For a cross flow heat exchanger hot air atmosphere pressure

For a cross flow heat exchanger, hot air atmosphere pressure with an average velocity of 3m.s flows across a bank of tubes in an inline arrangement with X_1 = X_t = 5 cm. The tube diameter is 2.5 cm. The array has 20 rows in the direction of flow. The tube wall temperature is 30 degree C and the average air temp in the bundle is assumed to be 300 degree C. Calculate and repeat the calculation if the array has six rows in the flow direction.

Solution

The data is not sufficient, Any way I shall give what best can be done.

Area of air passing over the pipes is taken as (20x .025) x .05 = .025 sq m

The mass of air flowing = Rhox A xV = .0.616 kg/m3 x .025 m2 x 3 m/s = .0462 m3/sec

The heat carried by air = .0462 x .024 W/mK x 270 = 1.197 W/mK

Q = UA Delta T where U is the heat tranfer coefficient , A is the area of pipe (3.14x2.5 x20 = 1.57 m2 ), T is the logrithemic temp. gradient ( this cannot be calculated since data is missing)assumed 270K /20 =13.5

so 1.197 = 0.462 x 1.57 x 13.5 x U hence U= 0.122

similarly for 6 rows the air passing over to be calculated by suitably reducing the cross section and the same way reduce the area exposed for the pipe for 6 rows.