A fairly long thin-wall double-pipe heat exchanger (DPHE) with an inner tube diameter of 1.0-cm and outer tube diameter 2.5-cm is used to cool refrigerant-134a with the water temperature at 20C. The refrigerant flows through the inner tube with the surface convectional heat transfer coefficient at 4100 W/m2K. Water flows through the annulus space at a rate of 0.3-kg/s. a) Calculate the overall heat transfer coefficient U for the heat exchanger. b) Repeat the calculation and define the overall heat transfer coefficient U for the case when surface fouling has taken place and a 2-mm thick uniform layer of limestone (k = 1.3 W/mK) has formed on top of the waterside of the heat transfer surface.