By Kevin Walter, Lutz Refrigeration
This project at Uitdraai Boerdery, Polokwane, included building the steel structure, electrical work, insulation panels and finally, the refrigeration plant.
One would not necessarily think about the dynamics around harvesting this crop type and what is involved in a year-long supply, nor of the needs around new seed potatoes.
The project was awarded in August 2020 and completed in November 2020. Timing was critical as the potatoes come out of the ground from late November/early December and the facility needed to be ready to receive stock. Lots of upfront planning by the client had gone into the project so that when it came time to turn the first sod, all contractors were organised, and preparation was complete.
The project ran smoothly according to the set schedule and was commissioned roughly a month before the first potatoes would enter the rooms during December. This was a preferred outcome allowing the plant to bed in before loading up the rooms over the Christmas period. There were no major teething issues, and the plant ran well from day one.
Design specifications/client brief
The facility would receive roughly 800 bins of potatoes daily weighing roughly 800kg each. The potatoes come straight from the fields in Polokwane where they are loaded into the staging area in front of the cold rooms to be cooled via ambient air during the night. The following morning the potatoes are loaded into the rooms.
The potatoes enter at roughly 20°C into the rooms. The rooms are loaded for a few weeks, one at a time. Once any room is fully loaded, room temperature is gradually dropped to 2°C to simulate the onset of winter and the hibernation period of the potato without shocking the seed.
Seed potatoes are stored for up to six months before being planted. Interestingly, the potatoes are planted during the winter months in Polokwane and in the colder Southern climates of the western Free State (also a major potato farming area) the potatoes are planted in summer. This is how the consumer can access fresh potatoes all year round in South Africa without cold storage required for the table variety.
To achieve the best product preservation, air flow to every corner of the rooms is imperative as well as keeping humidity up so as not to dry the potatoes out. Maximising airflow was achieved by installing kickplates to straighten the air off the coils and increase the air throw (the distance that air travels away) of the evaporator and prevent air recirculation as much as possible.
Increasing humidity in the rooms is achieved by over-sizing cooling coils thereby reducing the temperature difference over the coils which in turn reduces moisture removal from the air.
System and facility detail
Each room is approximately 30m x 20m x 6m high. Insulating panels are 150mm thick polystyrene with 0.5mm chromadek cladding on either side. The polystyrene is slightly thicker than traditional specs of 100mm for a cold room, but no doubt saves energy and improves the whole plant efficiency. Using this product range, we found that the advent of hot spots in the rooms were almost completely eliminated, improved through the combination of the correct vapour seal and thicker than normal panelling.
Each room has two standalone refrigeration systems that can provide roughly 70kW of cooling each (enough to keep the room on temperature individually). This is a relatively high capacity for a room of this size but is required for the pulldown load on the potatoes during the beginning of the season and means that in the event of a break down, the cold chain can be maintained with one plant running.
Additionally, a portion of the stock is rotated out of the rooms in the middle of the season and during this period the existing dormant potatoes cannot be warmed up and brought out of hibernation, the room has to remain on temperature even with the warm stock and open doors in the heat of summer.
The Recam condensers are heavily over-sized to cope with the commonly high ambient temperatures often experienced in Polokwane during summer.
Evaporator coils have four of 500mm fans each which, on paper, can throw air for up to 25m. In this room the required throw was 15m as the coils were placed on opposite ends of the rooms on the 20m walls blowing towards the centre with a gap left in between the crates for the air to enter. As mentioned, square kick plates on the front face of the coils were installed to ‘straighten’ the air and increase throw to the centre of room. The airflow can be felt beautifully as it washes all the spuds.
The final design touch which can often be overlooked but is critical to rooms like this, are fresh air fans which simply blast fresh air into the rooms with a fan and louvre system. These fans will be fired up periodically to flush out build-up of deleterious gases which are emitted from the potatoes during storage.
The gas is emitted more when the potatoes first enter the rooms and emissions reduce over time as the potato ages. The farmer knows his product best and will manually engage the fans more at the beginning of the cycle and less as time goes by, typically switching on the fresh fans early in the morning when ambient air is coolest works best.
The biggest challenge for the contractors on this project was the ‘out of town’ aspect. As a contractor you have to be well prepared and make sure you don’t forget any materials or equipment before mobilising on site. Fortunately, Uitdraai Boerdery is a very sophisticated and has a well-equipped operation – having almost enough in-house equipment to handle a project like this themselves.
Sustainability aspects in the project
The greatest energy efficiency will be gleaned from the fact that this is one of the first large scale seed potato storerooms in Polokwane with most of the country’s current capacity being found in towns much further south like Christiana. Historically these seed potatoes would travel south to Bethel for storage and then back north to be planted, a round trip of 1400kms. This will save transport costs and improve the cold chain too by shortening the time from field to storage.
Additionally, Uitdraai Boerdery has installed a solar system which provides roughly 80% of the electrical power required to drive these rooms during the day. Without batteries the plant uses a mix of solar and grid power as required. Additionally, Uitdraai Boerdery feeds surplus power back into the grid when the plant is not running thereby assisting Eskom and other local farmers with a more stable power grid. This surplus power fed into the grid is credited to Uitdraai Boerdery to give free power from the grid down the line making the feasibility of installing systems like this much more attractive.
Unique project aspects
Lutz Refrigeration installed its first potato room for Lubbe Cilliers in the early 80s. The principle of seed potato storage has not changed much since then. There has always been an emphasis on getting cold air to every inch of the room, ensuring an unbroken cold chain, changing fresh air periodically and maximising humidity in the room. Over the years Lutz has installed seed potato storerooms for companies like Wesgrow, Belerive Trust, Dehoek Trust and Uitdraai Boerdery more recently. Lutz has been fortunate enough to be able to fine tune our designs over the years to get to where we are today with a very tight knowledge for design capacity, unique product requirements and control system tweaking to keep those potatoes dormant and healthy waiting for the next planting season.
Speaking recently with Human and Pieter Du Preez (the father and son team that run the farm) they noted that the seeds that come out of these new rooms (compared with their existing older rooms) experienced much lower product losses and additionally the seeds took a full week longer (now 3 weeks) to sprout when taken out of the rooms than what they had achieved historically.