Industrial film greenhouses

How a multi-span film greenhouse is arranged

A multi-span film greenhouse joins several bays into a single production volume. There are no external gaps between the sections, so the site is used more efficiently and heating, irrigation and climate equipment can be combined into shared circuits.

The geometry is chosen for the crop and the technology: the bay width, the ridge height, the trellis height, the length of the block and the layout of the service aisles all matter. The frame is calculated for the loads of the specific region and for the weight of suspended equipment.

Geometry and scaling

The current range of multi-span greenhouses uses bays of different widths, including 8.0, 9.6 and 12.8 m. The particular grid is not chosen on its own, but together with the process layout: the arrangement of rows, service aisles, gutters, trellises and utility runs. The length and the number of bays are determined by the site and by the construction phasing.

For tall crops the usable volume above the plants is critical. The greater the air volume, the slower temperature and humidity change — but the higher the demands on the frame, the heating and the ventilation. The height is therefore chosen for the crop and the climate system, not simply out of a wish for more space inside.

Covering and envelope

The roof uses light-stabilized greenhouse film, including a double layer with inflation. The air gap between the sheets reduces heat loss and stabilizes the covering. Side and end walls can be made of polycarbonate where the project needs a more rigid envelope.

The material and the thickness of the covering are selected together with the frame. Junction details, drainage, sealing and the position of doors and service aisles are considered separately.

Active Climate and microclimate control

In a classic film greenhouse a large share of air exchange goes through side and roof vents. The Active Climate system supplements natural ventilation with recirculation and forced distribution of air through the internal volume.

Air ducts and climate equipment help even out temperature, humidity and CO2 concentration across the area. This matters especially in large blocks, where local hot spots and zones of high humidity directly affect the stability of the growing regime. The composition of the system is calculated for the crop, the climate and the heating capacity.

Crops grown in multi-span film greenhouses

Heating, irrigation and supplementary lighting

A multi-span greenhouse has the advantage that engineering systems can be designed for the whole area and split into controllable zones. Heating is calculated from heat losses and the regional climate, irrigation from the crop and the output, and fertigation from the growing technology in use. On large areas, pump redundancy and the ability to service equipment without shutting down the whole complex are assessed separately.

Year-round vegetable and leafy-greens projects may require assimilation lighting. It affects not only the electrical load but also the heat balance: part of the energy from the lamps becomes heat, so lighting, heating and ventilation have to be calculated together.

Operation in cold and snowy regions

For northern sites the key factors are the snow load, heat loss and heating capacity calculations. Double film and rigid end walls help reduce heat loss, but they do not replace thermal calculations.

For year-round operation the available electrical capacity, the heat source, redundancy of pumps and boiler equipment, the need for supplementary lighting and the scenarios for an emergency temperature drop are all assessed as well.

What the cost and the economics of the project depend on

Payback cannot be judged by the cost of the frame alone. It is affected by the crop, the yield, the selling price, energy costs, how fully the facility is used and the level of automation. The economics are therefore best calculated together with the technical concept.

What data is needed for an estimate

• the crop and the planned production cycle
• the area and the dimensions of the site
• the region of construction and the site conditions
• the required trellis height and the row layout
• seasonality and heating requirements
• irrigation, fertigation, supplementary lighting, CO2 and climate control
• available water, electricity and heat capacity
• the planned construction phasing