Compressor Amp Draw After Refrigerant Conversion

[intellitech]

Gentlemen,

I have been throwing a thought around in my head for a while so I thought I would turn to my fellow refrigeration techs to see what you all think:

Let's say a system is changed from R-12 to MP-39 or Hotshot. So now let's say I want to check the compressor's amp draw via the manufacture's performance chart..........but with the new gas in the system how would I determine what the correct amp draw should be in light of the different pressure readings?

For example: R-12 at 25 degrees is 25#, MP-39 is 22# and Hotshot is 28#. Slight difference at best. But the high side readings are a different story: R-12 at 100 degrees is 117#, MP-39 is 124# and Hotshot is 134#.

So how could I figure out what the correct amp draw should be when using the performance chart with data for R-12 and not MP-39 or Hotshot for example?

Look forward to your answers......

[icemeister]

I think what may be puzzling you here is you should be looking at the saturated condensing temperature of the refrigerant and not the pressure.

For example, say a given replacement refrigerant runs 40# higher than R12 at the same conditions. If you were to go into the R12 performance data using the new refrigerant's pressure, you'd see at a much higher expected amp draw than if you had used its corresponding saturated temperature.

(BTW, I always try to think in terms of saturated temperatures, not the pressures. ;))

I have found measured amperages with the HCFC service replacement refrigerants for R12, like MP39, R409A, etc., are fairly close to Copeland's Compressor Performance Calculator's numbers for the original refrigerant at the same saturated conditions. They will generally be a tad higher, but I'd say still within 5% or so.

[Dowadudda]

I recently was at a supplier hosted refrigerant seminar here locally where they were getting us up to speed on the next few years of bull crap going down. And I mean bull crap. :D. It's ridiculous.

But one thing I got from this was that, when looking at retro's for say a rack, there can be a savings based on the BTU's per pound, depending on if the gas new safe gas will work. As well it can also leave you with choosing a gas that will create a penalty. Just found that interesting. Just a BS post, talking out loud.

Also. This is stupid. Get this.

GWP. Global Warming Potential. I remember this seminar as being the first I heard of this too. Ridiculous. Our country has gone crazy.

[intellitech]

I think what may be puzzling you here is you should be looking at the saturated condensing temperature of the refrigerant and not the pressure.

For example, say a given replacement refrigerant runs 40# higher than R12 at the same conditions. If you were to go into the R12 performance data using the new refrigerant's pressure, you'd see at a much higher expected amp draw than if you had used its corresponding saturated temperature.

(BTW, I always try to think in terms of saturated temperatures, not the pressures. ;))

I have found measured amperages with the HCFC service replacement refrigerants for R12, like MP39, R409A, etc., are fairly close to Copeland's Compressor Performance Calculator's numbers for the original refrigerant at the same saturated conditions. They will generally be a tad higher, but I'd say still within 5% or so.

I was hoping you would chime in on my question :D.

Yes I am guilty of thinking in terms of pressures and not temps in my question........but that is why i asked it knowing I would get some good responses. I do not have access to the charts in the field (if I had a laptop I would have them at the tips of my fingers) but have printed some of them out at home when questioning a pump's amp draw.

I will keep 5% in mind.....thanks Ice!

One more thing......the link i am leaving is an article that has always had me thinking in terms of pressures when it comes to using a performance chart.

http://www.achrnews.com/Articles/Feature_Article/f170a045d9f5a010VgnVCM100000f932a8c0____

[basser]

Gentlemen here are my thoughts. If you convert an R-12 system to a replacement refrigerant then use the R-12 curve information for your amp draw. This is because most compressor manufacturers don't have the data published for all refrigerants or even all the ones they do approve. With that being said, I typically was taught to look at pressures instead of temperatures. Either is OK, and I'll try to explain why.

Let's look at a KALB-0150-CAB, medium temperature application, with pressures of 15/185. If I look at an R-12 sheet for 230 volts, I see 6.8 amps for that condition. If I have 401A in the system and it has the same pressures 15/185# my amp draw should be the same, because the load (pressure) on the motor is the same. 134a 15/185# the amp should be the same, because of my pressure load.

