The AHUs stationed outside the space at the back of the warehouse. Image credit: © Parsons and Lumsden Consulting Engineers
By Richard Gibson, Pr Tech Eng at Parsons and Lumsden Consulting Engineers | All image credits: © Parsons and Lumsden Consulting Engineers
This project involved a temperature controlled room inside the warehouse facility where temperature-sensitive pharmaceuticals and products get stored.
This facility, in Durban, is a distribution centre (DC) for a large pharmaceutical wholesaler. The project commenced in February 2019 with initial proposals and preliminary designs, and was subsequently completed in April of 2021.
The warehouse facility itself is approximately 200m in length, 50m wide and 16m high. The temperature controlled room is constructed from 150mm thick cold room panel-type walls and ceiling. The area of this room is 125m in length, 25m in width and 14m in height.
Design specifications/client brief of the project
The requirements for this project were that the temperature controlled room must be kept between 22°C and 25°C, and relative humidity (RH) needed to be less than 55% all year round.
The existing HVAC system, which was installed when the warehouse was built in 2016, had been unable to meet these specified conditions. Our brief then included to design and manage the installation of a new HVAC system that could meet and maintain these conditions. The control of internal conditions is critical to ensuring optimum shelf life of stored goods and in this application the guidelines that were followed were set out by SAHPRA (South African Health Products Regulatory Authority).
The warehouse undergoes a temperature mapping exercise once a year to determine temperature and RH distribution and compliance.
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The HVAC system and installation
The system that was designed consisted of four air handling units (AHUs) each linked to a dedicated air-cooled condenser with compressor, controls, and so on. Each AHU has two independent refrigerant circuits. These units are stationed outside the warehouse on steel platforms just above ground level. Supply air from these units is ducted into the temperature controlled room and air is then distributed throughout the internal space.
Being a retrofit project, care was taken not to interfere with the existing high rise racking systems. To this end, jet nozzle diffusers were used on the ducts for distribution of air internally.
The air conditioning load is covered by three of the units, while the fourth unit offers redundancy. In normal operation, all four units operate continuously, 24/7. Each AHU provides 102kWs of cooling and 31kWs re-heat capacity.
The systems are fully monitored and controlled via a Siemens building management system (BMS).
Difficulty in meeting design specification and solutions
Durban’s sub-tropical coastal climate means that RH is often extremely high, especially in the summer months. The challenge was in keeping the internal space below 55% RH. This meant keeping infiltration under control as well as dehumidifying the internal air.
To manage infiltration, the entire volume of the space received outside air at 0.5 air changes per hour (ACH). This may not sound like much but when one considers that the ceiling height is around 14m, this equates to a lot of outside air. With this volume of fresh air input, the design is dominated by the need to condition this air adequately, this then meant dealing with vast differences in on-coil conditions – both from a diurnal and a seasonal annual perspective.
It is important to remember that no amount of positive pressurisation within the space can stop the inflow of vapour pressure. Vapour pressure is only interrupted by vapour barriers, in the case of a building this means impervious building materials and very limited airgaps. Moisture will move against airflow if there is a vapour pressure differential. The existing building construction was very well sealed and relatively air tight, which assisted in limiting the inflow of moisture.
It can thus be seen that the constant flow of conditioned fresh air into the building is the critical factor in maintaining low relative humidities. To this end it is also important to note that the system is operated at as high a dry bulb (DB) temperature as possible, within the prescribed limits, in order to reduce the relative humidity and save on cooling energy requirements.
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The other benefit of constantly maintaining a positive pressure within the controlled space is that there is very little dust ingress since intake air is filtered through primary and secondary filter banks.
The AHUs were designed to cool the conditioned air down to a leaving coil condition of 11.5 °C DB and 11.3°C wet buld (WB). This “overcooling” drops the absolute humidity to the design point and then a condenser re-heat coil is used to re-heat the air back to 20°C DB which results in 50% RH inside the zone.
System components used
Standalone AHUs with dedicated direct expansion (DX) systems were selected. A chilled water system with AHUs would have been a good option, however it was not viable due to the distance that separated each AHU.
Jet nozzle diffusers were selected for air distribution in the room. These diffusers allowed the use of minimal ducting and also allowed us to steer clear of existing racking and rather use open space to run ducting and rely on the good throw-characteristics of the diffusers to get air distributed.
Efficiency and sustainability aspects of the system
Heat recovery from the refrigerant was used in the dehumidification cycle. Hot gas leaving the compressors was diverted through the re-heat coil before it entered the condenser. This provided “free” heat for re-heating the air before it was supplied to the space.
Special/unique project elements
The traditional approach to humidity control in industrial applications has been to reduce fresh air input to an absolute minimum and, in many cases, to exclude it entirely. This results in spaces that are difficult to use and to maintain due to airlock and sealing maintenance issues.
It also results in spaces that are designed to run at negative pressures. This is due to the cooling of the air within the space, reducing its volume and creating a vacuum that sucks uncontrolled and unfiltered air into the space. Essentially you cannot exclude fresh air from such a space, it will find its way in anyway, so it’s far better to control it, condition it and provide a clean, comfortable and safe environment with excellent air quality for the occupants. In essence, don’t make buildings with a poor quality indoor environment.
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