Refrigeration cycle efficiency breakdown?

[R600a]

I am trying to fully understand how different variables affect refrigeration cycle efficiency.

[R600a]

So i understand basically how using bigger evaporator and condensor coils can help efficiency because they can bring down the compression ratio. But what other variables are there either theoretical or real world?

[R600a]

Well i have had some time to think about this some more.
I was thinking about varying the evaporator size and coil temperature.
So suppose i have a 3 ton 14 seer ac that has a matching evaporator coil with txv running 400 cfm per ton with a suction saturation temperature of 40* and 10* superheat.
Now suppose i changed that evaporator out for a 5 ton (to take this thought experiment to the extreme) but i use the same 3 ton txv and adjust the air flow and charge to get a 40* suction saturation temperature with 10* superheat.

What would change?
Would my eer change and if so how?
Would my latent sensible ratio change and if so how?
How would it affect my head pressure and subcooling?

So now what if i changed the evaporator out for a 2 ton again with 40* suction saturation temp and 10* superheat what affect would that have on the aforementioned parameters?

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[coolcoil]

I will take a stab that this.

First, I am going to assume that when you say that you will keep the 3-ton TXV that means that I will have the same refrigerant mass flow. If I adjust airflow to get the same evap temperature and superheat, the refrigerant will absorb the same amount of heat with the 5-ton evaporator as it did with the 3-ton. Since the split between evap temp and condensing temp are the same and the mass flow of the refrigerant is the same, the refrigeration side of the system will not have any change in efficiency.

But, since I did have to reduce airflow to maintain the evap temperature and same superheat my fan energy will be reduced, so there will be system energy savings. Note that since I am taking the same amount of heat from the reduced airflow my discharge temperature will go down, so lower cfm/ton.

[R600a]

That is what i would expect also.
How do you think it would affect latent capacity?
I think it would increase latent capacity vs a smaller evaporator.

I will take a stab that this.

First, I am going to assume that when you say that you will keep the 3-ton TXV that means that I will have the same refrigerant mass flow. If I adjust airflow to get the same evap temperature and superheat, the refrigerant will absorb the same amount of heat with the 5-ton evaporator as it did with the 3-ton. Since the split between evap temp and condensing temp are the same and the mass flow of the refrigerant is the same, the refrigeration side of the system will not have any change in efficiency.

But, since I did have to reduce airflow to maintain the evap temperature and same superheat my fan energy will be reduced, so there will be system energy savings. Note that since I am taking the same amount of heat from the reduced airflow my discharge temperature will go down, so lower cfm/ton.

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[coolcoil]

Agreed. Lower discharge temp causes lower SHR. That’s assuming, of course, that your initial conditions had an SHR <1.0. If that was not the case, then you would have to know the entering air conditions and flows to determine if latent capacity changed.

[R600a]

Now what if we take that same base 3 ton system and recover some of that cooling that is lost down the drain by using the cold condensate to cool the liquid line below ambient?
How what affect would that have?


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[R600a]

Would the cooler refrigerant have more cooling power thus causing the txv to close more increasing head pressure and load on the compressor?
Would increasing airflow fix that problem resulting in no increase in load while increasing the btu output?


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[coolcoil]

That would have some positive effect on both capacity and efficiency. Assuming you were getting some flashing at the TXV you would now get less flashing so more liquid to evaporate in the condenser. The improvement would depend entirely on how well subcooled the liquid already is at the condenser.

I am going to guess that it is not all that much as manufacturers are continually investigating methods to improve subcooling, and I am sure that somebody in the industry has tried this and found it not worthwhile sometime in the last 70 years*. When I worked for a manufacturer we often looked at ways to use condensate, and the only viable use that I've ever heard of is collecting the water for other uses - feeding back to the cooling tower or watering the lawn - and that is only done for very large systems where there is enough water generated to cover the cost of piping somewhere.

*For what it's worth, I am always coming up with ingenious new HVAC ideas only to discover, (1) it is actually a dumb idea, or (2) the idea was patented 40 years ago.

[coolcoil]

Would the cooler refrigerant have more cooling power thus causing the txv to close more increasing head pressure and load on the compressor?
Would increasing airflow fix that problem resulting in no increase in load while increasing the btu output?

