When the exhaust air temperature at the tail end of a drying line is relatively high and the space for waste heat recovery is limited, finned tube heat exchangers are often included in the selection plan. Procurement personnel need to clarify which parameters to provide during the submission stage and how to judge the operating status. The following Q&A can serve as a reference.
The four data items—medium composition, flow rate, inlet and outlet temperatures, and allowable pressure drop—are the prerequisites for thermal calculation. The medium composition determines the finned tube material and fin spacing; if dust-containing or corrosive components are present, this must be stated separately. Flow rate and inlet/outlet temperatures are used to determine the heat exchange rate and logarithmic mean temperature difference. The allowable pressure drop constrains the selection of flow velocities on the air side and medium side; if the pressure drop requirement is too strict, the frontal area must be increased or the number of tube rows adjusted. If any one of these is missing, the supplier can only estimate based on experience, and the selection result is likely to deviate from the actual operating conditions.
Taking waste heat recovery on a certain drying line as an example, the required air temperature difference between inlet and outlet is not less than 25°C, the ambient temperature is -20 to +60°C, and the relative humidity is less than 80%. The temperature difference index directly corresponds to the heat exchange rate, and the humidity condition affects whether condensation forms on the fin surface. If the exhaust air humidity is close to saturation, condensate drainage and fin anti-corrosion measures should be evaluated to avoid corrosion at the fin root after long-term operation.
Finned tubes commonly use steel-aluminum composite, stainless steel, or copper-aluminum composite forms. When the medium side is corrosive, stainless steel tubes should be preferred; when the air side is relatively clean, steel-aluminum composite finned tubes can be used to control costs. Fin spacing is determined by dust content; for operating conditions with an obvious tendency to accumulate dust, a larger spacing should be used and space for purging should be reserved. The number of tube rows, frontal area, and fan air volume need to be calculated together; a decrease in air volume will directly weaken the heat exchange effect.
Recording changes in inlet/outlet temperature difference and air volume is the main means of judging equipment status. A reduced temperature difference or decreased air volume often indicates dust accumulation or scaling, and the fin surface and inner tube wall should be inspected promptly. Regularly purging the fins and arranging inner-tube cleaning according to water quality conditions helps maintain the design heat exchange capacity. During shutdown, the accumulated liquid on the medium side should be drained to prevent freezing and cracking at low temperatures.
After arrival, check the finned tube specifications, number of tube rows, and interface orientation against the drawings, and retain material certificates and pressure test records. On-site installation should ensure uniform air distribution on the fin side to avoid local short-circuiting that affects overall heat exchange efficiency.