Now let's look at temperature, same compresssor, same load, 15/185#. With R-12 this equals 13°/132°, look at the R-12 compressor data and the amp draw is 6.8 amps for these saturated temperatures. Look at 401A saturated temperatures for those same pressures, 13°/115°. Now if you look at the R-12 data sheet, because that is all we have, 13°/115° equals 6.4 amps. 134a equals 13°/125° for 6.7 amps. If you want to use the saturated temperature, I believe you can, but convert the system pressures to R-12 temperatures for use on the curve sheet. To me this can set up alot of confusion.

I hope this helps. As you can see, we are not talking major amp differences on this small compressor, but it is around 6%. I believe either way is OK, I just believe it can be confusing to why we switch refrigerant information.

I'm interested in the respose this will generate. Please respond with your thoughts. This is a subject that has caused some confusion in the past, but I don't remember seeing a thread like this on here. Great topic.

[icemeister]

Let's look at a KALB-0150-CAB, medium temperature application, with pressures of 15/185. If I look at an R-12 sheet for 230 volts, I see 6.8 amps for that condition. If I have 401A in the system and it has the same pressures 15/185# my amp draw should be the same, because the load (pressure) on the motor is the same. 134a 15/185# the amp should be the same, because of my pressure load.

I want to address a couple of points here.

First, for most all R12 replacement refrigerants the suction pressure will run pretty close to that of R12, so the low side numbers don't really affect this discussion much at all.

However the discharge pressures are typically significantly higher at the same saturated condensing temperature. (R416A would be an exception to this rule.) When analyzing a retrofitted system I base it on the fact that the compressor capacity and the power requirement....and therefore the total heat of rejection...should be roughly the same as with R12. This then leads me to say that since the resulting condenser TD would also be about the same then the resulting saturated condensing temperature (SCT) should also be quite close to that of the original R12.

So when I have an operating system and I want to check the compressor performance to the curves, I look at my gauges and note the pressures, but I go into the curves with the corresponding saturated temperatures. (The published curves and the Performance Calculator program use both press & temp) As I see it, using the temperatures vs the pressures gets you closer to what the new refrigerant is formulated to do....ie, act like R12.

Secondly, it's a minor point but....because the "pressure load" is the same doesn't necessarily mean the amps would be the same. The power requirement also depends upon refrigerant density and mass flow.



Now let's look at temperature, same compresssor, same load, 15/185#. With R-12 this equals 13°/132°, look at the R-12 compressor data and the amp draw is 6.8 amps for these saturated temperatures. Look at 401A saturated temperatures for those same pressures, 13°/115°. Now if you look at the R-12 data sheet, because that is all we have, 13°/115° equals 6.4 amps. 134a equals 13°/125° for 6.7 amps. If you want to use the saturated temperature, I believe you can, but convert the system pressures to R-12 temperatures for use on the curve sheet. To me this can set up alot of confusion.

I can see how this method can be confusing.

If you follow my equivalent SCT approach I explained above and just go into the charts using saturated temperatures, it's really not all that confusing at all.

My point is this. The basic heat transfer from the condensing temp to ambient temp (the TD) remains about the same regardless of the refrigerant if the evap load and power input is also about the same (the THR). We know the pressures will be much different, so just use the same saturated temperatures for both the replacement and the original refrigerants. It's much easier.:D

Basser, I agree this is indeed a great topic. :)

[coolhand55]

I thought it was the compression ratio that determined how hard the motor worked?

[basser]

Icemeister I agree with your post.


So when I have an operating system and I want to check the compressor performance to the curves, I look at my gauges and note the pressures, but I go into the curves with the corresponding saturated temperatures.

By using the gauges, in this case you would be looking at the R12 saturated temperatures. I believe we are saying the same thing here.


Secondly, it's a minor point but....because the "pressure load" is the same doesn't necessarily mean the amps would be the same. The power requirement also depends upon refrigerant density and mass flow.

You're exactly right here. Density and mass flow (lbs/hr) do indeed come into play here. I didn't mention it because my post was getting long enough and that is getting more in depth.

Also, the point you make on total heat of rejection is very important. If the THR increases capacity certainly will decrease along with possibly creating other problems.

[stanbyyourword]

good post and subject , i think a few people were getting side tracked operating pressures definitely
effect work done by a compressor, but the original topic was comparing drop in refrigerants via comp perf curve , where using sat temps is the way this would be done as icemeister said , just thought id clear the difference in topics a bit regards!! stan and the massflows w new hfc dropins is a whole other topic which ive had nightmares with!!! ;)