Heat Pump Cycles

Ch 6, Lesson B, Page 8 - Heat Pump Cycles

• The key to working with these traditional tie-fighter or heat engine diagrams is the sign convention.
• ALL work and heat flows are POSITIVE in the direction in which the arrows are drawn ON THE DIAGRAM.
• This sign convention is the traditional choice and most engineers accept it and so will we.
• This choice for the sign convention makes it absolutely critical that you put the arrows in the right directions on your diagram.
• That usually isn’t difficult, but it does require some thought and some self-discipline.
• Once you accept the sign convention, the application of the 1st Law shouldn’t be any problem.
• The measure of performance of any cycle is the ratio of the desired output or effect divided by the required input.
• Heat engines are also known as power cycles and I use the names interchangeably.
• For power cycles, the performance metric is called the efficiency.  We use the greek letter eta, which looks like a squiggly N to represent the efficiency.
• When we use the 1st Law to eliminate work, we get the nice simple equation for efficiency that is shown in the big yellow box.
• This should all be review for you since we covered it in chapter 4.
• But now let’s look at something new.  Let’s look inside the “SYSTEM” circle.