By Ron Burns
Let’s look into the dynamics of openings in the vertical face of building elements; whether they are automated ventilators, fusible link operated ventilators, static louvres, or my all-time favourite: an operable window.
It is not often that I get invited to lunch where I do not need to foot the bill … It wasn’t long before we were knee-deep in the merits of smoke control and I was having to answer a few questions on the last article published in RACA Journal.
It was a great Friday afternoon in Durban, the rain was falling gently, and the discussion around excessive cooling of the smoke plume was gaining momentum. We were tossing comments around on wind loading and the importance of selecting the correct ventilator, when out of left field I was asked why we did not install the ventilator in the vertical. Although the discussion was based largely on common sense, the old cliché was blatantly apparent, “Common sense is not so common” (Voltaire, 1764).
Let’s consider the dynamics of openings in the vertical face of building elements. We all know the old-time classic joke from our youth, “When is a door not a door? When it is ajar”, followed by fits of laughter as a young adolescent starts appreciating the finer nuances of English.
I am sure that if you lift your head up from where you are sitting now, you will be able to cast your eyes on a door (it may be closed or ajar), but the real question would be, is the door an exit or an entry point? The door that opens into the fire escape, is that an entrance or exit door? It’s amazing how a simple question can seem so silly. I can imagine a seasoned fire engineer shaking his head and wondering if I had finally played my last card and am simply waiting for the people in the white coats to collect me. After all, everyone knows that the direction in which the door opens determines the description of the door. The door opening into the staircase is the entrance door; the door opening outwards at ground floor level would be the exit door.
You decide to look at the door you entered into your office and you check how the door is hinged. You are so relieved to see that the door is hinged in such a way that you were able to enter into your office and as you smile and shake your head, the panic suddenly sets in. How are you going to get out of the office; this is an entrance door. You look around at your colleagues in the open plan office and you sheepishly wonder if they will notice; will they think less of you if you use the entrance door to exit your office? Should you take the chance, or is it unimportant because the building is not on fire? Seems silly? How silly if we had been discussing air blowing on the door?
I wonder if the penny has dropped yet. The name of the door, entrance or exit, has nothing to do with the hinges and the way the door opens. Yes, that is critical if you are trying to enter a fire escape; however, to determine if the door is an entrance or exit door is simple. Watch the direction that people travel through the door. Let’s explore the logic a little further. The louvre in the gable end of the building: is that an exhaust or inlet point? Be slow to answer; remember it is similar to the door of the fire escape. It makes no difference what the intention of the louvre is — it has to do with what happens around the louvre.
In the presentation to the local authority, it was easy to explain that during a fire in this building — which is not sprinkled — the smoke will rise up to the roof level, build down, and the energy from the fire will create sufficient buoyancy to drive the smoke out the vertical louvre. The local authority was convinced, the theory correct, and the plan approved. You had been sitting in a controlled and comfortable environment. Do you think you would have received approval had you been on site and could not study the design drawings since a gentle breeze kept blowing the drawing around? Would the fire safety officer not have been concerned about the vertical louvre? I sure would be.
When the gentle breeze hits the face of the vertical louvre, the air pressure from the breeze is going to ensure that the louvre becomes an inlet louvre. We all know how fast our cars go, and we all know that if we are selecting a fresh air inlet louvre for an external weather louvre to keep the inlet velocity below 2.5m/s, it will prevent water from being carried over into the plantroom, pooling and perhaps leaking through the slab. This phenomenon always intrigues me; we take all this care to prevent water from dripping through a porous slab; however, we use brick shafts — which is not air or water tight — in an office building to transport toxic deadly smoke. But this is another discussion for another day.
How fast is 2.5m/s? Would this trouble Lewis Hamilton? Would this only occur on windy days? Converted, 2.5m/s is only 9km/h; marginally faster than a Sunday afternoon stroll. The EN 12101 code, Part 4, addresses the wind speed through vertical louvres, for inlet air only. The level of concern to the authors of EN 12101 is a wind speed of 1.5m/s. EN 12101 says that the air speed and air direction needs to be monitored, and should the air speed exceed 1.5m/s the louvre shall be driven closed. The air-conditioning fraternity will never allow filters to be placed over the inlets of a natural ventilation system, as the sensitivity of the system to draw fresh air through a filtered louvre is such that the system will not function as per design. Such is the sensitivity of a natural system. However, all these principles are too easily forgotten when we are dealing with smoke and hot toxic gases.
