Fun quest...

[subzero*psia]

Okay... here is an easy one for everybody. Let's say you put a new valve plate in a compressor, you also change the head gasket with material you have in the shop.

The tech is an above average technician... all electrical is correct, system was evacuated, no leaks and the charge weighed in with a clear sight glass. What is the probability that something may have changed and what is it?

Sheesh, I make these WAY too easy.

~

[Dan]

Too easy? We will see about that. First thing that comes to mind is the thickness of the gasket. If it is thicker, you can lose volumetric efficiency.

Dan

[WebRam]

And the second thing that comes to mind is

What is the gasket material ??

[subzero*psia]

Hey you guys are the one's repairing this thing... its your gasket material that you had in your shop not mine! LOL!!

[subzero*psia]

Okay then, Dan is going into what I had been thinking. This isn't a problem... just seeing where you take it. What if this system were borderline when it was installed?

[Dan]

Apparently, I am not much better at easy problems than I am with difficult ones. But as it is with the thickness of otherwise identical valve plates, as well as the size of their discharge ports, what is otherwise an identical compressor, is not equally suitable for, lets say low temperature operation as it might be for medium temperature operation. Volumetric efficiency being one culprit. Further, I believe that the thickness (possibly material, too?) of the gasket could affect the volumetric efficiency by creating a larger volume of discharged gas on the underside of the valve plate.

So, if we have a 10 to 1 compression ratio with a low temperature application - originally design for a 1/32nd inch gasket, but replace it with a 1/16 inch gasket, we have an influence that tends toward doubling the distance the piston must travel before the re-expanding gas achieves low enough pressures to permit the suction valve to open.

It doesn't seem like much, when you stare at the gaskets, but it makes a larger difference to what a given bore and stroke can do than meets the eye.

Why we might have different thicknesses of gaskets for a given valve plate, I just imagined more than remember.

Shucks, Dean, the bag o' gasket had "Returned by Dez - Okay to use" written on the bag! You ordered the damn valve plate less gasket, as I recall.

[Dan]

Originally posted by fridgetech
To predict volumetric effeciency:



But compression is isentropic rather than adiabatic, as the prof explains in the fundamentals section, so residual gas in the clearance pocket doesn't necessarily add to isentropic losses, it mostly helps to drive the piston back and in so doing it helps the other pistons with their compression.

Isentropic efficiency is somewhat proportional to volumetric efficiency, especially where non unloading comps are concerned. It is proportional in the way that we could hold compressor efficiency losses fairly constant while effective displaced volume drops with increased clearance pocket and so the constant efficiency losses just become relatively large in comparison to a changing displaced volume.

I used to drill holes into over sized compressor pistons and/or valve plates to derate them by a calculated amount, obviously did this when they were over sized. Cheaper than EPR's where I came from

If I may restate your wonderful observation (and expect me to restate it wrong): A change in volumetric efficiency, is not so much a change in compressor efficiency, but definitely impacts the capacity of the compressor? In an almost linear fashion with a given compression ratio?

So one has to wonder why the introduction of the Copeland discus compressor was so highly touted as an efficiency gain, only to have them invent an unloader that decreases by many times the volumetric efficiency of the reed compressors it replaced. I have some confusion there. Is discus unloading (moduload) a more efficient way of unloading compared to previous designs, for example.

The differences between isentropic and adiabatic are still too subtle for me. Perhaps there is a ratio to be struck there that helps provide why having smaller re-expansion pockets increases efficiency of the compressor.

But re-expansion helping to push a cylinder down is a wonderful observation, and so simple that I wonder why I never thought of it that way.

[Prof Sporlan]

<i>But compression is isentropic rather than adiabatic</i>

Not that the Prof wants to be particular, but in Marc's case, he'll make an exception...

An adiabatic process is simply a process in which energy does not cross the predefined system boundary. An isentropic process is, by normal definition, an adiabatic process. Perhaps it is possible to conceive of an isentropic process which is not adiabatic, but this would place us well beyond applied thermodynamics...

<i>so residual gas in the clearance pocket doesn't necessarily add to isentropic losses, it mostly helps to drive the piston back and in so doing it helps the other pistons with their compression</i>

A very good point! An isentropic process is also reversible, by definition, which is to say entropy does not change, and that energy expended in the process can be recovered. With an ideal compressor, the work expended to compress the gas can be recovered by simply allowing it to reexpand.

