Question

In gas refrigeration cycles, can we replace the turbine by an expansion valve as we did in vapor-compression refrigeration cycles? Why?

Short answer

Why or why not? Answer: No, it is not advisable to replace the turbine in a gas refrigeration cycle with an expansion valve because the cooling effect would be diminished and the overall efficiency of the cycle would likely decrease due to the loss of work produced during the expansion process. This is due to the differences between gas refrigeration cycles and vapor-compression refrigeration cycles, where phase change and cooling effect in vapor-compression cycles is achieved by throttling through an expansion valve, while in gas refrigeration cycles, cooling is achieved by expanding the gas through a turbine.

Step-by-step solution

Differences between gas refrigeration and vapor-compression refrigeration cycles

Firstly, it is important to understand the differences between gas refrigeration cycles and vapor-compression refrigeration cycles. Gas refrigeration cycles involve a working fluid that remains a gas throughout the cycle, whereas vapor-compression refrigeration cycles involve a fluid that transitions between vapor and liquid states. In a vapor-compression refrigeration cycle, the working fluid undergoes phase change, which is the primary reason for cooling, whereas in a gas refrigeration cycle, the cooling is achieved by expanding the gas.

Expansion process using a valve vs. a turbine

In a vapor-compression refrigeration cycle, the expansion process (from a high-pressure liquid to a low-pressure mixture of liquid and vapor) takes place in an expansion valve. This is a throttling process, in which the working fluid undergoes a significant drop in temperature with little or no change in enthalpy. In a gas refrigeration cycle, however, the expansion process takes place in a turbine, where the working fluid expands and undergoes a significant reduction in temperature and pressure, while also producing work that can be utilized for various purposes.

Advantages of using a turbine in a gas refrigeration cycle

A turbine used in a gas refrigeration cycle has several advantages over an expansion valve. One such advantage is that it produces work during the expansion process, which can be utilized for other purposes, such as driving the compressor. This can improve the overall efficiency of the refrigeration cycle. In contrast, an expansion valve in a vapor-compression cycle does not produce any work and results in a loss of exergy (useful work potential) in the system.

Feasibility of replacing a turbine with an expansion valve in a gas refrigeration cycle

Replacing a turbine with an expansion valve in a gas refrigeration cycle would result in a throttling process, similar to what occurs in a vapor-compression cycle. However, throttling in a gas refrigeration cycle would not result in the same cooling effect as it does in a vapor-compression cycle because there is no phase change occurring. Additionally, the work produced by the turbine would be lost, and the overall efficiency of the gas refrigeration cycle would likely decrease.

Conclusion

In summary, replacing a turbine with an expansion valve in a gas refrigeration cycle is not advisable because the cooling effect would be diminished, and the overall efficiency of the cycle would likely decrease due to the loss of work produced during the expansion process. This would be counterproductive to the cooling effect desired in a refrigeration cycle, making it an unsuitable option for gas refrigeration cycles.