Thermodynamics

[Prof Sporlan]

The Prof has been reading a thermodynamics book published in 1913, "Temperature-Entropy Diagram", by Charles W. Berry, an MIT Professor of Heat Engineering. Published by John Wiley and Sons of New York and Chapman and Hall Ltd of London. This is his third edition of the book. The first edition came out in 1905.

It is interesting to see how this book presents thermodynamics not unlike any modern book on this subject. Ok, the thermodynamic cycles and relationships were figured out in the 19th century. But this book was published when carbon dioxide, ammonia, and sulphur dioxide were the only refrigerants in use. Thermostatic expansion valves didn't exist. And psychrometrics and air conditioning were just being figured out by Willis Carrier. But this book would not have been a bad thermodynamics text for current engineerings students.

But a couple of topics covered by the author were unknown to the Prof. The author, of course, covered the subject of the Carnot cycle, and showed it to be the most efficient heat engine. But he also showed modifications of the Carnot cycle that have exactly the same efficiency as the Carnot cycle. He refers to them as isodiabatic cycles. Sorry Marc, a Google search won't do you much good with this one. Perhaps it is a concept that has been lost to time. If anyone has an interest in the Carnot cycle, the Prof will also present the isodiabatic cycle.

The book also had a chapter on Kelvin's warming engine. The concept here is instead of using steam to warm a building, use steam to drive a compressor which will allow heat to convert to work and then back to heat by alternate compression and expansion, thereby achieving higher heating efficiencies. This concept seems to have some familiarity to it. But it wasn't until the Prof did a Google search that he realized Kelvin was talking about a heat pump!

[Prof Sporlan]

Basic thermodynamics teaches us that the reversed Carnot cycle is our most efficient refrigerator:

e to b: isentropic compression
b to a: isothermal heat removal (condensation)
a to f: isentropic expansion
f to e: isothermal heat addition (evaporation)

And the coefficient of performance of this cycle is calculated thusly:

COP = T1 / (T2 – T1)

So if we had a 100°F condensing and 20°F evaporating temperature, the Carnot cycle will give us a COP of: (20 + 460) / (100 – 20) = 6.0.

Unfortunately, the reverse Carnot cycle is not practical to construct, primarily due to the complex linkwork necessary for the isentropic expansion, much to the chagrin of the Prof! One day, he shall figure out an inexpensive combination expansion valve and compressor to simulate the Carnot cycle!

So if we use an modern day expansion valve with our Carnot cycle, we end up with a “wet” vapor compression cycle, where:

a to d: isenthalpic expansion

But is it possible to construct (or at least think of) a cycle with equivalent efficiencies to the Carnot cycle? Absolutely! Consider our above a to d isenthalpic expansion

But to maintain Carnot efficiencies, the expansion would need to be capable of being isentropic, but have it extract heat from our compression process,c to b, which also must be capable of being isentropic.

[shogun7]

So prof , to reiterate what you said so I understand The refrigeration cycle you just described; in essence is isentropic compression; isobaric/isothermal heat rejection; isentropic expansion; isobaric/ isothermal heat addition; is this correct? Is this idealized or not?
Roger

[Prof Sporlan]

You have described the classic Carnot refrigerator, which is ideal, and the most efficient refrigerator possible.

The author of this text was pointing out that it is possible to construct alternate cycles that have the same efficiency as Carnot, i.e., the isodiabatic cycles

For this to happen, we must have a Carnot type cycle where heat exchange takes place from the compression process to the expansion process. How one would do this (with a secondary fluid and enormous heat exchange surface perhaps?) would be a bit problematic. But achieving isentropic expansion is problematic in itself. Sometimes the Prof thinks his many types of expansion valves are the limiting factor in achieving Carnot type efficiencies. Fortunately, there are enough irreversibilities in the compressor, condensor, evaporator, and refrigerant lines for him not to take all the responsibility of system inefficiency.

The Carnot cycle has both of these processes adiabatic (no heat exchange) and isentropic.

It is worth noting that the ideal VC (vapor compression) cycle also has adiabatic compression and expansion process. But the expansion process in not isentropic (it is isenthalpic), only the compression process is isentropic. As a result, it can never be as efficient as Carnot.

The Prof found the isodiabatic cycle of interest because it suggests compressor discharge gas desuperheating may possibly be done without loss of system efficiency.

The isodiabatic heat exchanger. How about it Marc? Your friends at Hysave may be able to pick up on this one. The Prof get credit for thinking about it first!

[shogun7]

So professor, tell me how much responsibility are you willing to take with regards to inefficiencies? ( just kidding)
Even if it were possible to get the isodiabatic cycle Practically speaking would it be worth it and would it be a practical refrigeration application?
Roger

[Prof Sporlan]

Idealized thermodynamic cycles are useful in understanding the limits of system efficiency, and providing insight as to how to improve real world systems.

The Carnot and isodiabatic cycles, unfortunately, aren't very practical, so they'll likely remain textbook subjects for engineering and hvac/r students.

Perhaps the best use of understanding idealized thermodynamic cycles is the ability to confront claims of huge efficiency gains by salesmen promoting the various energy saving devices to this industry.

[Mark C]

I, too, relish and collect old textbooks, engineering manuals, and literature on refrigeration. I find the older books fascinating... (Need to know the bearing spec from that 1912 6X6 Frick?) In fact, I have just about every edition of "Ice & Refrigeration Illustrated" Magazine from the start in the late 1880s to its demise around WWII. ...And at least one or more copies!



I find these a source information. In fact, I have used these older texts for some of my lectures in the courses I teach.

I find it interesting to read about some of my clients' original installations. Like one in Fresno that was installed in 1924, and it talks of what was installed and how big it was. The magazines detail new patents, equipment, and such.

It just goes to prove that the basic principles of our field have not changed much! The only "new" things are the electronics and controls, as well as the "new" refrigerants and oils.

[Peter_1]

Originally posted by Mark C
I, ...I have just about every edition of "Ice & Refrigeration Illustrated" Magazine from the start in the late 1880s to its demise around WWII. ...And at least one or more copies!

Can you post some pictures of what they described then?

Peter

[Mark C]

I'll have to dig some out from the cabinet. I'll get some posted in the next day or so.

[Prof Sporlan]

The Nickerson and Collins publishers rings a bell with the Prof. Didn't they help get RSES (Refrigeration Service Engineers Society) started?

[Mark C]

I think so, but I'm not positive on that.