Steady State, Open-System Devices

                                                      A Chemical Engineering Lesson
                                                      Developed by Dr. Don Richards
                                                      Rose-Hulman Institute of Technology
 Open System Devices

 

Many different technologically important devices can be modeled as steady-state, open systems.    Some of the major classes of devices are shown below:

Turbines Pumps, compressors, and fans (or blowers) Nozzles Diffusers Throttling devices, Heat exchangers without mixing Heat exchangers with mixing (mixing devices)

To help you understand these devices and learn how to model them, we will study each device to identify its purpose, essential design (or physical) features, and typical operating conditions (or modeling assumptions).  In addition, you will find that there are common schematic diagrams used to represent devices.  Questions that you might ask to identify these things for a specific device are described below:

Purpose ·         What is this device supposed to do?  ·         Why would you need one? ·         What happens to the fluid flowing through this device? Physical or Design Factors ·         What are the unique physical characteristics of this device, e.g. number of inlets and outlets?  ·         What physical features does every one of these devices have? ·         What physical features do most of these devices have? ·         What can you say about the work term in the energy balance for this device, e.g. direction and magnitude? Operating Conditions or Modeling Assumptions   ·         What are the typical operating conditions for these devices, e.g. changes in kinetic energy negligible, constant pressure, adiabatic, one-dimensional flow, etc.? ·         What are the typical modeling assumptions that one would make in constructing a mathematical model to predict the performance of this device?

Note that the physical or design factors along with a device’s purpose are essential features of the device.  These are characteristics that should spring to your mind each this device is mentioned.  Operating conditions or modeling assumptions on the other hand are indicative of how the device usually operates.  For example, the assumption of an adiabatic system is rarely a physical or design factor; however, it is frequently an operating condition or modeling assumption.  One test to help you decide whether some attribute is a design factor or an operating condition is to ask yourself the following question, "Would this device still be a (device name) if this condition was not satisfied?"  If the condition is not essential, then you are probably considering an operating condition.

                      Lesson   |   Instruction to Students |  Open-System Devices   |   Open-System Devices Sheet

©2002, Don Richards                                                                                                                                   
These materials may be duplicated for educational purposes if properly credited.