Fire Science Show

116 - Fire Fundamentals pt. 4 - Natural and Powered Smoke Vents with Wojciech

• Wojciech Wegrzynski

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It is time for some engineering fundamentals in the show. This time in the fire fundamentals series we delve into the details of natural and powered smoke ventilators - what they are, how they work, how they are tested and what interesting mechanics impact their performance in fire.

I hope this episode is valuable for all engineers who would love to know how the devices they place in their design are tested and qualified for use in fire safety. It should also be a great way for fire scientists to broaden their horizons and learn about very intricate details of natural and powered vents, which you learn only through experience in the design.

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Fire Science Show podcast is produced in partnership with OFR Consultants

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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.

Speaker 1:

Hello everybody, welcome to the fire science show. Today I'm taking you into another episode of fire fundamentals. Previously, we've covered some aspects of combustion, ignition and other aspects that are related to how stuff burns. We've also covered how smoke is produced and how it creates smoke blooms. So they thought maybe let's try some engineering instead of pure fire science. I think it's absolutely fundamental for all engineers to understand the basics and the limitations of the systems that we are working with, even if you are designing them or not. I think it's fundamental for scientists to understand how fire engineering works and how different devices we put in our buildings work. So, yeah, perhaps this is interesting for all of you and let's try it. Let's try it with something that I am the most familiar, something that I've spent my entire professional career working with, that is, smoke extraction devices, smoke exhaust systems, and we are going to cover natural and powered smoke exhaust ventilators. We will focus on how these devices are built, how they operate, how they are tested, how they achieve their classes and what you can expect of such a device, and what interesting caveats are there related to using these devices on your buildings. So, hopefully, something interesting for you all. I don't think it needs much more advertisement. So let's spin the intro and jump into the episode. Welcome to the Fire Science Show. My name is Vojci Wigzynski and I will be your host. Fire Science Show is brought to you in collaboration with OFR Consultants, a multi-award winning independent consultancy dedicated to addressing fire safety challenges. Ofr is the UK's leading fire risk consultancy. Its globally established team has developed their petition for preeminent fire engineering expertise with colleagues working across the world to help protect people, property and environment. I've just learned they are nominated for a building magazine award, where they are shortlisted in the engineering consultant of the year category. Congratulations to OFR or another nominees. I'm crossing my fingers for this one. And if you would like to consider joining OFR's team, they're always open to hear from industry professionals who would like to collaborate on fire safety features. Get in touch at OFRConsultantscom.

Speaker 1:

And now back to natural and powered smoke ventilators. Okay, let's go. Natural and powered smoke ventilators. So what are these devices? Well, the name is quite self-explanatory they are used to remove smoke from our buildings, to ventilate them. That's pretty good explanation of how they operate. However, there are some quite important details in operation of both of these devices which we will try to explore in this podcast episode. In this episode, we're not gonna talk too much about smoke control design, in terms of how to choose how many of these devices you need in your building to successfully remove the smoke from it. Well, we'll touch a little bit about their performance and what makes or breaks it, but we're not gonna talk design. However, we will focus very much on the devices themselves how they are constructed, how exactly do they operate and what impacts their performance.

Speaker 1:

So let's start with the natural ventilators. I guess this already brings us to some controversies and let's start with that. What I have in mind is the naming conventions. I know in different parts of the world you would name them quite differently. I once heard an opinion that there is nothing natural in putting an automated opening in your roof, so you're not allowed to call it natural ventilator. Well, actually, let's agree in this podcast to follow EN 1211, part 2, which is the European standard for natural smoke ventilators convention, and let's call them natural ventilators. I like that. Actually, in Polish we have a very nice word, klappa d'imova, which means literally something like a smoke damper, and we like that naming a lot. It actually creates quite little issues when we translate the English based EN standards into Polish, because the wording you have to be aware of the little wording nuances.

