Oil at 15°c is to be heated by saturated steam at 1 atm in a double-pipe heat exchanger to a temperature of 25°c. the inner and outer diameters of the annular space are 3 cm and 5 cm, respectively, and oil enters with a mean velocity of 0.8 m/s. the inner tube may be assumed to be isothermal at 100°c, and the outer tube is well insulated. assuming fully developed flow for oil, determine the tube length required to heat the oil to the indicated temperature. in reality, will you need a shorter or longer tube? explain.

Determine the time of the day and month of the year at which the peak sensible cooling load occurs for a top floor, north-eastern corner room of an office building in durban for the following conditions: floor area: 8 x 8 x 3 m east and north walls: 115 mm face brick (outside), 20 mm air space, 115 mm ordinary brick (inside) with 15 mm plaster roof construction: suspended plasterboard ceiling, 450 mm air space, 150 mm concrete, 75mm screed, waterproofing no heat transfer across other surfaces window area 3x 1,5 m high in north wall only. ordinary glass with venetian blinds. lights and occupants : from 07: 00 to 18: 00 average light density: 25 w/m2 number of occupants : 5 seated, light office work room temperature 24°c