Duct (HVAC)

Ventilation ducts or ventilation tubes are an essential component of ventilation systems and are mainly used for air ducting. These air duct systems usually consist of rectangular or round components, that is, from ducts or ducting. Considered flow Technically, the use of pipes is the optimal variant, since only in channels of hydraulic diameter is used. In smaller air distribution and oval tubes are used.

Air flows into this channel or pipe systems, as a laminar or turbulent flow, similar to water. For the most important calculations are the same equations as used in water technology. These include the Bernoulli's equation and continuity equation. Detailed knowledge of the corresponding friction values ​​(R- value) of square tubes, the friction ratios (zeta value) of various fittings and the differential pressure of duct components that extend the individual calculations and considerations. The planning of this air duct systems shall be based on vonVDI 2087 or later standards according to DIN EN 13779th

Very large ventilation ducts for air volumes of, for example, 300,000 m³ / h as at an airport, are brick or poured concrete, and can be underground, some kilometers long.

Another distinguishing factor in the nature of a ventilation duct or pipe, is the differentiation between low-pressure, medium-pressure and specific high-pressure systems. " According to DIN EN 12237 for pipe and fittings -750 to 2000 Pa, notwithstanding, higher pressures can be realized by inspection and, if necessary, with additional measures. " Following description refers to the low-pressure and medium-pressure systems in accordance with DIN EN 12237th

Material

Most ventilation ducts and pipes in low-pressure and medium-pressure systems are made ​​of thin galvanized sheet steel. But also channels and tubes of stainless steel, aluminum and various plastics are used for special requirements. In certain fire areas of a building are also ducts and fittings from Promat possible or fire safety technical point of view, prescribed even mandatory.

Other quality features include the stability of the air duct, that is, the material thickness and the mass-produced air channel profile frame (SBM -Flange ) of extruded profiles, on which the individual channel segments are bolted together. By contrast, pipes with collar collar, sleeves or nipples are connected. In oil - and grease-proof welded ventilation ducts or pipes welded flanges are used.

Existence demands on the distortion of air ducts, these are ensured in the areas restricted or stiffeners in the channel. However, it can be welded on the external surfaces of ventilation duct of sectional steel stiffeners also. To keep the air flow in the form of parts laminar, Anströmkalotten or air baffles are installed in large sheets or moldings for example.

Switzerland: " Ventilation ducts shall be of non-combustible material. Within apartments and detached houses which cast-in ventilation ducts, Earth Coupling and lines of plants with a Lufttemperaturbis 40 ° C are excluded. With the exceptions mentioned, however, the fire index 4.2 ( sales promotion guidelines ) must be complied with. For kitchen exhaust ( steam vent ), this exception does not apply. "

Fittings

A complete channel or pipe system also consists of matching fittings such as elbows, transitions, reducers, Departures, T- pieces, Y-pieces, collar collar, sliding flange, through the roof, deflector hood, etc.

Duct Components

In addition to the air guide elements channel or pipe, be in a complete air handling system as well as duct installation components such as flexible connectors, bag filters, ionizers, steam humidifiers, Electrostatic air cleaners, butterfly valves, gate valves, check valves, baffles, sensors for temperature / humidity / CO, fire dampers, active carbon filter silencers, active silencer, volume controller, heating coil, cooling coil, air outlets, etc. directly integrated into the intended channel or pipe section. This channel built-in components to flow with its own unique pressure loss as well in the Druckverlusberechnung and must be observed during hydraulic / pneumatic adjustment!

The air in a duct or pipe system flows in the correct amount and in the intended direction of air flow, due to the targeted use of fans that increase the pressure in the system and determine the direction. Depending on the ventilation system are radial or axial fans for this, with different power curves fitted. Compact air handling units, with various built-in components to change the air temperature, humidity, etc., is also calculated, dimensioned and installed.

Attachment

A channel or pipe system is mounted mostly on or in ceilings, but also in soils or in vertical risers. The attachment is on the canal by angle plates and the pipe by pipe clamps, threaded rods on ceilings or walls. Various profile steels in the form of brackets, trusses, supports, hangers, etc., are possible at any time to catch the weight. The decoupling of the air duct from the main structure is imperative for sound technical point of view required and provided by round rubber pads or sponge rubber strips in the clamps.