Speaker 1:

Anyway, natural ventilators, the automated openings that you put on your roofs or in your facades that are used to relieve the smoke from your building. How do they operate? It's very simple. It's literally a window or a roof exit, some kind of a dome box. It can also be a part of a continuous roof light or another type of roof structure that automatically opens when the fire is detected. They operate by the force of buoyancy of the smoke. So the smoke is hotter than surrounding air, which means it has lower density than surrounding air, which means it flies upwards, as we discussed in the previous Fire Fundamental episode, and the force that it creates is also sufficient to push it through openings in the roof. As you can imagine, the hotter the smoke, the more efficient this extraction will be. Also, to have this action working, you need to have some air replacement makeup air sources in your building that supply fresh air that takes place of the hot smoke that you've exhausted. So when we're talking about smoke control, we always talk about extraction and inlets all together, because just extraction will not be enough. So I said, they operate when the fire is detected.

Speaker 1:

You actually have two ways how these devices can be started, and this already creates quite some issues in some buildings. So the first one probably the more used one, is that you equip each smoke vent, later each individual opening, with its own mechanism. It's very similar to how sprinklers mechanisms work. In this case, just like in sprinklers, the vent would have a thermal element that breaks when hot temperatures around it, and they also are color coded to different temperatures. So when the temperature is reached, the link breaks and releases gas in the container that's just next to it. The gas fills up the propelling mechanism which opens the vent. So as soon as you have high temperature nearby your vent, you automatically open it. Very simple.

Speaker 1:

Now this creates some issues because if you think about smoke control design, we don't necessarily design for a single vent. We design for systems which are compromise of multiple vents. So if you have only individual operation of vents because you only have this mode of activation, it means that you're not really triggering the whole system. You're triggering vents individually. Perhaps if you have a very big fire you will trigger all of them, perhaps you will not. So that's quite the challenge and it has to be understood and taken into account when the designer designs the system.

Speaker 1:

Another way of operating these systems require you to have some sort of smoke alarm. Technically you can also do this with sprinklers, combining the alarm valve of the sprinkler system with the control panel of the smoke extraction system. But the simpler way would be just use a smoke alarm. When the fire is detected, the alarm goes into some sort of control panel that controls all the vents in certain zone where the fire is detected, issues either an electrical signal to all of them and you have electrical actuators that will open the openings, or you may have a main gas bottle connected with pipes for the gas to all individual vents and you just like with the individual action of the gas bottle at the vent, you just overpressure the actuators and they force open the openings. In this case the vents open all together, simultaneously, all over the zone that they are designed for, perhaps your whole roof or just one smoke control zone, but you have operation of the whole array of vents and in my opinion this is much better because we are designing for the area of the whole array of vents, not just individual vent, and in this way we make sure that the operation is as the designer has expected.

Speaker 1:

I also think this difference in operation between individual vent operation and zone operation is something often misunderstood in many designs and perhaps misunderstood by many scientists working in the area of fire and fire science. And yeah, for me it's critical. And in our law in Poland you have the requirement that your vents operate automatically, and there is a big discussion in what that means. It doesn't mean that if single vent opens automatically, does this mean the system operated automatically or not? I mean it's very interesting discussions and some very technical problems that arise from a very simple choice to want your vent to be operated individually or be connected into a bigger system. Of course, it's a decision that has a lot of money involved in it, because if you can place your vents independently with no interconnection to anything just put a vent in the roof, make sure that the fused link is maintained, you check it every now and then as per the requirements of the manufacturer and your vent is good. And if you want to build a system, well, well, that's obviously a bigger investment because you need control panels and connection of this system to the other systems in the building, which you may not even have if you don't have smoke control systems in Poland. In many cases the smoke alarm system is designed purely for the reason that you need something to trigger the smoke vents. So yeah, a small difference in the structure, a little difference in nomenclature, but a hell of a difference in the costs and in how the systems operate.

Speaker 1:

Now let's talk about the devices themselves. These devices are fairly simple, I mean, as again it's a small dome, closing of a box or a window that is propelled by some sort of actuator, either electrical or gas, co2 powered, but their only purpose is to open fairly simple mechanism. But they go into some quite robust testing. I must say we are performing this test at my laboratory at ITB and I've been doing that for more than a decade. So the main thing we obviously test is the performance of vent at high temperature, which allows us to assign it to a class of 300, 600 degrees. As you can expect.

