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Mushroom Farms - Technical Information

Mushroom market

Poland is one of the largest international manufacturers of mushrooms, with the total annual output reaching 175,000 tons. Apart from mushroom manufacturers, the mushroom market encompasses also consulting companies, manufacturers of surfaces, mycelia, soil and peat, suppliers of cooling and air-conditioning appliances, as well as food processors.

Cultivation systems

In Poland, there are two distinct mushroom cultivation systems (including several variations), differing in the scope of cultivation stages carried out in single rooms. The example of the first system is cultivation on racks, in the case of which all stages from pasteurising of the bed (stage 2) to harvesting take place in the same room. The first stage, i.e. bed forming takes place outside the mushroom growing house. In the case of the second system, each stage, i.e. monitored fermentation (stage 2), incubation (stage 3), mycelia development and harvesting, takes place in a different room. Today, more and more Polish manufacturers chooses to purchase the bed after the stage 2 or 3 has been completed, transport it in blocks or loose to the facility and pull the bed on plastic mats onto the racks, thus concentrating exclusively on the cultivation and distribution of mushrooms. Such a solution enables the manufacturers to maximise the annual output per m² (30-32 kg of manually harvested mushrooms in one cycle per m², with 7,2 cultivation cycles per year).

Technological conditions

Designers and constructors of mushroom farms have to take into consideration the fact that the mushroom growing areas are exposed to:

  • high temperature (700°C) and humidity (100% + vapour pressure at high temperatures);
  • corrosive impact of water, continuously used during the entire cultivation process;
  • chemicals.

Temperature and humidity parameters depend on the cultivation stage, so the mushroom growing area is exposed to changing temperature and humidity.

It is assumed that one indoor cultivation cycle, from the aforementioned stage 3 to cleaning of the facility takes 60-64 days. Throughout this period, the temperature of the bed range between 200°C (temperature of air flowing to the chamber: 170°C) in the yielding stage and 290°C (temperature of air flowing to the chamber: 240°C) in the mycelia restoration stage. Air humidity also varies depending on the cultivation stage between 75% and 95%; during some stages, e.g. the fructification stage, the walls and floor are sprinkled with water in order to keep high humidity in the chamber.

In recent years, high importance for the mushroom cultivation process has been attributed to one of the components of the atmosphere and the product of the metabolism, i.e. carbon dioxide (CO2). Its condensation in the atmosphere reaches about 350 ppm (0.035%), whereas in the indoor mushroom growing room it is much higher; e.g. in the incubation stage, this concentration exceeds 5000 ppm (0.5%). Humidity conditions that are critical for anti-corrosion coatings of walls and structure of the mushroom growing room are typical for one of the cultivation stages involving cleaning and heat sterilisation of the room. After the cultivation process has finished, the entire room must be thermally sterilised and the bed and cover must be removed forthwith. In order to sterilise the room, all air inlets and outlets are carefully sealed and water vapour is let in, so that the temperature remains at the level of about 700°C for 12 hours. Next, the chamber is slowly cooled down to the ambient temperature. In practice, heating to the temperature level of 700°C should also be performed gradually, so the entire process of heating, heat sterilisation and cooling down the mushroom growing room interior takes about 3 days.

Components of a mushroom farm

A mushroom farm building consists of the following components:

  • mushroom growing rooms with all necessary facilities and a corridor,
  • boiler room,
  • packaging room,
  • cold room,
  • rest and refreshment rooms,
  • office,
  • packaging storage area,
  • settling and septic tanks

Mushroom farm construction

Size of the mushroom growing area (total rack growing area). Due to the necessity to install air-conditioning systems, the control of technological parameters and lower costs, the investors tend to choose to construct mushroom growing facilities whose area exceeds the former standards of 200 m². Mushroom growing rooms of the area between 300 m² and 400 m² are becoming the new industry standard. It is assumed that an optimum-sized mushroom farm consists of 7-14 rooms of the area of 350 m² each equipped with 4-level racks.

Standard size of the interior of the mushroom growing chamber of the area of about 185 m² is as follows: width: 6 m, length: 16,50 m and height at the eaves: 3,70 m. Cubic capacity of such a room is about 350 m³, whereas the total area of the walls, ceiling and floor reaches 360 m².

In such rooms, two rows of racks of the width of 1,40 m are arranged. If the room width is 6 m and the external width of the racks is 1,40 m, the central corridor can have the width of 1,20 m, while side corridors along the walls: 1 m each. The distance from the rack to the front wall (sliding gate) should be 1 m, while to the wall adjacent to the corridor – 2 m. Therefore, if the length of the rack is 13,50 m, the cultivation area of one shelf is 18,90 m². Provided that there are two rows of 5-level racks, the entire chamber has 189 m² cultivation area.

