6-57. A single-stage centrifugal blower is to be selected for an engineering design application. Suppliers have been consulted, and the choice has been narrowed down to two new models, both made by the same company and both having the same rated capacity and pressure. Both are driven at 3,600 rpm by identical 90-hp electric motors (output). One blower has a guaranteed efficiency of 72% at full load and is offered for $42,000. The other is more expensive because of aerodynamic refinement, which gives it a guaranteed efficiency of 81% at full load. Except for these differences in efficiency and installed price, the units are equally desirable in other operating characteristics such as durability, maintenance, ease of operation, and quietness. In both cases, plots of efficiency versus amount of air handled are flat in the vicinity of full rated load. The application is such that, whenever the blower is running, it will be at full load. Assume that both blowers have negligible market values at the end of the useful life, and the firm’s MARR is 20% per year. Develop a formula for calculating how much the user could afford to pay for the more efficient unit. (Hint: You need to specify important parameters and use them in your formula, and remember 1 hp = 0.746 kW.) (6.4)