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,

 

[saeedplc]

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

 

[3DDave]

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

 

[saeedplc]

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%

 

[3DDave]

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.

 

[saeedplc]

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.

 

[saeedplc]

Any reply is appreciated

 

[LittleInch]

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.

 

[saeedplc]

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.

 

[BronYrAur]

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.

 

[saeedplc]

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,

 

[EnergyProfessional]

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.

 

[saeedplc]

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.

 

[LittleInch]

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.

 

[EnergyProfessional]

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.

 

[BronYrAur]

@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.

 

[saeedplc]

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/

 

[BrianPetersen]

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?

 

[BronYrAur]

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

 

[EnergyProfessional]

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.

 

[saeedplc]

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.