www . refrigerationandaircon . co . za RACA Journal I March 2024 23
Feature
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 .
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 .
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 .
www . refrigerationandaircon . co . za RACA Journal I March 2024 23