Transnet Inyanda: retrofitted comfort and efficiency

By Ilana Koegelenberg and Luca Campodonico (CKR)

How do you up the comfort levels in an existing 12m atrium with a glass façade and great open spaces — and save on energy costs?

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The head office of Transnet Freight Rail is located in Johannesburg, comprising multiple buildings for the various functions within this large organisation. The Inyanda building is where the monitoring of the trains happens, with the heart of the operation based inside a 12m open-volume atrium space.

In 2016, Growthpoint (representing their tenant Transnet) approached CKR Consulting Engineers to present a possible solution for this Inyanda building where the existing HVAC design was causing the workforce some challenges in terms of comfort levels.

The underfloor air distribution system provides a more effective means of ventilation and so the fresh air quantity could be reduced, improving system efficiency as less unconditioned air has to be introduced into the system.

 CKR’s presentation highlighted the problem statement and provided possible solutions to the low comfort levels experienced in the atrium. The solutions suggested both active and passive means that would improve occupant comfort.

As per CKR’s recommendation, the client opted to implement the proposed passive solution first, which was adding insulation to the outside of the atrium roof. Extensive computational fluid dynamics (CFD) modelling was undertaken to optimise this solution.

Once the roof works were completed, upgrading of the HVAC system installation commenced. The project was completed within 139 days.

Project brief

The tenant’s brief was quite simple: increase the comfort levels inside the atrium by using as much of the existing HVAC system’s equipment as possible to contain costs.

As it stood, the atrium temperatures were not stable and previous attempts to cool the area resulted in draughts that were unpleasant and distracting to the occupants.

The tenant explained that the large screen at the front of the atrium had to be visible at all times, which eliminated the opportunity to install overhead ducting, as this would obstruct their view. The system also had to be operational 24 hours a day.

The tenant requested that some sort of individual control be provided, as the occupants’ required temperatures on the floor would be diverse.

Existing system

The existing HVAC system used a heat pump chiller to provide two air handling units (AHUs) with chilled or warm water (depending on the requirement). These AHUs distributed conditioned air to the space via two large supply air ducts spanning the outside perimeter of the atrium. One duct was positioned at approximately 4m and the other at about 12m above the floor level. Air was also supplied to a third ring on the upper third-level walkway.

As the ducts were a fair distance away from the occupants seated at the centre of the floor plate, double deflection grilles were used to disburse air at high velocities to reach the intended zone. Air required for the upper walkway was disbursed with the use of swirl diffusers.

The ‘upgrade’

Through several concept analysis sessions with the team at CKR Consulting Engineers, they opted to design on an underfloor HVAC system. The system consists of the following equipment:

With the use of shut-off valves, the existing chiller could still be used in the event of the newly installed chiller running into issues. This chiller feeds the modified AHUs. The AHUs then pressured a newly installed underfloor plenum through underfloor ducting reticulation, which ensures that conditioned air is evenly distributed.

Inyanda00 19Simulation of the results of the new underfloor system.
Image credit: CKR

The air is then supplied to the occupants via adjustable Trox floor grilles at extremely low velocities and moderate temperatures. This airflow initiates the stratification process and air is finally returned to the plant rooms via the repurposed upper supply air duct, which is now used for the return air.

These floor grilles were selected since they offer airflow adjustability. Airflow variation is provided with a height-adjustable dirt tray. When a screw on the top face of the grille is tightened, the tray would move closer to the outlet. Figure 1 illustrates the operation.

Figure1 Ajustable floor grille used MMFigure 1: Adjustable floor grille used.
Image credit: Trox

As the plant rooms are used as mixing plenums, the negative pressure procured in these rooms allow fresh air to be introduced via wall-mounted weather louvres and filters.

Challenges

The first challenge was to efficiently provide air to the occupants at temperatures and velocities that would not cause occupant discomfort. To increase the off-coil temperatures during cooling, a bypass damper system was implemented to allow the air to be reheated as well as sufficiently dehumidified without wasting energy. (Using a bypass damper to reheat off-coil air is not common practice; the temperature is usually regulated with the water flow through the heating/cooling coil.)

The system operates with a motorised damper positioned upstream of the cooling coil, allowing return air to be mixed with supply air. The damper motor is controlled to maintain a final off-coil temperature of 16°C, as recommended by the ASHRAE underfloor system design guide. This system is shown in the Figure 2 schematic.

Figure2Figure 2: Airflow and control schematic.
Image credit: CKR

Additionally, the airflow required by the AHUs had to be reduced. This was achieved with a combination of the existing variable speed drives (VSDs) and new driven pulleys. The new pulleys were implemented to disable the system from producing air velocities that would cause occupant discomfort.

Control of the system was done by reducing the airflow to the space, rather than varying the supply air temperature of the air. This type of control was selected, as a reduction in airflow to achieve the minimum required for fresh air ventilation would result in a very passive system and eliminate the risk of draughts.

Cost savings

A saving was presented to the client, with an overall project cost index of 0.81 for all works completed on the project. This was mainly achieved through the eventual re-use of the access floor system through modification only. 

Saving energy

A total energy reduction of 50% was achieved due to the active and passive elements deployed. The main contributing factors were as follows:

A heat pump chiller replaced the in-duct electric heating elements that were originally used to heat the space when required. The refrigeration cycle used by the chiller offers a more efficient solution for heating the space.

The underfloor air distribution system provides a more effective means of ventilation and so the fresh air quantity could be reduced, improving system efficiency as less unconditioned air has to be introduced into the system. Supply air temperatures were increased.

Inyanda00 18The new underfloor system.
Image credit: CKR

As air is distributed from the floor level, it presented the opportunity to only condition the occupied area of the atrium. This meant that the system only needs to condition a volume with a height of 2m, rather than the entire volume, which is 12m high. This resulted in a reduction of the energy required.

The use of a supply air bypass damper allows the underfloor plenum to be pressurised with drier air. This reduces the risk of mould growth and provides the occupants with a healthier working environment.

List of professionals

Owner  

Growthpoint Properties

Tenant

Transnet

Architect / Designer 

BILD Architects 

Project manager  

CKR Consulting Engineers

Consulting engineer 

Mechanical

CKR Consulting Engineers

Contractors 

HVAC&R

B&D Airconditioning

Controls

Ushaka

Product suppliers

Floor grilles 

Trox

Chiller

Ciat

DX hideaway units

Daikin

Actuators 

Belimo


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