Question

A vapor-compression refrigeration system operating continuously is being considered to provide a minimum of 80 tons of refrigeration for an industrial refrigerator maintaining a space at 28C. The surroundings to which the system rejects energy by heat transfer reach a maximum temperature of 408C. For effective heat transfer, the system requires a temperature difference of at least 208C between the condensing refrigerant and surroundings and between the vaporizing refrigerant and refrigerated space. The project manager wishes to install a system that minimizes the annual cost for electricity (monthly electricity cost is fixed at 5.692 cents for the first 250 kW h and 6.006 cents for any usage above 250 kW h). You are asked to evaluate two alternative designs: a standard vapor-compression refrigeration cycle and a vapor-compression refrigeration cycle that employs a power-recovery turbine in lieu of an expansion valve. For each alternative, consider three refrigerants: ammonia, Refrigerant 22, and Refrigerant 134a. Based on electricity cost, recommend the better choice between the two alternatives and a suitable refrigerant. Other than electricity cost, what additional factors should the manager consider in making a final selection? Prepare a written report including results, conclusions, and recommendations.

Answer 1

The power-recovery turbine design for a vapor-compression refrigeration cycle is the best to be more energy-efficient compared to a standard design, as it recovers some of the energy during expansion.

The report is titled: Evaluation of Vapor-Compression Refrigeration Systems for Industrial Refrigerator

The Objective: To evaluate two alternative vapor-compression refrigeration systems for an industrial refrigerator and recommend the most cost-effective option.

The Analysis that was done:

1. Standard Vapor-Compression Refrigeration Cycle: Ammonia is the most efficient refrigerant based on COP. However, ammonia's toxicity and flammability pose safety concerns.

2. Vapor-Compression Refrigeration Cycle with Power-Recovery Turbine: Ammonia remains the most efficient refrigerant based on net power input and COP.

So, my Recommendation is the Vapor-compression refrigeration cycle with power-recovery turbine and ammonia as the refrigerant is the most cost-effective option based on electricity cost.