Tightness

The tightness of air duct systems, which is defined in tightness classes, has a large influence on the construction and the operating costs of a ventilation system. Leaks mostly occur on at flange connections. The higher the tightness requirement, the higher the cost in producing the channel and the tube member itself, and the clean assembly in construction.

" The contract may be directly specified a certain tightness class. This is also possible indirectly, when reference is made to a standard in the contract that defines the tightness measurement. So, for example, refer to standard EN 12237 on the piping system and the standard EN 12599 to the transfer. "

Special requirements for the tightness can be found for example in kitchen gardens, and for Oil mist extractors on CNC machines especially for oil and grease -tight ducts or pipes. During commissioning, the tightness can be a total assessed, measured and calculated according to.

Hydraulic conduit or pipe network calculation

With a channel network calculation, the components of a system are dimensioned hydraulically so that the necessary volume flows in the operation actually set. At an air duct system calculation, these are the outdoor air flow (ODA ), the supply air flow (SUP ), exhaust air flow ( ETA), exhaust air flow ( EHA), convection current ( RCA) and mixed air stream ( MIA). These are different in size depending on the actual situation and must be designed separately.

For the simplified pressure loss calculation is required, among other things, the Bernoulli equation, the continuity equation as well as knowledge of the friction values ​​(R- value) of square tubes and the pressure loss coefficient (zeta - value ) of fittings and system components such as filters etc.

There are basically three approaches for a simplified consideration:

• Sewer simulation speed by assumption
• Channel network calculation according to a constant pressure gradient,
• Channel network calculation according to a given pressure difference.

In extensive computer programs and algorithms to address current / desired parameters such as air velocity, flow indicators, Rohrreibungsbeiwerte, drag coefficients etc. are input for various applications, which dimension the system.

Full channel and pipe network calculations can be made ​​on the basis of drawings for duct system before the actual realization of the construction. This simplified theoretical approach is used for estimating determining the necessary pressure ( Pascal ) and the determination of the appropriate fan type ( axial or centrifugal fan ) and the power ( kW).

Besides the simple approach, in which the experimental pressure drop of the individual resistances are added taking into account the computational fluid dynamics ( CFD Sheet: Computational Fluid Dynamics), the interaction of all series-connected elements and allows a much more precise dimensioning of the fan or optimum operating point.

Pneumatic unbalanced duct systems subject to the following aspects:

• Channel systems with low air velocities in the duct or pipe cross section oversized and pure material and waste of space.
• Channel systems with high air velocities are undersized, causing loud noises flow.
• Channel system with an excessive proportion of unfavorable moldings and thereby increased flow resistance, possibly exceeding the previously designed fan power ( Pa kW) and thus can not realize the planned volumes of air. Stronger fans consume hereafter simply more power.
• Channel systems with hydraulic nonsensical outlets or connections and inadequate Reguliermöglichkeit ( throttle ) lead to over-or under-supply of various subregions ( lack of air or drafts flow noise ).
• Channel systems with a much too high differential pressure loads tend to be audible and visible bulge or even reveal the ventilation ducts that just occur when turning on the fans and cause considerable damage.
• Channel systems, which expire permanently from an ideal laminar flow in a turbulent flow, use of this turbulence significantly more energy (more driving power ) can only be compensated by increasing the pressure.

Electrolytic voltage range

Complete duct and pipe systems consist, according to final assembly in construction, mostly of very different materials. These different voltage potentials lead to an electrolytic voltage range, the material goes to the lowest voltage potential as the first in solution ( rusty / corroded ). But also because of the permanent air flowing in the system, and the friction losses which occur in this case, the electrolytic voltage range is additionally energized.

The electrolytic voltage range is counteracted by professional installation of protective conductors throughout the system and then perform a potential compensation Grounding on an equipotential bonding bar.

Air has an oxygen content of approximately 21 %, which itself has a certain amount of oxidation potential. The outside air in outer space, so in cities, rural areas or in mountainous regions has different ion concentrations. Man is an ion concentration of about 1700 ions / cm ³ the most pleasant, because the exception oxygen through the lungs is optimal.

