Exhaust gases flowing through a double-pipe heat exchanger at a rate of 0.3 kg/s to be cooled from 400°c to 120°c by water in the annulus initially at 10°c. the inner pipe has a diameter of 33 cm and negligible thickness. the specific heat capacities of the gases and water may be taken as 1130 and 4190 j/kgk respectively, and the gas and water heat transfer coefficients are 150 and 665 w/m2k. find the heat exchanger length required when the cooling water flow is 0.4 kg/s, (a) for parallel flow and (b) for counter flow.