Mechanical HVAC and smoke management system for new Hilton hotel

By VMG Team – Nigel Pengelly, Patrick Gwitirwa, Sarah Vera Cruz, Marcus dos Reis,Victor Dzwinamurungu and Vishal Patel
‘Ship out of the ocean’ is what the locals of Eswatini call the newly developed Hilton Hotel.

Located in the heart of the central business district of Mbabane, the capital and largest city in Eswatini, and within walking distance of the Mbabane government hospital and two shopping malls, the hotel offers 124 rooms, a restaurant, a bar, a lounge, a fitness centre and conferencing facilities to its guests. Central to establishing itself as the city’s most iconic building, the heating, ventilation and air conditioning (HVAC) system, along with the smoke management system, play a key role in providing distinguished comfort and safety to all those who enter through its doors.

Images supplied by VMG Mechanical Engineering Consultants

HVAC Design

The HVAC service designs developed for the building were carried out by VMG Mechanical Engineering Consultants in accordance with the Middle East and Africa brand standards set by Hilton Garden Inn, a group of mid-priced, focused-service hotels owned by Hilton Worldwide.

The primary function of the HVAC system is to maintain conditions conducive to human comfort in the building. With the ever-increasing price of electricity, achieving the required levels of comfort without using exorbitant amounts of electricity calls for a design that supports increased building efficiency. Elements crucial to the efficient energy usage within a modern-day hotel include the following:

  • Air conditioning and comfort heating via chilled and hot water systems respectively;
  • Centralised control of mechanical services via a building-management system (BMS); and
  • An energy-management system.

Looking deeper into the HVAC design, the cooling and heating loads were achieved using two Climaveneta NECS-Q/CA 1414 chillers. With a drive towards efficiency, the use of electric elements was limited to redundancy-only usage to achieve the heating requirements and fresh air that was pre-conditioned before entering the building.

The heating system was designed to allow the hot-water circuit from the chillers to provide pre-heating to the water supplied to the Climaveneta water-to-water heat pump via a closed-loop heat exchanger circuit. In the event that the chillers and heat pump were out of service, the hot-water tanks were fitted with electric elements to pick up the slack. The planned downtime of the chillers and heat pump would not warrant extensive use of electric elements.

Strict adherence was paid to the Hilton Garden Inn indoor environmental matrix in determining fresh-air and space-pressure dynamics.

To achieve design synergy between the aesthetics of the building and mechanical elements, a lot of coordination had to happen. Colour schemes and shapes for outside grilles had to blend in with outside facades. Furthermore, to comply with the brand standards, the design catered for fresh air to be pre-conditioned before entering the building to limit the impact of infiltration on the total cooling load.
Three fresh-air air-handling units (by Airventfil) feed into three shafts through the building, with take-off points at every floor. Booster fans complete with variable-speed drives are located at each take-off point to assist airflow to a four-pipe fan-coil unit (supplied by Sinko and Climaveneta).

Each guest room has its own unit to allow for individual comfort control. Each room is linked to a world class BMS system as well as a Vingcard energy-saving unit.

For a hotel of this size a well-designed kitchen was incorporated that is efficiently ventilated by a series of canopies. The kitchen canopies were designed so that they extended beyond the cooking appliances to capture the expanding thermal currents rising from the appliance. These overhangs vary with the style of the hood, the distance between the hood and the cooking appliance and the characteristic of the cooking equipment.

The exhaust-volume flow-rate requirement was based on the group of equipment under the hood. In the case of this Hilton Hotel, there is more than one group, in other words, a mix of gas and electric cookers. We found it prudent to base the Volumatic flow rate on the predominant group, which is gas. The hotel has a massive atrium that houses the reception and concierge areas.

With a large front glass façade, comfort conditions are achieved through a series of fan-coil units along the entire bulkhead on the ground floor. Supply air is via long and thin strip grilles.

The atrium offers a smooth transition onto the first-floor staff offices directly above the ground floor that are served by individual fan-coil units that feed off the main chillers.

The transition progresses into the guest rooms on the second floor up to the seventh floor.

The main plant area is located on the roof where scenic mountainous backdrops provide panoramic views of Mbabane.

With the operators trying to maximise room space on a fixed GLA box, coordination with other services in the service zones was critical and often complex. This was achieved through precise modelling on Revit by Sarah Vera Cruz, together with constant separate coordination meetings with the architects (Paragon).

The choice of system was based on the following philosophies:

  • Simplicity – the system uses water as a heating and cooling medium in order to achieve space comfort;
  • Lifespan – any well-serviced chilled-water system has a lifespan of up to 30 years compared to other systems, which average 15 years;
  • Maintenance – a chilled-water system derives its extended life span from its ease of maintenance ability; and
  • Significant energy and therefore cost savings over the lifetime of the building.

Smoke-management system

The Hilton smoke-management system comprises six extraction fans that serve the iconic atrium at the hotel. The atrium is open to all the corridors that serve the hotel rooms, from the ground floor lobby all the way up to the seventh floor.

Atrium smoke extraction is not a new concept, but the reason for placing emphasis on it in this project was due to the use of a computational fluid dynamics (CFD) analysis and special-application fire-control systems, such as drencher systems and flame detectors to accommodate the unique atrium. The atrium ceiling has three rounded, triangular skylight features that drop into the atrium like a bulkhead. The perimeter of these three feature bulkheads is what is used as the smoke extraction intake.
The need for CFD analysis and special fire-control systems was due to the following characteristics of the atrium:

The fact that the corridors serving the rooms were communicating spaces with the atrium and therefore the fire zone, meant that there was no allowance for a smoke reservoir and very little time allowed for smoke build-up.

The open ground floor area, under the atrium ceiling, could not be served by ceiling fire-suppression systems, as they would be ineffective in activation and suppression at such a height (27 metres).

The skylight perimeter, which served as a smoke intake, created a choking point for the air intake, and could have effects such as plug holing (high-velocity make-up air pulled through the smoke).

To provide effective fire suppression of the ground floor in the event of a fire, a side wall drencher system was installed along the atrium perimeter of the ground floor.

A total escape time required was determined for the hotel occupants. The strategy of the smoke-management system was to ensure that the smoke produced by a fire did not breach the requirement of a tenable environment to allow occupants to escape to places of safety.

The smoke build-up time on the seventh floor was first determined through theoretical means and then analysed through CFD, whereby it showed that the extinguishment of the design fire would allow for the successful escape of occupants before a tenable environment was breached.

Figure 1 shows a graph of the heat-release rate (HRR) of the design fire, and then the decay after drencher activation.

A combination of flame detectors and beam detectors were installed on the perimeter of the atrium ground floor to ensure detection of a fire, as standard smoke detectors could not be used in this area due to no direct ceiling covering. The detection controls were set up to allow for a double-knock activation to ensure that a false activation of the drencher system was avoided.

Figure 2 shows an example of some of the graphical outputs obtained, which indicated the smoke visibility on the seventh floor at a specific time.

The design and validation of this smoke management system was a challenging rationale and required the use of new tools and techniques. In fact, it promoted a major learning experience for the project team and VMG team involved on this project.

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Figure 1: Ground floor fire HRR graph

 

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Figure  2: Ground floor fire smoke visibility on seventh floor at 140s.

References:

  1. https://hiltongardeninn3.hilton.com/en/hotels/swaziland/hilton-garden-inn-mbabane-QMNGIGI/index.html
  2. https://paragon.co.za/content/mbabane-hilton-garden-inn

    Prof list Nov2019
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