Evaporator performance question

[sahinoz]

I have two freezer evaporators (134a) which are identical by design (geometry). However, when I test these for energy, one of them always consumes more kWh. I am pretty sure that I am doing the test right, but I just can't tell why do I get different results? Any ideas?
Thanks,

 

[IRstuff]

What is the variance?

Do you actually know whether the variance is outside of the norm?


Why would you expect the wear and actual construction to be identical?

TTFN

 

[sahinoz]

Yes the variance is outside of the norm. I would say numbers vary by 25%. They are both tested under the same conditions though. Basically, it is a unique design manufactured by two different companies. when you look at them you won't be able to distinguis one from the other. Material that they are made out of is the same (with 90% confidence) I don't know what would affect the performance?? I am lost, i can't think of anything

 

[Yorkman]

Man! that leaves the field wide open, You could be talking slight difference in the refrigerant charge. One condenser or evaporator dirtier than the other.
Could you be more detailed in the test numbers are the pressures, superheats, and amount of subcooling the same, do you know for sure they are both seeing the same load? Are these reciprocating or scroll type compressors. Are they feed from the same electrical service is it an issue with power factoring? 25% is a pretty big difference.
Have you tried to plot their performance on a pressure enthalpy diagram maybe something will stand out there.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[HVACctrl]

Could one be insulated differently than the other? Perhaps you wouldn't be able to tell visually if so.

There has to be some difference between the two.

Ed

http://www.engineerboards.com

 

[sahinoz]

Basicallay what I do is, I take a freezer, and test it with one of the evaporators, and take that one out and install the second one in. test conditions are the same for both. I do the same thing for 5 pairs. Didn't test just one unit. Visually the only difference is that one looks more shiny than the other one. Thanks all for the answers, any more ideas where I should look at?

 

[sahinoz]

One more thing, tubing and fins are made out of Al. But not quite sure if they are both the same Al alloys. Would this has something to do with the performance. (I mean the source of my 25%)

 

[Yorkman]

Obviously there is more here than meets the eye. You did not say if the pressures and temperatures of the two evaporators were the same ie; superheats, subcooling, ect. Which measurements are leading you to the conclusion that one of the evaporators is underperforming. Is it possible when you connected the evaporator in question the evacuation was less than perfect and you are fighting air or maybe moisture restricting the metering device of inline filter.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[imok2]

Why can't you calculate it out by Q(total heat) = 4.5 x cfm x Enthalpy difference. if you take the beginning dry bulb and wet bulb temperature and ending dry bulb and wet bulb you could calculate total heat removal and at least it would tell you that 1 coil is more efficient and therefor is consuming more KW

 

[pennpoint]

sahinoz
Hard to detect but a true fact.
If the tubes are not swedged to the fins properly you can expect inconsistent and very poor performance. Stick to the manufacture that provide the better performing coil. Some manufactures draw a mandrill through the tube (best) others use hydraulic pressure to expand the tubes (a poor method <MHO>).
Good Luck

Best regards
pennpoint

 

[sahinoz]

I found out that the fins and the tubing has different surface roughness. The one that performs better has a better surface finish hence the water can hold on to surface easily. (ice build up clogging the air flow?) What do you guys think about the significance of surface finish on evaps?

 

[Yorkman]

The ice that formed on the coil face, was this on the smooth or rough surface in your test? I could not tell from your post. If the ice formed on the rough surface it could be because of reduced air flow because of increased resistance to air flow.
If you compared a rough surface to a smooth surface I'd say it's likely that the rough suface might have an extended surface area, providing a greater rate of heat transfer. As long as the air flows across both coils was measured and found the same.
I would caution that with a rough surface you might see a greater air flow resistance and reduced air flow, for accuracy I'd want to verify the air flows as being the same. That way you could calculate the heat transfer of both styles and be assured that you were comparing true coil performance, and not a greater [&Delta]H because of reduced air flow.


I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Yorkman]

Oh by the way Sahinoz,
thanks for returning to this forum with some details to your findings. It's always good to hear the outcome of an investigation.
OOpps I see I made a typo I wanted [&Delta;]H not, [&Delta]H. Oh well you got the drift.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[imok2]

Sahino, When dealing with fin geometry you can improve the overall heat-transfer coefficient, by lessening the airside film coefficient. Like velocity, fin geometry can be specified as part of the design of the refrigeration system. For evaporator applications, coil fins are usually stamped into waveforms resembling corrugated cardboard. These waveforms create turbulence in the passing air stream, which lessens the resistance to heat transfer. More exaggerated waveforms produce more turbulence thus better heat transfer and more KW

 

[sahinoz]

Yorkman and all others, thanks a bunch for all the comments. I was busy with something else and finally was able to find some time to get back to this post. In the mean time, I've conducted an EDS analysis on both tubes and fins. Here's what I found;Tubing material for both evaps are identical, they both contain 99.99% Al, I am guessing it is Al 1060 (no difference in thermal conductuvuty). However, fins vary a little, fins on the one evaporator that performs better contain some Zn and Fe. The other one is again Al 1060, no Zn, Fe, nothing. One of them could be anodized for numerous reasons. I'll look into this.
On the other hand, I immersed both evaporators into a water tank and then pull them out. The one evaporator with a rougher surface retained much much more water than the one with a smooth surface finish. Can you imagine these two evaporators running in a freezer, these parts sweat pretty much and if that water doesn't drain off the surface, it will remain on evaporator and end up freezing. I think this is one of the cons of rough surface. Also, rougher surface means poor surface contact with fins as well, you would want to create snug fit and it is easier with a smoother surface. Anyways,these are just my opinions ofcourse, any suggestion, comments are most welcome.

 

[insult2injury]

There are also advantages to rougher surfaces as well. The rougher surfaces induce more turbulence and thus higher heat transfer coefficients as well produce a larger surface area.

 

[sahinoz]

How come I am not getting a better performance from my evaporator that has rougher surface then?

 

[Yorkman]

It could be that the coil with the rough surface has a greater pressure drop across the face of the coil(air side). If this is the case then perhaps you did not compensate for this and you are actually moving less air acrossed the coil than you thought.
This is all speculation, as you have yet to give any body here any real numbers to work with.
design CFM
entering DB WB temperatures
leaving DB WB temperatures
load in the space BTU/Hr
type of refrigerant
suction pressure
discharge pressure
superheat
subcooling

With some additional data maybe we could come up with some ideas.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI