By Andrew Minnaar Pr Eng, AMC Engineers, edited by Eamonn Ryan | All photos by AMC Engineers
Scale was the unique characteristic of the Pick ‘n Pay Distribution Centre refrigeration project, being probably the largest such project of the past 12 months and the largest distribution centre in the southern hemisphere.
The development covers 36ha of state-of-the-art infrastructure and is currently being commissioned. It is part of a larger project to position the Eastport Logistics Park, near OR Tambo International Airport in Gauteng, and the broader R21 area, as South Africa’s prime logistics hub as allied industries and businesses seek proximity and rational integration into the country’s leading logistics ecosystem.
However, the complexity of the HVAC&R project stemmed less from its size as in having multiple different cold rooms, multiple different temperatures and multiple different smaller areas each with different considerations. This is a big distribution centre with large areas catering to massive amounts of product.
The consulting engineer’s description
The facility is one of the largest, if not the largest, chilled distribution centre in South Africa. The overall site is 360 000m², of which 150 000m² is distribution area. Of that, 45 000m² is the chilled distribution area and forms the portion of the development that AMC was involved in. For this project AMC completed the design, tender and site supervision of the Refrigeration, Insulated Structures as well as the HVAC to all ancillary buildings.
The ammonia plant is a 7.2 MW installed refrigeration plant with a total of 9.2 MW of heat rejection to the evaporative condensers.
The plant is divided into two suction regimes, namely minus 8.75°C and +3.25°C, to optimise energy efficiency. The -8.75°C system is a 4.5 MW chilled glycol system supplying the 2°C rooms with cooling, while the +3.25°C system is a 2.15 MW plant supplying the 14°C and chocolate box rooms with cooling.
The additional high stage capacity is utilised to economise the medium (-8.75°C) plant by means of open flash economising in the high stage vessel. A liquid subcooler is installed on the high-pressure liquid feed to the MT vessel to reduce flash gas and utilise incoming / top-up condenser water to subcool the HPL before entering the MT vessel / flash economiser.
The heat from the spaces (with the future ambient area incorporated) is removed by means of 80 coils of 74kWR each in the 2°C space, 24 coils of 76kWR each in the 14°C space and four coils of 73kWR each in the chocolate box.
The heat required to defrost the 2°C coils is recovered from the discharge side of the system by means of two 200kW condensing PHEs installed in the condenser yard. This heats the warmer glycol defrost circuit from 8°C to 15°C to facilitate the defrosting of the coils in the field with waste heat from the refrigeration plant.
High quality heat is recovered from the oil systems on the screw compressors. The waste heat recovery from plant is designed for a peak heat recovery of some 317kW, although initially only 200kW will be recoverable due to the plant capacity ramping up over the life of the facility. The hot compressor oil from the discharge side is used to heat a closed loop hot water circuit from 47°C to 63°C. This is then pumped via a hotwell to the crate washer to heat the crate washer water.
Provision is made for a future heat pump, utilising the discharge gas from the primary ammonia plant and compressing it to 70°C saturated conditions in order to generate hot water in future, if required.
The plant has been equipped with VSD on all major component motors to ensure that part load conditions are optimised and energy savings are being realised during part loads conditions.
High efficiency EC fans were specified for the cold room. There being a total of 104 blower coils and 416 fans with a total peak power consumption of 650kW. It was extremely important to optimise the fan speed and control to reduce power consumption to an absolute minimum.
Due to safety and as a result of the size of the facility, the decision was taken early on in the design phase to use secondary refrigerants in the field and limit the ammonia charge to the plant room. Noting the congestion in the roof void and the travel distances, it was a prudent decision and greatly increased the safety aspects of the overall facility and roof void.
Rainwater harvesting forms a key component of the overall system and facility design. This rainwater is treated to a high quality, to make it suitable for use in the evaporative condensers, as well as site potable consumption. This greatly reduces potable municipal water consumption and also provides a large back-up water source requiring municipal water as back-up/top-up only. Of the total 350 000m² site area,
250 000m² is utilised for water harvesting. The total estimated water consumption for the whole site is 125 M litres, of which 69 M litres are consumed by the refrigeration plant. Although a total of 133 M litres are available for harvesting during a year, due to peak rainfall and demand mismatch it effectively results in a total amount of harvested water that can be consumed on the site of 75 M litres per annum.
The professional team worked together in an incredibly efficient fashion and were all dedicated to the effective design and co-ordination of all aspects of the project. The use of BIM was effectively implemented to ensure good co-ordination and integration of all design aspects. This ensured that site clashes and problems were timeously identified and addressed and mostly resolved before the installation took place on site.
The client required an efficient, robust and sustainable design solution that speaks to Pick ‘n Pay’s journey of sustainability while also providing a world class facility for their centralised distribution operations.
The facility is divided into two massive chambers. The air distribution in these chambers was critical and formed part of a detailed design investigation. Heat loads were sub-divided into design compartments for various areas such as the docking bays and then central corridors. This informed the placement of the coils and air changes per hour in the various sub sections and regions of the large open areas.
The greatest challenge of this facility design was the appropriate selection of the refrigerant. During the concept design process a multitude of options were investigated. These ranged from primary ammonia to all areas, CO2 central versus decentralised / distributed plants and even remote or multiple low charge ammonia plants with massive air handling units. Personnel safety, sustainability, efficiency and capex cost guided the final decision.
In the end the most cost effective and efficient solution was a primary ammonia plant operating at two temperature regimes with the lower of the two open flash economised on the high stage. The secondary refrigerants were water on the high temp side and a propylene glycol solution on the medium temperature side.
Efficiencies and reliability
There were some specific efficiency decisions that AMC took on this project to ensure optimal energy efficiency. Two of the most notable aspects were a reduced condensing temperature and a tight approach on the secondary PHEs. The condensing temperature was selected at 30°C, resulting in a 9K approach on the design ambient wet bulb condition. All condenser fans were equipped with VSDs to ensure fan law efficiencies were maximised during low load conditions, exponentially increasing efficiencies as load decreased.
The PHEs were selected with a very tight 2K approach resulting in a 4K LMTD for the HT and MT systems. What this further allowed is for the system to be able to operate with a PHE out of service by reducing the suction temperature by only 0.7K on the MT system and 2K on the HT system.
System reliability and redundancy is absolutely critical for this type of facility considering the centralisation of the client’s distribution and the associated business and product value that moves through this facility. Redundancy was incorporated as responsibly as possible while ensuring that single points of failure were eliminated as far as possible. A further aspect that was implemented in the design is that where redundancy was required, it was employed to increase efficiency during normal operation, that is running the compressors at lower speeds for recips, running a reduced number of screws at full load on the MT system and reducing condensing temps and fan speeds on condensers. These energy savings elicit significant paybacks on the required redundancies and make the overall installation significantly efficient.
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