subcooling

[chillyblue]

Hi
today i was at a job with a air-cooled Bitzer condensing unit, i measured the discharge pressure which was 155psi (23deg C), and i measured the liquid temperature at the condenser outlet before the liquid reciever and it was also around 22 to 23deg C indicating little or no subcooling.
The liquid reciever has two sight glasses on it the bottom one was full and the top one was halve full, if this is the case why am i not getting any subcooling??, (the system must be fully charged)
Air temperature onto the condenser was about 12deg C, but i did'nt measure the air off temperature.

CB

[Gary]

Some systems have subcooling at the condenser outlet and some do not.

What was the subcooling at the receiver outlet?

[chillyblue]

Hi Gary
I did not check that.
Could you explain why some systems don't have subcooling at the cond outlet?? Could there be subcooling at the reciever outlet? And how's this produced?
CB

[Gary]

Hi Gary
I did not check that.
Could you explain why some systems don't have subcooling at the cond outlet?? Could there be subcooling at the reciever outlet? And how's this produced?
CB

On some systems, the receiver is below the condenser so there is no place for liquid to gather and cool before entering the receiver.

In addition, keep in mind that a liquid line can be no cooler than its surroundings... so if the liquid line is in the condenser leaving air stream it cannot cool sufficiently for an accurate subcooling measurement.

It is generally more accurate to measure the liquid line temp after the receiver and in an area that is cool enough to achieve the subcooling target.

[chillyblue]

Thanks Gary
just to clarify, the system I was working on has a vertical reciever at the same level as the cond outlet, the pipe enters halve way up the reciever which is higher than the condenser outlet, the reciever is in the airflow from the condenser, does this arrangment stop any subcooling be produced?
CB

[Gary]

If the air surrounding the receiver is only a couple degrees below the SCT, then the liquid at its outlet can only be (at most) a couple degrees below SCT. There is no cool air to provide further subcooling.

[Gary]

It should be noted that high (receiver outlet) subcooling is an indicator of overcharge or non-condensables.

But contrary to popular opinion, low subcooling does not reliably indicate anything.

[chillyblue]

[QUOTE

But contrary to popular opinion, low subcooling does not reliably indicate anything.[/QUOTE]

if that's the case, what's the best indication that the system was fully charged if there were no sight glasses on the reciever??

[mad fridgie]

Sub cooling, is not always what you think it is. In a free draining condensor into a reciever, then little sub cooling actually occurs. to know your actual sub cooling you need to know the liquid pressure at the point you are measureing the temp.
How do you know if the system is correctly charged (presume TXV). When your system is close to temperature, you need to measure your superheat, is the TXV controlling. If it is then charge is OK. A sight glass is an aid not the be all and end all.

[Gary]

Yep... minimum charge is enough to control the superheat. Maximum is when the receiver outlet subcooling reaches 15F/8.5K.

[chillyblue]

thanks chaps

CB

[Ernie]

ok, i ran into an issue today. was wondering if i could have some help.
reach in freezer. 2 seperate boxes, 1 txv in each, one evap in each one 4 door, one three door. condenser unit in compressor room hp-80.
suction at compressor-19psi
liquid at receiver outlet - 170 psi
liquid line temp. at receiver outlet -103 F
sight glass would not fill. was not able to obtain more than about 1 F subcooling.
added alot more refrigerant than i thought wise, total charge is about 30 lbs, i added 15 lbs , but i was just trying to make something change.
was able to get liquid pressure to 180 psi. ,but liquid line temp. followed with pressure. sight galss still not full.
receiver is below condenser coil and compressor.

issue was one of the freezer's would maintain 0 F while the other would sit around 15 F. any suggestion's, i plan to stop in tomorrw to check on it. could not watch if temp. in box fell because it is hot here and this unit is for ice cream at a grocery store.

any suggestion's would be a big help. thanks.

[nike123]

Pressures, without knowing what is refrigerant, doesn't mean anything.

[Ernie]

1) i wrote the refrigerant, just not clearly enough. HP-80
2) the pressure's on high side were supposed to be 270 psi and 280 psi not 170 and 180. sorry.

[NoNickName]

HP80 = R402A which is a replacement for R502