Potter & Somerton Chapter 4: The First Law of Thermodynamics

b) Control Volumes - Steam Power Plants and Refrigeration

This is a supplement to Section 4.8 (General Formulation for Control Volumes) and 4.9 (Applications of the Energy Equation). We will mainly consider steam power plants, vapor-compression refrigerators and heat pumps. We will also develop the use of the P-h diagram for both the steam power plant and refrigeration systems.

Recommended Supplementary Problems for this section - do all of the following 12 problems (All answers to the Supplementary Problems are given at the end of the chapter)

• Throttle (water, steam) 4.89, 4.90 (plot both problems on a P-h diagram)
• Compressor (air) 4.92, 4.93 (you will need to review The Adiabatic Process before attempting both problems. (Problem 4.93 is stated in a confusing manner. Delete the word "minimum" and add the condition that the velocity at the inlet is negligible. You will need to consider kinetic energy change to obtain the answer given)
• Compressor (steam) 4.94 (do a P-h diagram plot of this problem)
• Pump (water) 4.96
• Turbine (hydroelectric) 4.97, 4.98; (air) 4.101, 4.102
• Heat Exchanger (water/air) 4.107
• Steam Power Plant 4.108 (you MUST do a P-h diagram plot for this problem. This is a typical final exam problem, and the P-h diagram will typically be worth 25% of the grade - refer to the example of a P-h diagram drawn for Solved Problem 4.29)
• Gas Turbine 4.110 (again, review The Adiabatic Process before solving this problem)

We were surprised that in Chapter 4 of Potter & Somerton there are no examples or problems on refrigeration, air-conditioning or heat pump systems, nor any components using R134a refrigerant. Thus we have provided additional composite problems to fill in this gap. Be sure to do all four of the problems, any of which could typically be used in a quiz or final exam. Before doing these you should review the example solution of Refrigerators and Heat Pumps.

• Additional Problems 4.1 & 4.2 - Problem 4.1 (home refrigerator) and Problem 4.2 (home refrigerator with an internal heat exchanger)
• Additional Problem 4.3 (home heat pump system for space heating)
• Additional Problem 4.4 (home air conditioner and hot water heating system)

When dealing with closed systems we found that sketching T-v or P-v diagrams was a significant aid in describing and understanding the various processes. In steady flow systems we find that the Pressure-Enthalpy (P-h) diagrams serve a similar purpose, and we will use them extensively. Unfortunately our text book does not provide P-h diagrams for steam or for refrigerant R134a, thus we have provided them here. Print them out and use them as required, and we will illustrate their use in the following examples. Study the P-h diagrams carefully and try to understand the significance of the distinctive shapes of the constant temperature curves in the compressed liquid, saturated mixture (quality region) and superheated vapor regions.

More on Steam Power Plants

Unlike the situation with steam power plants it is common practice to design and analyze refrigeration and heat pump systems in terms of the P-h diagram. In fact, in the official Reference Handbook supplied by the NCEES to be used in the Fundamentals of Engineering exam, only the P-h diagram is presented for R134a and during the exam you are expected to answer all questions on refrigeration with the aid of this P-h diagram. We have provided one here, and once again you should print it out and copy it as required. We will use it extensively.

More on Refrigerators and Heat Pumps

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