By Mohamed Saleh (WSP)
WSP Group Africa (Pty) Ltd was appointed to design, tender, adjudicate, implement site quality control, and review and witness commissioning testing of the HVAC system for Rand Mutual Assurance’s (RMA) new headquarters in Parktown, Johannesburg.
Although this was the new home for RMA, the project consisted of the refurbishment of an existing building with an entirely new HVAC system. Being an existing building, there were various limitations and constraints such as electrical supply, plant room area, structural penetrations and shafts, as well as existing services coordination to name a few.
The brief from the client was clear, design an efficient HVAC system that was suitable for an office application, that would provide the best solution from a financial (capital and operating) and energy efficiency perspective, which yielded a sleek and stylish HVAC design as the services would be exposed.
HVAC system and design overview
The analysis of selecting the correct HVAC system for the building consisted of (to name a few):
- Capital cost vs operational cost
- System efficiency (coefficient of performance (COP) and energy efficiency ratios (EER))
- System flexibility
- Building application
Through various simulations and pros and cons analysis, the HVAC system selected for this project was a variable refrigerant flow (VRF) heat recovery system, with pre-treated fresh air. The system offers a flexible and reliable solution, with high energy efficiency and was found to be a suitable option for the buildings application.
VRF systems are energy efficient as the system is designed to run on the exact capacity required for the current cooling conditions. The VRF system detects what the zone requirements are and directs the appropriate amount of refrigerant to meet the requirements. Their efficiency and low energy consumption means lower utility bills, which cannot be bypassed especially in these challenging economic circumstances .The glass specifications were reviewed and the most efficient combination between u-value and cost was selected.
The building design makes clever use of the cladding to hide various components, leaving a neat and retro finish.
As a result of the people-density in the building, the fresh air is pre-treated via a roof top packaged unit (RTPU) before entering the building to reduce the fresh air load on the internal hide away units (HAU).
This resulted in a reduction in the HAUs capacity, and overall VRF systems capacity, which aided in the design of the constrained place room size on the roof. As a result of the structural constraints of the building, the outdoor unit’s plant was positioned around the parimeter of the roof top plant room as a result of the structural integrity decreasing towards the middle of the roof plant:
The fresh air system also contained pressure differential sensors over the filter banks to indicate to the facilities management team when the filters were dirty and required to be cleaned. This was some of the many systems implemented to ensure the maintenance of the system was as user friendly and efficient as possible.
The building utilises a common mechanical extraction system for the ablution facilities. The extraction system, along with the VRF outdoor units, is located on the roof plant room. Although the common fresh air system is also located on the roof plant room, the system was designed in such a way to prevent any cross contamination by analysing predominant wind flow direction, as well as sufficient distance away from one another.
Indoor hide away units (HAUs) and cassettes are supported from the slab on each floor within the HVAC zone and the indoor units were thermally zoned to serve the perimeter and internal zones individually. This arrangement prevents the building from over cooling in certain areas and under cooling in other areas, as a result of building orientation and various external loads throughout the day.
Although the indoor units were thermally zoned, the heat recovery (HR) boxes were optimised to ensure that heat recovery is effective at any time of the day. The heat exchange from cooling a particular zone is redirected to a zone that may be required to be in heating. To ensure that this operation was effective, parameter and internal zones were connected to the same HR box increasing the potential of having a zone in heating and a zone in cooling connected to the same HR box. This then further enable the outdoor units plant size to be optimally sized and diversified through this heat recovery design optimisation.
The third floor contained an auditorium, which naturally had to adhere to strict decibel levels. The auditorium was designed as a stand alone system, and only switches on when occupied. This further made the building more energy efficient and economical.
Apart from the third floor, the buildings internal services were exposed as the building has no ceilings. This meant that extra attention was required when selecting the indoor units as ceilings provide an element of noise attenuation, as well as superior quality of the workmanship on site as the services are all exposed.
Over and above the aforementioned, the correct second fix air terminals are imperative as diffuser selection and orientation has a major impact on the comfort of the occupants. Incorrect positioning of diffusers may result in cold and hot spots, causing discomfort to the occupants. To ensure that this risk was mitigated, the diffusers were carefully positioned based on their throw.
Ceilings also play an important role in the distribution of air within the space. In general, the air travelling from the diffuser sticks to the ceiling, this reduces the effects of air dumping and is known as the Coanda effect. With majority of the building being that of exposed services no ceilings were present, as a result constant volume swirl diffusers were selected for its good radial distribution and good air distribution performance index (ADPI).
Each hideaway and cassette unit have individual controllers which are clearly linked to each other, this allows individual control. Over and above this, there is a centralised controller located in the facility managers office which enables the facility manager to view and control the temperature of any unit within the building. To accommodate all employees within the ideal office temperature it was decided that all unit controllers within open place spaces would be locked and would only be adjustable from the centralised controller.
The building design incorporates various pause areas with a retro look – exposed ducting and hanging bulkheads.
The centralised controller is also linked to the manufacturer, they will be able to monitor the system from their offices and advise if there are any malfunctioning units. The centralised controller also offers additional services, for example, the lighting systems can be linked to the central controller. The central controller has the potential to act as a Building Management System.
WSP worked extensively on BIM 360 and invited the HVAC subcontractor on the project, to work on the platform as well. This enabled both WSP and the HVAC subcontractor to accurately indicate queries and issues on the latest drawings, and ensure that these queries and issues were timelessly attended to and closed out. The platform assisted us in generating up-to-date quality reports.
Challenges with HVAC exposed look
Having the services of the building exposed meant that the coordination between the professional teams was extremely important. Regular design and coordination sessions were held to achieve a favourable outcome. Further challenges were experienced during the construction phase as down-stand beams present on site and not on the model meant that HVAC duct dimensions had to change while ensuring the air velocity and pressure drop were still acceptable.
In certain areas, HVAC ductwork had to be re-routed while still ensuring that the overall aesthetic look was still pleasing. All supply air ductwork as well as the treated fresh air ductwork were internally insulated to prevent condensate from forming on the outer surface of the ductwork. Internal insulation was preferred to over external insulation as the finished product looked more aesthetically pleasing.
Designing an exposed HVAC system comes with many challenges, however, if the challenges are overcome the completion of the project is rewarding. The success of the project is hugely governed by an adequate amount of coordination between services and the professional team. It is important to ensure that velocity and pressure drops are within comfortable limits as noise levels are higher when no ceilings are present.
To ensure comfort of the occupants the correct diffusers need to be selected, the building should also be correctly thermally zoned by keeping parameter and internal zones on separate AC units, this enables more accurate control higher level of occupancy comfort.
Craig Blankers (Head of Division: Mechanical Services) and Mohamed Saleh (Mechanical Engineer) were both involved in the design, implementation, construction monitoring, commissioning review and close-out of the Heating, Ventilation and Air-Conditioning (HVAC) system for the refurbishment of an existing building. While the aforementioned was reviewed by Craig Blankers, Mohamed Saleh spearheaded as the project lead and successfully delivered another office block refurbishment project.
List of professionals
|Developer||Koen and Associates|
|Architect||Koen and Associates|
|Project Manager||Koen and Associates|
|Consulting Engineers||Electrical||Quad Africa Consulting|
|Contractors||Main Contractor||Koen and Associates|
|HVAC||Midrand Airconditioning and Ventilation (Pty) Ltd|
|Suppliers||Rooftop packaged unit||HC Heat-Exchangers (Pty) Ltd|
|DX-Split midwall unts (4 off)||Samsung|
|Diffusers and grilles||Trox|