Chapter 3

Greenhouse Curtain Systems

Opening Strategies of Greenhouse Curtains

The opening and closing of thermal curtains can be controlled manually or automatically. In small greenhouses, the curtain can be opened and closed manually with a hand crank device to reduce costs. Motorized curtains have the advantage of using automated controls to open and close the curtain, reducing labor and possibly being more, timely. Motorized systems have the advantage of being controlled by air temperature, sunlight level, or a time clock.

Motorized Units

Drive units consisting of an electric motor and gearbox physically move the curtain using one of two common drive methods: push-pull and cable/drum. The gear motor in these systems can damage the greenhouse should a limit switch fail, or the cable become jammed. This is because of the motor’s low speed and high torque.

Push-Pull System

This design is used in truss-to-truss systems. Rotating pinion gears move toothed racks that span the distance between two parallel trusses. These racks attach to rigid metal tubes (push tubes, push pipes, or push-pull pipes) that run the length of the greenhouse (minus one truss-to-truss spacing to allow the tube to move).

Cable/Drum Drive

A cable/drum drive system works with either the gutter-to-gutter or truss-to-truss method (Figure 3.6). Closed loops of galvanized or stainless-steel wire rope are wrapped around drums at one end of the system and pulleys at the other.

Chain and Cable Drive

A chain and cable drive system can move a curtain from gutter to gutter or from truss to truss. In this system, lengths of roller chain are spliced into loops of wire rope that span the length or width of the greenhouse. The length of roller chain equals the truss to truss or gutter to gutter distance that the curtain panels must span.

Operation of Mechanical Drive Systems

Deciding when to open and close the screen can be done several ways. Curtains can be controlled manually, by a central controller, or by a stand-alone controller. If needed, the controller can be manually overridden with toggle switches. The simplest is manual control by flipping a switch to turn on the gear drive motor. As with other control devices such as heaters or vents, this decision may not always be made at the best time.

Stratification of Air Temperature

Fully deployed curtains create a barrier between the attic area above the curtains and the growing area below the curtains resulting in differences in air temperature and humidity. This climate heterogeneity within the greenhouse (variations in climate between the upper and lower portions of the interior) can lead to abnormal or inefficient growth. During cold weather periods with curtains deployed, attic air can become much cooler overnight than air below the curtains in the growing area.

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