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Coil and dehumidification
5

Coil and dehumidification

Coil and dehumidification

(OP)
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,

RE: Coil and dehumidification

Does the air cool to its dew point?

RE: Coil and dehumidification

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

RE: Coil and dehumidification

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).

RE: Coil and dehumidification

(OP)
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

RE: Coil and dehumidification

And the input and output humidity is?

RE: Coil and dehumidification

(OP)
Input humidity is 75% and output humidity 74%

RE: Coil and dehumidification

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.

RE: Coil and dehumidification

(OP)
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?

RE: Coil and dehumidification

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-point-calculator.ht... 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.

RE: Coil and dehumidification

(OP)
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.

RE: Coil and dehumidification

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?

RE: Coil and dehumidification

(OP)
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,

RE: Coil and dehumidification

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.

RE: Coil and dehumidification

Quote (saeedplc)

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?

Quote:

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.

Quote:

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.

Quote:

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.

RE: Coil and dehumidification

"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.

RE: Coil and dehumidification

(OP)
@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

RE: Coil and dehumidification

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

RE: Coil and dehumidification

(OP)
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.

RE: Coil and dehumidification

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!

RE: Coil and dehumidification

(OP)
should the leaving air temperature from coil be lower than dewpoint in such an extent that it reach to 100% RH or at other temperature near above the dewpoint the dehumidification take place? I mean if in calculation the reletive humidity is for example 85% the dehumidification is done( but in lower rate) or it should be exactly 100% ?

Thanks

RE: Coil and dehumidification

RH 100% = condensation. If not, then no condensation takes place. The larger the volume of air reaching that point the more moisture is removed.

RE: Coil and dehumidification

(OP)
Thank you for the reply
Something is vague for me please correct me
Assume there is a cooling coil that input and output air is as follow :

Air in temperature:30'c
Humidity: 50%
Dewpoint:18.5 'c


Leaving air temperature: 18 'c
Humidity: 86.7%

By referring to the psychrometric the absolute humitity is diminished from 13.3g/kg to 11.2g/kg that means the dehumidification has been done although the leaving air humidity is not 100%

RE: Coil and dehumidification

The leaving air is not important for dehumidification. Only the air in direct contact with the coils and only for as long as that contact lasts. No humidity can be removed before that and no humidity can be removed after that.

RE: Coil and dehumidification

(OP)
I am confused a little
then What about the dewpoint role and water temperature through the coil?

In the example that i mentioned although leaving air humidity is 87% not 100% but dehumidification is done but it is not too much. Am i right?
This means if the leaving temperature decrease more the amount of dehumidification become more.right?

Could you please explain more about dehumidification process.

RE: Coil and dehumidification

(OP)
Any reply is appreciated

RE: Coil and dehumidification

I'm pretty sure you said the air was reheated after leaving the coil so the humidity level will decrease from 100%.

I haven't checked the charts, but it looks like there is some dehumidifying going on, but not a lot.

As to your general question it has been answered many time above. To get water to condense on the chiller plates the air temperature must fall to below the dewpoint.

Your post keep flicking between normal air cooling coils and this dehumidifier.

Also your control system seems to be poor if you cannot cope with lower cooling water temperature. If the air is then too cold, then reduce the water flow tot he AHU's and keep it at 3-5C for the dehumdifier which then heats the air back up to a comfortable temperature and not like a freezer.

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

RE: Coil and dehumidification

(OP)
Thank you for the reply LittleInch

My main question is that although the air leaving the coling coil is cooled lower than dewpoint but if temperature is decresed more, the relative humidity is more close to 100% so does it mean if the leaving air temperature from the cooling coil is very near blow dewpoint and Rh% is not 100%( for example 85%) no dehumidification is done?
I mean for dehumidification the relative humidity from the cooling coil should be 100% and is it a must?

Thanks a lot.

