An air handling unit, commonly called an AHU, is a device that is used to treat, condition and circulate air as part of a heating, ventilation and air conditioning (HVAC) system.
Vincent Mlangeni, AHI Carrier South Africa estimation and design controller.
Vincent Mlangeni, AHI Carrier South Africa estimation and design controller, adds: “It does this by collecting the outdoor air to meet ventilation requirements, mixes it with recirculated air from occupied spaces if required, passes the mixed air through stages of filters to remove contaminants and to protect equipment, thermal conditions the air, controls the moisture and discharges the conditioned air to occupied spaces.”
They are usually installed on the roof of buildings, and through ducts, the air is circulated to reach each of the rooms in the building in question. AHUs can be constructed to include various integral components such as mixing box section, filter section, heat exchanger section and supply air fan section.
Basic AHU configurations
“An AHU is not a self-contained packaged product,” explains Mlangeni. “It needs to be linked to additional equipment to meet the needs of the space, unless it is a fan filter unit used for ventilation purpose only. For comfort cooling application, the AHU will have a hydronic or direct expansion coil. If it is equipped with a hydronic coil, the system will contain a separate chiller that will be piped directly to its coil. If it is equipped with a DX coil, the system will contain a separate condensing unit which will also be piped to the coil of the air handler.
“AHUS are highly customisable and have countless combinations of filters, coils, fans and other section combinations to meet application requirements. Below is typical in-line and stacked configurations of units that AHI-Carrier can supply,” he says.
Mlangeni says that the two types of AHUs that AHI-Carrier can offer are packaged and central station units.
Packaged AHUs: Packaged AHUs have a more limited set of options than central station AHUs. They are generally composed of cooling/heating coils, mixing boxes, and discharge plenums. Most often, they are matched to split system condensing units and heat pumps. Several manufacturers provide matched performance ratings with their condensing units.
“As a result, packaged AHUs are usually classified by nominal cooling kilowattage, rather than airflow. They are typically found in the 20–140kW range. They are popular due to their low first cost, ease of selection and installation. In addition, manufacturers generally stock them, so they are good for fast track or emergency replacement jobs,” says Mlangeni.
Carrier 38ZS/ZF Air Cooled Condensing units that can connect to the 1 or 2 Carrier 48ZS/ZF Indoor Units.
Central Station AHUs: Central station units are semi-custom built-up air handlers that offer a wide range of component options to cover various application needs for commercial and industrial applications. Carrier central station AHU ranges in size from 0.5 m3/s for small single-zone applications, to large air handlers capable of delivering over 50 m3/s to constant volume or variable air volume systems. Although the size range is large, the majority of comfort air conditioning applications fall into the 0.5 m3/s to 25 m3/s size range.
“To simplify the design process and keep the time required to design, build and deliver the air handler to a minimum, most manufacturers offer a ‘standard’ catalogue offering of pre-engineered sizes and components. These components are assembled by the designer in building-block fashion to suit the job requirements. The designer must understand the function of each component in order to arrange them in the proper sequence for the application. In addition to traditional catalogues, some manufacturers offer computer selection software that facilitates the configuration and selection of a unit, along with detailed engineering drawings and data for submittal and ordering purposes,” he adds.
Manufacturers do not keep stock of central station units but produce them as ordered due to their variety and complexity.
“A third type of units are custom AHUs. These units are used where standard unit designs cannot easily be applied, like where there are space restrictions or a special materials requirement. These units are designed to meet the exact requirements of a given project and can include a staggering array of options and accessories on each project.
“Custom units generally are not certified because of their unique nature. Their designers must rely on the accuracy of the component manufacturer’s data and the design expertise of the manufacturer to assemble them into a functional unit. AHI-Carrier does not offer these types of units. Local manufacturing companies like HC Heat-Exchangers and Systemair specialise in them,” says Mlangeni.
Carrier 39HQ AHU. Supplied by AHI Carrier
Main functions of an AHU
In addition to managing the proper ventilation of the interior with outside air, the AHU performs other functions:
Filtration and control of the quality of the air that will reach the interior, thanks to the air purification filters, and depending on the retention of these filters, the air will be clean.
Control of the air temperature that regulates the air-conditioning system in cold or hot, so that the thermal sensation in the interior is the desired one.
Relative humidity monitoring for greater indoor comfort.
For its part, the places for which the AHU is intended are those in which the flow of people is large, accommodates many people at the same time and whose natural ventilation is limited: such as hotel dining rooms, function rooms, restaurants, convention halls and the like. It is also a suitable option for those spaces with high hygiene requirements: laboratories, clean rooms or operating theatres, among others. An AHU can also be used to ventilate places where air conditioning is provided by radiators or underfloor heating, for example.
AHU typical configuration. Supplied by AHI Carrier
What components does an AHU consist of?
