An effective method to recover water used for regeneration of ion exchange resins is to use a reverse osmosis system in a batch-treatment mode.

Such a system involves recirculation of the partially treated water back into the feed tank, causing the water to heat up. The water could be cooled using a single-pass heat exchanger or a closed-loop heat exchange system. The single-pass system would require a small chiller costing $920, plus stainless steel tubing, connectors, valves, etc., costing $360, and it could be used for 3 years.

The cost of water, sewer charges, electricity, etc., will be $3.10 per hour. The closed-loop system will cost $3850 to buy, have a useful life of 5 years, and will cost $1.28 per hour of operation.

The interest rate is 10% per year and the salvage values are negligible.

(a) What is the breakeven number of hours per year that the cooling system must be needed to justify purchase of the closedloop system?

(b) What is the difference in annual costs if 400 hours per year is the expected usage level?