Speaker 1:

Put the vent on the furnace. We heat up the furnace in five minutes to the temperature that you are required to obtain. Then the vent has to open and then for the next 25 minutes we observe what happens with the vent and hopefully no damage is done. The throat area of the vent does not change and if it's like that, then the vent has passed the test. So the test is pretty much a confirmation that the vent can open at elevated temperature and that it will remain its more or less integrity over the cause of the fire and removing the hot gases from the building.

Speaker 1:

I wouldn't say it's the most demanding test on the planet. Most vents usually pass it. But it's also pretty robust check that these vents are not made from bad materials or combustible materials. They're not like fully plastic made. They will not degrade in elevated temperatures, that we are sure that they will sustain their role in fire. So I would say this is the easier of the tests. Much more difficult tests are the loading tests that we also need to carry for this fence. So, as you can imagine, the vents are placed on the roof or on the windows and there will be some forces acting on these devices when they have to operate. We cannot expect that the vents will operate in windless weather, 20 degrees outside sunshine, great time for a fire. They must be ready any time of the year in any conditions they may have.

Speaker 1:

So we have some additional classes that are related to snow and wind load. Snow is something we must consider. In Poland, as a funny story, we had a client for much more southern part of the world who came to the lab and was shocked that they have to test for 500 newtons of snow and they were not ready for that. Under a fence, the vent was very, let's say, not prepared for Polish snow conditions. So the tests of wind and snow load means that we apply some loading dead loading on the vent. You have classes that tell you how much newtons the vent can carry and you just dead load the vent to the number that is specified in the class and then it has to open multiple times. When we are doing the snow load, it's acting from top down, so it's pressing, and when we are doing wind load, the action is obviously reversed because we are simulating the wind suction effect on the vent. The fail pass criteria are very simple for snow, it just has to open with the load applied. For wind, it should remain closed for prolonged time to make sure that it doesn't accidentally open on the wind. So these tests are made to make sure that the device is reliable under severe weather conditions. We also have a reliability test.

Speaker 1:

This is a test where we open the vent multiple times. The highest class is 10,000 cycles. So if the vent is for the natural venting of your building and for the fire operation, you would go for 10,000 class, which means that the device has to open and close 10,000 times. This puts quite severe loads on the mechanisms of the device, so it must be a really high quality to pass this, otherwise their mechanisms will just get damaged, locked and the device will break the fun part in this test. And it's some sort of running joke in my laboratories that sometimes very, very rarely you can find devices that are certified for a thousand or 10,000 cycles but they do not include automated closing system, which literally means there has to be a person that manually closes it 10,000 times after it automatically opened 10,000 times. And this is crazy. I have not done that myself, but two of my colleagues have had tests like that and, yeah, that's two weeks of fun sitting at the vent and helping it close out. Manufacturers, don't save your money. Please provide closing mechanisms for the ones that are supposed to open 10,000 times.

Speaker 1:

The next environmental class it's related to under zero temperatures, also very important for countries where you have severe winters, like Poland. So the class means that the vent can operate in very low temperatures, which means that there will be no adverse effects of freezing on the mechanisms. That's very important because you can have ice clutter your mechanisms and prevent the vent from operation. So yeah, they must be ready to operate in under zero conditions.

