# Thread: Which system is correctly charged?

1. Originally Posted by icemeister
It's much simpler to understand if you think of it as "any subcooling that is done outside of the system represents an increase in capacity of the system"

Then what defines that capacity increase? You can easily define it by looking up the heat content (Enthalpy) values in a thermodynamic properties table for that refrigerant. The increase becomes the enthalpy difference, or the enthalpy before minus the enthalpy after. Enthalpy is usually signified by the small letter "h".

Here's the thermodynamic properties for R404A: https://www.chemours.com/Refrigerant...o_prop_eng.pdf

Lets say we have a system running at -20°F SST with 100°F liquid leaving the condenser coil which then enters our subcooler heat exchanger where it's cooler to 50°F. We can describe the difference as Δh(liq) or hf(100) - hg(50), where the "f" represent the fluid and the "g" for the vapor.

Then we need to know what the original capacity was to determine the % increase. This is defined basically as the Net Refrigeration Effect (NRE) or the enthalpy of the vapor leaving the evaporator minus the enthalpy of the liquid entering the evaporator, or hg(-20) - hf(100).

The actual values for this example using R404A then would be:

Δh(liq) - NRE = hf100 - hf(50) / hg(-20) - Hf(100) = (49.1 - 29.7) / (88.4 - 49.1) = 19.4 / 39.3 = .494 ...or 49.4% Capacity Increase

This can be shown graphically using a PH diagram like this one: https://icemeister.net/backroom/wp-c...e-Analysis.pdf

If you look at the point "H" on the diagram, that represent the state of the refrigerant liquid leaving the condenser. Point "G" to the right of it is the refrigerant liquid in its saturated state, so the distance from G to H is the subcooling. Move H to the left would indicate more subcooling.

Since the enthalpy at H is always the same as the enthalpy at A (as represented by the vertical line connecting them), this is where it can be seen how if heat is removed from the liquid, an equal amount of heat is removed from the evaporator load.

NOTE: A to K on the diagram is the net refrigerating effect (NRE) which includes the evaporator superheat, but for simplicity, in this discussion we're calling the evaporator superheat zero and used A to B as the NRE.
thank you for the response ....

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Originally Posted by hvacskills
suppose it depends on what time of year I am working on the system ...... in the cooler weather I see it as the higher pressure forcing the vapor to condense, in the warmer weather I see it more as the condensing of vapor due to heat removal ....

so in regards to a fan cycling control that is how I see it too .... higher pressure forcing the vapor to condense ...
This is incorrect. Both in cool weather and warm weather condensation occurs because of heat removal. Beer explained the bubbles from fan cycling simply and correctly, I suggest you go back and read it.

3. Originally Posted by Poodle Head Mikey

Isn't it:

1. solid glass with the condenser fan cycled off?

2. bubbles appearing after the condenser fan re-starts?

Which suggests to me that it is the higher head pressure forcing more vapor into a liquid form than it is the condensing of vapor to liquid due to heat removal.

PHM
---------
It's the higher pressure that creates a higher condensing temperature and in turn, that increases the heat removal due to the greater TD.

If for example you have a two-fan condenser with one fan off and the SG is clear, when the second fan starts, the capacity of the condenser coil suddenly increases, so heat removal increases. Therefore the pressure drops suddenly. If the pressure drops more than its equivalent subcooling at the sightglass, then the liquid will flash. This flashing will tend to absorb heat from the liquid itself, the increased rate of heat transfer from the condenser causes as drop in the pressure, the system will shortly re-balance and clear the SG at the new condition with both fans running.

This is incorrect. Both in cool weather and warm weather condensation occurs because of heat removal. Beer explained the bubbles from fan cycling simply and correctly, I suggest you go back and read it.
what universe do you live in where refrigerant cannot condense due to an increase of pressure ? when the fan stops there is an increase in pressure, heat removal at that point has been greatly reduced ... I suggest you are the one that needs to go back and do some reading ....
Last edited by hvacskills; Yesterday at 05:28 PM.

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Originally Posted by hvacskills
what universe do you live in where refrigerant cannot condense due to an increase of pressure ? when the fan stops there is an increase in pressure, heat removal at that point has been greatly reduced ... I suggest you are the one that needs to go back and do some reading ....
Increasing the head pressure does nothing other than raising condensing temperature. The key to refrigeration is temperatures, not pressures.

Increasing the head pressure does nothing other than raising condensing temperature. The key to refrigeration is temperatures, not pressures.
the key to refrigeration is realizing how both temperature and pressure affect the refrigerant .... refrigerant without pressure is simply a vapor and would be USELESS

dont believe me ? take a jug of R22, keeping it the same temperature, and then open the valve to reduce the pressure and see what happens ....

however I did hear there were some kind of gas fired air conditioners back in the day .....

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kills, do a test, take a condenser and completely block it off and insulate it... see how much refrigerant condenses.

If you take a gas and increase the pressure enough, it will condense, but that is not what happens in refrigeration. Gases are specifically chosen to avoid that situation under the required conditions. For every system we touch, if there is no heat removed, no condensation will take place. After all, all we do is move heat... so if there is no heat removed at the condenser, what is the point?

kills, do a test, take a condenser and completely block it off and insulate it... see how much refrigerant condenses.

If you take a gas and increase the pressure enough, it will condense, but that is not what happens in refrigeration. Gases are specifically chosen to avoid that situation under the required conditions. For every system we touch, if there is no heat removed, no condensation will take place. After all, all we do is move heat... so if there is no heat removed at the condenser, what is the point?
kills ? you honestly think we can have a reasonable conversation after that ? not ... do whatever you like, block whatever you like, take whatever temperature reading you like at this point I could careless ....

not to mention, AGAIN, when you have a fan cycling control and the fan STOPS the heat removal DECREASES, it is the INCREASE in pressure that forces the refrigerant to condense .... why do you think they call it a P / T chart instead of just calling it a T chart ? ? ? ? ?

and you dont have to pressurize the refrigerant " enough " you only have to pressurize the refrigerant above its corresponding P / T saturation point which could be as little as 1 PSI, it ll condense ...
Last edited by hvacskills; Yesterday at 06:11 PM.

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Good luck, hopefully someone else will have more patience with you.

10. dont need luck I got skills

dont even make no sense .... the fan stops, so the removal of heat obviously has been reduced, but yet the sight glass clears ? ? ? must have been some other force at play ? ? ? ?

and when the fan turns back on the bubbles re appear ? so much for heat removal causing the refrigerant to condense ....
Last edited by hvacskills; Yesterday at 06:30 PM.

11. Originally Posted by BBeerme
Bubbles appear after the fan cycles on. And after the pressure of the liquid is below the point of subcooling. Then the liquid needs to boil to cool itself. Most techs don't get that they're watching liquid boil in the liquid line.
when talking about a fan cycling control ....

I tend to look at it as a " few bubbles " in the sight glass means the refrigerant has not fully condensed ... it started to condense in the condenser coil, it did in the condensor coil, and now is trying to un-condense because of the pressure drop when the fan re-started but it is unable to fully un-condense because there is too much subcooled liquid around it ...... so back to that whole point ICE made about a both a subcooled liquid occupying the same space as a vapor because I imagine a sight glass with a " few bubbles " probably has subcooled liquid refrigerant in it .....

to continue with this ....

12. we got another kux...

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