How do you find worse dehumidification day?

[illuzionb18c1]

Im trying to size a 100% OA unit. What is the typical way to do this? I always thought you would pick the point where the humidity ratio was the highest for that region. Is this the correct way?

 

[DrRTU]

It should be 100% saturated air (raining) on a design day.

 

[VerneE]

In general design to the 97% dew point temperature (i.e. does not exceed that dew point for 97% of the time) or if it is critical, design for 99% dew point. Even on a rainy day the air is not saturated. ASHRAE Fundamental book gives those values for most areas of the world. I have never seen the wet bulb temp exceed 80F in Chicago, even on 95F db days when it was raining (i.e. dew point was 75F).

 

[Zesti]

I guess it depends on the highest enthalpy value you get in your climate.

Over here we use 30°C/50%.

If it starts to rain in those weather conditions temperatures will drop several degrees.

 

[IRstuff]

Seems to me that would depend on where you want to do this. Florida and the Arabian Gulf are quite different than the rest of continental US.

TTFN

FAQ731-376

 

[AbbyNormal]

The weather data with the ASHRAE Handbook of fundamentals will give dehumidification data - basically the grains of moisture and the corresponding dry bulb that is associated with it for 0.4%, 1% and probably 2.5% design levels.

It is also found and easy to read in the appendix of the ASHRAE Design Guide for Humidity Control which you can get from ASHRAE or the author. Here is a link to some samples from the author, it gives a free chapter from the Design Guide for Humidity Control

http://www.masongrant.com/html/samples.html

if your area is not listed in the references I mentioned, perhaps try weather underground. You can look at a lot of data there, can go back some years and keep an eye out for what seems to be a re-occuring high dew point.

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

 

[walz]

take the highest WB temperature of the year. one easy way to get it is ASHRAE fundamentals. Select the 0.4% WB temp. thats the day when it is max enthalpy and coil has max dehumidification that day.