For SCD-220 shell-and-tube heat exchangers operating in steam heating or cooling water circuits, the decline in heat transfer performance over extended operating periods is often not due to equipment failure itself, but rather the combined effect of tube bundle fouling and thermal stress accumulation. Design and selection personnel need to understand the causes of these problems so that maintenance space can be reserved at the planning stage.
Tube bundle fouling leads to degradation of heat transfer capacity. The heat exchange tubes are Φ25×2.5mm in specification with a tube length of 6000mm, and the equilateral triangular arrangement makes the inter-tube passages relatively compact, so suspended solids or hardness components in the medium tend to deposit on the tube walls. The fouling layer increases thermal resistance, and the pressure drops on both the shell side and tube side rise simultaneously. In terms of design, the system heat load and safety margin can be incorporated into the selection calculation, and a removable tube header structure may be appropriately considered to facilitate later mechanical or chemical cleaning; for fouling-prone media, the distribution between tube side and shell side should give priority to routing the dirty medium to the side that is easier to clean.
Protection of thermal stress and tube-to-tubesheet connection areas. Shell-and-tube heat exchangers use the tube bundle wall surface as the heat transfer surface. When the temperature difference between hot and cold fluids is large, the difference in thermal expansion between the shell and the tube bundle creates stress at the tubesheet connections. At the selection stage, the design temperature and material combination should be verified, and the shell, tube bundle, and tubesheet materials should be matched according to medium characteristics to reduce fatigue at connection areas caused by uneven expansion. Avoiding rapid temperature rises and drops during operation is a standard practice for maintaining tubesheet sealing.
Determination of maintenance intervals and inspection items. Maintenance intervals should not simply follow a fixed schedule but should be determined based on medium cleanliness, operating temperature, and pressure drop trends. Key inspection points include the degree of fouling on the outer wall of the tube bundle, erosion conditions at baffle locations, and the sealing status of tube openings. For tube bundles in equilateral triangular arrangement, attention must be paid during cleaning to uniform coverage of the inter-tube passages to prevent locally residual fouling layers from becoming the starting point for fouling in the next cycle.
