Charging a/c unit using R407C

[carlfoster]

Hi, I'm new to the forum and I haven't worked in the refrigeration industry for 5 years.

Anyhow, I've started up again and need to work on some new a/c units that use R407C, a refrigerants I've not had experience with.

I've posted this in the fundamentals section as I feel it is and I just need a refresher on this;)

I've recovered the refrigerant of a packaged unit, as it lost some gas, and recharged from scratch, with liquid of course.

Now, the system uses a capillary tube rather than expansion valve and apparently is 2.5KW capacity unit. After charging a kg of refrigerant, and checking the temperatures, I believe that it is undercharged, because the pressures are too low (3bar LP and 14 bar HP) and because the first part of the evap coil is icing up.

The thing that throws me, is that the condenser fan has a potentiometer that varies the fan speed according to the head pressure and if you adjust this down the pressures obviously go up.

Thsi really confuses me as not only do I have to get the right charge but also the right fan settings. So what is the best way to do it?

[Andy]

Hi, I'm new to the forum and I haven't worked in the refrigeration industry for 5 years.

Anyhow, I've started up again and need to work on some new a/c units that use R407C, a refrigerants I've not had experience with.

I've posted this in the fundamentals section as I feel it is and I just need a refresher on this;)

I've recovered the refrigerant of a packaged unit, as it lost some gas, and recharged from scratch, with liquid of course.

Now, the system uses a capillary tube rather than expansion valve and apparently is 2.5KW capacity unit. After charging a kg of refrigerant, and checking the temperatures, I believe that it is undercharged, because the pressures are too low (3bar LP and 14 bar HP) and because the first part of the evap coil is icing up.

The thing that throws me, is that the condenser fan has a potentiometer that varies the fan speed according to the head pressure and if you adjust this down the pressures obviously go up.

Thsi really confuses me as not only do I have to get the right charge but also the right fan settings. So what is the best way to do it?

Hi Carl:)

welcome:)

If your working on a split it is best to pull all the charge, fix the leaks and ad in the correct charge by weight for both the standard pipe run and the addtional pipe run. There is no short cut this is the short cut;)

Now if it is a new unit you would need to evacuate and calculate the addtional charge for the extra pipe work.

Always charge as refrigerant liquid

Kind Regards Andy:)

[carlfoster]

Thanks for the quick reply Andy.

The unit isn't a split, it is a complete packaged unit. The company I've started working for actually manufacture these units, and therefore, we are the ones who have to determine and weigh in the proper refrigerant.

So, I have no info on what weight it requires. I've done some reading and I believe the best way is to charge the system based on the superheat calculations( which I think is the difference between the suction pressure-temperature and the actual temperature of the suction line after the evap.

I read though, that R407C has a rather large "glide" which would throw the superheat out by 5 or so degrees. The condenser fan speed also throws some of these figures off:confused:

Hope you can help.

Carl

[Andy]

Thanks for the quick reply Andy.

The unit isn't a split, it is a complete packaged unit. The company I've started working for actually manufacture these units, and therefore, we are the ones who have to determine and weigh in the proper refrigerant.

So, I have no info on what weight it requires. I've done some reading and I believe the best way is to charge the system based on the superheat calculations( which I think is the difference between the suction pressure-temperature and the actual temperature of the suction line after the evap.

I read though, that R407C has a rather large "glide" which would throw the superheat out by 5 or so degrees. The condenser fan speed also throws some of these figures off:confused:

Hope you can help.

Carl

Hi Carl:)
when using R400 series refrigerants dew point and bubble point are used instead of saturation. Your gauges take account of this (mostly) dew point I think. So evaporators ect are designed on bubble (mean average) point and when you take over dew point is what you use.
Your suction pressure design(bubble point) should be about 6K (design td) below the air on temperature + about 1k line loss.

The low pressure your reading maybe OK but check if it is dew or bubble point you are reading.

Kind Regards Andy:)

[carlfoster]

Thanks again Andy, I'll use a slide rule as that has both the dew and bubble points.

I have a pressure - enthalpy graph for 407C and, armed with this, I'll try and get the settings perfect.

Regards Carl

[Pooh]

Carl
what subcooling have you got, this is a better guide than superheat as you can guarantee youy have sufficient liquid going to the capillary, not a perfect method but as good as any. If your company designs them surely somebody has done the calcs to work out the charge.

Ian

[Andy]

Carl
what subcooling have you got, this is a better guide than superheat as you can guarantee youy have sufficient liquid going to the capillary, not a perfect method but as good as any. If your company designs them surely somebody has done the calcs to work out the charge.

Ian

and superheat;) probaby 1-2k of subcooling and 6-10k superheat dependant on expansion valve type and where you measure it.

Kind Regards Andy:)

[carlfoster]

and superheat;) probaby 1-2k of subcooling and 6-10k superheat dependant on expansion valve type and where you measure it.

Kind Regards Andy:)

Well, there is no expansion valve as it uses a capilliary tube to reduce the pressure. As to the subcool, we're able to get a high subcool due to the condenser fan being variable and I could get the liquid line down to 10 degrees if I wanted.

With regards to the superheat and settings, I've been plotting a Pressure-enthalpy graph, and the problem I'm having is where to read the temperature for the evaporator, as it seems to be a bit high directly after the cap tube, and then drops further after the distributor.

