R407C Sub Cooling

[rocport]

Hi wonder if someone can assist: We have fitted bigger outside condensers to enable operation in higher ambient temperatures. Refrigerant 407C originally sub cooling speed controllers set to maintain 40 deg C approx at the moment with an ambient temp of 19 deg C sub cool temp is 25 deg C. Will this cause a problem or should we try and raise to 40 Deg C?

Copeland compressers approx 30KW original charge 10.7 KG now 18kg after fitting bigger condensers.

[momo]

Interesting one: knowing the glide and vagaries of R407C I would try to keep to original design parameters and pressures otherwise odd things start to happen with the different evap/cond points of the refrigerant.

[rocport]

Thanks for the feedback, I do not have a record of the original settings but the current running pressures are 14.3 and 4.2 bar approximately

[Lowrider]

14,3 bar is 32dgr c. So is the subcooling 25 K or is the liquid 25 dgr C?

Does the condensor have an extra subcooler or is it just a larger condensor?

[nimamu]

Check the operation as follows first:
If you are condensing at 40deg C, first try to keep the sub cooling to about 5K (i.e 35 deg C liquid temperature).

I believe that sub cooling the liquid should not cause a problem. If the system works, then gradually increase the sub cooling temperature, while checking the operation of the system.

Regards

[rocport]

Hi,I am pretty new to the refrigeration but have been quoted the condensers have a capacity correction factor of 0.46 8K dt1 which is used to calculate the final rating of the condesers @ 47kw. could you explain what 5k at 35deg c relates to I have been informed of the glide regarding 407c. Sorry for asking the basics.

[Lowrider]

Well if your condensation temperature is 32 dgrc and you have a 5K subcool then the liquid temperature is 27 dgr C.

anything from 5 to 10K will be ok, if you have a larger condensor with subcoolcircuit you can even go as far as 12 - 14K subcool.

Keep an eye on the superheat and the discharge superheat and see if they stay in range.

[The Viking]

OK,
Let us all take a step back and look at this problem.

Rocoport, I assume that you have a manifold with gauges scaled for R407c, yes?

Then fit the HP gauge (via suitable hoses/connectors and so on) to the liquid pipe after the condensor (but before any receivers fitted).
Now take your electronic thermometer of reasonable quality and measure the temperature of the liquid pipe beside the schraeder (what's the correct spelling of that?).
From the temperature read from the R407c scale on your gauge, deduct the temperature you measured on the pipe.
This will give you your sub-cooling.

The above just to ensure that we are talking about the same things.

Now, if you have increased the size of the condensor, then the system will need a similar amount of refrigerant as with the original condensor, during the summer (read: warm periods). BUT during the winter (read: colder periods) you will need additional refrigerant in the system, how much depends on the change of internal volume of the condensor amongst other things.
To overcome this the receiver might have to be up-sized or a receiver will have to be fitted if none was there originally.

From your original post I also see that your condensors has fan speed controllers, these are not "sub cooling controllers" they are discharge pressure controls.

It's not just me being picky pointing out this difference, there is a reason just bear with me.

These controls comes in two types, pressure sensing and temperature sensing.

For your installation with oversized (compared with original design) condensors, my suggestion is that you should use pressure sensing ones.

The pressure sensing fan speed controller will keep the head/discharge/high pressure constant, depending on your system design this would normally be set at 18-22 bar.

The sub-cooling is then a resultant of how much liquid refrigerant you have in the bottom of your condensor. The more liquid there is, the higher the sub cooling will be. Normally an expansion device requires a sub-cooling of 4-6K to operate properly.

So, you have commissioned your system on the hottest day of the year. Set the fan speed control up to maintain 20 bar and charged with refrigerant until the sight-glass gone clear and you ended up with a sub-cooling of 2-6K, HEY, everything runs perfectly.

My guess is that you will get a call-out on the first cold night in November.

Why?

Because all of a sudden a much smaller condensor is needed in order to maintain the 20 Bar HP and the system will store a larger amount of liquid in the bottom of the condensor, your sub-cooling will also increase and you will see bubbles in your sight-glass.
The system have now dropped out on LP or the evaporator iced up.
This is not a fault but this is why you need the additional receiver volume.

