Hello there,

I know that an Air Handling Unit (AHU) has a sensible load and a latent load.
The sensible load changes to temperature and the latent load changes the status of water vapour in the air.

I have an installation with several rooms cooled by Fan Coil Units (FCU).
These FCUs have been specifically sized to offset the sensible and latent loads inside the corresponding rooms.

When u cool the air, its relative humidity increases (this is why the unit condensate).
Normally, and as can be seen on the Psychometric Chart, in normal applications, the supply air (straight at the grills) has a low temperature, say 12C (53.6F), and high Relative Humidity (RH) , about 98%.
But after mixture, and as the air heats up to 24C (75.2 F), its RH drops to the comfortable 50%.

Now, I need to install an AHU to supply all these rooms with cool fresh air - straight into the rooms.
I will size the AHU to supply the air at 24 C (75.2 F) (at the grills).
Now, I have seen catalogues for special AHU for fresh air. They have high latent loads.
So why would I want to invest more latent energy, to take out latent energy from the air, forcing the water vapour to draw near condensation, when I know that the air I am suplying will not heat back up, so the RH will not drop?

Any Thoughts ?

Paul

I am not sure I understand your question, it sounds like you are saying that you think the rh changes as it heats up (which it does) due to mixing with the room air. (which it might) The relative humidity (which is a percent number ) will change as the temperature changes go up or down. The amount of moisture (grains of water) do not change, only the percentage of water that the air will suspend before it drops (rain - condensate) As the air heats up the amount of moisture it can suspend increases, (lower percent rh) As the air cools ,the rh increases , when the air reachs its due point it condenses back to a liquid (condensate)

As the air cools ,the rh increases

Indeed.. and this is why, normally, when cool air is supplied in any room, its RH (exactly at the grill, before mixing with the room air) is about 98%.
You can see that on the Psychometric Chart of the software u are using.
After mixing, the RH goes down to about 50% as the air heats up after offseting the Sensible load in the room.

Now, in my case, another machine is already offseting this sensible load.
So, in theory, this fresh air being introduced (at 24C, and.. probably at 98% RH), will not heat up. So its RH will remain high??

So I ask again, why do these Fesh air units have high latent loads, if these loads actually increase the RH of the air before introducing it to the room?

If I understand your question, the latent load is at the A-coil where water condenses out of the air. The cooled air at the grill is still near 100 % RH higher than the warm moist air started, but contains fewer grains of water or however you want to measure it.

If you induce air from out side at 75 degrees with a RH of 90 percent to the air inside at 75 degrees and 50 percent RH (With no refrigeration in operation) the air inside will naturally bring the RH up. This is the price you have to pay to induce outside air (free cooling) There are dehumidifiers made which could be incorporated into this type of system to drop the humidity. This is simply a refrigeration unit cooling the air with a reheat coil mounted in front of the evaporator . As the air is refrigerated the moisture condenses then the air is heated back up , so that it does not drop the temperature . I only provided this information in the hopes of assisting you, not to demean your knowledge of the subject.

If I understand your question, the latent load is at the A-coil where water condenses out of the air. The cooled air at the grill is still near 100 % RH higher than the warm moist air started, but contains fewer grains of water or however you want to measure it.

Correct.
Long time Labman, hope u're doing good.


There are dehumidifiers made which could be incorporated into this type of system

Sure there are. However I was exploring the possibility of getting the job done without adding heating coils to the machine. But if no other solution prevails, I will eventually adopt it.


I only provided this information in the hopes of assisting you, not to demean your knowledge of the subject.

I fail to see how you demeaned my knowledge of anything.
I am asking because I am open to other people's suggestions... and I appreciate yours.

Have you been forced to make such an installation before (Seperate Fesh Air AHU with dehumedifier) ?

Oh, and just for the record, the first quote was from Labman and the last two ones were by acetec.