Speaker 1:

And finally, the perhaps most important from the market perspective, characteristic of the vent, which is its aerodynamic free area. Aerodynamic free area is a value that you get by multiplying the area of the opening times, the discharge coefficient of your ventilator and the district coefficient of the ventilator is well, you can simplify it into the ratio of actual flow through the vent in specific conditions, to the maximum possible flow in idealized conditions where the resistance would be none. So it's a number that tells you how efficient the vent is in letting air through it, and there are some generic values that usually are between 0.6 to 0.7 for typical vents. I think the standard allows you to take 0.4 without testing. So there are some generic values, but of course every manufacturer goes through very complicated aerodynamic testing to make sure their vent has the highest value, because this is the value that gets put into your Excel spreadsheet when you calculate how many devices you need on your roof to achieve the smoke venting that you want to have in your project. So of course the manufacturers are very incentivized to have as high value of this district coefficient as possible and again, as we are doing this test, we know exactly how important it is for them to maximize this parameter of the vent. The parameter itself is estimated in the wind tunnel. So you put the vent inside, you have a chamber underneath the vent where you put air inside with a known mass flow rate. In the wind tunnel you have wind with 10 meters per second velocity or higher and basically you rotate the vent in your tunnel, having pressure difference between 3 and 12 Pascal between the vent and the interior of the tunnel, and you measure the mass flow rate. From that you can calculate the discharge coefficient as a function of the angle of the vent against the wind and, of course, for the lowest value that you find you assign that as the discharge coefficient value for the vent.

Speaker 1:

An interesting caveat of this testing is manufacturers. Of course they don't produce a single type of event. They usually would have large ranges of products available because designers need to choose from something. So we obviously do not test all the vents in their family. Some of the vents are as representative for the family. There are quite good rules on how to pick them and, based on those vents that are chosen for the test, the value of the discharge coefficient for the whole family is approximated. So it's not even a direct measurement. It's more or less like a mathematical estimation of what the discharge coefficient of the family would be. And yeah, with that we can tell you what is the standardized certified discharge coefficient of a vent.

Speaker 1:

As here we're talking about small control systems and they're routinely designed with CFD methods used, I'm often asked by colleagues what to do with this aerodynamic free area in relation to the numerical modeling. Should they model the vents with their geometric area, like the real opening size, and they relate on the aerodynamic free area? As the simplification in the worst wind conditions, I would say just go with the geometrical area of your opening, but make sure to include some space outside of your vent. So just don't put an open boundary on your roof. You have to model the vent and then some space outside of it and that should allow you to pretty well capture the flow characteristics of the vent in your roof. And if you care about the wind action, well you should to see if they with wind aerodynamic free area is not an answer.

Speaker 1:

And if you ask me what I personally think about the discharge coefficients and aerodynamic free areas of vents, I hate it because it gives you very shallow information about the vent. Perhaps someone will get angry with me, but these things are quite generic. I mean the little details that give or take 0.05 aerodynamic discharge coefficient value. They make a really tiny difference compared to the differences that you find where, for example, a real building is shielded by another building in the winter, or if wind is hitting your building from a very specific direction where there is your inlet openings located. I mean, even if you take the vents and you put them on the roof, the vents of the exact same type, exact same discharge coefficient. If they're on the roof, each of them will be exposed to slightly different wind conditions. It will have a different discharge coefficient. In fact that they will all operate on the different values of flow rates through them and they all have the same parameter. I mean, in the end it's the system that operates, not the properties of single vented rule. If the system will work out or not, it's how the system is built, how vents are located, what wind effects are taken into account, what countermeasure you take on your roof to counterwind.

Speaker 1:

And now, if you look at how the products are delivered, the manufacturers are not incentivized to give you that advice on how to build your roof correctly, because they're not roof manufacturers. They're not saying roofs, they are certifying the vents. So the vent is the only thing you get and the little tricks they put on the vent to improve the flow rate through is the only thing they can do and the only thing they can sell you. And I can tell you, just putting a little already dynamic element in front of the vent on the roof, if you do it correctly, it can double the flow rate through the vent, not 5% increased, it can double it. And yeah, you cannot do that with just elements on the vent. But it's very difficult to design like that and I honestly don't know any projects outside of ITB where such approach would be taken. We have a ton of papers on that and I'm rambling on that for ages. I'll link them into show notes if I build some of your interests. You can read more about the air dynamic effects of the roofs and how different vents operate in different conditions and what that means for the whole system. Anyway, in the end, in the real world, you will be faced with the choice of vents. You will be given a brochure by the manufacturer that will point you to the air dynamic area or discharge coefficient of the vent and in the end that's the information you have on the properties of the device and that's the one that you use to choose your system.

