Vertical U-shaped tubes have been used in some waste heat boilers of targe synthetic ammonia plants. In operation

accident of tube failure at the bend due to overheating occurs frequently. The present tests showed that under the action of both centrifugal and gravitational forces

tratification of steam-water flow occurs

leading to heat transfer deterioration on the inner side of the bend. In order to solve the problem

three types of inserted turbulator-a single twisted tape

a double twisted tape and a spiral coil-were tested in a high-pressure electrically heated water-loop experimental installation. Boiling heat transfer and pressure drop characteristics were investigated. The experimental loop consisted of a φ25 × 2 mm stainless steel tube with a 180° bend of 700mm radius

under an operating pressure of 106 × 105Pa. These tests showed that boiling heat transfer deterioration might be eliminated effectively by use of any of the three types of inserted turbulator. The pressure drop of the turbulators were determined

and then the single-phase friction factor relationships were obtained. The Chisholm equation was used to correlate the data of the two-phase pressure drop

and acorrelation for determining the two-phase frictional multiplier was proposed. In order to understand the operating mechanism of the inserted turbulator

flow pattern observations of the single twisted tape were carried out in a transparent tube for air-water flow and an overall flow-pattern map was established in terms of superficial liquid and gas velocities. The single twi-sted tape

which was found to have the lowest frictional pressure drop

is recommended for waste heat boilers.