Advances In Air Handling
In buildings, where the hygienic requirements for air quality are lower, some of the air from the rooms can be re-circulated by a mixing chamber, and that results in significant energy savings...
Air handling means that the air will be delivered into a space with desired thermo-hygrometric parameters and quality. With the growing levels of industrialization in different fields like comfort applications, agriculture, horticulture etc., air handling is playing an important role. It is no exaggeration of facts to say that technological advances in air handling have really enabled to enhance reliability, efficiencies and profitability in various sectors.
When looking at comfort applications, i.e., applications designed for the comfort of people, livestock, etc. and not the protection of computer-based electrical systems or other process operations requiring close control of the environment, the temperature should ideally be between 20 and 24°C. Various energy efficient and advanced air handling devices are surfacing in the market every day. All of air handling units will allow considering the main aspects of indoor air quality for the application. These include particulate and odour filtration, temperature, humidity, noise, draught and fresh air supply. Equally are the requirements for service and maintenance as well as the sizable pressures of capital and operational cost.
An air handler, or Air Handling Unit (often abbreviated to AHU), is a device used to regulate and circulate air as part of a Heating, Ventilating, and Air-Conditioning (HVAC) system. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU. The basic function of the AHU is take in outside air, condition it and supply fresh air to a building. All exhaust air is discharged, which secures an acceptable indoor air quality. Depending on the required temperature of the conditioned air, the fresh air is either heated by a recovery unit or heating coil, or cooled by a cooling coil. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork. Small air handlers, for local use, are called terminal units, and may only include an air filter, coil, and blower; these simple terminal units are called blower coils or fan coil units. A larger air handler that conditions 100% outside air, and no recirculated air, is known as a Makeup Air Unit (MAU). An air handler designed for outdoor use, typically on roofs, is known as a Packaged Unit (PU) or Rooftop Unit (RTU). The accuracy of the air treatment will depend from the specificity of each project (offices, schools, swimming-pools, laboratories, factories with industrial processes, etc). This means, the AHU treats air by filtering, cooling and/or heating, humidifying and/or dehumidifying.
There are several types of air handling units: compact, modular, residential, DX integrated, low profile (ceiling), packaged, rooftop mounted (typically on the roofs of buildings, with special weather protection), etc. Usually the air handling units have a casing (also known box) constructed by a framing system and double skin insulated panels (also known as insulated sandwich panel). The most common framing materials are galvanized steel, AluZinc or aluminium.
Regarding the panels skin, the most common materials are galvanized steel and AluZinc. In hygienic AHU’s the inner skin usually is made from stainless steal or AluZinc with a special painting finishing. The materials used to insulate the panels are 99% of the times mineral wool (also known as stone wool or mineral fiber) or PU (Polyurethane). Some manufacturers choose the mineral wool and some the PU, it all depends on the compromise between thermal characteristics, acoustic attenuation, mechanical strength and production costs that each one is looking for the final product. All the components will be installed inside the casing. The casing is installed on top of a base (or chassis).
• In buildings, where the hygienic requirements for air quality are lower, some of the air from the rooms can be re-circulated by a mixing chamber, and result in significant energy savings. A mixing chamber has dampers for controlling the ratio between the return, outside, and exhaust air.
• A heat/cooling recovery exchanger is normally fitted to the AHU for energy savings and increasing capacity.
• A heat recovery device heat exchanger of many types, may be fitted to the air handler between supply and extract airstreams for energy savings and increasing capacity. These types more commonly include for:
Recuperator, or Plate Heat exchanger: a sandwich of plastic or metal plates with interlaced air paths. Heat is transferred between airstreams from one side of the plate to the other. The plates are typically spaced at 4 to 6 mm apart, they can also be used to recover coolth. Heat recovery efficiency up to 70%.
Thermal Wheel, or Rotary heat exchanger: a slowly rotating matrix of finely corrugated metal, operating in both opposing airstreams. When the air handling unit is in heating mode, heat is absorbed as air passes through the matrix in the exhaust airstream, during one half rotation, and 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 efficiency up to 85%. Wheels are also available with a hydroscopic coating to provide latent heat transfer and also the drying or humidification of airstreams.
Run around coil: Two air to liquid heat exchanger coils, in opposing airstreams, piped together with a circulating pump and using water or a brine as the heat transfer medium. This device, although not very efficient, allows heat recovery between remote and sometimes multiple supply and exhaust airstreams. Heat recovery efficiency up to 50%.
Heat Pipe: Operating in both opposing air paths, using a confined refrigerant as a heat transfer medium. The heat pipe uses multiple sealed pipes mounted in a coil configuration with fins 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 flows by gravity to the first side of the pipe to repeat the process. Heat recovery efficiency up to 65%.
Problems to be tackled
• Un-balanced fans wobble and vibrate. For home AC fans, this can be a major problem: air circulation is greatly reduced at the vents (as wobble is lost energy), efficiency is compromised, and noise is increased. Another major problem in fans that are not balanced is longevity of the bearings (attached to the fan and shaft) is compromised. This can cause failure to occur long before the bearings life expectancy. Weights can be strategically placed to correct for a smooth spin (for a ceiling fan, trial and error placement typically resolves the problem). But for a home / central AC fan or big fan are typically taken to shops, which have special balancers for more complicated balancing (trial and error can cause damage before the correct points are found). The fan motor itself does not typically vibrate.
• Controls are necessary to regulate every aspect of an air handler, such as: flow rate of air, supply air temperature, mixed air temperature, humidity, air quality. They may be as simple as an off/on thermostat or as complex as a building automation system. Common control components include temperature sensors, humidity sensors, sail switches, actuators, motors and controllers.
• The blowers in an air handler can create substantial vibration and the large area of the duct system would transmit this noise and vibration to the occupants of the building. To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler and often also between the fan compartment and the rest of the AHU. The rubberized canvas-like material of these sections allows the air handler components to vibrate without transmitting this motion to the attached ducts. The fan compartment can be further isolated by placing it on a spring suspension, which will mitigate the transfer of vibration through the floor.
Selecting an air handler
Air handling unit selection is as much an art as it is a science. Air handling units come in all shapes and sizes. It is important to learn to balance and prioritize all of the choices related to performance, efficiency, maintainability, and space constraints. It’s important to realise that there will not be a ‘perfect’ selection for any AHU as many competing criteria, not the least being cost, will force compromises. It is the engineer’s job to balance and prioritize all of the decisions related to performance, efficiency, maintainability, and space constraints to select a unit that has the lowest lifecycle cost for a given application. One must start with the desired – or calculated – capacity of air conditioning and heating the air handing unit must provide. Then select which air handling unit manufacturer to base the design and specification. It is prudent to compare the dimensions and weights of several manufacturers to ensure the installed unit fits within its allotted space. Select which options, accessories, modifications, etc. the unit must have to meet the desired performance, maintenance, and control. And finally, schedule the unit – or units – on the construction document plans and in the specifications. Simultaneously during the design and selection, coordinate size, weight, configuration, duct and piping layouts with the Owner, Architect, Electrical Engineer, Plumbing Engineer, and Structural Engineer. Coordination with all the other design trades and buy-in by the owner are essential to the successful design and installation of an air handling unit system.
AUTHOR CREDIT AND PHOTOGRAPH
Dr. S. S. Verma
Department of Physics