Speaker 1:

Now I brought a little bit about wind interaction. There are many interesting wind effects that can occur with natural smoke ventilation systems. So, first of all, the wind on the roof creates a very complicated pattern. So you have vortices shedding out of the edges of the roof and just behind where the vortices have shed you may have a very high under pressures, very high negative pressures on your roofs. When the vortices reattach to the roof, you will have very high positive pressures on the roof. It depends where your vent is against those edges and what velocity and direction wind had. Unfortunately, for every wind direction and velocity, the location of these under and over pressure zones will be different. There are some not bad guidances, like your code for wind. They give quite good estimation of where those zones will be located on your roofs, especially for flat roofs. So you can take an informed decision.

Speaker 1:

A good way is to do a CFD. Of course, we have some interesting studies that show how these different ventilators located in different zones on your roof will behave. We also have done a very interesting study where we've modeled not just the building and put wind on that, but we've put a building inside the city with a complicated city architecture and we found some very interesting interactions with the surrounding buildings and the building we had. And another thing it's not just the vents on the roof that will be affected by the wind, but also the inlets, the points where you supply the air. Remember, I told you at the beginning you have to remove the smoke from the building but you have to put fresh air inside that replaces the air that you have just extracted. So where those openings are located against the wind makes a big difference for the operation of the system.

Speaker 1:

But finally, if you don't have roof ventilators but you have facade ventilators, where you have vertical smoke ventilators we call them the smoke windows these operations are even more complicated because these devices have very low discharge coefficients. They work not very well when they are faced with wind. Actually, you're not supposed to put them on a single facade of the building. You should put them on two facades and have some tool that will tell you from which direction the wind is blowing and which side should operate. It's actually written in the standard, but I don't think anyone has ever designed it like that properly and very disappointed with that. If you've seen a project like that, point me out to that. I would be rejoiced to learn that someone is doing that properly, but from my experience this is not happening. Well, you should have them on two facades, otherwise they will not work correctly. I could go on and on and wind in directions. Actually, if you like this subject, there's episode 50 where Guillermo was interviewing me on wind, so perhaps you'll find more information in that.

Speaker 1:

And the last thing I would like to touch about natural ventilators is their interaction with sprinklers. That's another Pandora's box and in some cases insurers do not want the events to operate where sprinklers are present, especially early suppression, fast response, sfr sprinklers. I to some extent understand that, in terms of these specific types of sprinklers, you do not want anything to delay the sprinkler operation. So for sure you need to delay venting action of your building, removing the smoke, until all the sprinklers that are supposed to be triggered are triggered. On the other hand, it's enough to have the ventilators operating in their sanitary mode, which means they're only tilted a little bit, and from my research it shows that this is enough to already disturb the sprinkler operation. So if we go that far that we prevent the vents from opening, we should also have automated closing mechanisms. I mean, if we cut that much to basically disable a whole safety system, we should go a little step further and make sure that nothing is blocking the sprinklers. The second thing is and there's also some benefits to having venting action in your building in terms of later on firefighter's operation and the control of the overall damage in the building. So I think there must be a compromise between the sprinkler people and the venting people. I guess that could be a whole episode on its own, and perhaps in the future I would do one, because that's a topic that, as long as I remember it comes back and back and back again, especially in Poland, where you are forced by law to have automated smoke extraction system in your building, which means it should operate, and having those vents closed does not count as an operating system. So that's a brief summary of what natural ventilators are how they are tested and what are their fundamental characteristics.

