Air at atmospheric pressure and 27C enters a tube bank from the outside and perpendicular to the tubes. The staggered system is composed of 6 rows of tubes in the direction of the airflow and 12 tubes in each. The diameter of the tubes is 2 cm, while the longitudinal step(SL) is 4 cm and the transverse step(St) is 5 cm. If the external surface temperature of the tubes is 80C and the air inlet velocity is 5 m/s, determine:

a) The air outlet temperature

b) The heat transfer of the tubes per unit length.