Air intake: air handling units collect air from outside, which is treated and distributed throughout the rooms; and/or indoor air that is ‘recycled’
Filter: depending on the air purity requirements, the filter applied will have a higher or lower particle, virus, bacteria, odour and other air pollutants retention
Fan: this is an electromechanical system that powers the air to expel it from the AHU to the ducts that distribute the air throughout the rooms
Heat exchangers: devices that transfer temperature between two fluids, in this case, coolant and air, separated by a solid barrier
Cooling coil: the air passing through this module is cooled. In this process, water droplets can be generated, which are collected in a condensate tray thanks to the built-in droplet separator
Silencer: coatings that considerably reduce the sound level of the installation
Plenums: empty spaces in which the air flow is homogenised
Energy efficiency of AHUs
The ultimate aim of an AHU is energy efficiency, and this is mandatory since 2016 by the European Ecodesign Regulation 1235/2014.
By having heat recovery units, the AHU reduces the use of energy required in air conditioning, as in the exchanger, the indoor and outdoor air is mixed, so that when the air reaches the coil the temperature contrast is lower, therefore, the climatic contribution is also lower and energy consumption is reduced.
Likewise, the variable regulation of the equipment means that the fans can work according to the flow rate needs, reducing their consumption.
Carrier 38ZS/ZF Air Cooled Condenser. Supplied by AHI Carrier
Synergy between AHU and VRV
The relationship between AHUs and variable refrigerant volume (VRV) plays a central role in ensuring efficient and effective climate control.
Ian Roelofse, Daikin South Africa industrial segment manager, says: “There are two sources of cooling associated with an AHU: chilled water and direct expansion. Chilled water is supplied from a water chiller to the AHU cooling coil. Direct expansion is the thermodynamic process that takes place in a refrigeration cycle, whereby low temperature low pressure refrigerant is supplied to the cooling coil (evaporator) withing the AHU. This low temperature refrigerant comes from the VRV condensing unit.”
Drawing parallels between chillers and VRV systems, Daikin emphasises that both operate on the same thermodynamic principles, albeit with different mediums. While chillers employ chilled water as a cooling medium, VRV systems utilise refrigerant – a fluid with distinct thermodynamic properties.
“In essence, the relationship between AHU and VRV mirrors that of AHU and chillers. The key distinction lies in the thermodynamic medium used for cooling – from an engineering and thermodynamics perspective, the fundamental principles remain the same. The AHU acts as the interface through which the air passes, undergoing a cooling process facilitated by the circulation of either chilled water or refrigerant within the heat exchanger.”
Various suppliers may use different refrigerants, but for VRV systems, R-410A is commonly employed as the refrigerant medium. This refrigerant undergoes a compression cycle and passes through the heat exchanger at low pressure and low temperature, effectively cooling the air as it interacts with the hot air moving over the coil.
“As the refrigerant enters the coil at low pressure and low temperature, the heat generated by the hot air causes the refrigerant to boil off. This phase change cools the air passing over the coil, much like the process in a refrigerator. Consequently, the air entering the coil at a higher temperature exits at a lower temperature, contributing to the overall cooling of the space,” explains Roelofse.
The best practice for selecting an AHU, is “clearly understanding the specific application and having the design engineer’s technical specification requirements at hand”.
He lists the benefits of using an AHU:
Improved indoor air quality (IAQ): AHUs filter and condition the air, removing dust, pollen and other contaminants. This results in a healthier environment for the occupants.
Temperature control: AHUs distribute conditioned air throughout the occupied space, ensuring consistent temperatures through heating and cooling.
Humidity control: Similarly, it maintains optimal humidity within the space for comfort and preventing mould.
Energy efficient: AHUs are designed to be efficient using variable speed motors, energy recovery systems, and smart controls to optimise energy consumption.
Zoning: AHUs allow for diversification within building, meaning one can control different areas independently. This saves energy by not conditioning air in unused spaces.
Fresh air integration: Fresh air can be mixed with return air in a mixing box to improve IAQ and prevent unpleasant odours.
Customisation: AHUs can be customised for each application, tailored to the specific needs, thus having great flexibility.
However, AHUs also come with a number of challenges, the first being space requirements, he says: AHUs require significant floor space (plantrooms). The large requirement for ducting in risers (shafts) and in ceilings also demands significant space.
“Regular maintenance is crucial to keep AHUs operating at peak performance, preventing issues that can compromise their effectiveness. One of the primary aspects of AHU maintenance is establishing a regular schedule for upkeep. Depending on the specific application, monthly maintenance is often recommended to address critical components. This routine check ensures the longevity and optimal functionality of the AHU, preventing potential breakdowns and system inefficiencies.”
