SteamWeeding: Technical

2010 March update - please also see the article Thermal weed management for crop production (621KB pdf file) written for eOrganic (USA). Please note that the article states that steam weeders are not widely available. The article was written for a USA audience and PhysicalWeeding does not yet have a presence in North America, however, if you are interested in steam weeders they can be purchased direct from us.

Introduction

Flame weeding is a long established technology used in organic horticulture and agriculture and also by municipal authorities. While flame weeders are useful and productive tools they have a number of drawbacks including:

  • They use large quantities of LPG, which can inconvenient in agricultural / horticultural situations;
  • The open flames are a fire hazard and they can not be used when fire bans are in place;
  • It is difficult to effectively transfer the heat produced into the weeds;
  • Wind, especially from the rear, can cause large decreases in effectiveness for many flame weeders with poorly designed hoods.

Scientific research has shown that steam is superior to open flames for transferring heat into weeds, principally due to the large latent heat of condensation of water (Figure 1) which contains a large amount of energy, and is therefore more efficient at transferring heat into weeds.

Figure 1. Heating/cooling curves for water. A: Rise in temperature as ice absorbs heat., B: Absorption of latent heat of fusion., C: Rise in temperature as liquid water absorbs heat., D: Water boils and absorbs latent heat of vaporization., E: Steam absorbs heat and thus increases its temperature.

Until recently it has not been practical to use steam for weed control (as opposes to soil steaming) in agricultural and horticultural settings because the standard steam boiler design is heavy, expensive, needs regular maintenance and often has to comply with a range of safety legislation. In the last few years new approaches of steam generation have been used to develop two commercial steam weeders for orchard and vineyard intrarow (under vine / tree) weed control the Atarus Stinger® and the Vaporjet® H1200, which overcome some of the problems of standard steam boiler design. However both these weeders superheat water within heat exchangers which is unnecessary complicated and can suffer from problems such as lime scale buildup and have the potential to produce a superheated steam explosion if safety systems fail. Fortunately there is now a safer and simpler alternative - direct-fired steam weeders.

Direct-fired steam weeders (DFSW) work by spraying water as a fine mist directly into the exhaust gasses from a burner, causing it to flash into steam. This simple but highly effective approach has a number of advantages over standard flame weeder designs and existing steam weeders. These include:

  • No open flames: the flame in the DFSW is contained in an insulated refractory chamber, the steam / hot gas mix it produces has a lower temperature than open flame and contains little oxygen which means it would be very difficult for the DFSW to start a fire;
  • The lower temperatures / low fire risk means that the DFSW can be used over polythene and paper based mulches;
  • A wide range of fuels can be used, including diesel, which is available on practically all farms and even bio-fuels such as those made from oil seed rape (canola) or even used chip shop oil!;
  • The separation of the steam production unit from how and where steam is applied creates huge flexibility. For example, it could be used under-vine, across a whole vegetable bed, or applied in strips over crop rows;
  • A wide range of heat outputs can be catered for e.g., from 100 - 600 kW per steam generator unit allowing accurate matching of heat output to the job in hand. Individual steam generator units can even have two, easily switchable heat outputs, e.g. 200 or 500 kilowatts;
  • As the burner and steam production are protected within combustion chambers they are totally unaffected by wind or other weather conditions, and, depending on the application method, e.g., closed hood over a vegetable bed, the application of the steam will also be unaffected by weather conditions;
  • Direct-fired steam avoids the use of heat exchangers containing superheated water, so reducing complexity, cost and increasing safety as there is no potential for a superheated steam explosion.

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