For the 2018 onion harvest Bennett and Co completed a 1700tonne onion store for J S Means (Terrington) Ltd. The building is situated on the edge of their existing farm yard and had to be carefully sited to allow for adequate apron space in front of the building for grading, loading and unloading. A high tension cable close to the proposed building was buried in a duct and a ditch piped to enable the structure to fit in the site and not compromise the irrigation reservoir behind the building or the client’s existing concrete standing for empty potato boxes.
Due to the nature of the marsh soil conditions and a redundant back filled pond on the site, the foundations were piled before a large reinforced base slab was constructed on top of the piles. Once this was achieved the steelwork could be erected.
The drying floor is a hardwood deep bearer timber floor supplied and constructed by Flach and Leroy, designed specifically for this type of application with the large airflows needed in the first stage of onion drying. The floor is constructed from a double bearer and the stability of the bearers is maintained by lateral struts along the length of the floor giving an overall height of over half a metre. Inside the main air duct there are air inlet doors with stepped wooden blocks allowing the doors to be partially opened to enable further control and flexibility in airflows used.
The timber necessitated a large reinforced concrete up stand around it. This in turn required two bridges constructed over the up stands to access the building to install the concrete panels for the outside walls, sides and roof of the central main air duct and then the installation of the timber floor. Once this was completed the bridges were removed.
Fläkt Woods supplied three 1400mm diameter fans with guide vanes and large silencers. These are controlled by inverters to keep the starting loads to a minimum but also to enable the airflows in the store to be closely controlled by air speed sensors in the crop and an air duct pressure sensor. This is achieved by the Vegtec controller which also controls the air mixing louvres and brings in the refrigeration when required for pull down and maintenance of crop temperatures during storage. The controller can be remotely accessed by GSM modem to a mobile phone or computer.
Heat for drying is from a Harvest Installations modulating gas burner that can be modulated for either temperature or relative humidity (used if drying grain). This controls the drying air temperature and with the use of the Farm Electronics motorised louvres the relative humidity of the air is controlled too during the drying phases. The louvres are also used to provide ambient air cooling and recirculation to bring down and maintain crop storage temperatures along with the refrigeration unit.
The Farm Electronics refrigeration unit has a large evaporator coil installed into a divider wall in the fan house with its own recirculation louvre to draw air off the top of the crop and into the plenum created behind the divider wall and sucked through the coil by the bottom fan. The compressors and condenser coil are put together in a factory constructed box unit with the condensers on the roof. This is mounted outside the fan house.
With the need to keep onions near to 0-1ºC the building cladding is insulated composite sheet with plastisol coating, with the insulation 100mm thick giving a U value of 0.2W/m²/ºC. The costs of running refrigeration are such that insulation of this level is economic. To improve the insulation properties of the building further the client chose to have a door with 80mm thick insulation panels.