Handling the Heat

A new resin not only stands up to high temperatures, but also resists the acid environments found in the gas inlet sections of flue gas scrubbers

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Despite its excellent thermal and mechanical properties and overall positive results in seven years of practical use, the resin is not a solution for all problems with temperature excursions of 390F to 535F (200C to 280C). Like its metal alternatives, the resin has a limited life span; however, in many composite designs, the new EVER composite alternative provides lifetimes that clearly outperform metal alloy alternatives.

Although other high-temperature-resistant resins such as the vinyl ester urethane hybrid and the highly cross-linked UP have been used for temperatures ranging from 320F to 430F (160C to 220C), in some cases these resins could not withstand the service conditions. In fact, in several cases the failed composite designs were re-made in the new novolac EVER. Subsequent field experience demonstrated that the new resin withstood the conditions.

FRP fabricators have invested considerable resources in problematic industrial applications and have found that the optimum balance of mechanical and thermal properties is crucial for a successful design.

Efforts with highly cross-linked resins, aimed to trim the molecular backbone in such a way that a higher thermal resistance is attained at the expense of a decreased elongation, proved in practice to be the wrong approach. The higher elongation of the EVER compared to competitive resins seems crucial in many designs, imparting a lower sensitivity to stress variations resulting from:

Actual thermal gradient.

Thermal shock (either from the process or the fast heat-up/cool-down that results from plant startup and shutdown).

Laminate design

At service temperatures above 340F (170C), higher FRP laminate quality becomes a prerequisite for success. Air bubbles, nonhomogeneous fiber content throughout the laminate and wall-thickness variations (commonly present when nozzles are laminated into the laminate wall and reinforcing rings are laminated onto the part) will result in significant thermal stresses. This could result in delaminations within the laminate during the heat cycle and when thermal shocks occur during service.

For this reason, emphasis is on the fabrication of good-quality laminates. Large wall-thickness variations (e.g., between reinforcing rings and laminate), as well as significant differences in coefficient of expansion (COE) between chemical-resistant layers and structural laminates, need to be prevented.

The laminate design proposed exploits the principle that a gas has a very low heat capacity and that FRP is a heat insulator. These two aspects combined illustrate that although the gas temperature might be high, the gas is unable to transfer sufficient heat to the FRP, keeping the material from reaching the appropriate gas temperature throughout its full thickness.

A laminate design often begins with a heat shield to protect the valuable structural wall of the vessel from excessive heat. The flow of gas through the inlet section is, in many cases, turbulent and often can result in some areas of the vessel becoming hotter than other areas. The difference in temperature will lead to a thermal expansion difference and could result in delamination. Carbon fiber and graphite powder can be included in the design; they will produce improved heat conductivity and allow a more even temperature distribution across the surface of the heat shield.

The thickness of the shield depends on the gas characteristics, but it is generally in the range of 3 millimeters (mm) to 5 mm. It is important for the laminate quality to be nearly perfect, requiring minimal air entrapment. To reach this quality goal, the authors suggest that a suitable air release agent such as BYK A -555 (0.1- 0.3 phr) be used. This additive enhances air release during impregnation of the glass reinforcement. The fiber content also should be watched closely to achieve a homogenous laminate. The authors recommend that highly experienced laminators complete the process.

The heat shield could be followed by an air insulation annulus to reinforce the heat protection afforded to the structural wall. This can be achieved by allowing the heat shield to hang as a loose sleeve, with suitable spacers, to keep it from touching the structural wall of the vessel.

The design is such that the temperature after the heat shield and the air insulation gap must be low enough for the structural wall of the vessel to be made using standard FRP fabrication techniques with standard novolac EVERs.

Laminate temperature distribution

To optimize the protection of the first two layers of the structural wall, the authors conducted a study of heat flow through FRP laminates and developed a computer program that can predict the temperature at any given point in the laminate. Based on the vessel dimensions, gas flow, gas temperature and heat capacity, the temperatures within the laminate can be estimated.

This temperature distribution then can be used to calculate the thickness of the heat shield and air insulation layers required to reach the degree of heat protection needed for the structural wall. The temperature profile also can be used to calculate the average temperature through the thickness of the structural laminate and to predict the need for insulation of the vessel's outer wall.

The following example looks at the temperature distribution through the laminate in more detail. It assumes:

A gas temperature of 430F (220C).

A target FRP surface temperature of 390F (200C).

The thickness of the first protective layer then can be calculated based on the individual application details, but will generally be in the 3-mm to 5-mm range. The temperature behind this layer should be 355F (180C). Again, the computer program can be used to calculate the thickness of the insulating layer, which is typically in the 5-mm to 10-mm range.

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