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I'm trying to include facts as they become necessary. I don't know a better way to do this. If you have a unit that discharges air at 55F and do not have humidity control how far below 55F will the temperature go? Keep in mind that to remove moisture the discharge air DB and WB temperatures have to be the same.
One thing to keep in mind we are doing evaluations on systems that are working as designed and the design is good. When you have a system that is poorly designed or not functioning as designed or both you have to identify the problem and a good knowledge of psychrometrics ls very useful. In essence we are discussing psychrometrics.
No man can be both ignorant and free.
Thomas Jefferson
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An important point is that dew point of the air leaving coil is dependent on the cooling coil temperature. With modern large coils that are used by mid to high SEER coils, the dew point is 4-5^F above the coil temperature.
Example: 75^F, 50%RH, a 55^F dew point passes through a 45^F coil. The Dew point of the air is typically 4-5^F above the actual coil temperature, while the air leaving the coil is 54-55^F, about 90%RH.
If the return air was 75^F, 45%RH, a 50^F dew point, the dew point of the leaving cold air would be 3-4^F lower than when it entered. As either sensible/latent decreases, the coil temperature and capacity decreases slightly. the amount of moisture continues to decline to none until the dew point of the return air approaches the coil temperature.
Thus increasing or decreasing the cooling capacity or increasing the flow of air through the coil are the two most common ways of maintaining specific %RH in the conditioned space. Moisture is forced to condense on the cooling surface of the coil at the dew point of the air passing through it. This is why only the air coming is contract with is the coil temperature. The air mixes as it leaves the coil and has a uniform dew point/%RH.
Hope this helps.
Regards Teddy Bear
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
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Thanks Teddy Bear your input is always appreciated. We hadn't got that far but it's good you brought it up.
No man can be both ignorant and free.
Thomas Jefferson
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there is a reason the weather geek on the local news stop using RH years ago. they found out dew point was an easier way to explain humidity to the masses. they knew the difference between relative and absolute humidity, but hell try and explain that to even a HVAC manager these days.
even at 100% RH off the coil after an extended run the absolute humidity will continue to decrease.
hot goes to cold, always
wet goes to dry, always
delta dictates the rate of change
why Willis Carrier coined the term relative humidity is beyond me (ok I don really know if Willis did or not, but I always thought the psych chart was his doing. maybe it was Mollier)
EDIT, I realized why away getting another cup of coffee that RH is in fact very important to us. but it only confuses those not doing design or troubleshooting.
Last edited by ch4man; 05-15-2020 at 09:07 AM.
my boss thinks its possible to repeal the laws of physics
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ch4man the things you pointed out in your post #64 is the very reason I started this thread. I have stayed away from terms like absolute humidity here and some other terms so as not to side track the subject.
I'm glad you brought it up the way you did.
No man can be both ignorant and free.
Thomas Jefferson
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I think most of us have a good understanding of RH, dewpoint, and so on, but it's more difficult to apply concepts in practice. A physicist might understand the thermodynamics of how the Otto cycle engine works, but doesn't have a clue how to do a tune-up on his car. We're talking a lot about the physics of the cooling coil, but haven't really answered the original question, about why an over-sized system doesn't perform as well in practice. A lot of our jobs involve evaluating mismatches between how a system was designed, vs. how it works in practice, so this is a good discussion to have.
I was looking around online and found a copy of Munter's Dehumidification Handbook. It talks a lot about dessicant based dehumidification. It looks interesting. IDK how useful.
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There is a limit to what a cold coil can do. Beyond that, you need desiccant to get lower. If anyone is interested, I'll post the chart. You can only do so much with a cold coil.
Originally Posted by
RobertStephens
I think most of us have a good understanding of RH, dewpoint, and so on, but it's more difficult to apply concepts in practice. A physicist might understand the thermodynamics of how the Otto cycle engine works, but doesn't have a clue how to do a tune-up on his car. We're talking a lot about the physics of the cooling coil, but haven't really answered the original question, about why an over-sized system doesn't perform as well in practice. A lot of our jobs involve evaluating mismatches between how a system was designed, vs. how it works in practice, so this is a good discussion to have.
