Flow restrictors for regulating capacity with alternate refrigerants

Hi all, this is my first post.

I'm interested in the topic of trying modern refrigerants in antique GE monitor top refrigerators that originally ran SO2 back in the 1920s and 1930s. These machines use a flooded evaporator, with a high-side float. Field experience has shown that they work quite well with modern refrigerants such as R134a or R152a. For R134a in particular, the performance characteristics (compressor noise, power draw) seem to best match the original SO2 when the suction line is restricted (i.e. crimped, or with a narrower diameter tube inserted).

I used CoolProp (a free alternative to REFPROP) to try to model the performance characteristics of several candidate refrigerants. Here is a thread in another (monitor top specific) forum where I crunched some numbers:
http://monitortop.freeforums.net/post/15636/thread

The gist is that all alternative refrigerants have a higher refrigeration capacity than SO2 at the same volumetric flow, and all have a higher mass flow (which directly correlates with compressor noise, as far as I can tell. Compressor noise is important in monitor tops, since the compressor dome and condenser are out in the open, at ear level).

Shaving the piston would theoretically reduce the compressor volumetric efficiency, and therefore reduce the mass flow of a given refrigerant. Reducing each refrigerant mass flow in the calculations until capacity equals the original results in some of the refrigerants (R152a in particular) being surprisingly close to match to the original SO2.

In the real world, though, shaving a piston is invasive and irreversible. Nobody has done it. Adding a restriction to the suction line is a more practical solution. I am puzzled, however, at how to model the effects of a flow restriction in theoretical computations.

My intuition is that a restriction on the suction line is *not* equivalent to simply reducing the mass flow. I'm imaging a restriction in the suction line as an isenthalpic process that ultimately results in the same enthalpy in the gas entering the compressor, but since it has a different entropy will result in a different enthalpy exiting the compressor, assuming isentropic compression. So instead of altering just one variable (mass flow), it alters two (mass flow, and Δh in the compressor). That would make it considerably harder (computationally) to normalize for capacity, but doable given numerical methods.

So I guess my question is: Am I on the right track in trying to model the effects of flow restriction in the suction line? Would it truly affect both ṁ and Δh, resulting in very different operating points than altering volumetric efficiency to change only ṁ?

Thanks!

Ah, I had it backwards, ans was accidentally trying to solve for compressor power draw rather than capacity.

In normalizing for capacity, you:
* Calculate the mass flow that gives the desired capacity
* Calculate the change in density that a flow restrictor would need to achieve
* Use the refrigerant's equation of state to find the entropy entering the compressor, given known enthalpies (which we know from evaporator conditions; the restrictor doesn't change this since it's isenthalpic) and densities (which we know from our mass flow, caused by the action of the restrictor)
* Use the refrigerant's equation of state to find the enthalpy and discharge temperatures exiting the compressor, given the entropy we calculated above (we assume isentropic compression) and condenser saturation pressure.
* Now that we know Δh for the compressor, we can calculate power draw.

Anyway, the results after doing this make sense. A suction line restriction meters the correct mass flow that we want, at the cost of greater compressor power draw (i.e less efficiency), and higher discharge temperatures.

Interestingly, most refrigerants I looked at would result in a 10-20% increase in efficiency compared to SO2 if the stroke of the piston were adjusted to match the desired mass flow. Using flow restriction to throttle down mass flow, most refrigerants ended up with a 20-30% *decrease* in efficiency compared to SO2. So throttling looks fairly costly.

The standouts when throttling were R227ea and R1234ze(E). R227ea hardly needed any throttling, so ended up about 8% more efficient than SO2. Its mass flow rate was still very high, suggesting that it could be unacceptably noisy in practice. R1234ze(E), one of the new HFO refrigerants, also needed fairly light throttling, and was theoretically only 2-3% less efficient than SO2, with mass flows comparable to R134a, which tends to produce an "acceptable" increase in compressor noise.

...........

Well if it were possible speed regulation would seem the best way of controlling capacity.
From my experience with SO2 this is what was done albeit with open drive compressors, not an option for you I know.
However all this speculation on performance assumes the very old compressor is in good enough shape to make the effort worth while.

Quote:

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Originally Posted by B G Scott

Well if it were possible speed regulation would seem the best way of controlling capacity.
From my experience with SO2 this is what was done albeit with open drive compressors, not an option for you I know.
However all this speculation on performance assumes the very old compressor is in good enough shape to make the effort worth while.