He lists the key components of maintenance within regular checks as follows:
Air filters: As the first line of defence against airborne particles, filters require close attention. Monthly inspections involve checking for dust and pollutants, promptly cleaning or replacing filters to maintain a clean airstream. Clogged filters can compromise the efficiency of the coil, hindering its ability to cool the air effectively.
Coils: Regular inspection of coils is essential to prevent clogging and maintain their effectiveness. Failure to replace filters can lead to particles adhering to cold coils with condensation, reducing the unit’s thermodynamic efficiency. Monthly checks ensure coils remain unobstructed and perform optimally.
Drain pan: The drain pan beneath the coil must be cleaned regularly to avoid blockages. Debris, such as food wrappers or packets (erroneously left inside the unit), can obstruct drainage, leading to overflow and potential damage. This simple yet crucial task prevents water damage within the unit.
Electrical components: For units with additional components like electric heating or humidifiers, a thorough visual check is recommended, as well as of all electrical terminals. This includes inspecting all fans and ensuring there are no bearing noises. Belt drive fans require attention to alignment and prevention of slipping and wear.
Overall component check: Basic AHUs may include components like fans, coils, heaters and variable frequency drives. Regular checks should encompass a comprehensive evaluation of each component’s functionality, examining connections, voltages and amperages to ensure balanced operation.
Airflow measurement: Measuring airflow from the supply fan is essential to verify that it falls within design parameters. This check helps maintain the intended airflow, contributing to the overall efficiency of the AHU.
External inspection: A visual inspection of both internal and external components and surfaces is crucial to identify any signs of damage or corrosion. This external assessment ensures the AHU’s structural integrity and helps pre-emptively address potential issues.
Mlangeni describes some of the common problems associated with AHUs:
Airflow obstructions: The main purpose of the filters is to remove dust and debris from the air and keep it out of system. If they get overloaded and clogged, this results in obstructions that limit the airflow causing the AHU to be less efficient.
Condensation blockages: When filters are overloaded, dust and other particles in the air enter the coil and drip out with the water as air passes through the coils. They block the condensate drain over time and cause water to accumulate in the drain pan.
Fan blade issues: Fan blades are likely to bend or become loose during operation because they are attached to a moving part. A damaged blade has a major impact on the air flow.
Carrier DBFP Suspended AHU. Supplied by AHI Carrier
What are the best practices for selecting an AHU?
Mlangeni explains that when selecting an air handling unit, the position and arrangement and type of components inside the AHU are important for reasons of cost, efficiency and for ensuring the trouble-free operation.
“First, carefully assess the specific requirements of the building to be served by the AHU. Determine the required airflow rate, temperature, humidity and filtration level – then the AHU can be selected to meet them.
“The type and efficiency of the AHU components will have a significant impact on overall performance. High efficiency fans, motors and heat recovery units can reduce energy consumption.
“The AHU should be equipped with appropriate controls to allow for precise and efficient operation. This may include different sensors and controllers that monitor and maintain desired temperature, humidity and filtration levels. These controls optimise performance and energy efficiency.
“The AHU should be designed for easy access and maintenance. This will help to ensure that the unit is operating at its peak performance and that any potential problems can be quickly identified and resolved,” he adds.
Carrier 38ZS/ZF Air Cooled Condenser. Supplied by AHI Carrier
A look at recent innovations in the AHU industry
There has been a significant transformation in the design and functionality of AHUs, says Roelofse. “One noteworthy area of innovation has been in the evolution of fans, particularly the adoption of electronically commutated fans (EC fans). Daikin and other leading manufacturers have been at the forefront of integrating these fans into AHU designs. They have proven to be a game-changer by addressing key challenges associated with traditional belt drive fans. They represent a more compact alternative to traditional belt drive fans. The elimination of pulleys and belts reduces the bulkiness of the fan and reduced maintenance, allowing for a more streamlined and space-efficient AHU design, reflecting the demand for efficient use of space in commercial and office environments. This is particularly crucial in areas where architects prioritise maximising usable space without compromising on HVAC performance.”
Additionally, notes Roelofse, “EC fans offer unprecedented flexibility and redundancy options. Rather than relying on a single large centrifugal fan, multiple EC fans of the same size can be employed. This not only enhances redundancy but also allows for more flexible and efficient unit configurations.
“EC fans can be controlled seamlessly across the entire operating spectrum without the need for external variable frequency drives. Integration with building management systems (BMS) enables direct control, providing increased efficiency and eliminating the need for additional electrical components.
“An additional benefit is their adaptability to external factors,” says Roelofse. “For instance, pressure sensors connected to filters can be utilised to monitor air quality. As filters accumulate dirt, the pressure drop increases. This information can be integrated into the control cycle, prompting the fans to adjust their speed to compensate for reduced airflow, ensuring consistent performance.
