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,

 

[EnergyProfessional]

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.

 

[LittleInch]

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.

 

[saeedplc]

Thank you all so much for the helpful replies

 

[saeedplc]

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.

 

[LittleInch]

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.

 

[EnergyProfessional]

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.

 

[saeedplc]

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.

 

[pierreick]

HI,
If you want to validate your calculation , you may want to use the document attached .
Good luck
Pierre