Repeat the process described in Problem 8.12 using a simple refrigeration loop to determine the cost of providing 1 GJ of refrigeration at –50°C using the following working fluids:

• Propylene
• Ethane
• Ammonia
• Determine whether any of the working fluids given above cannot be used in a simple (noncascaded) refrigeration loop. For these fluids, would using a cascaded refrigeration system to provide –50°C refrigerant make sense? Explain carefully your answers to these questions.

Problem 8.12

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.