Calculate the number of effects for the minimum total annual operating expense of a small evaporator system to concentrate a colloidal suspension. Steam costs $3.00/M Btu and each pound of steam will evaporate 0.8N lb of water, N being the number of effects. The capital cost of each effect is $40,000 and has an estimated life of 10 years. The annual maintenance expense is 10% of the capital cost. Labor and other expenses not mentioned may be considered as independent of the number of effects. The system will operate 300 days/yr with 100 M Btu/day evaporator duty. Depreciation may be taken as 7-year straight-line and the federal income tax rate is 35%. Annual net profit after taxes on the investment must be 15% for the installed equipment.