Back to the vertical louvre. Assume the wind is gently blowing at the louvre, at say a moderate 15km/h (4.17m/s). We are definitely not getting any smoke exhaust through that louvre then. It is important to acknowledge that the code does not address common sense items. I have been hearing the comment that the code does not prevent vertical venting for too long. A designer cannot simply ignore standard scientific principles, like overcoming wind resistance. Ignoring this principle is not good engineering; the real question would be to see if this is bordering on negligence.
With a smile, I get asked if we can split the louvre requirement and put an equal number of louvres on opposite ends of the building. Good question? Perhaps not. I think we should apply the common-sense rules here again.
On the back of a serviette, my colleague starts to scribble a section through the building we are discussing. Annotated with heights to the apex, 7.3m, and smoke layer requirement of 2.5m, the calculation is simple and an AVCV of 12.4m2 is calculated. For the discussion, we assumed a co-efficient of 55% and select a set of 1 500×1 500 weather louvres, five-off on each gable end. Sitting opposite, smiling, is a happy engineer — well momentarily anyway. I suggest we apply a wind on the vertical face and see what happens. The initial thought is of the wind entering the louvres on the east façade and exiting on the west façade. Have you considered the effects of the 20°C air entering your smoke plume, assuming a smoke plume of 170°C, I ask? Should we do a calculation and see what effect that has on the ventilation AVCV? We have now introduced a large volume of cool air into the hot plume. Fortunately for me, he did not need any more convincing that cooling the plume was a bad idea. I have never contemplated that calculation before and was not too sure how to determine the effect by means of calculation. The discussion did not end there; we agreed to ignore the cooling effect and look at the geometric effect on the calculations.
What had happened is that the original AVCV of 12.4m2 we supplied had now been reduced to 6.2m2. Where does that leave the smoke clear layer? Luckily, we had more serviettes and the reverse engineering calculations were scribbled post haste. We determined a clear layer of 1.6m above finished floor level, with an AVCV of 5.81m2. It was agreed, no more vertical venting. Even if not deliberately stipulated in the code, it is a simple application of common sense more than an engineering function to see whether vertical is permissible. Our conclusion: vertical venting is a no-no.
Now the conversation deepens and we drift into vertical vents with fusible links. Discussions around time delays for the opening of the vents seem apparent as we discuss the time required to heat a fusible link in the vertical. A little lamp was burning on the table so we decided to do a simple experiment. Placing a serviette over the flame — although entertaining – did prove a little too risky for the waiter. Holding our hands parallel with the flame, it soon became apparent that the heat would never become significant enough to require us to move our hands away from the flame. We discussed the result of having to pop a fusible link by dropping the smoke deep enough below the ventilator to generate sufficient heat, and decided that the risk related to this process would cause the ventilators to be placed on the roof where the heat accumulates easily.
The thought of closing the ventilators when the wind was driving directly at the ventilators was short-lived. This would result in emergency power, wind speed and direction sensing, fire-rated cabling, and then a ventilator — tested and found fit for purpose — able to open and close at elevated temperatures. All a little too complicated when measured against a simple slope mounted installation. The option of windows was discarded early on, as we discussed the thought of glass falling from an open window once the seals had deteriorated in the heat. Windows will be covered in a separate article.
The item that raised the greatest concern was natural building elements that had been placed in the building by other designers, impacting on the smoke ventilation. We agreed that these elements needed to be considered. The first element was the 4m-high roller shutter door. We agreed it would be responsible at design stage to select a smoke clear layer that was higher than the top of the roller shutter door. Ignoring the door in an open position could lead to the same scenario as a vertically installed ventilator. The second element was a vertical louvre installed in the smoke plume for day-to-day ventilation. We concluded that the suggestion should perhaps be made to the architect to replace the high-level day-to-day vertical ventilator with slope mounted roof ventilators, or perhaps a ridge ventilator. Although these two products provide a highly efficient ventilation solution, during elevated smoke temperatures these ventilators tend to buckle and close, thereby throttling the air. Therefore, the ventilation supplied by this equipment needs to be excluded from smoke AVCV requirements. In the early stages of the fire there will definitely be an advantage in having these openings; however, when we get down to the working portion of the fire we can ignore any benefit from this equipment.
Lunch is definitely a great opportunity to enter into technical discussions regarding the performance of equipment, fresh air routes, heat generation, and evacuation paths. Perhaps burning the odd piece of restaurant equipment is frowned upon, but the lessons learnt are invaluable. Lunch, anyone?