Determine the cost of producing a refrigerant stream at –50°C using propane as the working fluid in a noncascaded system. You may wish to refer to Example 8.5 to do this problem. The steps you should follow are as follows:

• Determine the pressure at which propane can be condensed at 45°C, which assumes that cooling water with a 5°C temperature approach will be used as the condensing medium.
• Determine the pressure to which the propane must be throttled in order to liquefy it at –50°C.
• Use the results of Parts (a) and (b) to set the approximate pressure levels in the condenser and evaporator in the refrigeration system.
• Determine the amount of propane necessary to extract 1 GJ of heat in the evaporator.
• Assuming a 5 kPa pressure drop in both heat exchangers and that a single-stage compressor is used with an efficiency of 75%, determine the cost of electricity to run the compressor, determine the cooling water cost, and from this determine the cost of providing refrigeration at –50°C using propane as the working fluid.

Example 8.5

Cooling water is priced on an energy basis: $/GJ. The temperature rise is usually assumed to be 10°C. Does the cooling water cost change if the return temperature changes? Are there any limitations to the return temperature? Explain.