“Initially perceived as a premium solution, the cost of EC fans has significantly decreased with the establishment of large-scale manufacturing facilities by leading brands such as ebm-papst and Ziehl. This reduction in cost has made EC fans a more economically viable choice, further accelerating their adoption across the industry.”
Carrier 40ZS/ZF Indoor Unit. Supplied by AHI Carrier
The rise of hybrid AHU systems
Continuing with the subject of innovations in AHUs, Mitsubishi Electric technical manager Johann Willemse and marketing director Marco Ferdinandi, discuss the evolution of AHUs, the impact of hybrid systems, and the critical role of maintenance and research and development (R&D).
Hybrid systems combine AHUs with variable refrigerant flow (VRF) systems. “This approach involves essentially utilising a box equipped with a heat exchanger, a fan and a coil. The flexibility of VRF systems, known for their energy-saving capabilities, has made them a popular choice in the industry,” says Willemse.
Those energy-saving attributes of VRF systems are driven by inverter compressors that make the entire system variable in capacity. This versatility allows for the development of both small-scale and large-scale systems, enabling Mitsubishi Electric to handle projects ranging from modest installations to impressive 1 000kW systems, as well as in between – as exemplified by the company’s recent 200kW project for a university.
The company is actively involved in projects for hospital theatre systems and other specialised spaces. “Additionally, we frequently refurbish existing AHUs, extending their lifespan by incorporating modern advancements.”
Willemse notes that refurbishment work typically arises when AHUs are not regularly maintained, as regular upkeep maximises the lifecycle of AHUs. “Neglecting maintenance can lead to breakdowns and excessive wear and tear – common issues faced in the South African market. In contrast, a proactive approach to maintenance would prevent potential issues.”
Another instance where refurbishment is viable, is where the internal AHU unit is still in good condition but requires technological upgrades. “This often involves replacing outdated components, such as the heat exchanger, to comply with modern refrigerants. The refurbished unit undergoes a transformation, integrating new technology, more environmentally friendly gases, and energy-efficient features like EC fans and high efficiency filtration. This process ensures that the unit is revived and aligned with current industry standards,” he says.
While the return on investment (ROI) of refurbishment clearly varies with individual circumstances, Willemse notes that based on specific projects and Mitsubishi’s past experiences, payback periods typically range from three to five years with the energy savings being the driving force behind such a refurbishment decision, providing long-term benefits and sustainability.
Recognising the scarcity of skilled personnel in the industry, Mitsubishi Electric has taken a proactive stance by offering installation and maintenance courses. The company’s training programmes extend beyond contractors to end-users, ensuring that clients’ internal staff are equipped with the skills needed for effective AHU maintenance.
“Maintenance is more than just routine tasks like cleaning filters – it involves a deep understanding of the equipment. Mitsubishi’s systems training includes embedded functions that record operational data, enabling service technicians to proactively address potential issues. This is important knowledge for maintenance personnel, enabling them to leverage the full capabilities of the equipment,” says Willemse.
Mitsubishi Electric technical manager Johann Willemse and marketing director Marco Ferdinandi. ©RACA Journal
R&D outlook
Ferdinandi highlights two key priorities for Mitsubishi: energy-saving solutions and environmentally friendly systems. “One development in HVAC systems is utilising chilled water as a medium for heat transfer reflecting a commitment to efficiency and sustainability. The other priority is safety through ongoing efforts to create systems that are not only environmentally friendly but also safe for users.”
Because closed loop systems use water as a transfer medium for heat energy, it minimises leakages and provides a safer alternative, especially considering potential future regulations regarding refrigerants. Regarding these imminent changes in refrigerants, Ferdinandi notes that Mitsubishi is proactively developing systems that address environmental concerns and prioritise user safety.
Mitsubishi’s latest innovation is a generation of small heat pump chillers, capable of delivering as low as seven kilowatts. “These chillers can be employed to drive AHUs, offering an environmentally safe and efficient alternative which is cost-effective for sites such as hospital theatre systems.”
He highlights the shift from a time when the industry had to work with off-the-shelf solutions to the present, where consulting engineers and customers enjoy a more fortunate position. “The ability to customise and bespoke AHUs to meet specific needs has become a key advantage. Consulting engineers can now design systems tailored for various applications, from open-plan spaces to specialised environments like operating theatres. This flexibility in design allows for countless iterations and adaptations to unique requirements, creating a stimulating environment for R&D.
“This makes for an exciting time due to the variety of opportunities that arise from the flexibility and customisation now possible in AHU design. The ability to address diverse needs and specifications has opened up new avenues for innovation, making R&D a dynamic and evolving field within the industry,” says Ferdinandi.
References:
- Airnectics
- Daikin
- HC Airconditioners
- Carrier resources: https://www.ahi-carriersa.co.za
- Carrier Technical Development Programs – TDP 611 Central Station Air Handlers: https://www.alarko-carrier.com.tr/en/air-handling-u
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