Is this a contradiction?
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GB151-1999 No. 1 5.6.3.2 The problem of the center distance of heat exchange tubes: The commonly used center distance tables of heat exchange tubes indicate that the outer diameters of heat exchange tubes are 10mm, 12mm and 14mm, and the center distances of heat exchange tubes are 13mm~14mm, 16mm and 19mm. And below the table it says: a) When mechanical cleaning is required between heat exchanger tubes, they should be arranged in a square shape. The clearance distance (Sd) between two adjacent tubes should not be less than 6mm. For heat exchanger tubes with outer diameters of 10, 12 and 14mm, the center distance should not be less than 17, 19 and 21mm respectively. What's going on?
There is no contradiction. (1) The former requires the minimum distance and is the commonly used center distance. (2) From the perspective of practicality and maintenance, the required center distance of the latter is generally larger than the conventional center distance.
There is no contradiction. The center distance is generally considered as 1.25 times the outer diameter of the pipe. ； When there is a cleaning requirement, the clearance distance between the two pipes (the outer diameter of the two adjacent pipes) should not be less than 6 mm.
There is no contradiction. 151 stipulates that the center distance of heat exchange tubes should be no less than 1.25 times the outer diameter of the heat exchange tube. In actual design, the center distance is generally considered as 1.25 times the outer diameter of the tube. However, the latter one is a requirement when the heat exchanger needs to be cleaned. It requires that the clearance distance (Sd) between two adjacent tubes should not be less than 6mm.