Basic question about refrigerant to use

Hello, I am a chemical engineer trying to repair a broken laboratory chiller bath, and I am trying to understand how the refrigerant I use will affect the lowest temperature I can obtain with it (R12 vs R22)

I have a pretty good understanding of thermodynamics so that's not a problem. From what I have been able to find by researching refrigeration systems, R22 can provide more cooling capacity for a given compressor, which does not mean that it can cool something to a lower temperature. R12 is a no-brainer if the system was designed for R12 because of the damage that R22 causes to rubber seals, resulting in leaks.

Basically I am understanding that the minumum temperature possible is not as important as cooling capacity. If I may make an anology, it's like the difference between using a liquid coolant line that is -40° but flowing slowly, versus using a coolant line that is -10° with a higher flow rate. The amount of heat energy (enyhalpy) that can be removed from a system is the same for either one, if you ignore the boost in compressor capacity provided via R22.

Am I making any sense here?

Thanks in advance,
Craig

If you want the best of both worlds, good capacity yet low head pressures I'd go the HC route and use R-290.

Or if you want to get fancy you can use HFC HC blends or even just HC blends.

How ever it is a four part equation, what capacity/temp/and flow/ and delta are you needing?

Quote:

---

Originally Posted by cbfull

Am I making any sense here?

---

Short answer Craig,

NO....


If you are looking at just repair something, then keep the gas the same.
In order to convert a system to run on a different refrigerant there are numerous factors to consider, the efficiency being one but it is quite far down on the list.

You are talking about R12 and R22, both these gasses are old technology and should not even be considered to use as a replacement or in new systems. They are both considered to damage our environment.
R12 has been phased out and should not be used under any circumstance, R22 is being phased out and should not be available after 2010 (currently you can recharge existing R22 systems but not install new systems running on R22)
This is true in the "civilised" world, I am not so sure about the legislation "over there" but, regardless of legislation, both R12 and R22 are in the past.

As a basic guide: any refrigerant worth considering for using as a replacement will have 3 numbers (i.e. R134a, R407c, R410a and so on)

But, you will have to address a lot of issues before you get there, expansion device, pipework, compressor, oil, condensor size, evaporator size, running pressures, safety devices, to name but a few.

:cool:

Quote:

---

Originally Posted by The Viking

This is true in the "civilised" world, I am not so sure about the legislation "over there" but, regardless of legislation, both R12 and R22 are in the past.
:cool:

---

We are not civilized on this side of the Pond?:confused:
Excuse me I have to go, the Natives are attacking again. Those arrows HURT!:)

Quote:

---

Originally Posted by NH3LVR

We are not civilized on this side of the Pond?:confused:
Excuse me I have to go, the Natives are attacking again. Those arrows HURT!:)

---

You think you got problems with the natives??

Over here it's Englishmen everywhere!:eek:

:D

Quote:

---

Originally Posted by The Viking

But, you will have to address a lot of issues before you get there, expansion device, pipework, compressor, oil, condensor size, evaporator size, running pressures, safety devices, to name but a few.

:cool:

---

Especially the condenser needs to be changed due to the increased heat regain.

Quote:

---

Originally Posted by Peter_1

Especially the condenser needs to be changed due to the increased heat regain.

---

i think we've had this discussion before under condensing would it be or what ever you tech people call it.:D:D:D:D

Lrac

Thanks for your responses guys, while I do have a degree in engineering, I am not qualified to repair this Lauda refrigerated recirculator myself. I have an experienced and qualified HVAC friend who knows his stuff working on it for me. He recommends the same as you guys, keep the gas the same.

The problem is, this unit is fairly old and it is not clear what refrigerant was originally in it. He tells me that by looking at the capillary design, it's his bet that it was R12, which he has on hand. He has already located and repaired the leak, and he has determined that the compressor needs replaced as well. I have sourced a replacement from Brinkmann, but it's a bit pricey and I am forced to wonder if there is a cross-reference for the part so that I might be able to find it as a standard part from another retailer ($270 is a bit steep, but I'll pay it if I have no other choice). Based on those two details (leak and bad compressor) the diagnosis would appear to be that it was run continuously for who knows how long with the refrig slowly leaking out. I'm not sure exactly how that would cause the compressor to fail unless the oil was leaking out as well or air was leaking in, it's not something I am familiar enough with to guess, and really not that important anyway.

I was originally wanting to modify the unit to provide cooling down to something like -40°C, but I think that's excessive and not really even worth the trouble.

Any other thoughts you guys might have are of couse appreciated.

Craig

Thats actualy cheap for a compressor.

As for why a leak can kill it? As the gas leaks out the gas that would normaly cool the compressor gets thinner and thinner, as a lab tech you should know a vacuum is a good insulator but all so decreases the resistance of the wire insulater. So over heating + decreased electrical resistance = dead compressor, to over simplify it.