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Right, speed regulation is not really an option. Internally, they have an oil pump and pressurized lubrication. This is one of the reasons why the compressor parts tend to be in excellent condition after many decades of use. They also have an unloader which is operated by oil pressure, so reducing the speed (and reducing the pressure opposing the unloader) might destabilize the unloader circuit.

Interestingly, in the late 1930s they switched a few models to R12. The R12 models were mostly identical, except they used a shorter piston stroke. The units had the same refrigeration capacity. The stroke on the R12 units 20% shorter, and the bore was kept the same. In order to have the same capacity as the unit on SO2, the volumetric flow of R12 would need to be about 44% less. So for 20% less stroke to pump 44% less fluid, volumetric efficiency would also need to drop too. I think they must have done this by increasing the clearance volume as well. "shaving the piston" would only reduce volumetric efficiency, as it wouldn't change the stroke. Still, I think it might theoretically work, and not suffer the losses of flow restriction.

Just as interesting, they shortly went back to SO2, due to the objectionable compressor noise on R12, until a completely new design came out after the war that was engineered from the start for R12.

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What is the designed evaporating temperature?
Restriction inside the suction will result in a pressure
drop so would it not be possible to restrict the liquid
entering the float chamber and and restrict the liquid
Line?

Rob

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Rob very interesting, I would like to know how this would play, if the load on the evaporator increases and the HS float calls for more refrigerant which is restricted there will be a delay in satisfying the demand on the evaporator.
I guess this will result in a lower evaporating temperature until the HS float can restore the balance.
Interesting thoughts on a very old fridge system, what does it say about us oldies?
In any event every system will achieve a balance point some where even if it is totally inefficient for the application.
Radical thought, buy some SO2 (don't know where these days) and recharge the system.
The low operating pressures and slow speeds of the plant is what made it so reliable.
I think I have still got my gas mask some where, in all probability it still smells of SO2
"I love the smell of SO2 in the morning" adulterated stolen quote I know, but who's counting.

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:D

"I love the smell of SO2 in the morning" adulterated stolen quote I know, but who's counting.

And here I am thinking it was the smell of Ammonia in the morning :D .

If you restrict the suction, the pressure drop will lower the
pressure of the refrigerant in the compressor but it will raise
the pressure inside the evaporator. It will act as an Evaporator
Pressure Regulator (EPR).

If you restrict the liquid supplying the evaporator that will reduce
the evaporating temperature.

Mass flow is mass flow, so what passes down one pipe returns back
up the other. It is the volume that changes, so whatever the difference
is between the different refrigerants the balance always has to be
achieved in both the compressor and restriction device.

No easy solution to your problem because if the refrigerant is more
efficient then the compromise will have to be made in either the
compressor or the restriction device. The restriction device might be
the easiest??

You say your compressor is mechanically sound? If there is any wear
or "slack" in it then that might automatically take up the difference you
need.

Have you actually tried it with the new refrigerant?

Rob

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Double post

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Hi all, thanks for the replies!

The temperature control for these things is based on evaporator temperature - i.e. a bellows tube is stuck to the evaporator and operates the main contacts. They operate in a range of 10F (e.g. cut in at evap temp of 10F, cut off at 20F). A dial shifts this range up or down.

This works great with suction restrictors; the net effect is to make the inlet pressure less than the evaporator pressure - which achieves the effect of lower mass flow. After I finally got the calculations right, it proved that the costs of a restrictor were loss of efficiency and higher discharge temperatures.

Regulating the mass flow of the inlet may not work in practice, I don't think anybody has ever tried. These things use a flooded evaporator, different models have something like 2-13lbs of refrigerant pool boiling in their evaps. So by restricting mass flow on the inlet, I'd guess that the evaporation would still continue at the same rate, albeit with the liquid line slowly dropping while running, and filling back up when off?

In practice, the restrictor ends up being easiest; though some of these have bolt-on domes that can be "easily" removed to provide access to the compressor internals. Typically, the only justification to do that is burnt out motor windings, or a stuck compressor.

By the way, here's a link to one that is running on R134a, for a sample of what it can sound like:
https://youtu.be/7nyPBSxAjTU?t=406

Here's another R134a one on a bench:
http://i556.photobucket.com/albums/s...psi6wa9lvj.mp4

For comparison, an original one on SO2:
https://youtu.be/ld8-y6g9jeM?t=18

A slightly different model on R152a
https://youtu.be/F3t0rDcO00A

Anyway, you can see why some people say "nothing sounds like SO2"

interesting, Good process for the refrigerator.Esmo invite this process