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Temprite
05-10-2005, 01:38 PM
G'day all

Sorry to bring up an old topic that has already been thrashed to death, but looking through archives I couldnt find an answer to my question.

We all know that a partial restriction in a cap tube causes low suction pressure and high superheat. But so does short of gas and they can both look similar on the low side gauge when the system is running. Also an icing cappilary line could be another indication..

What if the particular system doesnt have a name plate charge or a gauge connection on the high side so you can check subcooling to determine whether there is a partial restriction.?

Does taking the temperature in the mid point of the condenser coil give you a proper indication of SCT good enough to do a subcool measurement?

I wonder how many mechanics stuff cap tube systems full of refrigerant until the suction line gets colder thinking they are short when it is actually a restriction.

Does anyone know of a foolproof method that will enable you to get it right first time without trying to hunt down a non existant leak.

Thanks in advance.

tonto
05-10-2005, 02:51 PM
Something that has always plagued me as well temprite, would be keen to see what sort of replies come back,

chemi-cool
05-10-2005, 06:47 PM
Hi guys,

Cap tube can and is tricky at times. You have to trust your experience when charging these systems.

Most cap tube systems, operate above 0C so its easy to see when the frost clears at the end of it.

you have to pay attention to pressures, both suction and discharge, any problem with the refrigerant flow, will affect the pressures. So keep your eyes open for any change.

If there is a doubt, don't mess with it and replace it.
Cap tube systems require experience and patient.

Chemi:)

shanes696
07-10-2005, 12:57 AM
i have found that the best way of diagnosing this problem is to run the unit with your suction gague fitted , if there is a blockage or if the system is short of gas ,the system will behave in the same manner by pumping to a low pressure or even a vacume , hovever , if you switch the unit off , if there is a blockage at the cap or drier the suction pressure will stay where it is or rise very slowly, whereas if the system is just short of gas, the pressure will rise to at least 0 psi quite quickly, also when the unit is switched off if there is a blockage the drier or cap will get v cold or even start to ice up which will not happen if the unit is just short of gas

frosty74
07-10-2005, 08:11 PM
i have to agree with shanes696
like people have said in the past caps can be very tricky at times and take time to diagnos properly with plenty of patience.
i would go about it the same switching off the unit and checking the reaction of pressures also temp of copper drier etc.
i was going to put this point forward last night but i was interested to see how other people deal with this situation.
as sombody on the site says better to keep quiet and look stupid than speak up and remove all doubt.
but yes this is how i usally deal with this diagnosis would anybody else like to agree or advise on other methods.

Peter_1
07-10-2005, 10:15 PM
You were calling me?

frosty74
07-10-2005, 10:26 PM
thats the one peter good phrase i like that one

Temprite
10-10-2005, 12:02 PM
Thanks for your replies people.I have taken it on board.

The reason I started this thread was I was working on a freezer which I suspect of a blocked capillary.

Some information.
Model:Zannoti slide in BGM220394.
Compressor:Hermitique FH2480Z.
Refrigerant:404A
Hot gas defrost.

Upon arriving at job check all coils evap and condenser were clear.
Suction Pressure:15psi
Room temp: -12 degrees c
Suction line temp just before comp -4.4 degrees c.

Initially thought that unit was short of refrigerant but after looking and not finding a leak tried adding refrigerant but situation did not improve. Then I suspected a restriction.

Reclaimed charge and evacuated.Then weighed in nameplate charge (0.84kg). After weighing in.
Suction Pressure:14psi
Room temp: -14 degrees c
Suction line temp just before comp: -4.3 degrees c.

At that room temp the suction pressure should be higher. Therefore the only conclusion is a restriction.

Any help would be appreciated.:)

Cofreth
11-10-2005, 07:26 AM
A restricted cap tube causes the condenser coil flooded with liquid, will have a higher condensing pressures and higher compressor running amperes.

chemi-cool
11-10-2005, 10:08 PM
No, it wont.
Think why.

Chemi:)

chillin out
12-10-2005, 12:37 AM
Hi Temprite,

Ive had one of those zannoti`s do this as well, and I found it short of gas.
I know the nameplate says what it says but I checked my capillary and it was ok.
I just charged the thing untill it 'felt' right and all was ok.

Dont forget to check the evap fan direction and if the blade is on the right way.

:) :)

Abe
12-10-2005, 09:57 AM
Refrigerant:404A
Hot gas defrost.

Upon arriving at job check all coils evap and condenser were clear.
Suction Pressure:15psi
Room temp: -12 degrees c
Suction line temp just before comp -4.4 degrees c.


Reclaimed charge and evacuated.Then weighed in nameplate charge (0.84kg). After weighing in.
Suction Pressure:14psi
Room temp: -14 degrees c
Suction line temp just before comp: -4.3 degrees c.

Any help would be appreciated.:)


Im thinking on this one
Its not a restriction I dont think
If it was a restriction you would be in vacuum
I know, youre thinking , maybe its a partial vacuum

Your back pressure 15psi is correct
This is my advice

Switch off your evaporator fans
Add refrigerant till the whole evaporator coil is iced
At this point stop adding refrigerant

Make sure your condensor coil is clean

See how it goes

frank
12-10-2005, 08:30 PM
No one yet has mentioned checking the compressor running current - it's a must when charging a system.

Too little refrigerant and low amps, too much refrigerant and high amps. All part of the overall picture.

chemi-cool
12-10-2005, 09:38 PM
Yes Frank, But if there is a partial blocked cap tube, then your amps will not show the truth.

Chemi:)

angryk
13-10-2005, 02:10 AM
What works best for me is something I picked up off this forum. A restriction in the liquid line will have an initial high pressure (liquid), at start up or when adding gas in an attempt to compensate for the low suction. then it will work its way down. Lots of sub cooling, usually cold drier.

Temprite
21-10-2005, 01:44 PM
Just an update.

Turns out chillin was right.

After checking capillary and weighing in charge system was still short.

Nameplate 0.84kg.

System took about 1.15kg but I also installed slightly larger dryer.

Working much better.

Suction pressure about 24 Psi.]

Thanks all for your valuable comments.:)

Gary
21-10-2005, 02:45 PM
G'day all

Does taking the temperature in the mid point of the condenser coil give you a proper indication of SCT good enough to do a subcool measurement?

Thanks in advance.


Try using a small piece of cardboard. Poke a hole in the center for your temperature sensor and hold the cardboard against the condenser fins to block airflow around the sensor. This will give you a more accurate estimate of SCT.

rbratlett
21-10-2005, 11:35 PM
generally using a surface probe on any of the u bends around the coil's middle third will give you an indication of saturation temperature.

Temprite
22-10-2005, 01:20 PM
Thanks for the tip Gary.

Richard.
I have a pipe clamp, a wire bead, and a digital thermometer. Good contact thermometer is next on the shopping list.