary side, or condenser section, each pipe contacts a cold process fluid, causing the gaseous working fluid to simultaneously condense.

The ends of the heat pipe are free to expand and contract, preventing mechanical stress on the equipment. Furthermore, heat pipe APHs are highly customisable: the number of pipes, their spacing, dimensions, orientation, material of construction, type of working fluid and casing dimensions can all be tailored to meet specific application requirements.

Case study

In SER’s case, the heat pipe APH installed at its power station in Pennsylvania was split on the secondary side to heat both primary and secondary air – comprising 39 rows, each containing 56 pipes arranged in an inline pipe layout for a total of 2,184 pipes.

Initially, the scope of the repairs was to address only the primary air heater. However, the product manufacturer had discontinued support on Q -Dot heat pipe APH, so SER contacted Econotherm, a UK-based company that specialises in the design and manufacturing of industrial waste heat recovery heat exchangers based on heat pipe technology.

An inspection and engineering review highlighted critical issues with the initial design:

• An insufficient number of heat pipes were installed, meaning the unit’s design prohibited SER from realising full heat recovery potential.
• The unit had reached its maximum pipe working temperature (PWT). The max PWT in the design condition was 347°C (656°F), which exceeded the critical temperature of toluene (working fluid in the original pipes) of 319°C (605°F). When the temperature reached or exceeded critical levels, the evaporation-condensation cycle ceased. This rendered the heat pipes inoperative.
• The minimum PWT was lower than stated. The design conditions allowed for 90.4°C (194.72°F), which was lower than the stated minimum metal temperature of 103.3°C (218°F). This increased the likelihood of cold end corrosion issues within the unit
• The length of the heat pipes in the design conditions requires accurate design to avoid hitting heat pipe limits. In the case of SER’s existing unit, the design conditions had led to the counter-current limit being exceeded in some areas.

As a result, the Q-Dot heat pipe APH was only achieving 85% of the design performance (See Table 1). Furthermore, the effect of corrosion, removed pipes and poor pipe function had compromised the current APH by 75% relative to its theoretical built capability.

Table 1

The solution

Econotherm’s sizing tools indicated that the unit’s performance could be improved significantly, even with considerably fewer pipes. Design considerations included:

1. Pipe working temperature:  The use of asymmetric fin densities enables control of the PWT. Thus, the typical approach would be to elevate the cold end PWT to minimise cold end corrosion problems such as those experienced in the current Q-Dot APH. Conversely in hot applications, asymmetric fin densities could lower the PWT in the hot end of the unit.
2. Number of heat pipes: Designing to minimise the number of required heat pipes comes with compromises – in this case, the low-end PWT.
3. Single heat pipe type: It’s typical with an Econotherm APH to utilise multiple heat pipe types in a single unit, with PWT elevating configurations in the cold end of the unit, lowering configurations in the hot end and neutral configurations in the middle. Additionally, considerations regarding pipe material include using several rows of stainless steel-grade pipes at the cold end to enhance corrosion resistance during transient operating conditions. Distilled water rather than the original organic fluid can also be used.
4. Heat pipe style: The current unit utilised finned heat pipes, but consideration can be given to plain pipes. This approach would require more units, but has the advantage of being able to utilise a more robust pipe with lower fouling and simpler cleaning procedures.
5. Pipe sealing: The current welded approach was not practical and had been shown to be problematic in the current Q-Dot APH. A screw-in collar was preferred, but required a slightly greater fin to fin separation in the unit. This resulted in less pipes per row in the unit to remain within the available space.

Results

SER proceeded with the project in 2023. The heat pipe modules were fabricated in Bridgend, Wales, and shipped to site where they were installed in 2024. The resulting performance achieved is illustrated in Tables 2 and 3.

Table 2

Table 3

Savings realised:

• Reduction of 57.4 million BTU or 2.9 coal short tonnes per hour burned.
• Hourly savings of $173.23 USD based on price per tonne of $187.70/hour coal price based on MMBTU coal.
• Annual savings of $1.5 million.
• Reduction of 42,959 tonnes of carbon dioxide per annum.
• Substantial reduction in quench water consumed at the power station

Conclusion

As industrial operators continue to seek sustainable solutions, waste heat recovery and reutilisation has the potential to become a critical element in maximising operational efficiencies and turning waste heat into a valuable resource.

The success of technologies such as heat pipe air pre-heaters demonstrates that, when designed properly, can help those operators transformed overlooked energy streams into valuable resources. Furthermore, by addressing the challenges of aging systems and optimising their design to meet modern demands, operators can better position themselves for within the current energy transition.

FAQs: Waste Heat Recovery with Heat Pipe Air Pre-Heaters

What is a heat pipe air pre-heater (APH)?
A heat pipe APH is a heat exchanger that uses phase-changing working fluids in sealed tubes to transfer energy between two process streams

Why did SER replace its original APH?
The original unit suffered from corrosion leakage and poor design causing major heat loss and reduced performance over time

What company supplied the replacement APH?
Econotherm a UK-based heat recovery specialist provided the replacement system with stainless-steel heat pipes

What improvements did the new APH deliver?
The upgraded unit boosted recovered heat duty by over 14 MW cut water spray use and reduced carbon emissions by nearly 43 000 tonnes annually

How were the heat pipes improved?
They were redesigned using stainless steel with better fin structures for reduced fouling simpler cleaning and higher durability

What savings were realised?
The project saved $1.5 million per year by reducing fuel use and maintenance needs and improved overall energy efficiency

Where were the new heat pipes made?
They were fabricated in Bridgend Wales and installed at the power station in 2024

What role does waste heat recovery play in sustainability?
It helps reduce primary energy demand lower emissions and support the shift to more sustainable industrial operations