Speaker 1:

Now let's move to powered smoke ventilators. Powered ventilators, as the name suggests, use some sort of power to force this air movement, so we no longer rely simply on the buoyancy forces of the smoke, but we use mechanical devices to extract the smoke from the building. These are usually either axial or centrifugal fans. Axial are the ones where the fan is mounted behind the engine and they're on the same shaft, and the engine is pretty much in the stream of the air, which has some implications on the performance characteristics of this fans. In centrifugal fans you would have a very large rotor that sucks air from the bottom and spits the air to its sides, from which it is removed. The engine in this type of fan is usually mounted in a separate chamber, perhaps on the same shaft as the rotor, but definitely separated from it in a separated enclosure, which means it's not subject to the high temperatures of the smoke removed by the ventilator. The differences of the operation of those two types of fans are there are some centrifugal fans allow you to have higher pressures, but they have limits to their volumetric capacity. They typically would be used on top of the roofs because it's very easy to set them up directly to your smoke reservoir.

Speaker 1:

Axial fans you can build up to very large sizes. You can have multiple fans on them. You can have very high powered engines. The downside is that you usually have the engine in the flow of the hot gases, which has some implications on how hot fire they can take. The fans would be mounted, as I said, on the roof directly connecting to the smoke reservoir from which they're extracting the smoke. That's the simplest mode of operation, but you can also put them in ducts or shafts, where they are used as the pumps of the smoke extraction system. They suck the air through the ducts from your building and they release them on the other end of the duct, and in this case you of course need much higher operating pressures for them.

Speaker 1:

You also have another way of operation, and that's very common in car parks and tunnels, which is called jet fan systems. In this case you have usually axial fans, but they're also centrifugal jet fans, actually, but most of them are axial. So you have axial fan that's mounted under the roof of your building and it's used to create a flow of air that eventually pushes smoke in a specific direction. It's not fun that they're sucking the smoke on one end and throwing it further on the other end. It's a device that creates the flow of air that is meant to push the smoke away. It's actually often a very big misunderstanding of how jet fan systems operate. I'll come back to that on the end of this section.

Speaker 1:

So, as the natural ventilators are defined, with their discharge coefficient or our dynamic free area, in case of the powered ventilators we are usually speaking about their volumetric capacity and operating pressure. These two things are connected with each other through a concept called operating curve. It's a curve between the pressure and the volumetric flow through the vent, which tells you how much air you can remove with the fan at a certain pressure, and it's used by the MEP designers to choose the correct fan for a very specific point of operation of your duct system in your building. It's very important to have these things matched, otherwise you may end up with a system that has insufficient power to extract the smoke at a certain volumetric capacity. So this curve is the fundamental basis of the design, but there are two other curves that you should consider. One is the power curve, which tells you at which volumetric capacity how much power the fan will take. An interesting thing is that as the air gets hotter in your building, as you start removing smoke instead of cold air, the power demand on the fan will decrease because you're suddenly removing something which has small density than ambient air. So that's very interesting, that the power changes. The other one is frequency curve, which basically means that if your fan operates at different power frequencies, it will have different operating curves. In the end, what you get from the manufacturer is quite a cluttered plot of all different setups of either frequencies or perhaps angles of the blades in your in your fan, versus the pressure, versus the volumetric capacity, out of which you have to figure out which device will operate best in your building in your very specific conditions. It's perhaps something that not many fire engineers would ever touch. It's something that MEP designers would do for us, but you should be aware of how it works. Otherwise you may ask for devices on your buildings that are simply impossible to be introduced, or perhaps you would create systems that are very inefficient. So if you make a very complicated ductwork with multiple bends, multiple separators, many joints, multiple points, it's very easy to make a system which has to operate on very, very high pressures and that creates difficulties in choosing the correct fans. So yeah, understanding how the pressure and volume flow work together is critical for you to design nice, sleek and very efficient powered systems.

Speaker 1:

Now it is also very interesting to think about how the fan operates. How, why does it remove air? Actually, it gives much better insight into some more complicated aspects of how whole smoke control systems work. So you have an electric motor that is connected to a fan and this motor, this power, by AC current. So you have alternative current that goes into the motor, you have electromagnets that make the interior part of the motor spin and that spins the fan. Now your rotational speed will be defined by the number of poles you have in the engine and the frequency of the current that goes through the engine. So if the current goes through the engine at the specific frequency, you will get a very specific rotational velocity of your shaft and that means you will have a very specific rotational velocity of your fan and a very specific flow rate through the fan.

