Augmentation of forced convection condensation heat transfer inside a horizontal tube using spiral spring inserts

An experimental investigation has been carried out to study the augmentation of the heat-transfer coefficient during condensation of R-134a vapor inside a horizontal tube with different spiral spring inserts. The test condenser was a double-pipe counter-flow heat exchanger of 1040-mm length; the refrigerant flowed inside the inner tube and the cooling water flowed in the annulus. Four spiral springs of 1.0-mm diameter and different pitches of 5, 8, 10, and 13 mm were inserted, one by one, on the refrigerant side of a test-condenser tube. For each spiral spring insert, the data were acquired for the mass flow rates of 87, 105, 122, and 144 kg/m(2).s. The spiral spring of 10-mm pitch gave the highest enhancement in the heat-transfer coefficient, h, in a range of 65 to 75% in comparison to that for a plain tube. Subsequently, three more spiral spring inserts of 10-mm pitch and 0.5-, 0,7-, and 1.5-mm coil-wire diameter were also tested. The spiral spring with 1.5-mm wire diameter outperformed the other spiral spring inserts and increased the condensing side heat-transfer coefficient, h, in a range of 75 to 80% in comparison to that for a plain tube. In addition, the influence of vapor quality on the heat-transfer coefficient, h, is also investigated. An empirical correlation has been developed to predict the heat-transfer coefficient, h, during condensation inside a horizontal tube in the presence of a spiral spring insert.