Northgate refurb: ‘next generation’ efficiency

By Marcus Dos Reis of VMG Consultants

When a hypermarket in Northgate Mall recently underwent a revamp, its dated R22 evaporative condensing system was replaced with a more compact, modern, and efficient variable refrigerant flow (VRF) solution.

COVER Septebmer2018

The retail store in Northgate Mall in Johannesburg is a hypermarket and recently underwent a revamp with a reduction in size and transformation to the next generation store upgrade.

The existing air-conditioning system was over 20 years old and although it had been well maintained over this time, it was time to install a new technologically advanced plant. The new installation would serve the revamped store for the next 20 years.

This HVAC plant refurbishment potentially saw the two biggest fan walls in South Africa being installed.

The HVAC works was completed in July 2018.

A new solution

An old R22 evaporative condensing system served as the HVAC system for this store, split between three built-up air handling units (AHUs), two of which were situated at the back of the store and one at the front of the store.

The one positioned at the front of the store (feeding the front of the hypermarket) was demolished, while the remaining two (positioned at the back of the store) were replaced with a new Mitsubishi VRF system, making use of the existing masonry AHUs.

After considering various options and in consultation with the client, the consultants determined that a VRF-type installation would be the best solution in this particular instance.

The reasons for replacing the existing plant with a VRF system are as follows:

  • Existing plant spaces allowed for an easy retrofit to an air-cooled DX system.
  • The air-cooled DX system has no direct water usage, a request from the client.
  • Air-cooled systems are less maintenance intensive than water-cooled systems.
  • The inverter compressors and VRF control technology allow for the compressor to run at a reduced speed (5%) to match reduced heat/cooling loads and save on running costs.
  • VRF systems can accommodate the large cooling capacity required for the large store size at an acceptable outdoor plant space.
  • There are extended warranties on compressor failures.

 Air handling units (AHUs)

The entire VRF system is split between four AHUs. Two of these are the existing masonry AHUs that feed the main floor as well as the checkout areas (which previously were fed from the old masonry AHU that was demolished). The other two AHUs are new HC packaged units, much smaller in comparison, with one feeding the stock room and the other feeding the staff facilities.

The two renovated trading floor AHUs are a big feature of this renovation project, as the existing masonry shell of the AHUs was reused, but the internal components are all new.

Coils: Each one of these two AHUs were fitted with new interlaced R410A coils manufactured by HC Heat Exchangers. When a coil is supplied with refrigerant from multiple condensing units, interlaced coils allow for full coverage of the coil, so that in the event of one condensing unit or bank failing, there is still good coverage of refrigerant across the coil, reducing the bypass.

Each coil is connected to three VRF condensing banks (nine condensing units in total) with each bank providing 160kW of cooling. This brings the total cooling to 480kW.

The coil is split into three sections, each section approximately 1.1m high by 4m wide and connected to three electronic expansion valve kits. Each kit is connected to a different condensing unit from a different condensing bank.

The configuration in Figure 1 provides the full interlaced refrigerant coverage.

Figure 1 This figure illustrates the interlacing connection of the different condensing units across the entire coilFigure 1: This figure illustrates the interlacing connection of the different condensing units across the entire coil.

Fans: The original AHUs each contained a dual intake, backward curved centrifugal fan and was belt driven by a 55kW motor. These fans were chopped up and removed from the plant area and replaced with new centrifugal electronically commutated (EC) plug fans.

Each AHU contains a fan wall consisting of 15x ebm-papst EC plug fans. This is to accommodate an air supply volume flow rate of 30m3/s through each AHU.

The EC fans operate with a brushless DC motor and are controlled electronically, allowing for speed control by means of a 1 to 10V signal. The use of a DC motor allows for approximately 30% more efficiency compared with an AC motor.

In comparison to the old centrifugal fans, the EC fans are directly coupled and offer further efficiencies due to no belt losses.

The use of speed control allows for the fans to increase or decrease in their running speed to accommodate the required airflow to maintain a desired input parameter, such as static pressure and temperature.

