Air Flow Rate and Static Pressure

[Farid Kamal]

Good Day everyone,

I am very new into air conditioning and would like to improve myself in it.

I have read a few threads about condensation happening at supply air diffusers and got a few confusions.

How does "air flow rate" and "static pressure" of a system contributes to condensation at the supply diffusers (water dripping)?

Would be glad if someone can clear up my confusions as I am a newbie in this field.

Thank You ^_^

[frank]

The short answer is...it doesn't.
Condensation from the air occurs when the temperature of the air cools to a point where the moisture entrained within the air becomes saturated (100%RH) and water droplets form.

While the air is moving, the water droplets are carried along with it for the most part but can accumulate on the grille or diffuser, leading to drips of water.

When warm air passes over a cooling coil and it drops in temperature, the same thing happens and condensate is formed. This is called de-humidification.

Static pressure is the term used for a surface acting against the flow a
of air in a duct. Coupled with Velocity pressure, these add up to the Total pressure. Velocity Pressure (VP) + Static Pressure (SP) = Total Pressure (TP)

Air Flow Rate is the amount of air flowing in a system measured in M3/s or CFM

[Farid Kamal]

Thank You Frank..

I can seem to understand a bit now.
Seems like I have to master the Psychrometric Chart to fully understand this.

=)

[The Viking]

Sorry Frank,
We have to agree to disagree on this one as I actually seen it happened, sorry.

First, we need to understand that condensation occurs whenever air meets a temperature below the air's dew point. The dewpoint varies depending on humidity levels (and to a lesser degree also with air pressure), I won't go in to the details about it here but Google "psychrometric chart" to get an idea.

Now, if we got a ducted indoor unit designed for a ∆T of 10K that would give us an air off temp of 14ºC when the room temp is 24ºC, the supply grille is therefore likely to be at around 15-16ºC, well above the dewpoint for the cold and relatively dry air coming from the A/C unit. However, due to the vortexes and venturi effect created by the grill it will also be exposed to the relatively warm and moist air in the room. At 15-16ºC it is likely to be just above the room temp's dew point but what will happen if the airflow drop?

If the static pressure increases to outside the unit's design envelope, for example by bad duct design or a blocked grille, what would happen?
-The airflow will be reduced but what effect will this have?
-As the capacity of the unit stays the same, with less airflow the temperature of the supplied air will have to drop. All of a sudden you might find a ∆T of 15K, in the example above that would give us a supply air temperature of just 9ºC and that would be below the room air's dew point.

So if the grill is at 9ºC and in contact with room air (which it is) condensation will start to form...

In short, (too) high static pressure = reduced airflow = higher ∆T = lower supply air temperature, this chain of events might cause condensation to form on the supply air grille.



.

[Tesla]

Also if the static pressure drops due to a restriction on the inlet of fan like a blocked filter, this will reduce the air flow and a cooler supply air temp will cause condensation on the supply air grill. Unless there is supply air temp control that limits the supply air temp from cooling coil.