Low ambient charge

[Chuck]

If we restrict airflow, it stands to reason that we are rejecting less heat....So, wouldn't subcooling go down?

If you have a dirty condenser, your head pressure is high, and your subcooling low...Washing the coil brings the head pressure down and the subcooling would rise, yes?

Actually my experience has shown otherwise. If you raise the head pressure by restricting airflow you didn't change the entering air temp. The pressure goes up, but the refrigerant temp doesn't go up as much, resulting in higher SC. The approach temp (difference between entering air and leaving refrigerant) doesn't change much, but the head pressure went up.

But if you raise the head pressure by raising the entering air temp, both temp and pressure will go up together resulting in the same SC.

I will definitely be testing it this summer again to be certain.

[John Markl]

I will definitely be testing it this summer again to be certain.

You and me bof....

I'm sitting here thinking......Systems with sight glasses that I work on, that have fan cycle controls......as the head pressure rises, just before the pressure switch cuts in, the glass loses liquid and turns to vapor, and the liquid line gets warmer....when the fan comes on, within seconds, the glass is back to liquid and cooler.

Oh well, off to the theater....("Kurves")....catch ya later

[pressordrop1]

Remeber yor laws increas pressure increase the temp it should increase your SC

[itsiceman]

You and me bof....

I'm sitting here thinking......Systems with sight glasses that I work on, that have fan cycle controls......as the head pressure rises, just before the pressure switch cuts in, the glass loses liquid and turns to vapor, and the liquid line gets warmer....when the fan comes on, within seconds, the glass is back to liquid and cooler.

Oh well, off to the theater....("Kurves")....catch ya later

Its the opposite but I've seen it too
These units with receiver and fan cycling need more charge than a unit with a head pressure control valve so you don't loose the liquid at the dip tube

Also just because a sight glass isn't clear doesn't always mean there is no SC



Bottom line was liquid temp where they intersect the glass flashed

[Saturatedpsi]

Actually my experience has shown otherwise. If you raise the head pressure by restricting airflow you didn't change the entering air temp. The pressure goes up, but the refrigerant temp doesn't go up as much, resulting in higher SC.

Your observations are accurate. The explanation I've seen is low condenser air changes the "design" of the condenser coil, maybe suggesting it's about like removing a few rows of tubing, resulting in a smaller coil volume. The end result is a smaller coil with the same amount of refrigerant contained. The liquid would stack further back, increasing subcooling. I don't fully comprehend the reasoning, but it does sound good. And I can't come up with anything better.

[John Markl]

I attended a class put on by Rheem tonight after work....They gave us a book, and Table 23 says that "Low Condenser Air Flow = High Discharge Press ure and Low Subcooling"

[Saturatedpsi]

I attended a class put on by Rheem tonight after work....They gave us a book, and Table 23 says that "Low Condenser Air Flow = High Discharge Press ure and Low Subcooling"

So, who do you believe?

[John Markl]

So, who do you believe?

Obama....

[itsiceman]

Saw the Fieldpiece charging jacket in the RSES Journal so my measurements must be wrong.
Maybe I should try it again with a set of analog gauges and pocket thermometer

[Saturatedpsi]

I wouldn't argue the point, were it not for the fact I've "staged" low condenser air numerous times, and it always resulted in increased subcooling.

If we restrict airflow, it stands to reason that we are rejecting less heat....So, wouldn't subcooling go down? Wouldn't the condenser have LESS liquid in the bottom 1/3 ?

I believe the flaw in that reasoning lies in fact the condenser is rejecting essentially the same amount of heat. There is likely some decrease in net refrigeration due to elevated liquid temps, but whatever the amount of heat energy being taken in by the evaporating process, it is being rejected in the condenser. If it wasn’t, something would eventually blow up…

The condensing process has adjusted for the reduced rate of heat transfer by generating a higher set of saturated conditions for condensing to take place. The same adjustment takes place as OD temps increase. But for reasons I don’t fully understand, reduced condenser air volume affects subcooling, where increased condenser air temperature doesn’t.

To me, the intuitive conclusion is a relatively unchanged subcooling value in either case, assuming no change in the evaporator load.

In the YouTube link, Jim Bergman discusses the “irregularities” of restricted condenser airflow, 12 minutes or so into the video.

http://www.youtube.com/watch?v=SomBa...yer_embedded#!

[bluecool]

Raising the head pressure should be enough. Its not necessary to have a load on the evap. For example, if you had a perfectly charged working unit except the RA fiter was dirty to the point that the coil started to ice over, in other words a no load condition. Would you expect this to make the subcool low? Raising head press and checking subcool should be enough, although I am willing to be schooled on the subject.

