What is deflection of a free jet? When the inlet air has a different temperature than the air in the room, a free jet will have a deflection upwards or downwards due to difference in the air density.
A hot air jet will have a deflection upwards while a cool air jet will have a deflection downwards.
What is industrial cooling? Industrial cooling is a general description of cooling used in industrial applications where there is a requirement to the maximum air velocity in the room.
What is comfort ventilation? Comfort ventilation is a general description of ventilation in applications where people have comfort requirements (temperature, velocity, relative humidity, air quality etc.)
What is relative humidity? The relative humidity of air is defined as the ratio of the partial of water vapour in the air to the saturated vapour pressure of water at a defined temperature.
What is dew point temperature? The dew point temperature is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapour to condense into water. The dew point is a saturation point. When the dew point temperature falls below freezing it is often called the frost point, as the water vapour no longer creates dew but instead creates frost or hoarfrost by deposition.
What is activity level of persons? The activity level of the persons in the room describes how much the persons move, e.g. how much heat they supply to the room. The requirements to the air distribution is often dependent on the persons' activity level.
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.
What is thermal and atmospheric indoor climate? The thermal indoor climate is a measure of the air temperature, mean radiation temperature, air velocity, and relative humidity.
The atmospheric indoor climate is a measure of the air quality by means of organic vapours, bacteria, smoke etc.
What is CO2and ppm? Carbon Dioxide (CO2) is a surrogate for indoor pollutants emitted by humans and correlates with human metabolic activity. Unusual high indoor carbon dioxide levels may cause occupants to become drowsy, get headaches, or function at lower activity levels. Humans are the main indoor source of carbon dioxide.
What is geometric inlet area and effective inlet area? The geometric inlet area of an inlet (e.g. nozzles) is the geometric area of the opening in the inlet. In all types of air inlets there is a contraction of the flow, meaning that the effective inlet area will be smaller than the geometric inlet area. The ratio of the effective inlet area to the geometric inlet area will therefore always be below 1. The ratio is called the "contraction coefficient" for the inlet opening.
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 occupied zone? The occupied zone is the area in a room which people occupy for a long period of time and is defined as the area where efforts are made to maintain the indoor climate at a general level. The occupied zone is not a standardised area, but a zone which is defined from one project to another in consultation with the architect and client. The occupied zone is often defined as the zone from the floor up to a height of 1.8 m above people who are in a standing position doing their job, while this height is set to 1.1 m for people who are seated.
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 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 "Coandă effect"? If a jet is directed towards a ceiling surface it will try to "stick" to this surface because a negative pressure is generated between the jet and the ceiling as no replacement air can be supplied for the volume of room air entrained by the jet.
This phenomenon known as "Coandă effect" increases the throw length by a factor of √2 (1.4) in relation to the throw length for a free jet. The air velocity needs to be a minimum of 0.35 m/s to be able to utilise the Coandă effect.
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 air density? The air density is the "weight" of air given in kg/m³. The air density is dependent on the air temperature. For normal ventilation purposes the density is 1,205 kg/m³.
What is cooling and heating capacity? Cooling and heating capacity is the cooling and heating power of a central air handling unit. It is most commonly measured as the joules per second (Watts) of heat that the air handling unit can remove from or add to the air.
The cooling capacity is dependent on the air quantity and the difference in temperature. To determine the necessary cooling or heating capacity a load calculation of the room/building is needed. This calculation must take the internal heat, the load size of the room, the direction of windows, insulation efficiency etc. into account.
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 has a tendency to veer away and flow parallel to the main duct, unless the holes or nozzles are fitted with baffle plates.
In the vast majority of 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 have a tendency to deviate and flow along the duct.
The KE DireJet System never has problems with entrainment. The nozzles are designed at a length that ensures that the air is discharged perpendicularly to the main duct. In the KE Inject System the holes are cut directly in the textile, which means that the use of baffle plates is not an option. On the other hand, entrainment does not occur to such a noticeable extent in this case either. The reason for this is that the air velocity at the centre of the hole, i.e. perpendicular to the main duct, is usually higher in the KE Inject System than in conventional systems.
What is induction and induction number? Induction is a result of blowing air through a hole or nozzle with high velocity. The high air velocity will generate excess pressure, resulting in an inflow and entrainment of room air towards the air jet supplied. During the first air flow cycle the air velocity will be high, but as the quantity of room air that becomes mixed increases, the air velocity will decrease.
The induction number is the ratio of inlet air and induced room air in the jet. A high induction number is a measure of a good mixing capability of the ventilation system. The induction number will increase with increased distance to the air outlet.
What is thermal penetration length? The thermal penetration length is absolutely crucial in determining whether the air is being distributed as expected. The theory underlying wall jets is actually based on the fact that 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. This means that it will feel uncomfortable for anyone located in the zone affected by the jet.
What is the permeability after washing? Normally, a textile material gets up to 15% more open after washing. If the textile is not properly heat-set the permeability can be up to 35-40% higher as specified. It is recommended to follow the washing instructions of KE Fibertec AS.
What is the shrinkage after washing? If the washing procedures from KE Fibertec AS are observed, shrinkage will not exceed 0.5%.