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Old 20-04-2018, 15:03   #1
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Refrigeration - how many gms 134a?

Hi all,

I have returned to my fridge project but I am stumped ... so some questions if I may ....

The system is based on an A-B cold machine. Evaporator plate, about 10 ft of 1/4" OD copper connecting the suction side and 10 ft of 3/16" copper connecting to the compressed (liquid) side which connects to the cap tube which, in turn, disappears into the inside the other (suction) side which connects to the evap plate. I have a manifold gauge set attached to the system.

Questions:

(1) Roughly, in grams, how much R134a should I expect to need to properly fill the system? I put a vacuum pump on both sides of the system so it is starting from empty. I've put almost half a can (170 gms) into it so far with no effect. I have used soapy water to look for leaks and have found none.

(2) When the compressor is running, the high side gauge reads 65 psi and the low side reads 30 inHg vacuum. When feeding R134a into the vapor side, the pressure goes from 30 inHg vacuum to 10psi or more. After a few seconds I close the low side valve and the pressure slowly drops back to 30 inHg vacuum. Repeat multiple times and the same result. Vacuum on one side and pressure on the other means that the compressor is functioning --right?

(3) How long shall I expect to wait before the evaporator feels cold?

(4) I am beginning to believe that I have a massive leak but if that were the case the high pressure side would drop quickly and the low pressure side would rise when the compressor is shut off -- the don;'t (at least not quickly).

(5) When working, what pressure drop is expected: 65psi high side to ? low side.

Any insight would be much appreciated!

David
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Old 20-04-2018, 16:22   #2
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Re: Refrigeration - how many gms 134a?

Hi David

First , tell me about the evaporator plate . Size and type .

Next . You say you have a vacuum pump on the system ? Do you have a micron vacuum gauge ? Are you testing for leaks with your vacuum gauge ?

Regards John
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Old 21-04-2018, 04:04   #3
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Re: Refrigeration - how many gms 134a?

Hi John,

I'm away from the fridge but the evaporator is L-shaped pressed Aluminium probably of an area of 20" x 8" . (roughly). It will be familiar to you as this is the abandoned one that you had laying around and generously sent me last fall (to an address in Hamilton) to replace the one i had scavenged from a dorm fridge (separate thread entitled something like "frankenfridge ... "

I don't have a vacuum gauge that measures on the micron scale just the dial gauge on the manifold and a 0 - -25 psi gauge on the vacuum pump reservoir tank.
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Old 21-04-2018, 04:15   #4
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Re: Refrigeration - how many gms 134a?

The common method to charge a sealed system is, charging small amounts until -5 to -10 Hg vacuum levels established in the system.

Turn the system on, hook up your manifold gauge to the low side... attach another manifold hose to the R134a tank (while the manifold hose is closed)

You can charge refrigerant to the system by opening the manifold valve. Charge a small amount, wait a while.... until the pressure gauge becomes steady.... and repeat until you reach steady -5 Hg vacuum levels.

Don't forget... compressor must be running all the time during charging.

In your case, you may have a restriction in the system because it goes into the deep vacuum even after charged... look for a very cold spot by tracing tubing.

If you still suspect that there's a leak in the system .... Large leaks are easier to detect.... if you have access to a nitrogen tank, charge the system with nitrogen first and use a soapy water solution again (or a leak detector solution sold in appliance parts stores at $5) to detect the leak.

I hope this helps....
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Old 21-04-2018, 05:20   #5
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Re: Refrigeration - how many gms 134a?

Responsible mobile refrigeration companies offer different methods of servicing their units. I use a coverall refrigerant servicing method on systems with capillary tube refrigerant flow control and the Danfoss BD compressors. This 30 year old Adler service bulletin still works today.

Knowing compressor model and module number will make fine tuning refrigerant later much easier?

It sounds like there is presently some refrigerant in this unit. Your first step is to check for leaks by adding refrigerant gas vapor only with unit not running. Bottle refrigerant pressure on a warm day should raise pressure in system to a range from 55psi to 65 psi adequate for basic leak testing.

These are the steps needed on a BD2, BD2.5, BD35 or BD50 compressor capillary Tube unit to re-commission your system:
Note this servicing procedure should not be used on TXV or Eutectic Holding plates. If you need help with off brand compressors check system manufactures web site repair Bulletins instructions.
1. Check gauge set to see that gauges read zero when not attached to unit if not adjust or make a note of their errors.
2. With both gauge hand valves closed, connect 134a bottle to center yellow hose. Check to see that blue gauge has blue hose and lose end of blue hose has a valve core depressor inside. Before connecting blue hose to low pressure connection open blue gauge valve for 2 seconds to purge yellow and blue hose then close valve and connect blue hose to service fitting on top of compressor.
Inspect valve cap you removes to see that rubber seal inside cap is still good. Slow leaks are sometimes traced to a bad cap seal.
3. With refrigerant bottle upright open blue gauge valve to add refrigerant. Blue gauge should now be reading system pressure of 30 to 60 psi depending on temperature of complete system. After blue gauge is connected and system has pressure in it you will need a one inch paint brush and a small amount of water and liquid dishwashing soap mixed 50/50% to locate refrigerant leak.
4. With 50 psi of gas pressure in system, use soap and water mixture to locate leak. By using brush to apply mixture to only one point at a time stabbing brush into location until a shaving cream mixture surrounds it you will break down to liquid surface tension so any leak must come through foam generating bubbles. Leak testing is done with compressor off.
5. After leak is found and repaired unit can be serviced with correct amount of refrigerant. In your situation best way to get correct charge is again raise pressure 50 psi then run compressor for 10 minutes and then and only then add refrigerant if necessary or reach 6 to 8 psi suction pressure. It is important that you not add refrigerant after 20 minutes run time as correct pressure can not be determined after 20 minutes. For final fine tuning unit needs to run for a day or more. Correct refrigerant charge for your unit is when 90 to 100% of evaporators surface is covered with frost and no frost on line outside refrigerator on line returning to compressor.
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Old 21-04-2018, 05:22   #6
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Re: Refrigeration - how many gms 134a?