I was looking around online and found a copy of Munter's Dehumidification Handbook. It talks a lot about dessicant based dehumidification. It looks interesting. IDK how useful.
I do a triple evac with nitro to remove non condensables.
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Originally Posted by
BBeerme
There is a limit to what a cold coil can do. Beyond that, you need desiccant to get lower. If anyone is interested, I'll post the chart. You can only do so much with a cold coil.
Right now, my outdoor RH is 83%. I don't need it to be 5%, but I would like it below 70% indoors. Commercial buildings sometimes specify 50%. So around here, it's very relevant topic.
Last edited by RobertStephens; 05-15-2020 at 11:13 AM.
Reason: added one word
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I do a triple evac with nitro to remove non condensables.
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Originally Posted by
WAYNE3298
ch4man the things you pointed out in your post #64 is the very reason I started this thread. I have stayed away from terms like absolute humidity here and some other terms so as not to side track the subject.
I'm glad you brought it up the way you did.
i thought that was what you were getting at, but it took a day or two for me to add to the thread w/out sounding dumber than I am
my boss thinks its possible to repeal the laws of physics
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I haven't found anyone on this site that's dumb. If I thought that I wouldn't have started this thread. As previously said this subject is easy but complicated. It is easy to get confused.
I am a little surprised that nobody has ask BBeerme to post the coil info he has.
No man can be both ignorant and free.
Thomas Jefferson
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I think this is what you are referring to. As I mentioned, used to do a LOT of work on environmental chambers. And customers were always wanting to get just a bit more out of them. This is from empirical data and testing. These are the limits of removing moisture from air using mechanical refrigeration. If a customer wanted lower humidity's at a temp below the curve, told them not possible, you'll need to use desiccant driers.
Originally Posted by
WAYNE3298
I haven't found anyone on this site that's dumb. If I thought that I wouldn't have started this thread. As previously said this subject is easy but complicated. It is easy to get confused.
I am a little surprised that nobody has ask BBeerme to post the coil info he has.
I do a triple evac with nitro to remove non condensables.
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Here's a shopping mall in Malaysia that had shut down their HVAC for two months. 🙄
https://returntonow.net/2020/05/22/m...-conditioning/
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Interesting how the mold did not attack some things.
Originally Posted by
RobertStephens
I do a triple evac with nitro to remove non condensables.
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Originally Posted by
BBeerme
I think this is what you are referring to. As I mentioned, used to do a LOT of work on environmental chambers. And customers were always wanting to get just a bit more out of them. This is from empirical data and testing. These are the limits of removing moisture from air using mechanical refrigeration. If a customer wanted lower humidity's at a temp below the curve, told them not possible, you'll need to use desiccant driers.
i notice the curve intersects the 41* line twice. Why is that?
*********
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Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/
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Some sort of software plotting program using empirical data.
Originally Posted by
kdean1
i notice the curve intersects the 41* line twice. Why is that?
I do a triple evac with nitro to remove non condensables.
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Since the curve is drawn from empirical data it should be considered as a general curve. Oddly enough although not likely there could be an anomaly that causes the 41 line to be crossed two times.
No man can be both ignorant and free.
Thomas Jefferson
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My original one was hand drawn. Someone else did that.
Customers were always wanting low humidity's at low temps. I'd use that chart to let them know if it was possible or not. For example, at 20*C you are not going to be able to achieve 20% RH with mechanical refrigeration, you'd need to go with a desiccant air dryer.
I do a triple evac with nitro to remove non condensables.
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It's interesting that the mall photos show that the color of the items influence how they have reacted to the high humidity.
I'm surprised we haven't had any comments on that.
No man can be both ignorant and free.
Thomas Jefferson
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Originally Posted by
WAYNE3298
It's interesting that the mall photos show that the color of the items influence how they have reacted to the high humidity.
I'm surprised we haven't had any comments on that.
I'm not necessarily seeing a correlation to color and "growths". I figured maybe it had more to do with the material than color?
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