Steady-State Energy Balances

Ch 5, Lesson C, Page 2 - Steady-State Energy Balances

• We start with the definition of steady-state.  No process variable or property changes with time.
• The energy of the system is a property of the system and if it does not change with time, then the time derivative of Esys is zero !
• You’ve just gotta love it when any term in a big equation like the 1st Law is zero.
• Are there any other characteristics of systems undergoing processes at steady-state ?
• Yes, indeed.  Consider the diagram shown here.
• The little balls represent chunks of fluid flowing through a system.
• Steady-state does not mean that properties do not vary with position within a system.
• The properties of the fluid at position 1 are NOT necessarily the same as the properties of the fluid at position 4.
• Steady does mean that even though the balls move through the system, the properties of whatever ball occupies position 4 are always the same.
• For example, if this process were a water heater, the water would be hotter at position 4 than it would be at position 1, right ?
• But at steady state, the temperature of the water when it reaches position 4 does not change no matter how long the system operates.
• OK, now what about the feed and effluent streams?
• Their flow rates and properties must also remain constant with respect to time.
• Cool, now what about heat and work ?
• Well, heat and work are process variables, so at steady state they must also remain constant with respect to time.
• It seems like processes operating at steady-state are going to be a lot easier to analyze.
• The best news is that for most processes, the GOAL is to reach steady-state and stay there as long as possible.
• So, steady-state analyses are very common and very useful and generally easier than transient analyses.
• Transient problems are problems in which one or more property or process variable changes with time.
• That is the topic of Lessons D & E.
• But for now, let’s just stay focused on steady-state analysis.
• Next, we need to look a little closer at Wsys.
• Flip the page and let’s get started.