Hello,
In first place, I apologize for my English. I hope that my text will be quite understandable.
Our project is a mixed use hospital and laboratories of 1 400 000 ft² and 2 400 spaces in Canada. We are using the Ashrae Standard 55-2010 Comfort tool to calculate the PMV/PPD for proving that the design is good relative to Ashrae 55-2004.
For short, we have reduced the 2 400 spaces of the project into 60 spaces types.
According to Ashrae 55-2004, we have excluded the spaces where occupants are staying under 15 minutes and where they can sleep or bed rest, such as patient room or recovery room. So we have grouped the remaining areas under the same environmental characteristics:
- Air temperature set (for the 4 seasons)
- Humidity set
- Metabolic Rate
- Clothing level
- Mean Radiative Temperature ( We have seen that the MRT is the factor who have the lowest incidence on the PMV/PPD, so we have decided that we split this characteristic in 3 categories (internal area, space with one exterior side, space with two exterior sides))
That is why we are able to reduce the number of cases at 60, instead of 2 400.
Is our interpretation of the Ashrae 55 correct so far?
We are wondering how to define/justify the air speed in each space. The air speed must remain below 40 fpm in the Ashrae Standard 55-2010 Comfort tool to be consistent with the Ashrae 55.
How can we calculate the air speed at human height?
Have we to make a CFD simulation for each space? Or is there a formula with the air speed at the output from the diffuser capable of calculate the airspeed at human height?
Maybe can we use the same air speed for the 60 spaces on taking the most conservative air speed value for all the spaces?
Your help is more than appreciate on this,
Thank’s,
Cédric
Julia Weatherby
PresidentWeatherby Design & Co. Engineers
94 thumbs up
August 30, 2013 - 11:51 am
Hello, Cedric-
No, you shouldn't have to do a CFD simulation. In general, conventional diffuser selection and layout will result in being below 40 fpm air speed in the occupied area.
To get a better understanding of the air speed, you could look at the application guide from a diffuser manufacturer, such as Titus or Price.
Your approach sounds right to me.
-Julia G. Weatherby, P.E.
Cédric Marzolf
Specialist in energy modeling3 thumbs up
September 3, 2013 - 9:02 am
Thanks Julia to give me book references.
So, according to section 9.4 of "Engineer's HVAC handbook" from Price, if we can prouve that the throw of our diffusers is (much) larger than the throw of the manufacturer at 50 fpm, the project probably met the standard 55?
It would be more understandable if LEED council want us to prove that the air is blowing under 50 fpm (instead of 40 fpm) in the occupied zone..
Cédric
Julia Weatherby
PresidentWeatherby Design & Co. Engineers
94 thumbs up
September 3, 2013 - 9:31 am
Hi, Cedric-
I think you probably understand fine, but it is not specifically that you want your diffusers to have a large throw. It's that you want the air to slow down to 40 fpm or less by the time it reaches the people sitting or standing in the occupied space.
As long as your diffusers seem to be generally properly selected, it should be fine to assume a fixed velocity, such as 20 fpm average air velocity, in all your spaces. If you have any spaces with unusual characteristics, then it may make sense to do a specific air velocity estimate for that space.
-Julia
Cédric Marzolf
Specialist in energy modeling3 thumbs up
September 5, 2013 - 1:45 pm
Hi Julia,
Just to be more specific, our mechanical design team is selecting the diffusers in accordance with the manufactuer throw data at 50ppm. Fo now, I’ve made my calculation in the Ashrae Comfort Tool with an air velocity of 30ppm.
So, can we just saying, in our explanation letter who goes with the Letter type, that
“Our mechanical design team has selected the diffusers according to the generally accepted engineering practices and the manufacturer data throw at 50ppm, so we can consider that the air velocity to the occupant height is lower than 30 ppm (or 40ppm)”, to be consistent with the credit 7.1 requirements?
Thank’s a lot for your help
Cédric
Julia Weatherby
PresidentWeatherby Design & Co. Engineers
94 thumbs up
September 5, 2013 - 2:04 pm
That seems fine to me, Cedric. I'm not sure you really need to include any justification for the air velocity used unless specifically asked to do so by a reviewer, but it is fine to include the statement if you are more comfortable doing so.
Cédric Marzolf
Specialist in energy modeling3 thumbs up
September 5, 2013 - 2:25 pm
Ok, we will do that.
I m a beginner in the Leed project certification and honestly I'm not aware of the reviewers want for accuracy.. So i prefer to explain all my interpretation :)
Thank's a lot
Best regards,
Cédric