Speaker 1:

Because if you think about the fan, it is cutting and pushing little pieces of air with every turn of its blades and the interesting thing it is doing it is removing the same volume of air every time it moves, no matter what temperature. It okay, it changes a little bit, but it's not that big difference. Every time the blade moves it takes the same amount of air with it. Now, this means that the fan operates at constant volume flow rate. That's very important to understand that it's constant volume flow rate and this means that the mass flow rate that you have in your system will change depending on the temperature of the smoke that you're removing. If you do remove ambient air with a certain volumetric capacity, once you start removing hot smoke at the elevated temperature, you will remove the exact same amount volume of this smoke, but the mass that you've removed, because it has lower density, will be lower. This is important because if you have your natural makeup inlets, they will supply as much mass of the air as as much mass of the smoke you've removed, not not as much volume. So that's quite a little detail that perhaps is not very well understood. And and actually this loads me to discover a very efficient way of extracting smoke through very small shafts.

Speaker 1:

I've called it smart smoke control, but that's a material for another podcast episode. I'll link you to a paper in the show notes if if I got you intrigued. The important aspect for you is that the frequency defines the amount of air that you extract through a fan. Now what we often use to control these devices, because sometimes you want to change this value depending, for example, of which smoke control zone you operate. If I have my fan connected to multiple zones and in some of them I would love to have lower exhaust rate than in other ones, you can drive that through frequency inverters. So if you change the frequency of the power that you supply to the fan, you will change its rotational speed and because of the concept of operating curves, for different frequencies you will have a different operating curve and you will have different volumetric capacity. So you are able today to really well steer these devices in your systems. And, yeah, you can even do it very dynamically, as we do in pressurization systems, which were also covered in the fire shine show before. So now you know how the fans operate.

Speaker 1:

Let's discuss how they are given their temperature ratings. I've mentioned difference between centrifugal and axial fans in terms of how well they handle elevated temperatures because of where the engine is located against the stream of hot gases. So, as you can imagine, in the high temperature tests of these fans. We literally put the fan on the installation connected to the furnace and we said send the hot gases from the furnace through the fan at the temperature that the manufacturer chooses to certify for. So usually it's 300, 400 degrees that these are the typical temperatures that the fans are certified. Many fans are certified for 200, 250. There are fans that are certified for 600, so the choice is quite big out there.

Speaker 1:

But they're all tested in the same way. You operate the fan for some time in ambient conditions to get the background parameters of the flow and electrical properties of the motor and then you start heating up the air. After 15 minutes you shut down the fan for two minutes and then you restart the fan and then you keep pushing hot gases through the fan till the end of the test and you of course measure how much of those gases are transported. So if fan did not change the, the properties, the volumetric capacity that it's providing, and you also monitor the engine through the electrical power consumption and other properties, in case of higher temperature fans, the difference between low and high temperature fan is basically in the way how the engine is built, how the internal power connections are made, because you don't want to have short circuits coming in your engine because some of the cables got exposed through the high temperature. There are also differences in the lubrication of bearings. This is actually super important. You want your device to keep rotating in elevated temperature and you want your bearings to survive that, because if your bearings fail, the fan will fail very shortly after. So there are a lot of mechanical components that play a role in this.

Speaker 1:

From my experience, these tests are quite demanding on the fans. If I called the natural ventilator and tests easy, these ones are definitely the heart of, or maybe even the hardest ones to pass. So you really need a great engine and great construction of your fan to make sure it survives 400 degrees temperature for two hours. It's. It's not an easy test and, yeah, I've seen a lot of fans that did not did not pass one.

Speaker 1:

Now, if you have a fan of, let's say, 300 degrees rating, what does it mean if the fire is nearby? That's also a question that we often get like I have, for example, a car park, I have extraction point and nearby that extraction point I may have a car that is burning. Of course, the flame temperatures are much higher than those 300 400 degrees. What does it mean to my fan? Does the fan get destroyed? Do I need a 1000 degree fan? Such funds would be very hard to get.

