Please define "small temperature" in regard to HVAC 2

[kbparham]

I'm a chemical person not a mechanical person - please help me understand what the HVAC Chiller Mechanic is referring to when he speaks of "the small temperature" he refers to this differential (I assume) as a means of evaluating whether the condenser tubes are fouled due to poor water treatment and thus are inhibiting heat transfer. I did not think there was such a simplistic approach to determine if loss of efficiency was due to water treatment or via mechanical problems - refrigerant, bearings, pumps, etc.... We'd always pulled the end bell of the condenser and examined the tubes to make the determination if water treatment was the problem. Can anyone enlighten me?

Thanks!!
kbparham

 

[BigInch]

You should be able to correlate heat exchange efficiency by monitoring all flows and the temperatures of all streams going into and out from any heat exchanger.

Usually they are designed such that there is one large "process" flow volume, in the AC part of HVAC work that would be the air supply going into the building, and a smaller flow, in your case, of refrigerant that is cooling that air.

Typically the air might enter the exchanger at 80ºF and exit at 68ºF, the difference being 12ºF. The smaller flow of refrigerant usually requires a much greater temperature difference to affect the larger air volume, so its entry temperature might be 48ºF and exiting at 87ºF. That difference being 39ºF.

The efficiency of the heat transfer process can be correlated to any changes in the differences between the air flow temps and refrigerant temps over time. As efficiency worsens, the temperature difference of each stream as it enters and exits would get smaller and smaller. You could correlate efficiency to either or both streams and it appears that your tech has correlated efficiency in his mind to what would be the temperature differential of the air flow in the above example, since that was the "small dTemp"



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[TurbineInvention]

The greater the temperature differance the better the heat exchange. The smaller the differance the less work that is being done. One 1/16th of an inch of scale on the inside of the tubes can reduce efficiency by as much as 20%. When was the last time you sprayed the condesing coil off in your back yard. Cotton wood, grass, lint or dust will do the same thing to the efficiency of your home cooling system as well. Years ago when I did service work 30% of the time the only problem was the furnace filter was dirty. In this case not enough air moves accross evap coil. This causes the boiling point to drop below freezing. The coil freezes, the lack of air flow stops the boiling and liquid returns to the compressor destroying it. Hope this helps.

 

[Yorkman]

The small temperature difference could also be called "approach". It's the difference between the leaving condenser water temperature and the leaving refrigerant liquid temperature, or depending on the machines design saturated liguid refrigerant temperature. When tubes get fouled in the condenser the difference between these two mediums increases. Depending on the extent of the fouling you may also see a decrease in the [Δ]T for the entering and leaving condenser water. Air and or non-condensables in a machine can also cause an increase in the approach. Depending on the vintage of the machine 3-5 degrees F is about normal for an older 20+ years machine, 1-4 for a newer machine. If your running 6-10 degrees I would suspect fouling of the tubes or air.

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

 

[kbparham]

Thank you Yorkman, I'm not a real engineer either, but I've been staying the last few nights at a Holiday Inn Express.


KBParham

 

[chadl]

Yorkman,

I'm not an hvac guy so I could be wrong, but I always thought that approach was more like the inlet coolant temperature and the outlet air temperature (in a typical AC system) ie a good (low approach) gets the air going out as close to the temperature of the coolant coming in. with an ideal heat exchanger producing air at the same temperature as the coolant coming in. I guess in a condenser the coolant temperature should be constant (more or less) is that the difference, or I'm I wrong here?

chad

 

[TD2K]

If you aren't a mechanical person (and I'm not familar with refrigeration either), why don't you just ask the mechanic 'what do you mean by that?'. It's a quite reasonable question for someone who doesn't routinely work in this area. Use his knowledge to help you learn also. Engineers do not know everything.

 

[Yorkman]

Easy Guys,
Fair question/no foul! And you are right chadl in descibing the approach for a hydronic coil. It is a little different when relating it to a flooded evaporator or condenser.
Remember the inlet that you see at the cooling coil is actually the outlet that I see at the evaporator bundle. So my approach is figured differently. It just a matter of looking what the machine is trying to produce versus what your cooling coil is producing. Does that make sense?

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

 

[kbparham]

T2DK,
I appreciate the tip, but wanted to verify the information he provided. Just because your mechanic tells you something -- doesn't mean you should take their word as gospel right?? In any event, I wanted to know if this "diagnostic" was widely accepted, or an engineering "wives tale." I searched on the web at large for the term "small temperature" that he used and was unable to find any definitive information. It was then and only then that I opted to bother you guys on this forum. As it turns out I found that non condensables can also be responsible for the differential - not just condenser tube fouling as he had mentioned. The wide variety of responses on this forum is what makes it such an asset. I learn more from you guys as a group, than any one person could ever explain.

Thanks guys,
KBParham

 

[TurbineInvention]

Yes the approach is always the temp of the fluid transferring energy. If the refrigerant is 40 you never reach 40. In this case I think small temp is the difference between the inlet and outlet of the fluid being cooled. Less difference (small) less heat transfer. Anything coating the inside of the tubes will insulate the cooling fluid from the fluid we want to cool. Hope that helps.

The approach can also be the wet bulb temp when talking about evaporative cooling towers. This is in relation to relative humidity and can allow the cooled fluid to go lower than the ambient temp.

 

[BigInch]

TD2K, You know, I decided that I really really like your answer. I'm voting for you. 2 stars!

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