A 250 KVA, 6600/400V, 3 phase core type transformer has a total loss of 4800W at full load. The transformer tank is 1.25 m in height and 1mx0.5m in plan. Design suitable scheme for tubes if the average temperature rise is to be limited to $35^0C$. The diameter of tube is 50mm and are spaced 75 mm from each other. The average height of tube is 1.05m.

Specific heat dissipation due to radiation and convection is respectively 6 and $6.5 W/m^{2}°C$. Assume that convection is improved by 35% due to provision of tubes.