Chemical Process Engineering & Simulation

Problem Statement & Given Data: Warm water at 45°C is to be cooled to 30°C by countercurrent contact with air in a tower packed with wood slats . The inlet air has a dry-bulb temperature of 31°C and a wet-bulb temperature of 22°C. The mass flow rate of water is 6000 kg/m 2 .h and that of air is 1.4 times the minimum. The individual gas-phase mass transfer coefficient is k Y’ a = 6000 kg/m 3 .h.∆Y’. The volumetric water-side heat transfer coefficient is given by h L a = 0.059 x L 0.51 x G S , in kcal/m 3 .h.K, where L and G S are mass low rates of water and air (dry basis). Determine (a) the dry air flow rate to be used, (b) the height of packing. Solution: (a)                                                Inlet air: T G = 31°C; T W = 22°C = T as                            ...

Interference and Recirculation: A number of cooling towers may have to be used if the hot water load is large. The tower are very often installed at one place in the plant for convenience. Interference and recirculation are two common problems with cooling towers placed in proximity. Some of the moist air leaving a tower may be sucked into another tower installed “downwind” in the vicinity of the former [Figure 1(a)] affecting the performance. This is called “ interference ” . Sometimes a part of the moist air from a tower may be sucked into itself by the “forced-draft fan”. This is “recirculation” [Figure 1(b)]. The ratio of the “plume velocity” to the “wind velocity” is an important factor to determine the extent of recirculation. If this ratio is small, a forced-draft tower becomes more susceptible to recirculation [Figure 1(c)]. Both the phenomena of interference and recirculation reduce the enthalpy driving for cooling and adversely affect the performance of cooling towe...

            A cooling tower is a special type of heat exchanger in which the warm water and the air are brought in direct contact for evaporative cooling . It must provide a very good contact of air and water in terms of the contact area and mass transfer coefficient of water vapor while keeping the air pressure drop low.  Figure 1 A single deck spray pond cooling system.            In the early years of industrial development, cooling of warm water for reuse was done in “ spray ponds ”. In a spray pond, as the name implies, a spray system located about six to eight feet above the water surface creates small droplets of warm water that cool down by evaporation in contact with air. This is simple and easy process but required a large pond area. It is inefficient (the effective heat transfer coefficient is about 3.5 Btu/h.ft 2 .°F or 20 W/m 2 .K), and creates the problem of entrainment and carryover...