[Andy]

Well, there is no expansion valve as it uses a capilliary tube to reduce the pressure. As to the subcool, we're able to get a high subcool due to the condenser fan being variable and I could get the liquid line down to 10 degrees if I wanted.

With regards to the superheat and settings, I've been plotting a Pressure-enthalpy graph, and the problem I'm having is where to read the temperature for the evaporator, as it seems to be a bit high directly after the cap tube, and then drops further after the distributor.

Hi Carl:)

best not go too high with the subcooling, at least at the start:)

Take your pressure where you take your temperature, at the suction pipe at the evaporator outlet:)

It's nice to kno the pressure at the capillary outlet but not required for suction superheat.

Hope this helps.

Kind Regards Andy:)

[Quality]

You could maybe acheive satisfactory operation but could prove to be more succesfull if a better refrigerant was an option

[carlfoster]

Well, I've managed to achieve a measure of success on the unit, but I'm still a bit flumaxed:confused:

I've ended up charging about 1550 grams into the unit. We ran another unit next to it, exactly the same design, which only had 1350 grams of gas.

It ran at a much better back pressure, bringing nice cool gas back to cool the compressor. But the unit which had more refrigerant struggled to reach 5 bar LP and the suction was running cool but not as cool as the other, which had a smaller refrigerant charge:confused:

The only thing I can think of is that the gas hasn't been measured in accurately, or the capilliary tube is shorter on one.

We'll just have to test it longer to see if the compressor will cope.

[Andy]

Well, I've managed to achieve a measure of success on the unit, but I'm still a bit flumaxed:confused:

I've ended up charging about 1550 grams into the unit. We ran another unit next to it, exactly the same design, which only had 1350 grams of gas.

It ran at a much better back pressure, bringing nice cool gas back to cool the compressor. But the unit which had more refrigerant struggled to reach 5 bar LP and the suction was running cool but not as cool as the other, which had a smaller refrigerant charge:confused:

The only thing I can think of is that the gas hasn't been measured in accurately, or the capilliary tube is shorter on one.

We'll just have to test it longer to see if the compressor will cope.


In that situation I would pull out the charge, vac the system and recharge with virgin refrigerant liquid.

Kind Regards Andy:)

[carlfoster]

Thats actually what we did. We charged the unit up to 1kg, then slowly 100 grams at a time monitoring the pressures and temperatures each time. In the end, 1550 grams seemed to be the best.

[Brian_UK]

As you said Carl, maybe the capillaries are slightly different. Might be worth checking.

Perhaps run a contact thermometer along the capillary lengths to see what is happening and compare the two.

[g. amis]

Hello to all,

My problem is almost the same with the above subject. We replaced the compressor (3HP) of a split type aircon (3TR) and we decided to use R407C instead of R22. As standard practice, we removed mineral oil and change it with polyester oil. We also flushed the pipeline and replace filter drier. We charged the sytem same with the old charge (around 3Kg). When we run the system, low side pressure is only 10psi while the high side is only 160psi. We have tried to add R407C gradually but the pressure remains the same even the actual refrigerant charge goes up to 5KG. There is no cooling effect on the evaporator. The fins are very clean and the air flow is ok. Anybody who could explain and help us to solve the problem. Thanks...

[carlfoster]

Does it use a capilliary tube? If so, that could be blocked.

Being an older system, there could be a number of problems, oil in the evaporator, dirty pipes etc.

Hopefully you've thouroughly flushed the system through before fitting the new compressor.

[monkey spanners]

Maybe thats what caused the previous compressor to fail.

[Andy]

Does it use a capilliary tube? If so, that could be blocked.

Being an older system, there could be a number of problems, oil in the evaporator, dirty pipes etc.

Hopefully you've thouroughly flushed the system through before fitting the new compressor.


Hi would R407c not require a larger mass of refrigerant flow for the same capacity and being less dense would take up more area as a liquid, both meaning a larger diameter capillary line for the same capacity.

Kind Regards Andy:)

[carlfoster]

Interesting point Andy. I have no idea though and wouldn't know where to start to work that out.

[Brian_UK]

Hey Carl, download this, it'll keep you busy.

http://www.danfoss.com/United_Kingdom/BusinessAreas/Refrigeration+and+Air+Conditioning/Product+Selection+Tools+Details/DanCap.htm

[Tomking]

BrianUK you have good links. I've just joined and I've spent more time enjoying your links than reading the messages. I mean that in a good way. Thanks. (smile)

[Brian_UK]

BrianUK you have good links. I've just joined and I've spent more time enjoying your links than reading the messages. I mean that in a good way. Thanks. (smile)Thanks Tom, you are more than welcome.

It's the one problem that I find with the Web, once you start poking around trying to find information another page opens up even more information.

I feel that I need an automatic shutdown timer fitted sometimes :D

Not so much with the corporate websites but I'm always amazed at the amount of effort that has been put into some private websites to provide facts and figures.

[carlfoster]

Thanks Brian, I have downloaded it and input the parameters. I came up with a very different cap size to what is installed.

But then, I realised that it doesn't take into effect the distributer fitted to the evaporator.:(

It's still a good tool though.:)