Finding Coil Entering Wet Bulb Temperature

[EnOm]

Hi.
I want to ask about finding the Entering Wet Bulb Temperature of the air entering a Fan Coil Unit. The issue I have is:

Suppose that I have a zone design condition of 75/62.4 (DBT/WBT).
[ol 1]
[li]I can find the entering DB by adding on the sensible heat gains from the ducting or plenum and the outside air. (and any others)[/li]
[li]I can find the entering WB by adding on the moisture gains from the OA. (and any others)[/li]
[li]I can find the unit's sensible and latent capacity corresponding to these temperatures (from manufacturer literature/software)[/li]
[li]I get the supply temperature and use it with the unit's airflow to find the and latent sensible capacities reaching the zone[/li]
[/ol]

Now the issue is that the unit's capacities do not match my zone's calculated loads, and thus my initial conditions of 75/62.4 (DBT/WBT) are not met exactly. Now this won't affect design entering DBT estimates because the temperature inside the zone is controlled by the Thermostat and thus entering DBT won't deviate too much from the design condition. My main concern is about entering WBT because:
Firstly: It is not controlled (No Humidistat).
Secondly: As far as I know, it has a much more significant effect on coil performance than entering DBT.

Any insight?

Thank you

 

[Drazen]

according to what you described it looks as if you are bringing completely untreated fresh air to local fan coils?

i would assume you have window sill openings to allow direct entrance of fresh air to fc's but you are mentioning duct heat gains. if you have central fresh air supply i am wondering why do you not pretreat fresh air to at least neutral room conditions.

as regards to your question, you are right, you cannot control wb. actually if you do not have humidistat-controlled designed wet cooler all what you have is partial dehumidification.

you need to know whether it satisfied design taks. aside from very humid climates, for usual commercial/residential conditions it is often acceptable to have humidity variations in range of say 30-70%. if it is not, you need to design system which will include full humidity control.

 

[EnOm]

Thanks for the reply.

As for your questions:

Q1: Yes I am bringing fresh air directly to the fan coil units. I am considering treated air as well.
Q2: I was describing a general procedure, applicable to any fan selection scenario.

The thing is, I am trying to be as efficient as possible. I don't want to oversize the fan units due to assuming an unnecessarily high Entering WBT.
For now I'm working on residential applications, so intricate humidity control is not really a requirement. What would be a reasonable assumption for room WBT to use as a reference point in calculating entering conditions from? Should I use 62.54 F (with 75 DB it gives 50% RH zone condition)? or maybe 65.3 F (gives 60% RH), just in case?

Regards

 

[dbill74]

First do not get caught up trying to match the room load with equipment load exactly, it will never happen, there are too many variables involved.

The effect of wbT on coil performance? Let's take a look at that. First what does the wbT tell us? It is a second point that is needed along with dbT so we know where we are on the psychormetic chart, once we know where the entering air condition is on the psychrometric chart we can get Relative Humidity, Humidity Ratio (W) and Enthalpy. The Humidity Ratio (right side of the psychrometric chart) of the EAT is important to know because the difference between it and the Humidity Ratio of the design temperature is relative to the amount of moisture that must be removed from the air during the cooling process; this is the latent cooling (aka dehumidification) the system must do to get rid of moisture introduced by OA and people.

You want to be looking for a coil that has the ability to have the air coming off the coil at a Humidity Ratio comparable to the Humidity Ratio of your design point for the given EAT conditions; reheat or just introducing the air to the space will drop the RH. In manufacturer's literature, look for coils with EAT conditions that is as close to your calculated EAT condition as possible. Like I mentioned above with capacities, you will probably never match what is in the literature. But then if manufactures provided tables for all possible EAT conditions, their literature would be tens of pages for a single coil. Lastly, your wbT will change as dbT changes for a constant Humidity ratio (study your psychrometric chart to see how this works).

On humidity control, unless OA CFM is 25%+ of SA CFM, I wouldn't concern myself with adding special humidity control features even in a humid area such as the southeast in the USA. You simply are not introducing enough moisture through the outside air stream that the cooling coil can't remove it. That isn't to say that humidity control is never needed below 25% OA, but I look at those situations on a case by case basis.

 

[EnOm]

dbill74:
Thank you for the information. I agree with what you said.

I do no think humidity control would be a problem for me, especially that outside air has a 13% RH. But I still need a reference value for room WBT to start my psychometric process from. Should I just use my design design condition of 50% (even though I know this won't actually be met because of the difference between my unit's capacity and the actual heats entering the zone).

The reason I'm insisting on a specific value for entering WBT because what I have is an electronic catalogue. Its actually more of a simulator. I can put in any entering conditions (within the unit's limits) and get the corresponding latent and sensible capacities. I would be a lot more comfortable having worked from a reference point that just relying/hoping on the fact that I won't have any humidity problems. It's better to have some figures down.

So what do you recommend?

Thank you

 

[Drazen]

yes, use 50%, and see what you get. if you find that no unit can give satisfactory results, use 60%, and so on. up to 70%, as mentioned above.

 

[EnOm]

Drazen

I understand, thanks.

Regards