We recently worked on a project with the Newhurst Energy-from-Waste facility where the customer wanted to try a new design approach that led to some interesting modelling results we wanted to share with you.
To give some context around the building design, the Newhurst power generation facility is much like any other power generation building – a large, mainly open structure that is around 40 metres tall. Inside the building, large steam turbines, boilers and other processes are constantly producing high levels of heat that needs to be managed in order to keep the interior conditions stable for both the benefit of the staff and the energy-producing processes.
A typical climate control solution for this type of building would be a natural ventilation approach, consisting of natural ventilation openings in the external envelope of the buildings at both high and low-level. With this type of system, cooler outside air tends to flow into the building through ventilation openings in the lower half of the building, and warm air flows out of the building through natural ventilation openings in the upper half of the building. This is commonly referred to as displacement or stack ventilation system.
The ventilation systems are usually controlled so that they open and close according to the heat output of the equipment in the building, and the external ambient conditions. This additional control helps to ensure that the internal temperature will never become too warm or too cold as the ventilators will adjust themselves to deliver the correct level of ventilation at all times.
As a value engineering exercise, the client on this project asked us to investigate whether the ventilation system could operate effectively without having controls on the high-level ventilators. Their hope was that by closing all the vents on the bottom only and constantly keeping the top ones open, the system could be throttled sufficiently to continue operating effectively in those times where less ventilation was required. This would do away with the need for control mechanisms for the top vents, offering an immediate saving there, and also ongoing maintenance and repair cost savings, as high-level ventilators can be difficult to access.
As we had not previously used this approach and were unsure of what the exact outcome might be, our technical team created a CFD model to simulate what might happen in a ventilation scenario with the controls and systems set to these restrictions - the results that came back were interesting.
On this project, the exhaust air ventilators were located within the walls at high-level, and varied from 3m high to 5.6m high at different locations. What we found, was that the ventilators that were up at high level actually ended up supplying both the supply and the extract air for the system; in the bottom half of the ventilator, fresh, cool supply air was coming in, whilst warm air was simultaneously being released through the top half of the ventilator. As a result, the ventilation system continued to operate. The reason for this can be explained by looking at the fundamentals of natural ventilation design.
Heat loads within the building cause the indoor air to heat up, generating a “thermal lift“, which creates a difference in pressure within the room, resulting in negative pressure on the floor and positive pressure under the roof. The effect of this difference in pressure is that outside air streams in through any openings in the low-lying area of a building while air streams out through any openings in the upper area of the building.
Accordingly, an area of neutral pressure must exists between these two opposite pressures, where there is neither a positive or negative pressure – this is termed the neutral pressure plane (NPP). The location of the NPP is a result of the location of the heat source(s) and ventilator(s). On buildings with ventilation openings at the bottom and top, and a heat source focused toward the bottom of the building, the NPP is about mid-way between the sets of ventilators.
As a result of the low-level ventilators being closed off, the NPP moved up the building and prevailed at a mid-point through the high-level ventilators.
In addition to the ventilation system continuing to operate, a significant negative pressure was generated in the lower part of the building (around 30-40Pa). The suction created from such a high negative pressure would make simple tasks such as opening and closing doors difficult and dangerous and would create intermittent draughts, resulting in undesirable working conditions for staff. The energy-producing processes that are taking place in the building are also sensitive to temperature and pressure changes in the environment and require stable, constant conditions to work properly. The negative pressure produced would adversely affect these as well.
This realisation made it clear that leaving out the dampers on the top vents was not viable.
The discoveries from the CFD experiments added new insights to the project and meant that both our designers and the customer could clearly see the potential pitfalls with this new approach.
If you are considering a change to your ventilation setup or require a completely new system, talk to our experts – our free design service can help pinpoint potential problems before they arise and will ensure you get the best ventilation solution for your building.
If your building is too hot or too cold, if your process gives off fume or moisture, if your product requires specific conditions during its manufacture or storage, or if noise is a concern, then we may be able to help you.
We can survey your building using a range of techniques and equipment to identify your problem. Once established, we can then recommend a solution based on proven design work.
Laurence Cockman is a Technical Manager for Colt UK Climate Control Division and specialises in the design and product application of energy efficient HVAC systems.