RE: Coil and dehumidification

The air that comes in contact with the cooling coil that is below dew point will condense water and leave the coil at 100%RH. However, not all of the air comes directly in contact with the coil fins. Some air passes through the fins without touching them. So there is a certain amount of "coil bypass air" that mixes back in with the cool air. The slightly raises the temperature and slightly lowers the %RH. The resulting air leaving the coil should be upwards of 97%RH. The is RELATIVE humidity at the given temperature. Once the air passes through the reheat coil, the RELATIVE humidity will drop considerable, but no moisture is being removed. The absolute humidity remains the same, but the %RH decreases.

RE: Coil and dehumidification

(OP)
Thank you all so much for your replies

Could you please let me know what cooling coil power(kw) do i need to dehumidify the air with the following characteristic:

Air entering the coil: 18 'c 65% RH
Desired air leaving the coil: 50% RH
Air flow rate: 2000 m3/h
Entering Water temperature : 7 'c

Regards,

RE: Coil and dehumidification

I think you should take a psychrometric chart, learn how to use it, and plot what is going on.

you also may need to measure more air locations, like directly downstream of the CC.

Likely the air just doesn't get cold enough. If LWT is 11°C, the coil surface may be 13°C, so the air with 10°C dewpoint is too dry to condense.

also consider all your instruments are somewhat inaccurate. You may think you have 10°C dewpoint, but it could be 9 or 11. You think LWT is 11°C, but it could be 12°C.

And if you have condensation (air cooled to below dewpoint), RH will be 100% at that temperature. Note RH will change WITHOUT any mass transfer when you change drybulb temperature. So the 100% RH air at 10°C will be 50% at a warmer drybulb temperature. Look at the psychrometric chart and it will become obvious.

RE: Coil and dehumidification

(OP)
I have used online psychrometric calculator
And it gave me the following results:


So could you please let me know if dehumidification is done although RH after cooling coil is not exactly 100%?
Is the coil power correct to coumt on it?

Please correct me if i am wrong.

RE: Coil and dehumidification

I don't understand what that data is. It looks strange with the temperatures set as a Delta T

As it say 18C @ 65RH has a dew point of 11.4 C. Looks correct.
But a delta T of only 4C may not be enough to actually get water to drop out before it heats up and basically exits the coils at close to 100% RH. When you heat it to get back to your inlet temp, the RH decreases.



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

RE: Coil and dehumidification

Entering numbers in an online calculator doesn't necessarily explain why things happen. That calculator basically just shows you a single point of the huge 2D psychrometric chart. .... Reviewing and understanding the psychrometric chart will help you understand what is happening.

RE: Coil and dehumidification

@saeedplc. you said:
Air entering the coil: 18 'c 65% RH
Desired air leaving the coil: 50% RH

This is not possible with a cooling coil. 50%RH at what temperature? You can heat the air up to roughly 22 deg C and achieve 50%RH; however, you will still have the same amount of moisture in the air. No water will be removed, yet the %RH dropped. I don't think this is what you want.

I think you want the air to be colder, say 13 deg C for example, and you want 50% RH. Is that correct? If that is the case, you cannot achieve it with your current arrangement.

RE: Coil and dehumidification

(OP)
Excuse me if I couls not convey my mean

The entering air : 18 'c. 65% RH
Air flow: 2000 m3/h
The coil should cool it up to 7 'c and the the heating coil increase the temperature to 16 'c

The desired output air from the heating coil is 16 'c. 50% RH



My purpose from the dara is to know if dehumidification can be done and also kw of the cooling coil is correct.
If it is not correct please let me know what changes should be made and what is the problem in the online calculator.
Needless to say, the water temperature to cooling coil is 7'c
Link of the calculator:
https://www.mollier-diagram.com/

RE: Coil and dehumidification

If the cooling coil inlet water temperature is 7 C and you want to bring air temperature to 7 C then you either need surface area approaching infinity or contact time approaching infinity or some combination. Back up and restate more achievable objectives.

Why can't you get the inlet water temp lower?