Having a fan wall comprising multiple fans with speed control that are linked to a central controller also allows for redundancy. So, if one fan in the fan wall fails, the remaining fans will increase speed to maintain the desired airflow.

The total cooling capacity of the condensing units is 1 100kW and is split between four systems.

Currently, the EC plug fans are set to maintain a specific static pressur but VMG is working on a control scheme to allow full use of the EC fans to achieve better energy efficiency.

This control scheme will be adopted by the system once finalised, which will allow the fans to regulate their speed to match a specific store temperature as opposed to a static pressure alone.

The benefit of this is that when a static pressure is the input parameter, the system operates as a constant volume system, which means that even when there is no high heat load (that is, mild temperature day), the fans are still running at design value.

By switching the input parameter to the store temperature, the fans can be reduced in speed as the store set-point is maintained, thus accommodating the reduced load and saving on running costs.

The engineers considered an ideal fan and applied the cube law, which states that the power consumed by a fan is proportional to the speed of the fan cubed.

Capture

Halving the fan speed, for instance, allows it to consume one eighth of the power required at full speed. The relationship is an exponential one and allows for great energy savings.

Ventilation: Another energy-saving design addition is that of CO2 monitoring for the trading floor and the checkouts. This allows the fresh air being supplied to the AHUs to be controlled and therefore reduces ventilation load, except in instances where there are high CO2 levels. In this instance, the fresh air damper will open fully and provide the design amount of fresh air to the AHU until CO2 levels in the store drop to an acceptable level.

VRF system

The VRF condensing units are all situated on the plant room roof slab, directly above the two main AHUs.

The total cooling capacity of the condensing units is 1 100kW and is split between multiple systems.

The first two systems comprise three PUHY-P1500 heat pump condensing banks each (with nine condensing units) making up the bulk of the cooling capacity at 960kW and are connected to the two main AHUs as already discussed.

The third system is a 70kW heat recovery system that feeds a small AHU for the staff facilities and five cassettes for the pharmacy. The heat recovery system allows for simultaneous heating and cooling off the same system by use of a BC controller, which directs liquid or gas refrigerant from one system to another and reuses the absorbed energy from one space to accommodate another space.

The reason for connecting the pharmacy on a heat recovery system is that the various pharmacy consultation rooms are potentially operating in cooling and heating modes simultaneously. The staff facilities were then connected to the same system, so that the coefficient of performance (COP) of the entire system could be improved.

The upgrade of the refrigeration plant was handled as a separate contract by Matador Refrigeration.

Kitchen extraction

The bakery and deli were moved to the back of the store where an all new extraction and make-up air system would be required.

What would generally be a straightforward approach became rather challenging, because the new bakery and deli were positioned under the existing masonry plant floor slab, which left a very limited void space to route the ducting.

A total of five extraction canopies were installed and connected across three extraction fans, which are housed in a pyramid formation in the yard. The two make-up air fans were routed up to the masonry plant level, where they are positioned in front of the masonry AHUs.

Challenges

The challenges experienced on this project were the ones that inherently exist when performing revamps, such as making use of existing materials, and making those existing materials work for a new design.

There were some other key challenges too. The first was the positioning of the two new HC Heat Exchangers packaged units on the mezzanine slab inside the stockroom. With limited space, these units had to be manually hoisted with the assistance of a hydraulic lift.

The second challenge was the hoisting of the interlaced coils into place, which had to be carried through the fresh air intakes of the masonry AHUs and then built up inside the AHU on a custom frame.

Ultimately, everything came together and the revamp of the HVAC system at the hypermarket in Northgate was a success.

List of professionals

Architect / Designer  B-Arc Africa
Project manager   Betts Townsend
Consulting engineers Electrical ESE Consulting Engineers/Igoda
Mechanical VMG Consultants 
Contractors Main  HC Projects
HVAC Empire/Uniserve
Electrical Standard Electrical
Product suppliers  Fans Luft and S&P 
Main AHUs HC Heat Exchangers
VRF condensing units Mitsubishi
Split units  Dunham-Bush 
Cooking extraction canopies AMMS 

 


Click here to read the September 2018 issue of RACA Journal


 

 

 

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