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Graphite heat exchangers have a long and successful history and were introduced in the 1950s by Union Carbide of Cleveland, Ohio. They introduced impermeable graphite to the market under the trade name Karbate, which performs well in the phosphoric acid production market and any application including the use of hydrochloric acid. Graphite heat exchangers continue to be popular. By the 1960s, they were produced by several domestic and foreign companies. Today, the international graphite heat exchanger market is worth US$25-300 million. 

The greatness of graphite

For a long time, graphite has been a sought-after heat transfer material for harsh and corrosive process flows because it has both metallic and non-metallic properties, which contributes to the service life and reliability of equipment. Non-metallic properties include excellent corrosion resistance and low thermal expansion coefficient. Metal properties include high thermal conductivity, exceeding all common metals except aluminum.  

Graphite is inert, giving it excellent corrosion resistance under both static and dynamic conditions. However, the corrosion resistance of metals is affected by flow. Metals may perform well in laboratory tests, but fail to meet expectations in operation. Although in some corrosive environments, many nickel alloys and active metals have a lifespan similar to graphite, graphite is very suitable for strong reducing environments. Therefore, graphite is usually the first choice for hydrochloric acid applications. It is also a common choice for lower oxidizing environments, such as sulfuric acid concentrations below 85% by weight.

Graphite does have its disadvantages

In normal operation in hydrochloric acid, the average life expectancy of graphite heat exchangers can exceed 15-20 years. But since graphite is 100% resistant to HCl corrosion, why can't it last forever?

The main failure mode of graphite exchangers is tube rupture as the device ages. As the equipment ages, the tube will naturally lose strength. This is due to the aging and fatigue (vibration) load caused by phenolic impregnation.

The graphite raw material is impregnated with phenolic resin to make it impermeable to water. But just as importantly, the impregnation greatly increases the strength of the graphite material. As the unit ages, the resin continues to crosslink and eventually begins to detach from the graphite matrix on a microscopic level, resulting in a decrease in tube strength. At the same time, during years of operation, slight vibrations related to cycles or occasional system failures can cause random micro-cracks in the pipe. Over time, both will cause the material strength to decrease.

Since the yield strength and ultimate strength of graphite are the same, there is no significant deformation before fracture. Measurements, such as eddy current testing to measure tube wall thickness, cannot be effectively used to evaluate graphite tubes. Therefore, the graphite tube may look normal one day, but it will break the next day.  

In addition to unexpected failures, repairs are not easy to complete. The tubes in the graphite shell and tube heat exchanger are bonded to the tube sheet with phenolic cement. Whether to replace or repair a pipe, it requires drilling graphite and curing phenolic cement. Many operating sites do not have enough experience and/or skills to work confidently on graphite exchangers. Therefore, professional graphite repair shops are often hired to perform repair services. This can lead to longer turnaround times and higher costs.

Cleaning the graphite heat exchanger can be done chemically or mechanically, but care must be taken in either case. Graphite materials are generally softer than the dirt inside the tube. Expert knowledge is required to operate spray guns or other mechanical cleaning equipment to avoid damaging the graphite while cleaning the tube. Over time, the profile of the tube wall on the graphite tube will change due to corrosion, which usually leads to an increase in fouling rate. 

Is there a way to improve the service life of graphite heat exchangers?

Yes. Attention to detail and strict quality control are the keys to the service life of graphite heat exchangers. 

The fully graphitized substrate and high-quality impregnation process can reduce the impact of resin crosslinking and microcracks. Proper impregnation will ensure complete and uniform resin content throughout the thickness of the graphite sheet by eliminating glaze on the surface. A fully graphitized tube is less fragile than a resin bonded tube or a graphite-carbon composite tube, resulting in a tougher and more elastic tube.

Vibration and flow analysis should be performed with thermal dimensions to verify that there are no frequency or resonance issues. The shell diameter, baffle arrangement, connection size, etc. should be adjusted to eliminate potential vibration.

The O-ring is completely independent of the tube nut, which is tightened to compress the O-ring and keep it stationary between the two support rings. This design allows the tube to expand and contract independently of the shell, thereby eliminating excessive tensile and compressive loads across the tube that may occur in the graphite cemented design.

For a more complete discussion of these topics, please see https://www.bicmagazine.com/resources/sponsed-content/impervite-graphite-engineered-for-reliable-heat-transfer/

New technologies and designs provide options and solutions.

As mentioned above, the tubes in the graphite heat exchanger are glued into the tube sheet. In order to compensate for the difference in expansion rate between the graphite bundle and the steel shell, a floating tube sheet with a shell-side packing gland is used. CG Thermal has a proprietary O-ring design that eliminates the cementing process and makes maintenance fast, inexpensive, and easy to complete on site. This design uses two fixed tube plates instead of graphite floating tube plates, and no packing gland is required.

This O-ring design also allows for easy replacement of any damaged tubes without disturbing any other tubes in the heat exchanger. The end plate can be removed so that the pipe can be fully accessed for cleaning or inspection. CG Thermal's SiC Umax advanced ceramic heat exchanger and Impervite PPS-GR heat exchanger adopt this design.

In highly corrosive and harsh environments, the use of ceramic and polymer materials has proven to be superior to graphite. Tubes made of these materials are favored by many people because of their excellent thermal and mechanical properties and excellent corrosion resistance. Heat exchangers that combine the advantages of O-ring tube seals with these tube materials can be used. 

Graphite has excellent thermal conductivity. However, it is surpassed by Umax Advanced Ceramic materials. The Umax tube is made of alpha sintered silicon carbide, and because it contains no fillers or free silicon, it has universal corrosion resistance. It is 50% harder than tungsten carbide. This hardness eliminates the problems associated with cleaning graphite tubes, as well as the erosion problems that limit the life of graphite tubes. The SIC structure makes the surface less likely to foul. Combining the physical properties of Umax ceramics with a unique O-ring design makes these devices 100% resistant to thermal shock damage.

PPS-GR tube is graphite polyphenylene sulfide composite material. In its application range, its corrosion resistance is comparable to resin-impregnated graphite tubes, but it is light in weight and longer in length, and can be easily replaced on site. Due to the polymer composition, the contaminated material will not stick to the tube wall, and it is easy to remove, less production time loss, and less worry about tube damage. As a more ductile material, it will withstand system stress and has excellent thermal shock resistance. This material can be used for higher pressure designs than graphite.

Since the bending strength of PPS-GR pipe is lower than Umax and graphite, the baffle assembly is designed to minimize the bending load on the pipe. Although its conductivity is significantly lower than other materials, in most applications, only about 7-15% of additional surface area needs to be added.

In short, graphite heat exchangers are very suitable for harsh process fluids such as hydrochloric acid, sulfuric acid and phosphoric acid. However, when the operating life of graphite heat exchangers is less than 8 years, it is time to consider material and design alternatives to extend operating life, simplify maintenance, and increase resistance to corrosion and erosion.

CG Thermal will host a webinar on this topic on July 28th. To register, click here.

CG Thermal provides process technology solutions for demanding and corrosive process streams. To contact them for more information, click here.

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