Speaker 1:

So the response to that is that when you think about the system, it's more heat transfer problem, not simply max temperature problem. So, yeah, sure, flame will have a very high temperature, but the flame is not always there. The flame is not stationary, it pulsates, it moves in and out and in the end your temperature will. If you average it over time in space, it's going to be much lower than the flame temperature. You also have to transfer the heat from the gases to the parts of the fan which are quite heavy pieces of metal, so they have quite big thermal bulk. They do not heat in an instant, so it takes some time for heat transfer to actually heat them up. So I would say you have to investigate the average smoke temperature nearby your vent and by that you will know if the fan can survive or not. I actually like to do it in a reverse way. I like to calculate what is the required heat release rate to heat up air going through my fan to the certified temperature and I compare this theoretical heat release rate with my design fire and obviously, if my design fire is lower than the theoretical heat release rate, I would need to heat up the fan. Then my design is quite fail-safe and I can do that on a piece of paper without even CVD. If I'm close, I perhaps would like to do CVD to check how the architectural details and three-dimensional space plays a role in that. But this is a very simple check that allows me to know if my fan is more or less suitable to handle the high temperatures that I'm designing for. So finally, for the end, I've teased you with JetFence. So JetFence systems are definitely among my favorite systems that I'm designing. We've been doing that in Poland since like 2010,. We've designed more than a hundred for sure. We even wrote a book with my friend Gzers Kriewski, who was also the guest of the show, on how to design smoke control with JetFence systems in carbox.

Speaker 1:

The principle is that you put a lot of little JetFence axial fans that push the air and create the flow of air around the stream that you release. That's the point of it. It's like a Dyson hairdryer where you inject a stream of high velocity air to move a lot of surrounding air to have the action that you want. Jetfence operate in the same way. You inject high velocity air into your car park or tunnel and you move much larger amount of air with this stream. So in principle it's a momentum transfer technology. Actually, what you do is you create momentum at your fan, you transfer that momentum into high velocity stream in the JetFence and then this momentum is transferred from the high velocity stream into the surrounding air and then you have a large amount of air moving at fairly low velocity to three meters per second, which is just enough to move the smoke into the place that you want.

Speaker 1:

There are, of course, some caveats in designing those systems. It's not enough to just put a lot of tiny JetFence and overpower the smoke into a very small smoke extraction shaft with a small extraction fan and hope it will be great. It doesn't work like that. You have to match the extraction rate and the forces that you create in the car park, which perhaps is a challenge. Also depends where you place the extraction and inlet points for your system, so you create a very nice pathways for air to go through your car park. It's usually best to have reversible systems so you have more than one way that you can transport the smoke, and there are, of course, limits to how much of a fire you can handle with that. You have to calculate that. You have to understand that and, especially in tunnels, we would like to design them for quite high velocities that allow us to make sure that the flow is always in the direction that we want.

Speaker 1:

So, wow, it's already almost an hour of talking about ventilators. I'm a huge fan of this topic, that pun intended. I could probably talk about it for much longer, but I don't really want to bore you to the hell with this. I guess I will stop on this. I've opened like 3 or 4 subjects that require a separate podcast episodes. The Vent later, natural Vent later and Sprintless System is one for sure.

Speaker 1:

Perhaps the Jetfin design would be a nice podcast episode for you as well, and the Smart Smoke Control concept that we've designed. I should really cover that one. That was fun to discover research and field test. We actually have experimental proof that it works. If you're impatient, most of that are in papers linked in the show notes, but for now I'll stop here. Let me know if you enjoyed that. Let me know if I actually told you something new or it was just a reminder of what are we using and how are we using it. I hope with this episode, the world of smoke control is a little bit closer to you and you will enjoy it as much as I do in my professional career. So thank you very much for listening and see you here again next Wednesday. Cheers, bye.