In an industrial facility, a countercurrent double pipe heat exchanger is used to heat the glycerin flowing at a rate of 1.5 kg/s from 10°C to 50°C by passing hot water at an inlet temperature of 120°C. The hot water experiences a temperature drop of 50°C as it flows through the heat exchanger. The overall heat transfer coefficient of the heat exchanger may be assumed to be 950 W/m² · K. A similar arrangement is to be installed at another location and it is proposed to use two small heat exchangers of same surface area to be arranged in series instead of one single large heat exchanger. However, it is first required to compare the surface area of the two small heat exchangers against that of the single large heat exchanger, since the available space is a major constraint for the proposed design. The arrangement of the heat exchangers is as shown in Fig. P11–102. The water flow rate is split between the two heat exchangers such that 60% would go to the first heat exchanger and the remaining 40% would go to the second heat exchanger. The overall heat transfer coefficient of the two small heat exchangers is assumed to remain same as that of the large heat exchanger. As a design engineer, for both cases determine the heat exchanger

(a) Effectiveness,

(b) NTU,

(c) Surface area,

(d) Choice of the heat exchanger if the construction cost of the smaller heat exchangers is about 15% higher than a single large heat exchanger per unit surface area.