Towering above the rest

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By Noel de Villiers of Sutherland Engineers

The new 15-storey Rosebank Towers is the tallest building in the area and boasts an efficient ice storage system that has warranted it a 4-Star Green Star South Africa rating.

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Rosebank Towers creates a striking corporate address in the Rosebank CBD in Johannesburg, complemented by easy vehicular access, strong pedestrian links, and an identifiable visual approach. The landscaped pedestrian sidewalk integrates the building and the street environment.

The building was developed by Abland in joint venture with Redefine Properties and Ellwain Investments. The development team, headed by Abland in conjunction with Redefine, is responsible for the development and project management, and is also heading the leasing process.

The building’s facetted design consists of a combination of glass, composite spandrels, and plaster on top of a fragmented stone plinth. The canopy of trees on the plinth mediates between the activity at the ground level and the office floor above. This vegetation brings an organic softness to the structured built environment around it, and establishes a sense of a peaceful and productive environment in tune with the leafy neighbourhood of Rosebank beyond.

The building is the tallest in the area. The 15-storey height above ground adds to the building’s visibility and identity, from Oxford Road and the wider surrounding environment.

ABOUT THE BUILDING

The building is situated in Biermann Avenue, Rosebank, and comprises the following:

Rosebank Towers is the new home of Redefine Properties, which occupies 6 000m2. Overall, the building offers some 22 000m2 of lettable area, with a parking ratio of five bays per 100m2 of office space. It has a flexible plan configuration with floor plates of approximately 3 000m2. This allows for easy subdivision for tenants wishing to take spaces from 600m2 and up, without compromising the quality of the office space.

HVAC SYSTEM SUMMARY

The air-conditioning and ventilation system comprises the following, as per the client’s requirements:

DESIGN CRITERIA

All calculations and the selection of equipment are based on the site being at an altitude of 1 700m above sea level.

The system was designed based on the following conditions:

SYSTEM DESCRIPTION

The building’s air-conditioning and ventilation system was designed as follows in each of the areas.

Basement parking

The one level of basement parking is mechanically ventilated as follows:

Ground floor

Only the entrance foyer and security office on ground floor are air conditioned by means of a single ducted split unit. Note that this is a 100% outside air unit, to provide sufficient fresh air. This unit is a heat pump type unit for winter heating.

Above ground parking level

The five above ground parking levels are naturally ventilated by means of suitably sized, permanently open weather louvres, supplied and installed by the builder.

Kitchen canopy extract systems

Two tenants have kitchens that require canopy extract systems:

Toilet extract system

Two toilet extract systems serve the toilets on the seven office floors. One system provides conventional extract from the urinal/wash hand basin areas, while the second system is a Hygizone extract system connected to the male and female toilet pans (excluding the disabled toilet). The roof-mounted extract fans run at a constant speed during office hours, off a seven-day time clock.

At each branch duct into the toilets, a curtain type fire damper with fusible link is installed in the branch duct.

Office floor

The office floors are air conditioned by means of a central chilled water system with variable air volume (VAV) air handling units (AHUs) located in four plant rooms on each floor and air-cooled water chillers situated on the roof. An insulated chilled water reticulation system circulates chilled water to each AHU. 

The system uses two air-cooled chillers in conjunction with an ice storage system located on the roof of the building. Each chiller is complete with integral chilled water circulating pump and there are two secondary chilled water pumps, namely duty and standby.

Air distribution on the floors is via Rickard VAV swirl diffusers, complete with building management system (BMS) compatible controls. The ceiling void is used as a common return air plenum. Each plant room has an external louvre for fresh air intake. Motorised dampers facilitate the use of an economy cycle for energy saving as well as for night flushing of the building when appropriate.

The plant and equipment on each floor have been configured and zoned in such a way that it is possible to divide the floor into four separate tenancies. For this reason, there are four AHUs per floor: one for each possible tenancy.


The office floors are air conditioned by means of a central chilled water system with VAV AHUs located in four plant rooms on each floor and air-cooled water chillers situated on the roof.


Water chillers

There are two air-cooled chillers, each with a nominal cooling capacity of 440kW: water in/out 14°C/6°C in normal mode, and-2.5°C/-5.5°C in ice making mode.

Chillers run at 100% load most of the time and have been selected for optimum efficiency at full load.

Each chiller is complete with the following options:

Refrigerant with zero ozone depletion potential (ODP)
Suitable for standard daytime operation as well as ice making at night
Suitable for operation with 25% ethylene glycol
Integral dual chilled water circulating pumps
Full-function temperature controller, BMS compatible
Factory fitted over/under voltage relays, as well as compressor thermal overload relays
Condenser fan speed control
Refrigerant leak detection system (Green Star).
To reduce pumping costs, the system has been designed to have a large delta T across the chillers and the AHU coils. The chiller has therefore been selected for a leaving temperature of 6°C and a return water temperature of 14°C (8°C delta T).

The two chillers are connected to a common header that supplies a primary chilled water loop that can divert the glycol mix through the Calmac Ice tanks. The secondary chilled water loop includes a set of two pumps (one duty, one standby), feeding the four chilled water risers to the AHU plant rooms below. A differential pressure sensor is installed across the flow and return pipes on the roof, and pump speed is controlled from this sensor to maintain a constant differential pressure. Pipe sizing is such that the water pressure varies minimally throughout the system, especially at flows lower than design. This simplifies the balancing requirements.

