Pre Treatment of Fresh Air

[walz]

What is the basic objective in Pre-Treating the Fresh air. My question is for Cooling Case only for Hot and Humid Climates.

1. Is it removal of grains of moisture in outside air down to grains of room air.

2. or is it lowering the enthalpy of outside air down to room air??

if it is to lower the grains of moisture of outside air down to room air. the size of cooling coil becomes too high. and if the coil has to bring enthalpy of outside air down to room condition. then i can size my cooling coil with off coil condition at the intersection of room condition enthalpy and 95% rh. this way my size of coil will be less but treated fresh air will have more grains of moisture than room air.

 

[AbbyNormal]

An ERV would be a good example of pre-treating the fresh air.

It can shed some serious laod, especially since in a hot humid climate the lions share of the load is dehumidifying the ventilation air.

You can also have several cooling coils in series instead of a single coil with 8 rows to get the mositure out.

You can also go with a DOAS system that gets the fresh air down to having a dewpoint a couple degrees lower than you want in your space like 53F and use it for first stage cooling, duct this air directly to all the zones.

Sometimes it is dumb to reheat that air to a neutral temperature.

Take care of that outside air in the tropics and you will never have a humidity problem.



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[SAK9]

In a humid climate pretreatment of fresh air is required if you need to maintain humidity in the conditioned space within a reasonable band(say under 60%). The coil in a FCU or AHU could be operating in a low flow condition once the room temperature is achieved and therefore will not be capable of removing moisture from the fresh air and maintaining the room humidity within safe limits.This is very true in the case of tropical hotels where the room can have mould growth if the humidity drifts beyond 65%.

 

[ChasBean1]

Reduce the dew point temperature of the incoming air below any typical surface temperatures in your building.

1. Yes, basically, but add in humidity generation rate from the space. Stick with measurable parameters, typically you’d just remove moisture to below about a 55°F dew point limit in the supply airstream.

2. No, your supply air enthalpy might have to be lower than the room enthalpy.

E.g., if you need typical room humidity below about 60%, set your supply air temperature set point to 55°F any time the OA dew point is above 55°F.

For dehumidification control, set your OA make-up temperature down to 55°F whenever OA dew point is above 55°F and scale the make-up temperature during lower humidity conditions to work best with whatever temperature control strategy you have.