I would have to think about it, but off the top of my head I don't think the TXV would see a difference. The same net total amount of heat is being transferred into the refrigerant. The only difference is that with better subcooling it happens in the airstream rather than before getting into the coil.

[R600a]

Also maby you could set the charge for lower subcooling at the condensor and run lower head pressure.

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[R600a]

I was supposing that both systems had sufficient subcooling. Lets just go with 10* before the condensate heat exchanger.

I would have to think about it, but off the top of my head I don't think the TXV would see a difference. The same net total amount of heat is being transferred into the refrigerant. The only difference is that with better subcooling it happens in the airstream rather than before getting into the coil.

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[R600a]

Well this is not a new idea. My boss did it before i was born but he did not try to quantify the results.
Also i found a patent for this very thing on ptacs and window units along with using it to cool the discharge line before the condensor.

That would have some positive effect on both capacity and efficiency. Assuming you were getting some flashing at the TXV you would now get less flashing so more liquid to evaporate in the condenser. The improvement would depend entirely on how well subcooled the liquid already is at the condenser.

I am going to guess that it is not all that much as manufacturers are continually investigating methods to improve subcooling, and I am sure that somebody in the industry has tried this and found it not worthwhile sometime in the last 70 years*. When I worked for a manufacturer we often looked at ways to use condensate, and the only viable use that I've ever heard of is collecting the water for other uses - feeding back to the cooling tower or watering the lawn - and that is only done for very large systems where there is enough water generated to cover the cost of piping somewhere.

*For what it's worth, I am always coming up with ingenious new HVAC ideas only to discover, (1) it is actually a dumb idea, or (2) the idea was patented 40 years ago.

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[BBeerme]

I doubt you'd get any noticeable gain in efficiency by using the condensate to cool the liquid refrigerant. But, if we use an extreme example, and you were to cool the liquid too much, then your metering device would be over sized.

[R600a]

Why is that?

I doubt you'd get any noticeable gain in efficiency by using the condensate to cool the liquid refrigerant. But, if we use an extreme example, and you were to cool the liquid too much, then your metering device would be over sized.

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[R600a]

Lets just say that the heat exchanger is a miracle of engineering and cools the 100* liquid to 50 or lower. How would that affect the metering device?

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[R600a]

I am assuming that if the liquid is 100* or 50* the volume probably stays fairly consistent. And i am also going with a pressure of 110* for whatever refrigerant this theoretical system may be. So the 50* liquid has 60* subcooling. It can absorb enough heat to raise its temp 60*.

I know this probably splitting hairs since the amount of heat required to raise the temp of liquid is small compared to the amount it takes to boil it. But it gets the gears moving in my brain and sometimes i learn something totally new or finaly understand something i thought i knew

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[BBeerme]

At the outlet of the metering device is flash gas. That liquid refrigerant flashing [boiling] does nothing to cool the environment, all it does is cool the liquid to the saturation pressure.

If, as in my extreme example, the liquid is already at the saturation temperature, then you'll have too much liquid in the evap. And you would need a smaller metering device to throttle more of that liquid entering the evap.

Why is that?

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[R600a]

Ok i think i am getting your point now.
I addressed that in the original design by using a txv. That is why i was wondering if it would do the same amount of cooling with less mass flow and cause higher head pressure.
What do you think?

At the outlet of the metering device is flash gas. That liquid refrigerant flashing [boiling] does nothing to cool the environment, all it does is cool the liquid to the saturation pressure.

If, as in my extreme example, the liquid is already at the saturation temperature, then you'll have too much liquid in the evap. And you would need a smaller metering device to throttle more of that liquid entering the evap.

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[R600a]

I sort of think i answered my own question. I think that the amount of heat capacity gained by an extra 50* subcooling would be just a drop in the bucket compared to the latent heat of evaporation that the refrigerant already has since we are keeping the pressures constant i dont think the amount of work the compressor does would change and we would just have created a system that was far more sensitive to what we pour down the condensate line and the only benefit would be if we were low on charge we could maintain enough subcooling to not loose performance.
Am i right?

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