An experimental study was carried out on the boiling heat transfer of R22 flowing inside three internal thread tubes with different geometrical parameters. Three enhanced tubes have the same inner and outer diameters

which are 6.9 mm and 7.92 mm respectively. The outside surface of the tube is smooth

while the inside enhanced structure is different. The variation range of the helix angle is from14° to18°

the thread height is between 0.15 mm and 0.22 mm

and the thread width is from 0.1 mm to 0.2 mm. The relationship between the heat transfer performance and the mass velocity in three enhanced tubes and a smooth tube was tested by changing the mass velocity of refrigerant under given condition of import and export of refrigerant

and their performance is compared. Experimental results indicate that the heat transfer coefficients of three internal thread tubes are 60%—80%

80%—120% and 80% higher than that of smooth tube. Analysis suggests that the larger helix angle is advantageous to the heat transfer

when fluid is on laminar flow or the transition of laminar flow and annular flow

and more of thread amount is advantageous to the heat transfer

when the fluid is on annular flow.