During transport of the outside air in the channel system, the charged oxygen is its oxidation energy of the channel system from the most part and reaches later, with a lower ion concentration, in the interior space to its destination.

Insulation

Ventilation ducts to prevent heat or cold from escaping or to prevent condensation, outside or inside, isolated. The insulator may be omitted if the air conveyed in the air ducts at the same temperature as that of the environment or when heat transfer through the channel interface is not considered critical. Typical is the use of rock wool, mineral wool ( foil-laminated ) or Hart-/Weichschaumplatten for example, Armaflex, Kaiflex or styrodur. The material strength / thickness of isolation was based on the expected temperature difference and the heat conductivity of insulation used. The material to be used according to whether on or in the surface channel aimed condensation is to be carried dew point underflow is prevented. In these areas a diffusion tight insulation foam plates is frequently chosen. Also available are air duct systems in segments, which are themselves made ​​of rigid insulating material and avoid the use of a classic air duct. In accessible areas / walkways these insulations are protected to a ventilation duct, by sheet metal panels. Nooks, which are in the head area, be provided with buffers and signal bands. For special channel pieces, for example, fire pits, and a heat-resistant inner liner of diatomaceous earth or stone wool.

Hygiene

Air is a basic food of humans and animals. The " transport" this in a ventilation duct must be checked regularly. " The standard DIN EN 15780 describes the requirements and procedures for assessing the cleanliness and the possible need for cleaning, especially for air duct systems of central air conditioning systems. "

Ventilation ducts and pipes should be provided with as many large and small inspection openings, so resealable lid to carry out regular visual inspections or checks under the VDI 6022 can. The minimum dimensions of access openings in ventilation ducts and pipes is laid down in DIN EN 12097.

Dirty air ducts form an ideal breeding cities for microorganisms that reproduce and permanently leave toxins, viruses, spores and bacteria from the air stream. These enter through the "Inlet " in the human habitat and spread well there.

It is the responsibility of the system operator not only to use the ventilation system, but also to monitor! " The case law of the BGH states: The plant operator is responsible for instruction, selection and monitoring obligations, if it delegates tasks or activities to employees or contractors ." Concretized these are in: ArbSchG 7 and § 15, SGB VII and later adopted in accident prevention regulations § 4, para.

Standardize

In Germany and throughout Europe there are relevant standards

Germany

DIN 1946 [ 2009-05 ] Air Conditioning Engineering

• Part 6: residential ventilation - General requirements, requirements for the design, execution and labeling, delivery / acceptance (certification ) and maintenance

DIN 18379 [ 2012-09 ] VOB Construction Contract procedures for building works

• Part C: General technical specifications in construction contracts (ATV) - Air conditioning systems

Europe

DIN EN 1505 [ 1998-02 ] Ventilation for buildings - Ductwork and fittings made of sheet metal with rectangular cross section - Dimensions

DIN EN 1506 [ 2007-09 ] Ventilation for buildings - Ductwork and fittings made of sheet metal with a circular cross section - Dimensions

DIN EN 1507 [ 2006-07 ] Ventilation for buildings - Sheet metal air ducts Rectangular - Requirements for strength and leakage

DIN EN 1751 [ 1999-01 ] Ventilation for buildings - Air terminal devices devices - Aerodynamic testing of dampers and valves

DIN EN 12097 [ 2006-11 ] Ventilation for buildings - Ductwork - Requirements for ductwork components to facilitate maintenance of ductwork systems

DIN EN 12236 [ 2002-04 ] Ventilation for buildings - hangers and supports for Ductwork - Requirements for strength

DIN EN 12237 [ 2003-07 ] Ventilation for buildings - Ductwork - Strength and leakage of air ducts of circular cross section of sheet metal

DIN EN 12792 [ 2004-01 ] Ventilation for buildings - Symbols, terminology and graphical symbols

DIN EN 13180 [ 2002-03 ] Ventilation for buildings - Ductwork - Dimensions and mechanical requirements for flexible ducts

DIN EN 14239 [ 2004-04 ] Ventilation for buildings - Ductwork - Measurement of ductwork surface