evaporators and glide refrigerants

[PaulL]

Hi there
Hope you are fine and staying safe
Please, I am contacting you for following topic.
I was starting dealing with non azeotropic fluid, as r449a
I have following concern:
I need to select a cooler for a cold room
Many producers assert to utilize the same td for these refrigerants considering the average glide temperature as refernce TD.
Ex, if with R404a we would have 7K differnece, I am suggested to consider the same td but considering R449a average glide.
My concern is as follows.
If we consider the average glide, at the beginning of the evaporation the td between air and refrigerant will be bigger.
Ex with 449 A, @4.13 bar temperature on gas side is 0°C, while on liquid side is -5.7°C meaning a glide of 5.7K
Average glide will add ex, something around 5.7/2, meaning
At the end at beginning of evaporator we would have not, ex 7K, but almost 9.9K (meaning 7+5.7/2)
Can this difference affect the range of application of these refrigerants, since because of this higher temperature difference we may be in risk of drying unpacked goods in the cold room ?
Would it be enough to me to consider the capacity value or shall I consider also other parameters as the maximum temperature difference as well ?
I have read about fluids with even bigger glide, so that this effect will be augmented.

Please, has anybody any experience or any idea concerning my doubt ?

Thanks, happy Easter and stay safe !

[kiwifridgie]

Have been replacing R404a coolroom and freezer equipment with R449/448 for over a year now.
New txv (r22 /r407c) set superheat based on "midrange" of temp glide. No apparent difference in room humidity levels, no apparent issues with discharge temps - eg disch temp is still ok after retrofit.
System capacity is theoretically slightly better, but I base all sizing on the R404 numbers (suppliers are yet to fully update books).
Live a little, try it.

[kiwifridgie]

Also, I am using the Testo 557 for superheat checking.

[Glide]

Hi PaulL, I am so sorry I have missed this topic

I need to select a cooler for a cold room
Many producers assert to utilize the same td for these refrigerants considering the average glide temperature as refernce TD.
Ex, if with R404a we would have 7K differnece, I am suggested to consider the same td but considering R449a average glide.

Can you share a datasheets from those manufacturers?

If we consider the average glide, at the beginning of the evaporation the td between air and refrigerant will be bigger.
Ex with 449 A, @4.13 bar temperature on gas side is 0°C, while on liquid side is -5.7°C meaning a glide of 5.7K
Average glide will add ex, something around 5.7/2, meaning
At the end at beginning of evaporator we would have not, ex 7K, but almost 9.9K (meaning 7+5.7/2)
Can this difference affect the range of application of these refrigerants, since because of this higher temperature difference we may be in risk of drying unpacked goods in the cold room ?

I wouldn't be worried about drying if you have selected evaporator based on mid (mean) temperature difference.
If you have selected evaporator based on 7 K mean (mid) temp difference, than the difference would be bigger at the beginning (7K + half glide) but it would be lower at the end (7 K - half glide).

[Glide]

Have been replacing R404a coolroom and freezer equipment with R449/448 for over a year now.
New txv (r22 /r407c) set superheat based on "midrange" of temp glide. No apparent difference in room humidity levels, no apparent issues with discharge temps - eg disch temp is still ok after retrofit.
System capacity is theoretically slightly better, but I base all sizing on the R404 numbers (suppliers are yet to fully update books).
Live a little, try it.

Hi kiwifridgie, can you explain this bolded part a bit further?
Are you using TXV's when setting the superheat based on midrange of temp. glide?

[kiwifridgie]

https://www.hvacrschool.com/mid-poin...igerant-blend/
This explains it better than I can type ( 2 finger tapping rather than typing).
I normally just use my Testo 557 manifold for setting superheat. It's just easier as it calculates it for me.
But when one has to do the math, while sitting in the office, on the phone to the apprentice - midpoint is best.

[Glide]

Let's say you have a cold room with a 0 °C temperature and you have a evaporator selected with TD=10 K (mid).
That means that your mean evaporating temp. would be -10 °C and your dew point temp. would be -8 °C.
That means that your max. superheat could be 8 degrees, right?

[RANGER1]

Let's say you have a cold room with a 0 °C temperature and you have a evaporator selected with TD=10 K (mid).
That means that your mean evaporating temp. would be -10 °C and your dew point temp. would be -8 °C.
That means that your max. superheat could be 8 degrees, right?

I reckon forget dew point & just use air on evaporator to calculate maximum superheat at evaporator outlet.
So maximum 10° C superheat.

[Glide]

I am not sure this is correct.
The air entering evaporator comes into contact with evaporator pipes which carry completely evaporated (superheated) refrigerant, right?
Due to that, we should consider dew point when calculating SH?

[RANGER1]

I am not sure this is correct.
The air entering evaporator comes into contact with evaporator pipes which carry completely evaporated (superheated) refrigerant, right?
Due to that, we should consider dew point when calculating SH?

My interpretation would not take dew point into effect as the surface that the air is in contact with is not colder than the air.
That’s when superheat is taking place.
sensible heat or air on temperature.
Maybe I’m missing something?

[RANGER1]

Deleted, as off topic

[Glide]

Hopefilly we can continue this interesting discussion.

I'll post a link below:

https://www.achrnews.com/articles/13...perature-glide

Since superheat is defined as any sensible heat gained after the saturated vapor point in the evaporator...
When calculating superheat values, HVAC service technicians must always use the dew point values from the chart.

Here's another one:

https://www.fluorineproducts-honeywe...rochure-LR.pdf

In order to set the superheat, find the Dew Point temperature corresponding to the coil pressure. To get superheat compare the dew temperature from the chart to the actual temperature of the evaporator outlet piping. The difference in these two temperatures is
the superheat.

This makes a perfect sense to me. Very good explanations.

But what I don't understand is the following:

If we take a look at the following example:
- room temp. = 0 °C
- evaporating temp. = -8 °C
- dew temp. around = -6 °C

That means that the max superheat is 6 K, right? And that is not possible to accomplish with mechanical TXV's.