[Chuck]

I wouldn't argue the point, were it not for the fact I've "staged" low condenser air numerous times, and it always resulted in increased subcooling.



I believe the flaw in that reasoning lies in fact the condenser is rejecting essentially the same amount of heat. There is likely some decrease in net refrigeration due to elevated liquid temps, but whatever the amount of heat energy being taken in by the evaporating process, it is being rejected in the condenser. If it wasn’t, something would eventually blow up…

The condensing process has adjusted for the reduced rate of heat transfer by generating a higher set of saturated conditions for condensing to take place. The same adjustment takes place as OD temps increase. But for reasons I don’t fully understand, reduced condenser air volume affects subcooling, where increased condenser air temperature doesn’t.

To me, the intuitive conclusion is a relatively unchanged subcooling value in either case, assuming no change in the evaporator load.

In the YouTube link, Jim Bergman discusses the “irregularities” of restricted condenser airflow, 12 minutes or so into the video.

http://www.youtube.com/watch?v=SomBa...yer_embedded#!

Good video.

It really illustrates the problem with checking or adjusting charge with restricted condenser airflow.

[John Markl]

We have customers that require year-round A/C.....which necessitates the use of fan cycling.

It may be "irregular" but it works.

[Saturatedpsi]

Raising the head pressure should be enough. Its not necessary to have a load on the evap. For example, if you had a perfectly charged working unit except the RA fiter was dirty to the point that the coil started to ice over, in other words a no load condition. Would you expect this to make the subcool low? Raising head press and checking subcool should be enough, although I am willing to be schooled on the subject.

If the metering device is a TXV, the low evaporator load would result in the TXV closing down to maintain superheat which would create the same symptoms as a partial LL restriction, causing the subcooling to increase. If the head pressure is "artificially" raised by restricting condenser air, the subcooling would increase. Those results are reasonably predictable.

If however, the metering device is fixed, I can only guess at what might happen. The low evaporator load will result in low suction and probably low superheat (depending on head pressure).

If the head pressure is raised by restricting condenser air, the differential pressure across the orifice will increase, forcing more liquid through the orifice, probably reducing the amount of liquid in the condenser, which would lower the subcooling.

But, I don't know what "normal" subcooling values are with fixed orifice systems. If airflow is design across both coils, the pressures are "normal", and indoor / outdoor temps are within operating ranges of the superheat chart, the system is charged to the specified superheat value and the subcooling is whatever it is.

[Chuck]

We have customers that require year-round A/C.....which necessitates the use of fan cycling.

It may be "irregular" but it works.

It certainly works, and works fine for low ambient cooling. I have Telecom buildings that cool by fan cycling down below zero.

Just don't set the charge in low ambient by restricting airflow or you might be undercharged on a hot day.

[jim bergmann]

I ordered one of these to test it out. I have posted one video and am uploading a second and maybe a third.

http://youtu.be/pmFd8K-9hkU

I tested it on a 13 seer R22 system in the shop, at about 68 degrees indoor air temp at the condenser and about 73 degrees RADB,

[Tassajara]

Is the main purpose restricting air flow? Seems like it's just short cycling a portion of condenser air. I only got a quick look at the hood at the beginning of the video

Sent from my SPH-D710 using Tapatalk

[jim bergmann]

It actually just backs up the heat in the condenser. It is wrapped tight around the top so air does not recirculate.

[georgelass]

If the metering device is a TXV, the low evaporator load would result in the TXV closing down to maintain superheat which would create the same symptoms as a partial LL restriction, causing the subcooling to increase. If the head pressure is "artificially" raised by restricting condenser air, the subcooling would increase. Those results are reasonably predictable.

<snip>

So if in low-ambient and low-evap conditions and
trying to charge to the nameplate (TXV) subcooling value, the
subcooling will change drastically depending on whether the
condenser airflow is blocked off. (Right?)
Say, for R410, what does one shoot for? A certain
condenser pressure/temp range? (What might that be?)

(Today, I weighed in a charge and blocked off the condenser
such that condenser R410 sat temp got to about 84 degrees
and subcooling was, I think, almost 20 degrees, a line temp
of 64 F. Good thing I was weighing-in and not trying to shoot
for a specific SC value, as I was a bit unsure what would have
been correct under those conditions.)

Thank you.

[itsiceman]

I ordered one of these to test it out. I have posted one video and am uploading a second and maybe a third.

http://youtu.be/pmFd8K-9hkU

I tested it on a 13 seer R22 system in the shop, at about 68 degrees indoor air temp at the condenser and about 73 degrees RADB,

Interesting Jim
So that 13 seer unit calls for 10˚F SC
The one I tested in this thread (multi speed high seer) calls for 16˚F SC and ran to spec with no air restriction @40˚F ODA.