Defining Design Wet Bulb Temperature

[DerekLJ]

I know this seems to be a very easy question, but if i knew the answer, i wouldn't be asking, please forgive me for being so naive.

I understand that WBT is so critical in cooling processes, that we have begun to question how we define the DESIGN WBT.

WBT is not difficult to design, but what do we need to keep in mind when we use it for design purposes. ( WBT can change on any given day due to the humidity and temperature, so how do we define it for design use?)

Is there an accepted practice or industry standard that governs this process?

 

[MintJulep]

The ASHRAE design conditions tables give a "Mean Coincident Wet Bulb" temperature for the listed design conditions.

 

[DerekLJ]

Thanks MintJulep.

What does one do when they don't have the standard or those specific tables. I know i don't.

Is there a accepted practice for calculating (in the office... wrapping the sock around the bulb doesn't count)

Maybe what i'm getting at is what values does one use in the psycometeric chart?

maximum absolute dry bulb?
Maximum average dry bulb?
Maximimum absolute humidity?

a mixture between max abs temp and max avg humidity?

 

[MintJulep]

What does one do when they don't have the standard or those specific tables.

Buy the appropriate reference materials.

 

[KiwiMace]

The ASHRAE tables mentioned by mintjulep will provide you with a Design DBT, WBT for 1%, 2.5% conditions, which means, 99% and 97.5% of anticipated conditions.

If you have a remote location where there is no available information, you get as much weather data as you can, and derive these points yourself.

I once had to build annual weather files for a location in Antarctica for simulation use. It is no small task, as the data is usually sparse.

 

[MintJulep]

I've been to Antarctica. It's cold there. Pretty much all the time.

Just as a clarification on ASHRAE design conditions. The 99% design condition means that you can expect the actual conditions to exceed the design conditions 1% of the time. Same for the lower % conditions given.

 

[KiwiMace]

Did you need to know the 1% design conditions while you were there? I totally could have helped you.

 

[zerosum]

Derek L J,

What is the location that you need design wet bulb and associated conditions? Possibly we could just give you the design points out of ASHRAE. If you're going to do much specifying of mechanical equipment particularly that involves mass transfer, you will eventually need to be able to access local design (WB) conditions. NOAA, Ashville, NC, also has voluminous climatological data but last time I needed something from them it wasn't free. They probably have a website now, it's worth doing a search.

 

[DerekLJ]

Thanks everyone.

The location of the plant is in a remote area of argentina, near the end of the world. The basis of my question was if i should absolute tempertures and humidity to define the WTB or not.

When i did the calculation, the absolute DBT, gave me a WBT that was far beyond what is globably accepted.

When I used the maxmimum average DBT i got a WBT that was logical and close to the WBT of simular parts of the world.

So i guess the answer to my question is: you need to use the maximum average of the dry bulb and relative humidty to calculate an accurate WBT.

thanks again.

 

[zerosum]

Yeah, the hour-to hour variation of the local absolute humidity is quite small. The DBT swings appreciably during a 24-hr period, particularly in dry regions in response to the solar (day), and land (night) radiation, but the WBT moves much less.

I would agree to your final conclusion.

For cooling towers, for instance, you need to design for the 97.5th or 99th percentile WBT expected, as KiwiMace stated.

If this is a critical design, I would still try the NOAA Center in Ashville, NC. They have or can produce maps with isolines of things like WBT's worldwide I believe.

 

[KiwiMace]

Looking at the ASHRAE map on 2005 Fundamentals 28.3, there is data for several locations in the south of Argentina, Tierra del Fuego, likely Ushuaia as the station.

My ASHRAE cd stopped working a long time ago, so I can't pull the numbers for you.

 

[chicopee]

To your original question, you need the relative humidity and dry bulb temperature the latter of which is the regular temp. registered on a thermometer. Get to know on how to use a psychometric chart because there are a lot more info such moisture content and enthalpy beside the three items I mentioned.