HS Diagram for Multi-Stage Compression

Ch 8, Lesson C, Page 16 - HS Diagram for Multi-Stage Compression

• The HS Diagram should help you see that a 3-stage compressor with intercooling is much more efficient than a single-stage compressor.
• The effluent from a single-stage compressor has a high enthalpy. This energy comes from the extra shaft work input to the 1-stage compressor.
• A 3-stage compressor with 2 intercoolers is also more efficient than the 2-stage system because the enthalpy of the fluid leaving a 2-stage compressor is greater than the effluent from a 3-stage compressor.
• But adding stages yields diminishing returns. Adding the 2nd stage boosted the efficiency by more than the 3rd stage did.
• Adding another stage would move the outlet stream for the multi-stage compressor system closer to state 2T and improve the efficiency, but not by all that much.
• The isothermal compression is equivalent to a compression system with an infinite number of stages.
• An infinite number of stages would cost an infinite amount of money, so I am pretty sure that is not the most economical system to build.
• So, how many stages should we use in a compression system ?
• That depends on many, many factors, but the outcome is almost always either a 1-stage, 2-stage, or 3-stage compression system.
• This decision must be based on economic and possibly other factors. But this is beyond the scope of this course.
• There is one more interesting point about the design of multistage compression systems that is definitely in the domain of thermodynamics.
• In a multistage compression system, how do we decide what intermediate pressures to use ?
• In the HS Diagram shown here, how do we choose PA and PB?
• Does it make a difference? Yes!
• Is there a “best” or optimal choice? Yes!
• It gets too messy for a 3-stage compressor, but the solution isn’t all that bad for a 2-stage compressor.
• Flip the page and see for yourself!