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- To: aquatic-plants at actwin_com
- Subject: Entropy
- From: Paul Sears <psears at nrn1_NRCan.gc.ca>
- Date: Wed, 11 Jun 2003 09:24:06 -0400 (EDT)
I never thought I would use that title for a posting here!
Scott asked about the entropy involved in a chilling process.
I'm going to avoid _all_ the nineteenth century units (F, BTU,
hp, BTU/h, SEER). I'll stick to watts (joules/second), joules,
degrees C and K. Anyone who wants can do conversions!
A chiller might realistically remove 3 kW of heat from a coil at
5C, and dump heat to another coil at 40C, and use 1 kW of electrical
power in the process (COP 3). (I'll ignore motor losses).
3 kW heat is absorbed at 5C (278K), so the entropy lost here is:
3000/278 = 10.79 (joule/K) (entropy unit) per second
4 kW heat is emitted at 40C (313K) so the entropy gained here is:
4000/313 = 12.78 (joule/K) (entropy unit) per second
The electrical energy is pure work and has no entropy, so the
net increase in entropy is 1.99 joule/K per second. This is O.K.
by both the first and second laws of thermodynamics (energy balance,
and _increase_ in entropy).
One can see that the temperatures involved place a limit on the COP
possible. One can always get a _lower_ one!
Peltier effect coolers are neat, but have very low COP's
psears at nrn1_nrcan.gc.ca, 613-996-4171, facsimile / télécopieur 613-996-9400
Natural Resources Canada, 1, Haanel Drive, Nepean, Ontario K1A 1M1
Ressources naturelles Canada, 1, Haanel Drive, Nepean, Ontario K1A 1M1
Government of Canada / Gouvernement du Canada