The strength of steel is well known, as well as the corrosion-resistant properties of glass. Combining the two to produce a glass-lined steel surface (often referred to in the industry as ‘glass-lined steel’) creates material that is both strong and corrosion resistant, but the engineering challenges to produce this material were considerable. While glass has poor conductivity, steel is just the opposite. The two could be combined in a manner where the impact of glass on the overall heat transfer is minimized while taking advantage of its very high chemical resistance and ease of cleaning. But at the same time, using steel to provide structural strength, durability, and cost savings results in a construction material that has the best of both worlds.
This challenge of bonding glass to steel was overcome almost 150 years ago. The technology has been further improved and refined in the 21st century to overcome a wide range of process, operational, and cost issues.
By Tom Patnaik, Vice President, Sales & Service, THALETEC USA Inc., with contribution from Christian Stentzel, Head of Research & Development, THALETEC GmbH
What is Glass-Lined Steel?
Glass-lined steel is a composite material of glass particles (or frits) fused to a steel substrate. Glass is applied to steel in two ways:
1) either as a glass particulate slurry sprayed on the steel in six 0.25 mm layers with each layer followed by high temperature fusing in a furnace, or
2) as two layers of electrostatically charged glass particles applied to a specially treated steel pipe, with each application followed by firing in a furnace at high temperature.
Each layer is approximately 10 mil (0.25 mm) thick. As such, the wet method yields a glass thickness of 39 to 87 mils (or 1.0 to 2.2 mm) thick, whereas the dry electrostatic method yields glass thickness of just 20 mil (or 0.5 mm) thick. The wet method is usually applied to the inside of cylindrical vessels (such as chemical reactors or storage vessels) where the glass layers are under compression due to the larger coefficient of thermal expansion/contraction of the steel substrate. This compressive force and a chemical reaction between glass and carbon steel produces an extremely strong bond capable of absorbing significant impact. As such, glass-lined steel is very strong and durable and can handle an operating pressure of -1/+10 bar and temperatures of -29/+230 °C for standard applications (higher with some special design considerations).
In the wet slurry method of glass application, the first two layers comprise a ‘ground’ coat that provides the adhesion and transition from steel to glass, which is then applied in 3-5 cover coats. The ground coats are usually applied at a higher temperature (900 °C) and the cover coats at about 850 °C. The ground coats provide:
- adherence to the steel
- transition from the high thermal expansion of steel to the lesser expansion of glass
- suitable bubble structure (porosity) for outgassing during firing
The cover coats provide:
- the corrosion-resistance properties of glass
- the release properties (smooth surface) required by many pharma-chemical processes
Dry electrostatic coating is performed exclusively on steel pipes with a proprietary process that provides steel tubes with their inherent strength as well as the heat transfer properties of steel due to the very thin 20 mil thickness of glass. Combined with the corrosion-resistance properties inherent to glass, the end result is a shell-and-tube heat exchanger tube bundle that is strong, thermal conductive and corrosion resistant, and, due to the release properties of glass, not prone to fouling.
The glass-lined steel heat exchangers can range in size from 1 m2 to 53.3 m2 depending on the design of the tube bundle. In brief, there are two designs of the glass-lined heat exchanger:
1) the glass-lined U-tube with 2 or 4-passes and
2) the straight-tube made of silicon carbide with 1 or 2-passes. Within these designs are several sub-designs that increase the versatility and application range of glass-lined heat exchangers.
The glass-lined shell-and-tube bundle is one of the most common heat exchanger types with high chemical resistance. To achieve high chemical resistance on both shell and tube sides, silicon carbide tubes can also be used. A shrink fit system between glass-lined tubes and the ‘tube sheet’ ensures a tight connection without the use of gaskets, avoiding dead zones and temperature limitations that gaskets would present.
Why Glass-Lined Steel Is So Much Better Than Conventional Alloys or Tantalum
The answer is multifaceted. When selecting a material of construction, several aspects have to be considered. In addition to suitability in terms of chemical resistance, the investment cost is usually a primary consideration to the owner. Furthermore, the thermal and hydraulic application limits as well as the robustness (operational lifetime) must be considered. Finally, the resistance to abrasion and fouling of the surfaces can significantly influence the choice of material. Operational temperature limits are often determined by the sealing materials. Further selection criteria include the thermal conductivity of the material and the manufactured size, the GMP design, and the diffusion tightness. Finally, ecological aspects of raw material extraction and production should also be considered. In just about all these criteria, glass-lined steel as a construction material, especially when employing the modern method of applying two ultra-thin layers of glass to a thin substrate of steel, is starting to find success in a plethora of applications.
a. Corrosion-resistant properties
The corrosion-resistant properties of glass are well known. The Bishop-Stern Chart below is a well-known reference in the industry. It shows the relative position of glass-lined steel corrosion resistance versus other common metals and alloys such as tantalum, stainless steel, Hastelloy, etc.
b. Strength of glass-lined steel
With a yield strength of 235 N/mm2, glass-lined steel is stronger than most materials used in the construction of heat exchangers. The carbon steel tube therefore enables high pressurization and guarantees high strength. As long as the steel is deformed in the elastic range, no chip-offs will occur. The deformation and flexibility of a common steel tube is given.
