Beer cooler : have I charged it correctly?

[acnerd]

I put ice in water and let it stand for 10 minutes, then put more ice in and stirred. I used the liquid sensor, surface touch sensor and air sensor probes. All measured 2,5'C approx. If the ice water was 0'C, wouldn't it be frozen? I put a standard glass mercury thermometer in and it measured 1'C. I don't think any of the above is accurate enough for a definitive answer.........

[Gary]

Let's see how the new numbers work out:

old numbers:

Room setpoint: 13'C
Evaporator air on: 14'C
Evaporator air off: 9.5'C
Suction pressure: 30 PSI
SLT: 8'C to 12.5'C (TEV hunting?)
Condenser air on: 22.5'C
Condenser air off: 33.5'C
Liquid pressure: 110 PSI
LLT: 33'C to 35'C (why???)

new numbers (factoring in temp and pressure estimated errors):

Room setpoint: 13'C
Evaporator air on: 11.5'C
Evaporator air off: 7'C
Suction pressure: 23.2 PSI/-3C
SLT: 5.5'C to 10'C (TEV hunting?)
Condenser air on: 20'C
Condenser air off: 31'C
Liquid pressure: 103.2 PSI/32C
LLT: 30.5'C to 32.5'C (why???)

SC = 32-30.5 = 1.5K... to 32-32.5 = -0.5K
SH = 5.5--3 = 8.5K... to 10--3 = 13K

I would be tempted to say that the SC is low because the system is undercharged... but the sightglass is clear.

Seems we are still missing something.

I am running out of ideas... anyone have a fresh supply?

[Djorn]

As far as I know, any amount of SC, as long as you have some, will give solid liquid, but may still be undercharged for all “conditions”.
Is the probe use to measure LLT insulated? I have seen many instances where temp is measured in the airstream of the cond. Outlet, and the probe is picking up some of that heat. I usually put some “foam tape” around the probe to make sure what I’m measuring is just the LLT.
Does the unit have a CPR valve, a heat exchanger or any other device installed?

With calibration, as long as water and ice are in the glass, the temp will be approx. 00 C.

[acnerd]

It's a goof idea to shield the probe from the hot exhaust air from the condenser. I made sure of that.

The fridge unit has only the basic parts, no regulating valves whatsoever.

The fact that the system is running and cooling the product with a full sight-glass is enough for the customer.

Unfortunately for me I'm left with the niggling worry in the back of my head: is it me, the gauges or the sensors??

Thanks for all the help guys.

[Gary]

Unfortunately for me I'm left with the niggling worry in the back of my head: is it me, the gauges or the sensors??

I would calibrate the sensor to the ice water, then use the sensor on the refrigerant bottle to calibrate the gauges.

[acnerd]

Thanks Gary. I will look closely at the digital thermometer and see if I can do it myself.

[tbirdtbird]

The system does seem to work. The gauges and temp probe don't seem to be too far out. (water can be 0*C or ice can be 0*C...its all about latent heat. Ice water s/b 0*C as prev. stated). Gary has said in the past that the sight glass doesn't tell the whole story necessarily. I think Djorn has a point...wherever the temp probe is s/b very well insulated. Also liquid pressure seems low. I am not convinced there is a design issue.

[Gary]

Aside from the sightglass, this system says borderline charge. I would add a little and see if the LLT drops.

[RefrigNoob]

I would be tempted to say that the SC is low because the system is undercharged... but the sightglass is clear.

Seems we are still missing something.

I am running out of ideas... anyone have a fresh supply?

Could the orientation, flow direction and size of the sightglass have an influence? Similar to the filterdrier posts between yourself and desa?

Also, could the sightglass be clear because there is subcooling, but not enough to ensure a clear sightglass under various conditions. Isn't that why we charge to 5-10 deg SC? To ensure a clear sightglass under various conditions?

[Djorn]

Does the system have a receiver? If the unit has a receiver, than there should not be too much harm in adding some more gas, and after all, you can always take it out if you have too!
With Subcooling, I always visualize that if you back up a little liquid in the condenser, per se, you give more “time” to remove sensible heat from the subcooled liquid and thereby increase Subcooling.
The need for decent Subcooling helps with pressure drops in the piping. Pressure equals temperature and pipe risers and long runs and all kinds of things can rob you of sensible temperature and a solid column of liquid at the TEV.
If a device, like a sight glass perhaps, can change the pressure and you don’t have much Subcooling, that could cause bubbles in the sight glass I suppose.

[chillyblue]

Hi mate

Read this thread,
http://www.refrigeration-engineer.co...ad.php?t=11200
I had the same issues as you.
I'm still not convinced that having 7 deg C of subcooling does not cause excessive discharge pressures.
I guess i'm wrong but do cond unit maunfacturers actually design there systems big enough to cope with high amounts of subcooling.
I charged a system the other day with way more refrigerant than i would have normally charged, just to get subcooling to around 7 deg C.
The system was working well way before reaching 7 degC of subcooling.

Gary, why do you have to take the subcooling reading at the reciever outlet rather than condenser outlet.
I would guess the reicever this guy is using will be sitting in front of the condenser fan so air off the condenser will be heating the reciever surface raising it's temperature and lower the amount of subcooling.

[Gary]

Gary, why do you have to take the subcooling reading at the reciever outlet rather than condenser outlet.
I would guess the reicever this guy is using will be sitting in front of the condenser fan so air off the condenser will be heating the reciever surface raising it's temperature and lower the amount of subcooling.

This is a valid point... and something I had not considered. It is not possible for the liquid line to be cooler than its surroundings.

Therefore, the liquid line should not be measured in an area that is subjected to condenser leaving air, but should be measured further downstream where the line is subjected to cooler temperatures.

For the same reason, the sightglass should also be further downstream.

Good observation, Chillyblue

[tonyelian]

hi
it seems ther some thing missing here size of the room compressor &fan coil capacity so we can go from there