Quote:

---

Originally Posted by cbfull

I am a chemical engineer trying to repair a broken laboratory chiller bath

Craig

---

There is a reason that you are a chemical engineer and I am scientific refrigeration tech. So why are you trying to do my job? Does that mean that I get to do yours?:rolleyes:

Sorry to be an A$$ but I deal with this everyday. If the water bath is old enough to still have the mercury thermometer, with the dial on top, that you set the temp with. Stop what you are doing and get a new H2O bath. If you need -40c in the bath, stop what you are doing and get a new one that does what you need it to.

If you are going to use it for what it was designed for, then get me all of the numbers that are printed on the compressor and I will see if I can find you a compressor. Tell me where you are located and I will see if there is somebody I recommend using for the repair.

Ugh, I was actually trying to over-simplify my situation so that I didn't waste any time with details that have nothing to do with the reason I decided to ask for help.

I most certainly am not trying to do your job, I thought I was clear when I said above that I have an expert who is willing to repair it for me, and I am paying him (not that it is any of your business). I said nothing about mercury, it is a digital controller, and I see no reason to shell out a couple thousand bucks for a new one when I have one that can be repaired for far less.

I have located a replacement compressor and I plan to buy it, my question was about the type of refrigerant to use since the serial info on the unit has changed ownership a couple of times, they don't seem to know what was originally in the unit. The refrigeration repair guy tells me it is best to find out what used to be in it, and go from there. I just don't want to spend a bunch of money out of my own pocket (this unit belongs to me, I got it for a steal and I'm trying to get it working) and find out I made the wrong choice and then have the thing fail again. I'm being patient and doing research, and I sooo grateful that my HVAC friend is being so accomodating. I am absolutely lucky that he is willing to help me.


I don't know if I am being presumptuous or forgetting to acknowledge some basic respect for this community but I apologize If I have offended anyone here. I just want to educate myself so I thought I would ask some questions. That is what a forum is for, right?

I was just reading something today on the selection of refrigerants. The first part of the paper is enviromental considerations, but the last part and appendix has data on refrigerants.
http://www.airah.org.au/tec_pub.asp

Obviously if you are just repairing a system, the evap, condenser and flow control device is already there, so you'll be limited to refrigerants that run at similar pressures to the original refrigerant.

If it was capillary fed, then you will need the same gass, otherwise it won't work.

I suggest that if you need to replace the compressor anyhow, replace then the condensing unit and install a TEV on the evaporator.
You then can use any gass and the efficiency will also better.

If you don't know the compressor type, grind it open and measure bore x stroke x number of pistons to find the total theoretical displacement.

Quote:

---

Originally Posted by cbfull

I am paying him (not that it is any of your business).

---

You are correct, it is not my business. thank you for sharing.

Quote:

---

I said nothing about mercury, it is a digital controller,

---

You did not say anything at all. I have researchers wanting me to repair the old ones, with mercury, once a month. Since it is digital it is probably worth repairing.

Quote:

---

I have located a replacement compressor and I plan to buy it, my question was about the type of refrigerant to use since the serial info on the unit has changed ownership a couple of times, they don't seem to know what was originally in the unit. The refrigeration repair guy tells me it is best to find out what used to be in it, and go from there.

---

I can help if you will tell me the manufacturer, model number and serial number of the water bath. I can also tell from the old compressor manufacturer and model number on the compressor and the coldest temp that the water bath is supposed to reach.

Quote:

---

I don't know if I am being presumptuous

---

You are not, no problems

Quote:

---

I apologize If I have offended anyone here.

---

Speaking for myself, you have not and there is no need to.

PS. Also with the compressor numbers I can probably find you a cross so that you will not have to buy it from brinkman and I can save you a couple of bucks.

Quote:

---

Originally Posted by Peter_1

I suggest that if you need to replace the compressor anyhow, replace then the condensing unit and install a TEV on the evaporator.
You then can use any gass and the efficiency will also better.

---

On these types of H20 baths, they have everything crambed in there with no room to spare. Also the cap tube goes into a stainless steel tube evap. Brazing it back in is a bear.

Thanks so much guys, those responses are extremely helpful.

The unit is a Lauda RMT6 (more info to come, see below), the leak was buried in some rigid foam insulation but the awesome tech guy dug in and found a bad compression fitting, which he fixed.

It is a capillary type, and he said that he is almost certain that it was designed for R12. However, while researching the serial info, there was a blurb in an old manual suggesting that R22 was used for that unit. At that point I stopped everything so I could sort it out. I don't want to charge a gas and then find out it has to be removed, due to environmental concerns and risks. I certainly don't want to waste his valuable time by making mistakes that then have to be corrected.

I will get the model and compressor info posted early next week, and I must tell you guys that your assistance is very, very appreciated. I wish I could be more helpful to you guys with my engineering and thermodynamics education, but to be honest, it was all so incredibly confusing to me, I don't even know what I got out of it, other than the knowledge that I don't ever want to study engineering again.

Thanks SO much.

