Coil and dehumidification 5

[saeedplc]

Dear Experts

In one of the Air handling units in our company cooling coil is used for dehumidification but although the cooling coil valve is 100% open and its temperature is lower than dewpoint but dehumidification is not done at all.
Could you please let me know what parameters should i check and what parameters are vital for dehumidification?
If you need any data please let me know.

Regards,

 

[MintJulep]

Does the air cool to its dew point?

 

[saeedplc]

Yes as i mentioned the air is cooled to its dew point

 

[BrianPetersen]

So ... What is your basis for the statement that "dehumidification is not done at all"?

Is liquid water dripping off / out of the evaporator? It should be.

Does the liquid water dripping off the evaporator have someplace to go? Where does it go, and is it successfully getting there?

A subtlety in the question that was asked, may have slipped through. You stated that "the cooling coil ... temperature is lower than dewpoint" ... but that's a different question than "does the air cool to its dew point". Air temperature ... not cooling coil temperature.

What are the air temperature and humidity conditions (and flow rates and so forth) before and after the evaporator?

Dehumidification may be more effective with the fan speed kept low, to encourage the air to drop in temperature below the dewpoint, as opposed to blowing an excessive amount of air over the coil such that it hardly drops in temperature (and doesn't drop the water out).

 

[saeedplc]

Thank you for the replies

I tell dehumidificarion is not done because there no water out of the cooling coil to syphone

Input air temperature to the cooling coil is about 20'c and output is 15'c
The fan flow is 22000 m3/h

 

[MintJulep]

And the input and output humidity is?

 

[saeedplc]

Input humidity is 75% and output humidity 74%

 

[MintJulep]

Well,

Your exit condition is well above the dew point of the exiting air.

For dehumidification the exit air typically is very close to saturation.

In your case, the exiting air still has plenty of ability to retain water vapor, so it does.

It seems that the sensible heat ratio of your coil is too high for your application.

 

[saeedplc]

What should i do for dehumidification in this case? Lowering temperature of cooling water? Change setpoint s of the AHU or?

What factors are important and need to be considered for dehumidification?
Air flow,water flow,air speed
Or?


I dont have much data on dehumidification so please do me a favor and explain in general?

 

[3DDave]

Something appears wrong - assuming this is Relative Humidity that is discussed.

With that temperature drop the RH should increase, not decrease, if no water condenses.

The main factor to dehumidify air is to cool the air below the dew point.

Per this:

https://www.calculator.net/dew-poin...=75&dewpoint=&dewpointunit=fahrenheit&x=0&y=0

you should be seeing 100% RH on the output air.

What is the water volume, the water inlet temperature and the water outlet temperature? The energy to cool the air has to go somewhere and the energy that has to be removed to condense water also has to go somewhere.

 

[saeedplc]

The water inlet temperature is 7'c and water outlet temperature is 11'c
Dewpoint of air is about 10'c
Is not important flow of the air passing through the coil?
Should the relative humidity after cooling coil beexactly 100%? What does mean if it was 85% for example?
Something that is strange is that although cooling water in the coil is below dewpoint but no condense happen at all?
Which point should i check to find the root cause of the problem?

Ecxuse me if my questions are so a lot.

 

[3DDave]

I expect that a tiny amount of water condenses and then it evaporates before it drains out. I would look at it to see.

Is there water condensing on the water line on the way to the coil or not?

 

[saeedplc]

Unfortunately no water condense is coming out from the coil at all
Thank you so much for the time you gave me
I am going to install a temperature sensor on the cooling coil air side output to see at what temperature the air is leaving the coil pluse measuring relative humidity

Regards,

 

[LittleInch]

Maybe the coil is at below condensing temp, but it is so close that as the air then passes through the coil the water heats up and the air will just reabsorb the water.

However if your outlet air temp is 15C, kind of indicates you have too much air going through it. Try lowering the volume of air by say 50% or 75% and see what happens. Also if you can get inlet water temp down to closer to 3 or 4 C would also help.

Or buy a proper dehumidifier.

Dehumidifiers to me should then heat the air up to the temp it came in at, otherwise this is simply putting out cold air like any other coil no? What's different about this one compared to any other AHU?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MintJulep]

I am going to install a temperature sensor on the cooling coil air side output to see at what temperature the air is leaving the coil

So the 15°C at 74% RH air exiting condition that you provided earlier was what? A guess? Wishful thinking?

The water inlet temperature is 7'c and water outlet temperature is 11'c
Dewpoint of air is about 10'c

If these numbers are correct, then the air temperature cannot be below its dewpoint at the coil exit. See first law of thermodynamics.

Should the relative humidity after cooling coil be exactly 100%? What does mean if it was 85% for example?

The air leaving the coil will be very very close to 100% RH if dehumidification is happening. This is fundamental to the process. If the air leaving the coil is not saturated it means that dehumidification is not happening - which you already know.

Something that is strange is that although cooling water in the coil is below dewpoint but no condense happen at all?

This is not strange at all. The temperature of the air is what is important, not the temperature of the water.

Here I am writing the 15th post in this thread and we actually know less now than after your first question.

We need to know "everything", else all we can do is guess. Regardless, it seems that your need is greater than what could be expected for free from an internet forum.

Is this a new AHU designed and installed for the purpose of humidification? If yes, you need to talk with the company that provided it.

Did this AHU work as you wanted in the past, but doesn't now? If yes, you need to find out what changed.

Was this AHU repurposed from some other job, or some other location? If yes, talk to the person who made that decision.

 

[Rputvin]

"Yes as i mentioned the air is cooled to its dew point"
"Input air temperature to the cooling coil is about 20'c and output is 15'c"
"Dewpoint of air is about 10'c"

There is a conflict in these statements. If dew point is 10°C and the coldest your air is getting is 15°C where does the air get cooled to its dew point?

"Unfortunately no water condense is coming out from the coil at all"
There's likely some condensation from some sweating on the coil, but not enough to collect to drain and potentially re-evaporated since you're not saturated and still above dew point.

 

[saeedplc]

@littleInch
The 15'c air is the SA temperature of the air handling unit after passing the heating coil
Unfortunately making the water temperature lower is not viable because of tje comfort chiller limitation
All the AHUs are different in aspect of coil capacity,air flow and temperature setpoint

@MintJulep
15'c 74% is the SA temperature of the AHU
You are right,according to the temperature condense cn not be built
I agree with you to talk to the AHU manufacturer to get consult from them

@Rputvin
You are completely right anout my statement's conflict( i am absolitely confused)
It's because of my lack of enough knowledge
I need to read some books and enhance my knowledge of this field


Thank you so much dear experts for giving your precious time to me
I prefer to talk to the manufacturer and not bother you more than this
Bht if it is possible for you please summerize steps for troubleshooting of this case ( not dehumidify by cooling coil) what parameters a d parts need to be checked as a last post of this thread

Thank you very much again

 

[BronYrAur]

So what is the temperature of the air after leaving the cooling coil before it enters the heating coil?

 

[saeedplc]

Unfortunately there ia no temperature sensor just after the cooling coil.
Please forget this case and let me know only temperature below dewpoint is important for dehumidification or other factors such as speed of air and other things needs to be considered?

I am not good at mechanic at all and Am only enhancing my knowledge and experiance.

 

[BrianPetersen]

Temperature below dewpoint is what needs to be achieved. The "speed of air" - more properly, how long the air is in contact with the cold surface and how effectively the flow conditions promote heat transfer - is a factor in how that is achieved.

You can't get the temperature of the air below the temperature of the cold surface no matter how long it stays in contact!