Ice storage system

An ice storage system is included in this contract. The 15 Calmac Model 1190CV ice tanks as supplied by Airco are installed on the roof. This is a partial storage system and is used to supplement the cooling provided by the chillers during the day. Storage capacity is 7 500kWh in total.

The system includes the Calmac inventory meter as well as the glycol management system for make-up of glycol as and when required.

Air handling units

All AHUs are of the vertical, top discharge type with double-skin insulated casing panels. The fans are variable speed, plug fan types and chilled water control valves are Belimo two-way type, incorporating energy meters.

Cooling coils have been selected using a delta T of 8°C, entering at 7°C, a maximum air velocity of 2.5m/s through the coil, and a maximum fin density of 12 fins per 25mm. Condensate drain pans are stainless steel. Insulation used in the AHUs complies with the Green Star requirement EMI-04 and have zero ozone depletion substances associated with either the manufacture or the composition thereof.

Units are complete with washable panel filters, accessible from the front of the unit.

All units are complete with an electrical control panel, including BMS compatible temperature controller and variable speed drive for the fans, controlled from a static pressure sensor located in the supply air duct. 

Fresh air

Each AHU has a dedicated fresh air fan that has a manual speed controller so that the fan speed can be manually adjusted to ensure that the correct amount of fresh air is supplied to the AHU, regardless of any variation in the AHU fan speed.

Economy cycle

An economy cycle has been included. The fresh air damper in the façade of the plant room has been sized to provide 100% fresh air to the AHUs when the economy cycle is activated. In this case, the fresh air damper will open and the fresh air fans will switch off. At the same time, the return air dampers will close and the relief air dampers will open to relieve air to outside. The AHUs will then provide 100% outside air into the office space. The AHU controller initiates this function, based on the dry bulb outside air temperature.

This system will also be used to carry out night flushing of the building when appropriate. This function will be programmed by the BMS.

Air diffusion

On the office floors, air diffusion is by means of Rickard VAV swirl diffusers with MLM type controllers. Sizes and duties are shown on the floor layout drawings. All diffusers are fitted with an integral temperature sensor so that only a software operation is required to convert a slave into a master, or vice versa. Perimeter diffusers are fitted with integral re-heaters, complete with auto-reset as well as power reset overheat safeties.

Power supply units (PSU) and master control units (MCU) are located in the AHU plant rooms.

Duct static pressure is to be maintained at 50Pa by means of a duct mounted static pressure sensor linked to the VSD of the AHU. The static pressure shall be monitored and the set point adjusted via the BMS.

Ducting

All ducts are manufactured and installed to the latest SABS standards, with all joints sealed and clamps used on flanges above 400mm long.

Ducting used is the Piral HD Hydrotec panel with thickness of 20mm, as supplied by Voltas Technologies. This ducting is built to P3ductal standards and in conformity with the UNI EN 13403 standard.

Sound attenuators

Sound attenuators are Donkin Type PGL – C rectangular splitter attenuators.

Piping

The chilled water reticulation system is done in accordance to SABS 62-1971.

Chilled water control valves

Pressure independent control valves are used at the AHUs for temperature control. No manual balancing valves are mounted in the main, risers, or branches.

Valves are Belimo two-way valves wi th energy monitoring capability. 

Feed and expansion tank

The entire chilled water reticulation system is filled with a 25% mixture of ethylene glycol, and the make-up is a dedicated glycol management system located at the chillers on the roof.

BMS

The HVAC system includes a BMS, which monitors the central plant, including the AHUs.

PERFORMANCE

The system has performed well through the summer months and as the cooler months approach, it has been found that the ice that is generated at night is sufficient to take care of all the cooling during the day. This means that for the cooler months the chillers do not need to run at all during the day.

The ice storage system has allowed the maximum demand for the building to be reduced by 270kVA, with most of the chiller energy being consumed during off-peak hours when the electricity tariffs are much lower. This has resulted in significant cost savings for the client, and these cost savings will continue to increase for the life of the building as electricity tariffs go up.

LIST OF PROFESSIONALS

Owner   Abland 
Developer Abland 
Architect / Designer  GLH Architects
Project manager   Abland 
Consulting engineers  Electrical RWP Consulting
Mechanical Sutherland Engineers
Wet services GLH Architects / Sutherland Engineers
Fire Ronald Koekemoer & Associates
Sustainable building consultant WSP|Parsons Brinckerhoff
Contractors Main building WBHO
HVAC&R Luft
Product suppliers ·         Basement fans: SystemAir
·         Chilled water control valves: Belimo
·         AHU: Sunwind
·         Splits: Samsung (digital inverter) and Carrier
·         Ice storage: Calmac (Airco)
·         Chillers: BlueBox (Voltas)
·         Rooftop unit: Carrier
·         Pumps: KSB
·         Electric motors: Zest WEG
·         Fans on heat exchangers: ebm-papst
·         Ducting: P3ductal
·         Sound attenuators: Donkin

Click below to read the June 2017 issue of RACA Journal

RACA JUNE2017