birkie

19-05-2017, 05:23 PM

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!

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!