Where do you want to go ?

Introduction to Vapor and Gas Power Cycles

Now that we understand the 1st and

2nd Laws

of Thermodynamics we can consider some important applications and the equipment used to generate mechanical power from heat. In this chapter, we will consider

power cycles

and in the next chapter we will discuss

refrigeration cycles

and

heat pumps

In this chapter, we will study both

vapor

and

gas power cycles

in greater detail and consider how their performance can be improved.

During the cycle the working fluid exists as a vapor for part of the cycle and a liquid for the other part. Key components include a condenser and a boiler.

Vapor Cycles

During the entire cycle the working fluid remains in the gas phase. No phase changes occur.

Gas Cycles

After each pass through the cycle, the working fluid remains within the system.

Closed Cycles

In an open cycle, one heat exchanger is missing. After the working fluid passes through three processes, it is discarded and replaced by fresh working fluid.

Open Cycles

Heat
Engine

Lesson
9A Blog

Confused and have questions? We've got answers. With Chegg Study, you can get step-by-step solutions to your questions from an expert in the field. If you rather get 1:1 study help, try 30 minutes of free online tutoring with Chegg Tutors.

Roll your mouse over this box to close.

Join Learn Thermodynamics Advantage

Download Data Tables
Download Study Aids
Homework problem hints and answers
Get Help from Dr. B in the LT Blog
120 day membership

Click here to Log-In to your LTA account

Get it ALL for $5 US

Lesson 9A: page 1 of 8

Up to this point in the course, each chapter has brought a variety of new concepts and techniques for you to master.
This chapter and the following one break from this trend. In these two chapters, we will study types of thermodynamic cycles in more detail. We will discuss practical aspects of cycle operation and the equipment that is used to carry out the processes that make up the cycles. We will also discuss common techniques used to increase the efficiency of power cycles and the coefficient of performance of refrigeration and heat pump cycles.
I have divided this material into two chapters. In this chapter we will discuss power cycles and in the next chapter we will study refrigeration and heat pump cycles. This lesson provides a concise summary of some of the key aspects of cycle analysis that we have learned in the previous chapters.
We begin by characterizing thermodynamic cycles in two different ways. A thermodynamic cycle is either a gas cycle or a vapor cycle. In a gas cycle, the working fluid always remains in the gas phase. A vapor cycle usually includes a boiler or evaporator as well as a condenser so that the working fluid is a vapor at some points in the cycle and a liquid at others.
Basically, the path of a vapor cycle lies at least partially within the two-phase region on out PV or TS Diagrams, while the path of a gas cycle lies entirely above or to the right of the two-phase envelope.
We can also categorize thermodynamic cycles by the way that they use the working fluid.
The easiest way to see the difference between a closed cycle and an open cycle is to consider one of each.
A household refrigerator is perhaps the most common example of a closed cycle.
The refrigerant, perhaps R-134a, flows through all four processes in the cycle and never leaves the system.
The only reason that refrigerant is ever added to the system is when a significant amount of the refrigerant has leaked.
Compare the closed cycle of a household refrigerator to internal combustion engines, like the ones that power automobiles. Internal combustion engines are open cycles.
The working fluid is air. Fresh air is drawn in, compressed, heated and finally allowed to expand and do some work.
But, instead of using a heat exchanger to cool the air to its original state, the hot exhaust gas is discarded and fresh air is drawn in again at the beginning of the cycle.
The good news about open cycles is that they can be modeled as if they were really closed cycles.
We will NOT consider the details of combustion reactions.
We will simply consider the combustion reaction to be our traditional high temperature reservoir.
So, whether a cycle is open or closed and whether it includes internal combustion reactions or not, we will analyze it as if it were a closed system interacting with hot and cold reservoirs, just as we have in the past few chapters.
OK. If that’s all clear, let’s get started with the review or warm-up part of this chapter.