R600a Pressures and Temperatures in Domestic Refrigeration

[JSimpson]

Good Afternoon Everyone

First time member, so I hope my question wont offend anyone.

I have been a Domestic Appliance Engineer working for a small organisation for about 10 years, we deal with Washing Machines, Tumble Dryers, Dishwashers and Cookers (Electric Only). One of our contracts now wants us to take care of Domestic Refrigerators aswell, unfortunately I haven't worked on refrigeration for a long time and never on the Sealed System / Gas Side. I dont have any problems with the control side but the gas side has me somewhat apprehensive. The physical part, pipes and connections is not an issue and I am a pretty good at braising and for a health and safety point of view I will be looking into Lokrings, but as long as there is no gas (R600a specifically) in the system braising should not pose an issue.

So my main questions are on the R600a Gas Side, specifically temperatures and pressure readings on the suction / charging stub. I have had a look on google but some of the answers are conflicting. I know various factors will affect the readings so just looking for figures that guys with experience can advise on.

At the moment I have a single manifold R600a Gauge with temperature and pressure in psi and one with temperature and pressure and in BAR.

So using a set of piercing pliers tapped into the charging stub with the compressor on I should be getting a reading somewhere between -25 degrees to -30 degrees at 8 - 6.76 PSI or 0.55 - 0.47 Bar on a healthy system?

Now googling gave me some answers of -0.2 to -0.6 BAR and others -15 to -20 PSI for the pressure side which really confused me.

Oh one other thing, I dont have a vacuum gauge so how would I monitor a vacuum using the manifolds above.

So looking for advice.

(P.S I am looking into regassing courses, but just need to get my head around it somewhat)

Cheers

James

[seanf]

R600a pressure temperature chart
https://www.fsw.uk.com/sites/www.fsw.uk.com/files/r600a-iso-butane-pt-chart.pdf

You might be seeing pressures shown in absolute or in gauge. PSIA or PSIG, and Bar(g) or Bar(a), kPa.....

Some gauges go from (0)-to-(+whatever) others you can get that will show (vacuum)-to-(+whatever).
Really all you need is the pressure from the guage, and use a pressure temperature chart to find out what it means temperature-wise.

One thing to keep in mind is,
If your fridge is running and in a vacuum. When you put your gauge on itll suck in any air that is in the gauge and gauge hose.
If you switch the fridge off and the pressure reads positive on your gauge, when you take your gauge off youll be removing refrigerant that is held in the gauge and gauge hose.
When you look at how little amount of refrigerant is in a domestic fridge, the little losses and little additions could make a more noticable difference.

Like you said different factors means you see different pressure readings. Like,
Normally if the evaporators have a fan, the pressure youd expect to see will be higher than an evaporator that doesnt.
If you ever look at the discharge/high side of the system, a condenser with a fan will run at a lower pressure than one without a fan, normally.

Some people will say to avoid using compression type joints especially with a flammable refrigerant. Refrigerant leaks are a common fault.

Even if a system has leaked out all its gas. As in, when you put your gauge on and it reads 0 psig. You can still have some R600a in the pipework and in the compressor oil. So if your going to braze on or around it be sure to be purging the system with nitrogen.

I've probably managed to answer none of your questions, sorry.

[frank]

Good answer Sean. Another good point is to not rely on pressures, as in, what pressure am I looking for, but to consider what temperature are you looking for, then to see what the corresponding pressure should be.
Also, keep in mind that the system will take time to pull down to temperature, so your pressures will be above what you are looking for until the system is near set point.

[JSimpson]

Hi Guys

Thanks for the replys.

@seanf I found out the hard way with losing gas today, I erm removed the hose from the manifold before removing it from the charging point, goodbye mr R600a, haha

@frank thats probably easier as in all the dials the temperature is at least a constant factor unlike the pressure.

So after a little research in thermodynamics, arrrg for the evaporator to absorb the heat it must be in a colder state than what its trying to cool, so lets take a electronic fridge / freezer set at -22 Degrees in the freezer, then I would be looking roughly for a gas temperature of say -24 degrees on a cooling cycle. If we take -24 degrees, on a PT Chart R600a at -24 degrees should be 8.85 psi, but on my gauge with a psig I would be looking for -5.85 as you have to remove ambient pressure at 14.7 at sea level, then on my barg gauge 8.85 psi would be -0.4 barg.

"Even if a system has leaked out all its gas. As in, when you put your gauge on and it reads 0 psig."

