By Ron Burns
Last month, I decided we should compare a simple 2 000m2 and an 8 000m2 warehouse relating to the advantages of a natural ventilation system versus a powered ventilation system.
There seems to be a groundswell to move away from ventilators and provide fan solutions instead. A good place to start will be to look at the two systems in the base buildings and to see the impact on the financial and performance side of the solution.
2 000m2 building
Figure 1: Plan view of a 2 000m2 building. Image supplied
Figure 1 is a simple, single-storey warehouse with eaves at 10m above fixed floor level (AFFL) and a roof pitch of 5 degrees.
Figure 2: Pectional view of a 2 000m2 building. Image supplied
Figure 2 is a standard schematic sketch — the overanalytical must just breathe deep and visualize that the 8m clear layer is below the 10m eaves. For the analysis to work, we will need to establish a little more about our building. The building is fitted with both a smoke detection system and a sprinkler system. The sprinkler bulbs are rated at 141°C.
The first step is to assess the smoke zone requirements. The building is below 2 000m2, with both lengths shorter than 60m and therefore shall be treated as a single smoke zone. It is important to note that there are instances when a building’s geometry has one length longer than 60m. It is definitely worth the designer considering the option to install a smoke curtain in the building, separating the building into two smoke zones. There is an advantage in choosing this option, as it allows the designer the opportunity to feed the inlet air through the smoke zone that is not in a fire condition. We will keep the single smoke zone simple for this discussion. This smoke zone requirement will work for both the natural ventilation system (roof ventilators) and the powered extraction system (fans).
We are using the calculation formula in EN 12101:5, as required for a building of this nature. The area of ventilation required to satisfy the calculation is 18.2m2 AVCV. Should the designer select a ventilator with a 55% coefficient of performance, the total geometric ventilation area of 33.1m2 supplied will equal 1 655% of the floor area. When compared to the 3% rule in SANS 10400:2011 Part T 42, it becomes clear that there is a saving following the EN 12101 design methodology. The inlet air requirement equals the ventilation requirement, 18.2m2 AVCV.
A natural ventilation system is indeed the ‘go to’ system of choice for a designer looking for a smoke ventilation system that performs seamlessly without the requirement of any emergency backup systems. The system is self-balancing the ‘engine’ driving the smoke out the ventilators as the energy from the fire. Limited moving parts and with the correct configuration of a support service, smoke detection, the early detection allows for the use of general-purpose cabling, once again further reducing the cost of the system, should the designer elect to install power-closed, power-open ventilators. The removal of the sprinkler system would not in any way reduce the performance of the ventilation system.
Should the designer wish to achieve the same performance out of the smoke ventilation system and elect to install a powered system, the extraction rate required to achieve an 8m2 clear layer would be to extract 71.6m3/s. This would roughly equate to 37.5kW of power. The building would then require the installation of a standby generator set with a duty of approximately 400 volts, 75 amps, three-phase at 50Hz. The inlet air needs to be provided by supplying 15m2 of low-level ventilators. This is the only component in the powered system where a saving is realised when comparing the powered ventilation system to the natural ventilation system. The electrical reticulation between the fans and the smoke ventilation control panel needs to be armour cable-rated CWZ (900°C, with a water test and a shock load test). Cable reticulation and panel positions are often overlooked.
The code requires careful calculation of volt drops as the cable performance is hindered at elevated temperatures. Cable tray location, support, and securing of cables need to be considered, ensuring the cable tray does not deflect to a point where the cable tray loses its structural integrity and fails. Insofar as the panel location is concerned, the panel cannot be installed within the same smoke zone that the panel is protecting. Should the panel succumb to the heat of the fire, then the system ceases to operate.
Without doing too many cost calculations, it is safe to assume that the cost of eight ventilators is more economical than purchasing the correct number of certified smoke extraction fans to exhaust 71.6m3/s.
A quick note to the air-conditioning contractors who are selecting and installing smoke exhaust fans: the temperature rating of the fans — 300°C for 60 minutes — relates to the certification of the fan (EN 12101:3). The temperature corrections are carried out in the design calculations. Assuming the extraction rate of 71.6m3/s is calculated at 300°C and requesting the fan supplier to correct the fan duty to 20°C, is going to have a disastrous effect on the system in a fire condition. The extraction rate specified in the design is based on the fan being selected at 20°C.
8 000m2 building
Figure 3: Plan view of an 8 000m2 building. Image credit: Supplied
Figure 3 shows a simple, single-storey warehouse, with eaves at 10m AFFL and a roof pitch of 5 degrees.
Figure 4: Sectional view of an 8 000m2 building. Figure 4: Sectional view of an 8 000m2 building. Image supplied
Figure 4 is a standard schematic sketch. Visualise that the 9.5m clear layer is below the 10m eaves. The building is also fitted with both a smoke detection system and a sprinkler system. The sprinkler bulbs are rated at 141°C.
The first step is to assess the smoke zone requirements. The building at 8 000m2 requires the division into four smoke zones. This would be the requirement for both powered and natural ventilations in this particular building. With the building now separated into four smoke zones, the designer no longer has a requirement to provide a dedicated inlet air system. The designer may feed the inlet air through the adjacent three smoke zones that are not in a fire condition.
We are using the calculation formula in EN 12101:5, as required for a building of this nature. The area of ventilation required to satisfy the calculation is 18.1m2 AVCV. This is achieved by taking credit in the calculations for the inlet air to exhaust air ratio. Should the designer select a ventilator with a 55% coefficient of performance, the total geometric ventilation area of 33m2 supplied will equal 1 646% of the floor area. When compared to the 3% rule in SANS 10400:2011 Part T 42, it becomes clear that there is a saving following the EN 12101 design methodology.
Should the designer wish to achieve the same performance out of the smoke ventilation system and elect to install a powered system, the extraction rate required to achieve a 9.5m2 clear layer would be to extract 90m3/s. This would roughly equate to 52.5kW of power. The building would then require the installation of a standby generator set with a duty of approximately 400 volts, 105 amps, three-phase at 50Hz. The inlet air needs to be provided by supplying 18m2 of low-level ventilators. The electrical reticulation between the fans and the smoke ventilation control panel as detailed above needs to be maintained.
Again, without going into detail with the cost calculations, it is safe to assume that the cost of eight ventilators per smoke zone (32 in total) is more economical than purchasing the correct number of certified smoke extraction fans to exhaust 360m3/s. The designer would need to provide the 90m3/s per smoke zone, four off required. The requirement on the standby generator will be four times the capacity described above, allowing for smoke spillage into the adjacent smoke zones (approximately 400 volts, 420 amps, three-phase at 50Hz).
It is unlikely that the designer could motivate the additional financial expense in supplying a powered smoke ventilation as opposed to a roof ventilation system. It is important to ensure the roof ventilators specified have undergone rigorous, approved, and independent water testing to provide a waterproof installation to the building.