The cultivation area can be doubled by extending the length of the room to 30 m or arranging the racks in four rows. The latter solution is more advantageous, since it ensures nearly 400 m² of cultivation area in a 11-metre wide room. Optimum utilisation of the room depends also on the number of shelf in a rack. 5-level racks are the most popular solutions, but 4-level racks are much more comfortable, as they do not require any additional platforms or ladders for the personnel: it must be remembered that even work 1 m over the floor level requires appropriate safety measures. The height of the room and the rack supporting structure should enable the installation of five levels of shelves, but in the initial stages of manufacturing implementation, it is recommended that 4-level racks should be used and the cultivation might be maximised after gaining some experience and firm position in the market. A rational room cubic capacity / cultivation area ration should not be lower than 1,85:1 and, most preferably, should be equal to 2:1.

Walls and ceilings of mushroom growing rooms

The requirements of thermal insulation of walls and ceilings of mushroom growing rooms hitherto mentioned in the specialised reference materials were applicable to traditional materials; the recommended heat penetration rates were UC=0.7W/m²•K for walls and UC=0.35W/m²•K for ceilings.

In practical application, those requirements are most efficiently met by walls made of aerated concrete based on cement-lime mortar with an extra layer of bricks (12 cm), including 5-cm air insulation and traditional flat roofs made of hollow core slabs or troughed slabs insulated from the outside, e.g. with Styrofoam panels.

According to the currently applicable standards, the requirements of thermal insulation for outer walls of production facilities having contact with outside air, at the temperature inside the room at ti>160°C state that the heat penetration rate for walls should be 0.45 W/m² • K, while for ceilings: 0.30 W/m² • K.

Taking into account the specific manufacturing technology, particularly the heat sterilisation of the rooms, the temperature differences can be very extreme, between the temperature inside the chamber (700°C) and the outside temperature (-200°C), i.e. Δt=900 C. Thermal insulation of the wall expressed as heat stream density (i.e. the heat loss resulting from the temperature differences) is at the level of about 60 W/m² at UC=0.7 W/m² • K. To put it more simply, the heat loss is 50 kcal/m² per hour. It is a significant loss, if we consider the total area of walls of the chamber, but occurring within a short period, i.e. during 3 days per cycle, which makes 21 days per year.

In order to prevent those adverse circumstances, investors choose modern materials of the best parameters of thermal insulation and durability of the anti-corrosion layer. Currently, sandwich panels with polyurethane core in steel facing are getting more and more popular as a material both for walls and roofing.

Corridor

The corridor connecting the mushroom growing rooms should have the width of 4 m (at 6 growing rooms) or 5 m (at a larger number of rooms). In smaller mushroom farms, the corridor can also function as a packaging place.

Packaging room and cold room

A packaging room in a mushroom farm is a separate room adjacent to the cold room, the vacuum cooling system and the loading ramp.

Recommendations regarding the selection of sandwich panels with polyurethane core in steel facing.

Coating

In accordance with the PN-EN ISO 12944-2 standard: “Corrosion protection of steel structures by protective paint systems - Part 2: Classification of environments”, the environment inside the mushroom growing rooms may be classified regarding the risk of corrosion to C3 category, as manufacturing facilities of high humidity and certain level of air contamination, such as food processing plants. In accordance with the technical approval for BALEXTHERM and BALEXTHERM CH sandwich panels, those panels in galvanised steel facing (Z275 zinc coating) with organic coatings: SP25 PE or PVDF25 can be applied in environments classified regarding the risk of corrosion to C1, C2 and C3 categories. Heat resistance of both coatings is also sufficient for handling the heat sterilisation process (involving the exposure to water vapour of the temperature of 700°C), since the SP25 coating is resistant to temperature up to 800°C, while the PVDF25 – up to 1,100°C. White 9002- or 9010-coloured coatings on both sides of the panels are recommended. In the inspected mushroom farm constructed of sandwich panels with SP25 coating in 2003, the coating preserved its anti-corrosion properties and very few minor signs of corrosion were found only in spots where mechanical damages of the coating had occurred.

Panel thickness

Thickness of panels should be selected in accordance with the information set forth in the section titled “Mushroom farm construction”, as well as taking into account the general guidelines for designing facilities in which high heat losses may occur and in which technological processes in high temperatures are carried out. According to those guidelines, recommended thermal insulation (expressed as a heat stream) should be lower than 10 W/m². Therefore, if Δt=450C (temperature inside the room: +250°C, outside temperature: -200°C), outer walls of the mushroom growing room should be constructed of BALEXTHERM ST100 panels.

Following the aforementioned principles, the following panel thicknesses are recommended for diverse locations and construction components, taking also into account the slightly higher heat losses during a short part of the year, when heat sterilisation is being carried out:

The final decision on the panel thickness should be a compromise between the technical requirements and the planned expenses to be incurred by the investor. If you choose thinner panels, you can lower the total cost thus increasing future maintenance expenses, which will, however, be incurred over a longer period of time.

Assembly

During the assembly works, do not damage the varnish of the sandwich panels and cover any damages with protective varnish. All joints between the walls and ceilings in the corners and joints with the floor should be carefully filled with fitting foam in order to ensure full tightness of connections. Moreover, the panel contact areas should be filled with sealing compound or with silicon from the inside of the chamber. Pay particular attention to tight assembly of roof panels: if they are not sufficiently tight, in winter, as a result of evaporation, the water vapour might escape outside the chamber.

 
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