If you take the above reasoning a bit further, you will see that the sub-cooling will vary but you will have to ensure that when it is at it's minimum it is enough to provide your expansion device with the liquid it requires.

So, to recap. The sub-cooling is telling you how much refrigerant there is in the system and is necessary in order to ensure correct workings of the expansion device. It has nothing, per se, to do with condensor size.

[rocport]

The condensors have a speed controller that monitor the HP input pressure to the condensor. HP Discharge pressure measured on some compressers found to be just under 15bar. Range of fan speed controllers 14 - 24bar.

For info the old condenser fan speed controllers were temperature sensing and monitored the condenser output temperature.

[The Viking]

Good, then my previous post is valid.

Are they (the FSCs) Johnson / Penn by any chance?

[rocport]

Taking onboard your comments regarding pressures is would you consider 15bar discharge pressure a little low for 407C i.e. you indicated the normal pressures as being 18-22bar. The orginal hp pressure before the condenser upgrade was approximately 18 bar. Your feedback on in this matter is appreciated

[The Viking]

It all depends on the system.
But as a general rule I would try and get it to run at around 20 bar.
Depending on what type your controllers are, you would start with setting the pressure (normally this is done directly on the pressure sensor/switch) then you would proceed to set the "differential" or deadband (normally on PCB), on "posh" controllers you will have an adjustment of "gradient" (how fast the fan ramps up and down), this is important to get right as if it gets too narrow it might wear out the fanmotor. I normally keep this "dead band" above 1.5 bar.

After you set all this up, you will most likely find that you got one adjustment knob you still hasn't touched, if you do then this is the minimum fan speed adjustment (and you should have taken it all the way down to MIN before you started the adjustments). Set it up so that the fan runs as slow as possible without "humming".(this is easiest to do with the compressor turned off, then you know that the pressure is lower than the pressure you adjusted the fan to ramp up at)

(Just remembered, it is easier to adjust the initial pressure setting with all the other options set at MIN)

[rocport]

What are the potential drawbacks of leaving system discharge pressure at 15bar.

I would like to take this opportunity to say how impressed I am with the knowledge database provided by this site and to thank The Viking for his in depth feedback on this matter.

[The Viking]

Well, the idea with the FSCs is to keep the head pressure and running conditions "level", you probably will not be able to keep the head pressure at 15 bar in the summer..
The more variations you get, the more the required charge will change.
There are also problems with "over condensing" but this is a totally different chapter of it's own.



I just found a note about fan speed controllers I put together for my guys a couple of years ago, see attached.

[rocport]

I am not sure what the ambient temperature was when the condenser hp set at 15bar what could potentially occur if the ambient suddenly reached 37deg C or -4deg during winter.

[rocport]

We have a copeland scroll compressor which has low oil level we have a pump device that can be used to top up oil when compresser is running (artic22?) are these safe to use and how would we know the correct level.

[rocport]

How can I get a molinares chart for R407C

[The Viking]

Lots of questions now.

1. Personally I would raise the setpoint of the FSC.

2a. Yes but pump the system down first and it will be easier for you.

2b. How do you know that the oil level is low?

3. Your supplier should be able to get you one, otherwise have a look here

[rocport]

1. We were informed by the installers the compressors work less with a sp of 15bar, albiet condenser fans work harder.

2a. Lebert semi hermetic lp switch service v/vs pumpdown ok. How do you pump down a copeland scroll compresser.

2b. pipework repaired after repair pressure test - vac - refill no oil siteglass - prior to repair record shows 95%

3. we have a pump with sealed oil- instructions fill with machine running- prefer the off method

[rocport]

Hello, pumped down and filled with no problem and thanks for the chart.

Can you thinks of any reason not to leave the setpoint at 15bar -i.e. if really cold day will the fans just stay off - or if a very low ambient being could the increased sized condensers cause overcondensing what would be the the first symptoms of this