At my recent "Ventilation solutions for overheating corridors in apartment buildings" webinar, I received some interesting questions in the Q&A session. Here are my answers, which have been edited slightly for clarity.
Does the use of smoke shafts for day to day ventilation affect the performance as a smoke shaft?
Addressing the first part of that question, the basic environmental ventilation system places no furthers constraints on and has no implications for any aspect of the design of the smoke shaft.
The smoke shaft design takes precedence and the ventilation system works with the equipment installed for smoke ventilation. As mentioned it provides a nominal air flow. Since the ventilation system is likely to be limited by the size of the shaft and the sizes of the fans, as well as the location of the shafts and what are the sizes of grilles and dampers, this is why it is difficult to qualify the performance of the basic ventilation system using the smoke control equipment.
So, the use of the smoke shaft for day to day ventilation does not affect the performance of the smoke shaft.
Are there issues with maintenance/faults?
Up to now we’ve had no instances of any additional requirements for maintenance beyond what would be necessary for the smoke control system. We would generally select an actuator designed to life cycle more frequently. And we would generally select fans which as well as being rated at 300C for an hour are also rated for continuous operation at low ambient levels. So, other than that, there is no further maintenance or faults that we’ve seen.
Are there any advantages with using dual purpose smoke extract fans or would a smaller secondary fan always be better?
A smaller secondary fan is usually always better if the system is not too big. Once you need to start attenuating the smaller secondary fan, then it might become more cost effective to use the smoke extract fan. But usually, where the duty and the noise requirements allow you to use a small secondary fan, it’s the most cost effective solution.
Is the water supply for an evaporative cooling system normal mains and how much water is used?
Yes, its normal mains water. Once the system has consumed the water it just drains it out onto the roof and it will just go away in the rain water channels. How much water is used really depends on the location, the weather pattern, and at what set point you want to determine the air to be in the system. Using an evaporative cooling system, you could set the system to come on when the air got to 18C and therefore you are providing very cool air in the corridor and will consume more water. But generally speaking you’d probably be looking at just a few hundred litres of water per annum. So you use very little water, the cost of the water is almost irrelevant.
How do cavity barriers affect the airflow through the ceiling void?
Well, anything in the ceiling void affects the airflow. We cannot make the system work if there are barriers in the ceiling. So if you have a long corridor, with a cross corridor door and with fire stops above that, then you cannot use that as an air flow duct. You’d have to include further automatic dampers in the ceiling where there are any barriers so that there is a normal air flow and then close it off for day-to-day use. We do consider the amount of services in the ceiling void and our experience shows that there is always generally enough space to get the air flow through as it is not likely to be massive.
You mention that a push-pull system, where you have a smoke shaft either end of the corridor, would be more effective than a pull system, where you have a smoke shaft at one end and supply from the other. Was the example of 35°C achieved in the corridor from the 30°C ambient outside air an estimate for both types of system or is there a difference?
The estimate of 35°C just using the outside air is an estimate based on having a reasonable air flow through the corridor. So that’s where you’ve got ventilators and shafts that are positioned where you’ve got good coverage of air flow - good inlet which picks up a lot of the heat source, and extract, that picks up most of the air in the space. If you’ve got stagnant areas, then temperatures in these areas may increase beyond that. So the 30°C to 35°C is based on getting good coverage of the outside ambient air flow through the corridor and doesn’t take into account the higher temperatures in stagnant areas.
Where we only have a single smoke shaft pull system, our experience shows that you can carefully position ceiling grilles and balance the ceiling grilles to control which path the air will take through the ceiling void and then through the corridors. You can guide the air one way down across the ceiling void so that it comes out at one of the corridor and then it’s pulled out at the other end. It’s unlikely that there is system or corridor proposed even with a single shaft that we couldn’t provide a reasonable air flow throughout the corridor.
If the corridor is allowed to get to 40°C and the system was then turned on with an outside temperature of 30°C, is that why we end up with a inside temperature of 35°C?
The 5°C rise from the ambient air is simply because the heat load is so much greater in the corridor so that the air, as it gets brought in at 30°C, will automatically have a temperature rise of about 5°C before it gets exhausted at the other end. As the system is in continuous operation, the inside temperature would never get to 40°C. The rise in temperature is simply based on the fact that ambient air hasn’t got the capability to provide corridor cooling; it will always increase in temperature when it meets with a heat source.
What air changes are usually achieved in the corridor using the standard smoke shaft solution in a three storey building?
It’s normally around 1 to 2 air changes an hour using the smoke extract fans at a low speed. With a 3 storey building, you can perhaps look to increase that because the fans wouldn’t necessarily need to be running too quickly if you only have a few storeys, but generally, it’s 1 to 2 air changes an hour that you can achieve with general ventilation.
Laurence Cockman is a Senior Consultant for Colt UK and specialises in the design and product application of energy efficient HVAC systems.