c. Cleanability of glass-lined steel
Glass-lined steel is very smooth with an Ra of 4 to 8 micro-inch (0.1 to 0.2 µm), giving it excellent release properties and making it ideal for applications where cleanability is preferred or required, as in pharmaceuticals manufacturing or in the processing of substances that tend to stick to surfaces.
d. Cost of glass-lined steel
Glass-lined steel is among the least expensive of corrosion-resistant materials, as compared to tantalum or high nickel alloys used in the manufacturing of shell and tube heat exchangers. Glass-lined steel represents a good solution for durability, mechanical stability, chemical resistance, and cost.
e. Environmental impact of glass-lined steel
The impact of glass-lined steel on the environment as measured by CO2 emissions in its production is by far the most environmentally friendly, as shown in the chart below. Furthermore, glass-lined equipment can be re-glassed up to three or four times. When reusing the steel parts, about 50% of the energy demand can be saved during the manufacturing process compared to a new build. The carbon footprint of glass-lined equipment is considerably less than that of tantalum or any of the high-nickel alloys.
Where Is Glass-Lined Steel Used Currently?
Glass-lined reactors are used extensively in chemicals and pharmaceutical manufacturing with a variety of glass types such as true anti-static glass (CONDUSIST), ultra-smooth poly-glass (POLYSIST), abrasion resistant glass (ABRISIST), among others that enhance the glass-lined steel’s ability to perform more effectively in certain applications.
The glass-lined heat exchanger is used as the so-called PowerBaffle representing a glass-lined tube bundle inside a reactor combining the functions of baffling, heat transfer, and temperature probe. Glass-lined heat exchangers can be used in general as a liquid/liquid HEX, evaporator, and condenser. Therefore, the glass-lined HEX can be used as a stand-alone solution or as an integrated HEX in an equipment (evaporator “Kettle-type”, head condenser on reactor, short path evaporator, or cold trap). Furthermore, glass-lined tubes are used, for example, in power plants and waste-incineration plants to clean the flue gas and recover what is called the ‘deep waste heat’ where temperatures are close to the acid dew point (ADP) and present a far greater risk of corrosion.
Power Plants
In flue gas emissions from coal- or natural gas-fired power plants, the heat recovery at higher (>1,200 °F) or medium temperature (450 to 1200 °F) is widely carried out, but as the temperature drops below 450 °F, the ADP becomes a key parameter due to low-temperature corrosion of the heat exchanger material, as well as fouling due to the interaction of the chemicals in the flue gas and the construction material of the heat exchanger. Therefore, the low-temperature waste heat is often not captured and released into the atmosphere. With glass-lined steel tubes, this has changed, and more and more the low temperature deep waste heat is beginning to be captured, thereby considerably improving the overall efficiency. Each 20 °C reduction in flue gas temperature results in a 1% thermal efficiency improvement.
Conclusion
The glass-lined heat exchanger promises to upend not just the recovery of waste heat, but also a wide range of applications in the production of pharmaceuticals and agrochemicals. In waste heat recovery, a valuable alternative approach to improving overall energy efficiency is to capture and reuse the lost or ‘deep waste heat’ that is intrinsic to all industrial manufacturing. During these manufacturing processes, as much as 20 to 50% of the energy consumed is ultimately lost via waste heat contained in streams of hot exhaust gases and liquids, as well as through heat conduction, convection, and radiation from hot equipment surfaces and from heated product streams. In some cases, such as industrial furnaces, efficiency improvements resulting from waste heat recovery can improve energy efficiency by 10% to as much as 50%. However, due to the challenges presented by low-temperature corrosion at the acid dew point, the deep low-temperature waste heat recovery has been expensive and elusive. The glass-lined heat exchanger promises to change that, enabling recovery of waste heat at the ADP. In the quest for net zero and sustainability, this is a step in the right direction.
Glass-lined heat exchangers represent a perfect combination between chemical resistance, cleanability, durability and cost, with the thin layer of smooth glass lining on a highly conductive but strong steel substrate ensuring high heat flow with greater fouling resistance and lower total cost of ownership. Many applications where deep waste heat recovery was not previously a profitable return on investment are starting to experience change. As such, this is a promising new direction in heat-exchanger design in a multitude of ways.
ABOUT THE AUTHORS
Tom Patnaik serves as the Vice President of Sales & Service for Thaletec USA. He is responsible for creating awareness and recognition for the Thaletec brand of glass-lined reactors, heat exchangers, and columns, as well as growing the installed base in North America. Previously, Tom has served with industry leaders such as Bird Machine, Sanborn, Hosokawa Micron, Heinkel Drying & Separation, and Pfaudler, in various sales and business development capacities within the chemicals, pharmaceuticals and other industries where alloy and glass-lined process equipment is used. As such, Tom brings a wealth of experience in all areas of the pharma-chem process train from size reduction and granulation to filtration, drying, containment and packaging.
Dr-Ing. Christian Stentzel is a mechanical engineering graduate and holds a doctorate from TU Dresden. Dr-Ing. Stentzel is an accomplished researcher and serves as the Head of Research & Development at Thaletec GmbH.