AIR MOVEMENT
used in AHUs, axial fans or centrifugal ones. A centrifugal fan at constant speed moves a relatively constant volume of air rather than a constant mass. This means that the air velocity in a system is fi xed even though the mass fl ow rate through the fan is not. Centrifugal fans displace air radially, changing the direction of the air fl ow typically by 90°. Axial fans cause air to fl ow through it in an axial direction parallel to the fan shaft. The fl ow is axial at entry and exit and fans are designed to produce a pressure diff erence causing a fl ow through the fan. Flow rates are controlled by inlet vanes or outlet dampers of the fans.
Several fans may be present in AHUs typically placed at the end and the beginning of the ductwork. Such supply fans may be augmented by fans in the return air stream pushing air into the AHU.
Acoustic insulation In ventilation systems, noise comes from a variety of components such as fans or dampers. To comply with specifi ed noise levels, sound attenuators may be added to suitable locations in the AHU. Sound attenuators must have adequate silencing capabilities and low air resistance and to attenuate noise before it enters the ductwork by means of a duct accessory consisting of an inner perforated baffl e with sound-absorptive insulation. Beyond that the casing may be equipped with
panels with double thickness and proper insulation inside the panels.
Energy recovery
Energy recovery heat exchangers may be fi tted to an AHU between supply and return air streams for energy savings, increased capacity and better effi ciency. Plate Heat exchanger: A sandwich of metal plates
with interlaced air paths. Heat is transferred between air streams from one side of the plate to the other. Heat recovery effi ciency is up to 70%.
Rotary heat exchanger: A slowly rotating matrix of fi nely corrugated metal, operating in both opposing airstreams. When in heating mode, heat is absorbed as air passes through the matrix in the exhaust airstream, during one half rotation, and is released during the second half rotation into the supply airstream in a continuous process. When the air handling unit is in cooling mode, heat is released as air passes through the matrix in the exhaust airstream, during one half rotation, and absorbed during the second half rotation into the supply airstream. Heat recovery effi ciency is up to 85%.
Enthalpy wheels with a hydroscopic coating provide latent heat transfer and dry or humidify air streams. Run around coil: Two air-to-water heat exchang- er coils, in opposing air streams, piped together with a circulating pump and using water as heat transfer medium. This allows heat recovery between supply and exhaust airstreams. Heat recovery effi ciency is up to 50%.
Heat Pipe: Operating in both opposing air paths, using a confi ned refrigerant as heat transfer medium. The heat pipe uses multiple sealed pipes mounted in a coil confi guration with fi ns to increase heat transfer. Heat is absorbed on one side of the pipe, by evaporation of the refrigerant, and released at the other side, by condensation of the refrigerant. Condensed refrigerant fl ows by gravity to the fi rst side of the pipe to repeat the process. Heat recovery effi ciency is up to 65%.
Mixing sections To maintain high indoor air quality, AHUs have provisions for mixing room air with the right
amount of cooler outside air, with warmer return air to achieve the desired supply air temperature. Mixing sections have dampers controlling the ratio between return, outside and exhaust air.
Controls
Sophisticated controls are necessary to operate an AHU effi ciently e.g. air fl ow rate, supply air temperature, mixed air temperature, humidity, air quality. The range of available controls comprises simple thermostats or building automation system. Control components include temperature sensors, humidity sensors, various actuators, motors, and controllers.
Accessories
A huge variety of accessories is available and vibration isolators are in most cases mandatory. AHUs create substantial vibrations and the ductwork may transmit vibrations and related noise into the building. Vibrations isolators are inserted into the duct before and after the AHU and also between fan and adjacent sections. Fans may be further isolated by putting them on spring suspensions mitigating the transfer of vibrations and noise through AHU and ductwork. A number of protective devices are capable of contributing to the longevity of outdoor AHUs. Rain louvers prevent rain entering the AHUs. Roof covers helps to dispose of rainwater. Bird screens are wire meshes placed at the fresh air entrance to prevent insects and birds from entering the AHU.
Various designs
AHUs are used for a wide range of applications, however, there are applications which need to meet special requirements: AHUs for hygienic design conditions for specially conditioned air in hospitals or laboratories. AHUs which are designed for operation in hazardous areas.
Data centre cooling requires special designs of AHUs for cooling sensitive CPUs. AHUs in general combine sophisticated treatment of air with moving the air around in buildings and not only off er high comfort but also energy recovery if desired.
20 July 2021 •
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