The life of tubes in an evaporator is such that they need to replaced every 8 years if the evaporator is operated under scaling conditions; however, for the scaling operation there is a cleaning cost of $2000/yr. On the other hand, operating under essentially non-scaling conditions should reduce the cost of cleaning to $500/yr and give an estimated saving of $1200/yr because of better heat transfer. The nonscaling operation will shorten the life of the tubes. If tube replacement costs $8000, how long must the tubes last to justify the proposed nonscaling operation? Seven-year straight-line depreciation is to be used when replacing the tubes. Minimum acceptable return is 15% and the income tax rate is 35%.