<i>y = Isentropic index Cp/Cv</i>

Isentropic index? In North America, this term is called specific heat ratio....

[Dan]

An adiabatic process is simply a process in which energy does not cross the predefined system boundary. An
isentropic process is, by normal definition, an adiabatic process. Perhaps it is possible to conceive of an
isentropic process which is not adiabatic, but this would place us well beyond applied thermodynamics...
----------

There was a book I read that had the first line: "Call me Ishmael".

Call me "Sheesh." Please... please.... do not use the word "simply" and follow it up with "a process in which energy does not cross the predefined system boundary."

Is it possible that you could break a few rules, and equate these theoretical musings into hot, cold, metal, spinning, shaking and such? Maybe a beginning for discussions regarding scrolls, centrifugals, screws and other things I wonder about, if not others?

Maybe equate with what one sees in a machine room full of piping and noise and thousands of things to look at?

Is there a way to make this important to a person who services this equipment?

Dan

[Prof Sporlan]

<i>Is there a way to make this important to a person who services this equipment?</i>

LOL.... One who studies the various thermodynamic processes is generally more interested in how a particular system or cycle stands up to second law analysis, i.e., how its efficiency compares to the ideal cycle, and not if the system can actually be made to work.

This subject is a bit esoteric, though a well informed refrigeration system designer should have some familiarization with thermodynamic processes. One could solve problems like: what is the highest attainable EER for an R-22 a/c unit operating at a 40°F evaporator and a 100°F condenser? It also comes in handy when confronting claims of energy savings by the many energy saving devices being promoted today.

[Dan]

Welcome back from your vacation, Professor. The phrase you use.. "comparing to second law analysis" refers to what? I think of the Stirling cycle or somesuch? Darn, maybe the Carnot cycle? Where there is no temperature difference and the thing cannot be built to work but makes perfect sense otherwise? I would like to know more about how you compare to second law analysis.

I am intrigued. I am seeing a lot of flap over some new energy saving devices recently, one of which you asked about on some other thread. XDX? Publix is supposedly wanting to take a look at what they have to offer. I have asked the wholesaler for an audience with the promoters to see what they have to offer. I hope it happens.

Dan

[Prof Sporlan]

The Prof remembers as a student listening to his thermodynamics professor describe the three laws of thermodynamics as follows:

Law 1: you can't win
Law 2: you can't even break even
Law 3: you can break even if you can achieve absolute zero. Problem is, you can achieve absolute zero!

The Prof would later learn the laws more formally as:

Law 1: The change in internal energy of a system is the sum of work done on the system and the heat supplied to the system.
Law 2: For any spontaneous thermodynamic process in a isolated system, the change of entropy must be greater than or equal to zero.
Law 3: All elements in their most stable state have the same entropy at absolute zero.

The second law of thermodynamics is perhaps most profound, and it has been used to predict the end of the universe, refute the theory of evolution, and prove the existence of God among other things.

But for the more practically oriented, a process with zero entropy change is an interesting topic, as it is the most efficient process you can achieve. The Carnot cycle is an example of such a process. It is the ideal heat engine by definition. Reverse the Carnot cycle, and you get the ideal refrigerator. Unfortunately, it is not practical to build such an item. The Stirling cycle is another such animal that in its ideal form has the same efficiencies as the Carnot cycle. And one can actually build a Stirling cycle machine, albeit with less than perfect thermodynamic efficiency. Stirling cycle coolers were actually manufactured in the 1950's, though they never competed with mainstream refrigeration systems.

[Edited by Prof Sporlan on 30-05-2001 at 11:35 PM]

[Jack Lester]

[QUOTE]Originally posted by subzero*psia
[B]Okay then, Dan is going into what I had been thinking. This isn't a problem... just seeing where you take it. What if this system were borderline when it was installed?

I agree. I think you alll are getting to technical. Why was the valve plate thought to be the problem in the first place. Must be inefficent.... this didn;t fix it hummmm could be blow by on the piston. Same symptoms. hows the oil level?

[subzero*psia]

Is it time to put this scenario to bed? As I said this was an easy one and Dan nailed it immediately... no surprise there. I just wanted you all to run with it and see how much damage we could come up with... LOL!! You guys really get technical about this! I should have fed you some other info such as stroke, piston diameter, number of pistons, hertz, and original clearance...