RE: Coil and dehumidification

as BrianPetersen said, you need colder water. your diagram looks like is was based on 4 deg water.

RE: Coil and dehumidification

Your problem is the coil has a heat exchanger efficiency of less than 100% (there also is the NTU method etc.). There are 3 resistances to heat transfer: the convection from air to the metal, the conduction through metal (fins, tube, and the fin-tube interface) and then the convection to the water. If all 3 resistances would be zero, all would be better. But alas, we live in the real world..... to make up for the real World and you need colder water.

Since the convection resistances also depend on flow velocity (turbulence etc.) and the build of the fins (dimpled etc.) the flowrate will change that. You likely don't have that detail and your 80% efficiency is just a guess. Could be 70%, or 60%. Then you need even colder water.

RE: Coil and dehumidification

(OP)
Thank you all for the replies

The water tenperature can not be reduced because of some limitation on chiller.

Do you mean by decresing the water temperature dehumidification is done and
Using a coil with the KW given by the calculator is correct for the task?

Something that i want to enlighten for myself by giving the numbers is that
I was told for dehumidificatiin the RH% AFTER cooling coil should be 100% but as you can see in the numbers given by the calculator the RH% is about 93% but the temperature is lowee than dewpoint of air!
Please clarify it for me by your knowledge and experience.
I am confused a bit.

RE: Coil and dehumidification

It will be 100% IF you cool air down to dewpoint. if you cool above dewpoint, you have sensible cooling only (no vapor condensing). Of course, the fin and tube temperatures will be all over the place at each specific location. So some spots may be below dewpoint, many won't.

A coil (or any heat exchanger) doesn't really have a kW rating since the power depends on the fluid conditions (temperature, flowrate etc.). the Power number they tell is for the specific conditions.

RE: Coil and dehumidification

2
Forget that calculator - it doesn't make sense. The graph is far better.

Your assumption that the air will reach 7 degrees is your error IMHO. I think it is unlikely that the average air temp would even get to 11C, hence why it doesn't drop any liquid out.

There may be, as noted by EP, some bits of the coil which may get a bit damp, but you can't beat physics here - your incoming air is simply not humid enough to want to condense water at the temperature you can run your dehumidifier at.

It doesn't matter how many online calculators you put this into or how many times you ask the question, the answer will be the same - at the water temperature you have with the air temperatures and RH you have stated, you simply won't dehumidify the air to any noticeable extent.

So you can either lower the cooling water temp, increase the size of the coil for the same air flow or run at a much lower air flow in order to drop any significant amounts of water out. The choice is yours.

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

RE: Coil and dehumidification

(OP)
Thank you all so much for the helpful replies

RE: Coil and dehumidification

(OP)
Only my last question @LittleInch

What benefits does a psychrometric graph offer that the calculator does does not?

In my opinion something like coil real air outlet temperature and some practical thing is not offered even by the graph.


Thanks a lot.

RE: Coil and dehumidification

Because you can see exactly what it happening. A calculator you don't know what it is doing.
First understand the basics so that you can use the correct data and can sense check the results.
You only gave us a snap shot of the calculator so no one can figure out what it is actually trying to calculate.

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

RE: Coil and dehumidification

That calculator isn't bad since it also gives you the chart and that shows you the processes and multiple points like you would mark up a chart. It also shows the water temperature you need (4°C in your case) for the ESTIMATED coil efficiency. it is a good tool, IF you understand what it does. The calculator basically has the curves (or their equations) built in.

But you also should just use a chart and understand how it works and what it does. The calculator works much better once you know what it is doing.

There are free PDF of the charts and tutorials. Some charts have coil curves added (an average coil and not necessarily yours!). You also can practice and run your example on the chart.

RE: Coil and dehumidification

(OP)
Alright you are correct and I need to reinforce my knowledge for basics.
By studying the graph in more details i can get more info.
I would be appreciated to give me some examples to practice and know what is actually happening.
Dont want to take your precious time more than this.

Thank you all very much.

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