Thanks!

I've definitely screwed something up and I am leaning towards the seemingly contradictory idea of both a blockage in the capillary (since the low pressure side stays very low even after filing) and a massive leak to account for the large volume (filled with 174 gm, liq density 4.25 kg/m3, means 0.04 m3 of liquid) of 134a that has disappeared into the system.

My manifold is a cheapo harbor freight unit but I don't see how that could fail -- its just two valves (which seem to work) and two gauges (that seem to read correctly). The seals to the access points are a bit iffy with hard plastic bits that are compressed by tightening down a brass collar ... maybe it's not seated correctly ....

Ok, something to look at, I'm off to look for a leak ...

Thanks for for the help muddling through this!
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Old 21-04-2018, 05:28   #7
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Re: Refrigeration - how many gms 134a?

Thanks Richard, I posted before I saw your reply. What does the valve depressor look like? Is it a loose piece? That might be it, the vacuum sucks open the valve but without a depressor the gas isn't making it into the system (though where it went remains a mystery).
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Old 21-04-2018, 05:39   #8
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Re: Refrigeration - how many gms 134a?

Glad to see you are getting to your project . Was your system open to the air for a long time ? You may want to make sure it is dried out before starting the charging process.

Regards John
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Old 21-04-2018, 06:28   #9
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Re: Refrigeration - how many gms 134a?

Thanks John. I had it under vacuum for quite a while so I think the system should be dry. My plan is to see if I can make this thing work in my garage then reassemble it in the boat. When I do, I will replace the drier.

I see now that there is a valve depressor on the manifold gauge.

I have purged the lines as described in Richard's instructions.


Just to eliminate the possibility of something profoundly stupid ...

The low side, blue valve and hose on the manifold goes to the pipe that emerges from the top of the compressor (BD35) and sucks on the larger (vapor) pipe that comes from the evaporator. Right?
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Old 21-04-2018, 06:47   #10
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Re: Refrigeration - how many gms 134a?

You said you vacuumed the system. Now are you saying that valve core depressor is missing from Blue hose. Or are you saying the valve core in compressor service fitting is missing.

If problem is that Blue hose 40 degree angle end does not have a valve core depressor in it? Or is the correct end of blue hose without depressor connected to gauge as it should be?

You must find a way to put bottle pressure in system forget every thing else so you can leak check first. Unless there is a big leak it will only take 4 ounces of refrigerant.
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Old 21-04-2018, 07:41   #11
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Re: Refrigeration - how many gms 134a?

Thanks Richard, sorry for the confusion (all mine!).

I have now confirmed that there are depressors on the low and high pressure hoses (40 degree angle end connected to the BD35). Gas is making it into the compressor.

I have stopped the compressor and have pressurized the low side of the system (the high side is still under pressure from running the compressor earlier). I have swabbed soapy water at all the joints and there are no easily detectable leaks. I closed the blue valve and the low side holds pressure as does the high side.

I cracked the high pressure side 40 degree connector and saw 134a (cold fog) hissing out so I have put refrigerant gas into the system and it is pressurized well above what would have come out of the can.

So all I can think of now is that there is a capillary blockage. I was careful when assembling this thing but I suppose its possible. If ice, then I suppose I need to vacuum out the entire system (high side and low side) and leave it under vacuum for a few days in a warm place then start again.

As always, insight is much appreciated!
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Old 21-04-2018, 08:46   #12
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Re: Refrigeration - how many gms 134a?

OK you say there is pressure in system What is pressure reading on both high and low pressure gauges?

Now turn compressor on and what are both pressures after compressor has run 10 minutes?

Gauge valves must be closed.

If low pressure is in a very low vacuum at ten minutes turn off compressor and see how long it takes for the two gauges to equalize pressure.
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Old 21-04-2018, 09:08   #13
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Re: Refrigeration - how many gms 134a?

Richard:

Compressor off, 134a tank connected and blue valve open: 10 psi on blue gauge and 50 psi on red gauge (residual from when the compressor was on).

Compressor on, blue valve closed, low side starts at 10 psi and drops slowly (2-3 minutes) to -30 inHg, red gauge 50 psi.

Compressor off, blue valve still closed, no change is observed over 10-15 minutes. Blue gauge reads: -30 inHg, red gauge 50 psi.

I'm guessing complete capillary blockage?

Thanks for your time on this!

David
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Old 21-04-2018, 09:24   #14
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Re: Refrigeration - how many gms 134a?

also, yes, both gauges read zero when disconnected.
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Old 21-04-2018, 09:29   #15
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Re: Refrigeration - how many gms 134a?

Now we should try to locate blockage. Describe where in refrigerant flow lines are each of the service gauges connected.

Was any line connection soldered especially at compressor?
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