Opening window ventilation rates?

[Firthy]

Hi guys, new on here so please be gentle with me. I have a job in front of me which is an exam hall measuring 25m x 15m x 2.7m. There will be 140 people in this room and the specification is asking for 10 lps. Can I take into account the size of the opening windows when doing the calculations? There are 16 windows 900mm x 1000mm with restrictors on. How do I calculate the airflow through each window.

 

[flyrfan]

Check your local mechanical code.

Under the IMC, ventilation air can be delivered in 2 ways; mechanical ventilation and passive ventilation (operable windows). However, the free opening of the operable windows must be a minimum of 4% of the room's area. Otherwise you must mechanically ventilate.

Keep in mind that although the code permits passive ventilation, you should ensure that it is the right method for the application. In a school application, I would only use mechanical ventilation as it is best for IAQ in what I would call a critical application. I would use passive ventilation in an apartment building or small office application.

 

[GMcD]

Think about what forces will make air pass through an open window-

External wind forces around the building causing differential air pressures so air may be entering on one side, but exiting the open window on the other side of the room.

Stack effect- if the room is high enough and you have some type of ventilator/relief outlet at the high level, the warm bouyant air from the bodies can create some air movement.

Effects of mechanical HVAC systems- if the room has a powered HVAC system - and normally buildings are kept slightly pressurized to reduce infiltration, then how will that induce air in through an open window? Unless you have an exhaust fan in the room that can be controlled to keep the room at a slightly negative air pressure relative to the outdoors.

There are no rules of thumb or quick answers with natural ventilation. There are way too many variables, and the best you can assume is that it "might" be able to provide some supplemental ventilation, if the wind is right, or you use a little brute force powered exhaust. Also- "natural ventilation" is untreated, dust and pollen filled outdoor air at whatever temperature and humidity mother nature provides at that particular time. How will the windows be controlled? What if some idiot leaves them open overnight?

The only way natural ventilation systems can be modelled to get a rudimentary approximation of how opening windows might work is through a whole lotta CFD modelling with a variety of external effects. That's why most building Codes won't allow the use of solely natural ventilation or even a given amount of natural ventilation unless you can prove by modelling/calculations that you can meet the minimum ventilation requirements of ASHRAE-62. You can use CO2 sensors to monitor the system as one way of showing that the natural ventilation system complies, but you still need the proof first before you can allow it to be used effectively.

 

[KiwiMace]

Unless this is the meeting hall for the mid-latitude kite club, you can't rely on having sufficient ventilation for a design day. What are the temperatures when this is going to get used? You put 140 people into a room for 3 hours and it will be stifling. Are you going to air-condition with open windows? What's going to happen in winter?

I think you might want to look at having a system that runs with the windows closed, and then implement the off-switch-economiser and open up all the windows on the nice days.

and, 10 lps per... what?