Monday, May 21, 2007

Test #2 - Ch 5 to 8, May 22, 2007

Please post here any questions you might have about Test #2.

4 comments:

Dr. B said...

Etahn said:
In example problem 8D1 in the Thermo-CD notebook, you state that the lost work is the difference between the work the HE puts out and the work required to run the HP. Why isn't the lost work simply defined as the work that the HE puts out?

Dr. B says:
The lost work is the difference between the work output of the HE and the work requirement of the HP. This system must result in exactly the same heat transfer from the hot reservoir at 1000K and to the cold reservoir at 320K as the original heat "leak". This way we are comparing apples to apples. The hot and cold reservoirs cannot tell the difference between the heat leak and the HE/HP system. The only change on the universe is that we end up with a "free" net amount of work = WHE - WHP at zero cost because the net heat transfer from the dead state (surroundings at 298 in this problem) has no value and therefore no cost either.

Anonymous said...

will we need to know each individual part of a power cycle, refigeration cycle, etc???

Dr. B said...

Ethan said:
In example problem number 8C-1 in the Thermo-CD, part D, you use a compressor efficiency n=W(S,iso-T)/W(S,act). I looked through the book, but couldn't find that definition anywhere else. Where did this come from?

Dr. B says:
This efficiency is the isothermal efficiency and it is generally only applied to multiple compressors with intercoolers. It compares the work requirement of multiple compressors with intercoolers to the work requirement of an isothermal compressor (equivalent to an infinite number of compressors with intercoolers). This is not presented in the TCD book, just in the online portion of Thermo-CD.

Dr. B said...

Beast:
I am not sure what you mean. All the cycles are made up of the devices that we learned about in Ch 5. So, I think the answer to your question is YES, you will need to know how to analyze each of these parts.

You will also need to know what pieces of equipment make up each type of cycle. For example, vapor refrigeration cycle is made up of a compressor, a condenser, a throttling or expansion valve, and an evaporator.