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GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy ( ACRA ) in Edenvale . He holds a Bachelor of Business Administration and an associate degree in educational administration . He has a National Technical Diploma and completed an apprenticeship with Transnet . He has dual-trades status : refrigeration and electrical . He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018 . Currently he is the SAIRAC national treasurer .
WELCOME TO THE SOLUTIONS PAGE
By Grant Laidlaw
Many people ask for assistance in the understanding of theoretical and practical aspects of the industry . I will endeavour to enlighten . We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place .
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Aisha asks : Mr Grant , in air conditioning we have something called an AHU , this is a unit with a fan and filters . Please could you explain an AHU to us for our understanding and what do we know for service ? Thank you .
Hi Aisha , thanks for the question , the acronym AHU stands for air handling unit .
An air handling unit , also called an AHU , is part of an airconditioning system which contains the necessary equipment for conditioning and moving the air ( fans ), cooling coils or evaporators , air filters , heater and humidifiers all placed in an accessible box like structure in or outside a building . The AHU is the part of the system the handles the air , both fresh air and recirculated air , thus the name .
They are usually found on the roof of buildings , in ceilings , in plant rooms or specially designed rooms – plenum spaces in a building using ducting and sometimes ceiling voids , the air is
circulated to reach and return from each of the rooms of the building in question .
While not all AHUs have all of the components mentioned , when looking at one we normally find :
• A sealed , insulated box type structure , accessible through access hatches with electrical , controls , air , water drain and chilled water or refrigerant connections . In some cases , hot water or steam connections may be present . Sound deadening materials are often applied .
• Air intakes : AHUs collect air from outside ( fresh air ), which is distributed throughout the building ; and mixed return indoor air . Typically we always maintain a minimum of 10 % fresh air to ensure good air quality , maintain a positive static pressure , and maintain acceptable levels of oxygenation . Some units only use 100 % fresh air .
RACA Journal I March 2024
• Fans : these increase the air pressure to pump the air to from the AHU into the ducts that distribute the air as required throughout the building .
• Filters : The air is ‘ cleaned ’ depending on the air purity requirements , we may find primary , secondary HEPA , ultraviolet lighting etc to deal with airborne contaminants such as solid air borne particles , bacteria , viruses , odours and other air pollutants .
• Humidifiers , often bottle type steam generators or water sprays , are used to adjust relative humidity levels .
• Heaters , normally elements are used in South Africa but sometimes we may find steam or hot water coils .
• Heat recovery wheels : recover heat from air leaving the building for reuse .
• Evaporators : in direct expansion systems directly using the refrigerant to absorb heat . Sometimes even the reverse cycle for heating is utilised .
• Condensate pan with drain and P-trap to collect any condensate produced whilst cooling the air .
• Plenums : empty spaces in which the air flow is homogenised ( mixed ). Fresh air is mixed with return air . Mixing processes are performed to obtain the desired air temperature and humidity or to maintain energy-efficient performance .
• Control systems : control systems can vary from simple control systems to advanced control systems that use the latest technologies such programmable logic controllers . Usually controllers are used to control the temperatures and humidity of the supply air to the zones in the building . The system may control the damper systems in the AHU . Moreover , advanced control systems can even control the fan rotational speed and thus the rate of air supplied to the building based on the required parameters .
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