Comparative investigation on water fouling characteristics in 5 mm and 7 mm tubes with smooth type and screwed type

Application of copper tubes with the diameter of 5 mm instead of 7 mm in chillers of central air conditioning systems is the development tendency due to the requirement of copper material saving. However, the heat transfer deterioration by water fouling precipitated in 5 mm tubes may be more serious than that in 7 mm tubes. The purpose of this paper is to comparatively investigate the water fouling characteristics on both 5 mm and 7 mm tubes. An accelerated experimental method of water fouling process was designed to quickly evaluate the fouling degree. Four tube types are tested, i.e. a 7 mm smooth tube, a 7 mm screwed tube, a 5 mm smooth tube and a 5 mm screwed tube, and these tubes are fabricated as flexible coiled tubes with total length of 200 cm. The results show that the water fouling process experiences the initiation, growth and stabilization stages in sequence, and the crystal form of fouling is in the vaterite, aragonite and schistosity patterns respectively. The fouling masses in 5 mm smooth tube and 5 mm screwed tube are respectively 42.1% and 16.9% larger than those in 7 mm smooth tube and 7 mm screwed tube, meaning that 5 mm tubes are more prone to form water fouling compared with 7 mm tubes; while the averaged fouling mass in screwed tubes is 2.8% lower than that in smooth tubes since screwed tubes are benefit to destroy the integrality of fouling layer and enhance the removal of deposited particulates.