The cold face is comprised of the horizontal steel grid, perforated or expanded metal surface that carries the ceramic heat exchanger media and the vertical reinforcement support steel carrying the grid.
The media support grid is referred to as the cold face because this area is exposed only to inlet and exhaust gas temperatures as compared to the “hot face” temperatures in the combustion chamber of the oxidizer. Between the cold face and the hot face is the ceramic heat exchanger column.
Failure of cold face represents one of the most costly areas of reconditioning in PCWP RTOs. Replacement of the cold face requires complete removal and replacement of the ceramic media and typically a portion of the ceramic fiber insulation lining between the media and the RTO tower shell. Reinforcement of a failing cold face is possible in some cases. However, decline of original materials continues after reinforcements until a point at which entry into and repair of this area represents significant personnel safety risks.
PLANNED COLD FACE TEMPERATURE EXCURSIONS
RTO media supports have ironically been given the moniker of “cold face” since this area is not typically exposed to the > 1500 °F temperatures of the “hot face” combustion zone. However, the cold face must be designed to endure continuous temperatures of ~ 800 °F and intermittent temperatures of ~ 1000 °F as these temperatures are commonly reached during “bake-out” maintenance intervals.
During bake-out, conventional RTOs are operated off-line and on fresh air and the RTO valve cycle is purposely augmented to elevate the cold face temperatures to ~ 600 °F - 800 °F wherein organic particulate reaches its ignition temperature and burns off. The bake-out frees the ceramic media and cold face of organic particulate deposits restoring pressure drop and fan capacity to maintain desirable process exhaust rates.
The material selection for the cold face must consider these maintenance temperature variances. Exposure to these elevated temperatures after exposure to corrosive or acidic gas stream elements must also be considered in certain PCWP processes, such as, OSB dryer applications.
UN-PLANNED COLD FACE TEMPERATURE EXCURSIONS
Depending upon the PCWP process, the seasoned vendor may consider temperature exposures resulting from fires that can infrequently occur. For example, resin and slack wax deposits on the cold face of an OSB press exhaust RTO may ignite during a maintenance bake-out (particularly if production demands require that the equipment operate beyond controllable maintenance intervals). Short term temperature excursions exceeding the combustion zone temperatures can be reached at the cold face. Deflection and premature cold face failure in the absence of the appropriate heat resistant materials of construction may lead to damage and premature cold face failure.
Proper operation of the main RTO tower outlet valves is also important to avoiding un-planned temperature excursions at the cold face. When outlet valves do not seat properly and leakage across the valve occurs the energy stored in the ceramic is pulled downward. This not only augments the temperature gradient across the bed but unnecessarily exposes the cold face to elevated temperatures that are sometimes undetected until damage has already occurred. The elevated cold face temperature may not be detected since the leakage across the outlet valve may not elevate the overall blended exhaust manifold temperature high enough to set off a temperature detection device.
EXHAUST GAS COMPOSITION IMPLICATIONS
Acidic and corrosive elements contained in the exhaust gas stream of some PCWP processes require design considerations to discourage chemical attack of metals and subsequent materials fatigue. The most common failure of cold face support grids in OSB dryer and some MDF dryer RTOs has been chloride stress corrosion cracking (CSCC) where in microscopic fissures are observed in some grades of stainless steels. These fissures make the materials inherently brittle and subject to breakage under the force of human hands.
Changing processes and new formulations make the importance of defining actual exhaust gas composition paramount for proper cold face design. Process modifications to meet CARB phase 1 & 2 regulation may have end of pipe implications. For example, there have been nitric acid observations in the exhaust gas composition of some MDF facilities resulting from formaldehyde scavengers integral to the process. Where RTO cold faces are designed for resistance to the effects of chlorides, favorable conditions for decline may exist in the presence of Nitric acid.
COLD FACE DESIGN CONCLUSIONS
The RTO vendors’ knowledge of each specific PCWP process and the resulting gas composition specifics of each facility are critical to the metallurgical selections, surface treatments, and fabrication approaches of the cold face to ensure long term structural integrity.
Strategic metallurgical selections may include consideration for low carbon content, moly content, and specialty alloy deployment. Heat treat strategies may be necessary to relieve fabrication or installation stresses (where chloride attack will first occur) and fabrication approaches, such as, laser cutting of assembly materials in lieu of shearing to prevent microscopic metal fatigue can ensure the longevity of this critical internal RTO area.
The RTO industry is highly competitive and the business climate of the PCWP industry commonly dictates low cost purchase options. However, a premature cold face failure in even the smallest of PCWP RTOs can cost hundreds of thousands of dollars within a matter of months of initial operation. This is one design area where inexperience or thrift for cost savings will result in future expenditures and potential production losses that are not in the financial forecast.
Karl Walby is a Senior Key Account Manager and Wood Products Industry representative for Dürr Systems, Inc. – Clean Technology Systems in North America.