Craig

I just got two RMT 6 H2O baths in the shop this morning. It will be a day or two before I start on them. I will let you know what I find out on the refrigerants.

Quote:

---

Originally Posted by ultralo1

I just got two RMT 6 H2O baths in the shop this morning. It will be a day or two before I start on them. I will let you know what I find out on the refrigerants.

---

That is great! It's interesting that on the Brinkmann website they claim to have even dicontinued model manuals available, but it is apparently limited.

I know that the modern units use 134a, but it sounds as though yours may be before that time. Any idea how old they are?

Craig

Quote:

---

Originally Posted by paul_h

I was just reading something today on the selection of refrigerants. The first part of the paper is enviromental considerations, but the last part and appendix has data on refrigerants.
http://www.airah.org.au/tec_pub.asp<-->.

---

, Paul, I see no problem with posting a link direct to a pdf file although I would suggest that a note about the file size is useful. Not everyone has a fast internet connection so it's good to be warned.

Also, there is no harm in posting both links, the site and the pdf.

Cheers, Brian

I have not been following this whole thread, but just happened to notice something I would comment on.

Quote:

---

Originally Posted by paul_h

Obviously if you are just repairing a system, the evap, Condenser and flow Control device is already there, so you'll be limited to refrigerants that run at similar pressures to the original refrigerant.

---

Not quite right. Let me explain...

Pressure equivalency is important, but only part of the overall problem for obvious reasons.

The refrigerant selected is very important from the standpoint of it''s thermodynamic properties. Compressors just pump vapor, which they will do no matter what the refrigerant is.

The very important part of refrigerant selection is due to it's gas density and enthalpies (liquid and vapor).

You need to be aware of the heat absorbing capacity of the refrigerant since each pound or kilogram circulated by the compressor will only pick-up so much heat.

Secondly, the gas density determines how many pounds or kilograms of refrigerant each cubic foot or cubic meter of vapor will carry.

Remember the compressors pump volume (not kW cooling or Tons of Refrigeration), so if the gas density is lighter than another refrigerant you are pumping less mass flow.

If you pump less mass flow, and the enthalpy difference is less (than the original refrigerant) your refrigerating capacity goes down. Conversely, if the refrigerant has a higher density and greater enthalpy difference the cooling capacity can greatly increase.

Just got off the phone with brinkman. The RMT 6 family of H2O baths use R12. The alternate refrigerants are R134a and R409a. The original charge was R12 220 grams. Your HVAC buddy should be able to charge it up now. Also make sure that he replaces the filter dryer.

Awesome! Thanks for the info. My unit is also an RMT 6. Let me know how the units come along as you repair them. I am going to be ordering a new compressor very soon, but Brinkmann wants almost 300 dollars and I would like to get one from an alternative source if I can and save some money.

Also, if you have a recommedation as to where to get a new drier I'm all ears.

Thanks a bunch,

Craig

WAIT!!!!!!!!!
I got back to the office this afternoon and had a message from brinkman. They said to use R22, 160 grams. I called back for verification because that did not make any since to me. Of course everyone is gone for the day.

Also I have a price of $125USD for a replacement compressor. My supplier is checking the specs to makesure it will physically fit in the space. The original compressor is a Danfoss PW 3.5K7. Is that what yours is?

That's the same kind of confusion I got on my end. First they told me R12, then they told me R22. It is for this reason that I have halted repairs.

Did you get a copy of the manual for the RMT 6? If you need I can send you a pdf of it. It doesn't state what refrigerant to use, but it does specify it as "Super" RMT 6 (whatever that means) with a lower limit of -30°C.

I thought R12 was typically used for applications requiring temperatures down around 0 to -10°C. But I don't know squat yet I'm still sorting through all the basics just to educate myself as much as possible.

Since -30°C seems kinda low for a basic chiller, does that suggest a lower boiling refrigerant like R22?

I have no idea what compressor is in mine, neither does the expert. There is no writing anywhere on the compressor, and it's packed in there pretty tight. Did that quote for the compressor come from Brinkmann? I just looked through Danfoss' website and I couldn't find any model PW, must be discontinued. I suppose it's worth contacting them to see if they can supply specs/dimensions for that part number, if they recognize it (maybe even a substitution).

Keep me updated.

Craig

It will be monday before I get to talk to the brinkman engineer about the refrigerant.

You are correct that the comp is discontinued from danfoss. I have my suppiler making the calls to make sure the dimensions of the replacement will work. I will let you know the details.

Well, I called Danfoss and asked them if they have any specs on the PW 3.5 K7. They annoyed me by asking me what my temperatures are. First of all, that is so vague, there are so many temperatures I could specify, then they told me to find out what cooling capacity is required from Brinkmann.

I called my Brinkmann contact and he said he will try to find me any info he can, because the operators manual specs a heat removal capacity of ~950Btu/h@0°C, which is ludicrous. It would take a rather large compressor to manage that, not to mention the capillary design is fixed and likely isn't designed for a compressor like that.

Craig