Would this be with the compressor on or off, as on with no gas would I not it expect to pull into a vacuum when running?

Sorry if I am completely way of the mark here.

[seanf]

Hi Guys

Thanks for the replys.

@seanf I found out the hard way with losing gas today, I erm removed the hose from the manifold before removing it from the charging point, goodbye mr R600a, haha

@frank thats probably easier as in all the dials the temperature is at least a constant factor unlike the pressure.

So after a little research in thermodynamics, arrrg for the evaporator to absorb the heat it must be in a colder state than what its trying to cool, so lets take a electronic fridge / freezer set at -22 Degrees in the freezer, then I would be looking roughly for a gas temperature of say -24 degrees on a cooling cycle. If we take -24 degrees, on a PT Chart R600a at -24 degrees should be 8.85 psi, but on my gauge with a psig I would be looking for -5.85 as you have to remove ambient pressure at 14.7 at sea level, then on my barg gauge 8.85 psi would be -0.4 barg.

If the inside of the freezer is actually down to -22 DegC (you measuring the box air temperature, not just the setting on the display), then yer youd see your pressure of -24 DegC.

But your likely to see pressures that are about 5-10 DegC below the box air temperature, on units that have finned tube evaporators with a fan.
Then for units with just a plate evap and no fan, youll likely see pressures even lower.

"Even if a system has leaked out all its gas. As in, when you put your gauge on and it reads 0 psig."

Would this be with the compressor on or off, as on with no gas would I not it expect to pull into a vacuum when running?

Sorry, yer when the compressor is off.
When the comp is running and the system is low on gas its likely still to pull a vacuum.

Sorry if I am completely way of the mark here.

.
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See blue text

[JSimpson]

Hi @seanf, thanks for the reply

Sorry, just need to clarify, when you are saying likely to see pressures that are about 5-10 DegC below the box air temp, so if box air temp is -18, looking to see pressure readings in the -23 to -28 region?

Thanks for putting up with these stupid questions.

Cheers

James

[seanf]

Yes, about those sorts of pressures when the units box temp is running nearish the normal operating temp range.

If say a freezer was just switched on and the box was at +20 DegC youd probably see something different.
Youve got factors like different expansion devices, fan/no fan, single or multiple circuits, different designs.....

If you put your gauge on and a thermometer in the box of a fridge you know is working and has the correct gas charge ect.. just watch the pressure gauge and box temp as it pulls down from +20 to -20, thatll give you an idea of what to expect. You can then also do things like restict the air flow to the condenser or the evaporator and see what that does pressure and temp wise.

You should see the daft questions I still keep asking, and some crazy people think I know what I'm doing .

[JSimpson]

Hi @seanf

Thanks for the help, so I had an undercounter freezer where it was not cooling at all, compressor was running hot (70 DegC +) but not pumping and it was being supplied 240v, stuck on the gauge and the compressor wasnt pulling any kind of vacuum so I assumed seized motor or valve.

Decided to try a compressor replacement, had a like for like compressor both rated at 1/5 with the same CFM, changing it over and putting on a schraeder charge stub went ok, although I am out of practice on my braising.

Vacuumed and leak tested went fine, so no major leaks, charged with R600a and leak tested again, gauges read that it was charged and compressor was running fine.

Left for 24 hours, came in today, temp inside box was reading -24 degrees so thought yey, but the suction line had a huge build up of ice on it and the pressure on the gauge was showing 0 while the compressor was running, now I know I had no leaks as I tested it at the vacuum stage then after regassing.

Opening up the manifold valve connected to the charging stub also produced no gas, so I am a little confused, haha.

James

[seanf]

If your seeing 0 psig on your gauge, then it means its running at atmospheric pressure. Theres no pressure difference, so when you open your manifold to the ambient no or very little gas will come out. Or air sucked in.

Did you recharge the r600a by weight?

You may have over charge it. That would give you a higher than expected running pressue and too much cooling back to the compressor.
These wee fridges can be tricky to recharge even when your weighing it in, you need to account for gauge hose length and consider if its vapour or liquid. Use short hoses and sensitive scales.

[JSimpson]

@seanf

We will ignore my comment about my gauge readings, lets just say user error, haha.

Yeah I had overcharged it. Regassed it yesterday and all looks good with a box temp of -19.2 degrees today and no ice build up on the suction line. So I am going to put that down as a partial success, haha.

Thanks for all your help and I will apologise in advance for the likely many questions to come I will post on this forum.

James