Learn more about textile based ventilation
What is isothermal air?
Isothermal air is air that is not influenced by buoyancy forces, e.g. inlet air with the same temperature as the surrounding air.
What is Delta T (ΔT)?
The difference between the inlet temperature and the room temperature. If a room in a cooling situation has a ΔT of -3 °C then the room temp. could be 24 °C and the inlet temp. 21 °C.
What does stratification and temperature gradient mean?
Stratification is an expression to how the given temperatures are in a certain area. If hot air has short-circuited under the ceiling the stratification temperature gradient is said to be high in the room.
What is ventilation and temperature efficiency?
The ventilation efficiency is a measure to describe how efficient the ventilation system is. The ventilation efficiency describes the ratio of the average pollution in the occupied zone to the pollution in the extract air. The temperature efficiency describes the connection between the temperature level in the occupied zone and the temperature of the extract air.
If the extract opening is placed high in the room the temperature efficiency will indicate if hot air is short-circuiting in the room. In mixing ventilation the temperature efficiency can be maximum 1 meaning 100% air mixing. In displacement ventilation the temperature efficiency can be higher than 1 due to stratification of air. Therefore displacement ventilation can be used to save energy for ventilation.
Which type of fan is the best for TBV?
In ventilation systems there are two types of fans; axial-flow fans and centrifugal fans.
The axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate. Axial fans blow air across the axis of the fan (like a table fan). The rotation of the blades creates a subpressure on the backside of the fan, which creates a forward air flow. The efficiency of the axial-flow fan is high, but the fan has a lack of ability to build up external static pressure which often makes the fan less usable for TBV.
The centrifugal fan has a moving impeller that consists of a central shaft about which a set of blades are positioned. Centrifugal fans blow air at right angles to the intake of the fan and spin the air outwards to the outlet. As the air passes through the "paddle wheel", energy is supplied to the air causing the total pressure to rise. A centrifugal fan produces more pressure for a given air volume and can be adjusted by changing pulley. They are typically noisier than comparable axial fans, but the ability to produce higher external static pressure makes the fan advantageous in TBV systems.
What is 2-dimensional and 3-dimensional jets?
A 2-dimensional jet is characterized by a spread in two directions, which means that the jet induces room air from two sides of the jet. A 2-dimensional jet is also known as a plane jet. In TBV systems jets from slots and inject-JET systems can be characterized as 2-dimensional jets.
A 3-dimensional jet has a spread in 3 dimensions, which means that the jet induces room air from all sides of the jet (cone shaped jet). A 3-dimensional jet is also known as a circular jet. In TBV systems jets from nozzles, inject LV and MV and big holes can be characterized as 3-dimensional jets. However, nozzles or holes placed close to each other will be characterized as a 2-dimensional flow.
What is static pressure, dynamic pressure and total pressure?
The static pressure is measured in relation to atmospheric pressure. It has an identical impact in every direction and keeps the textile material inflated as well as pushing the air out through the holes/nozzles.
The dynamic pressure, or velocity pressure, has an impact on the direction of the air and carries it from A to B. The dynamic pressure is related to the mean air velocity in the duct.
The total pressure is the pressure that needs to be produced by the fan to overcome the total resistance in the ventilation system, i.e. the loss from the individual types of resistance, such as filters, cooling/heating surfaces, elbows, frictional loss and the static pressure in the system etc. The total pressure can be calculated anywhere in a textile based ventilation system as the sum of the static and dynamic pressures.
What is a free jet and a wall jet?
If the air jet is directed outwards into the open room, this is what is known in flow engineering as a free jet. The turbulent air jet entrains air from its surroundings and the jet's diameter increases in proportion to the distance from the duct, while the velocity in the jet decreases.
A wall jet is a jet that is pointed towards a surface. The flow can be regarded as a bisected free jet as the surface can be regarded as a plane of symmetry. The maximum velocity is achieved close to the surface and is 40% higher than the equivalent velocity for a free jet at the same distance from the duct.
What is throw length?
The throw length is defined as the largest distance from the supply duct to a specific point in the premises where the air velocity is precisely equal to the desired final velocity, e.g. viso= 0.20 m/s.
It is important to note that the throw length, by its definition, is valid in isothermal conditions. As a result, the calculations for air velocities must be corrected if the supplied air is either colder or warmer than the surrounding room air.
The throw length for a wall jet is 40% longer than for an equivalent free jet. The reason for this is that a wall jet sticks to the ceiling due to the Coandă effect and as a result, only half the volume of room air contributes to reducing the velocity rate.
What is entrainment?
It is well known in flow engineering that holes positioned with a perpendicular direction in a long main duct can cause problems. The main problem is that the air tends to veer away and flow parallel to the main duct unless the holes or nozzles are fitted with baffle plates.
In most cases, entrainment will cause problems with air distribution in the premises. At one end of the room (nearest the inlet), problems may arise with stagnant air, while at the other end of the room, entrainment may be the cause of major draught problems. Entrainment occurs if the inlet velocity is significantly higher than the outlet velocity through the holes in the textile material.
To counter this inconvenient deflection of flow (entrainment), KE Fibertec recommends that the outlet velocity through the holes be at least 35% higher than the inlet velocity in the duct.
Another factor which has an impact on entrainment is the size of the hole that has been cut out. If air is supplied through large holes, it will always tend to deviate and flow along the duct.
What is thermal penetration length?
The thermal penetration length is crucial in determining whether the air is being distributed as expected. The theory underlying wall jets is since the jet will not become so "heavy" that it will leave the ceiling's surface before it is supposed to. If it does, the air velocity at the entrance of the occupied zone will be higher than calculated.