In an amino acid fermentation factory, molasses solution must be heated from $10\backslash$deg C to $40\backslash$deg C in a double$-$pipe heat exchanger at a rate of at least $25$ kg min$-1.$ The heat is supplied by water at $70\backslash$deg C; an outlet water temperature of $30\backslash$deg C is desired. The overall heat transfer coefficient is $15$ kW m$-2$ C$-1$ and the exchanger is operated with cocurrent flow. The specific heat capacities of water and molasses solution are $4\mathrm{.}2$ kJ kg$-1$ C$-1$ and $3\mathrm{.}7$ kJ kg$-1\backslash$deg C$-1,$ respectively. a$)$ Calculate the heat transfer area. b$)$ Each stackable unit of a commercially available double$-$pipe assembly contains $0\mathrm{.}02$ m$2$ of heat transfer surface. How many units need to be purchased and connected together for this process? c$)$ If the flow direction is changed to countercurrent flow, calculate the heat transfer area and the number of stackable units that need to be purchased. d$)$ Based on your results for both flow direction, which one is more efficient and why? Explain.