seed bank design: seed drying rooms

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1. Millennium Seed Bank Project, Wakehurst Place, Ardingly,West Sussex RH17 6TN, UK Copyright 2009, Board of Trustees of the Royal Botanic Gardens, bank design: seeddrying roomsSimon Linington and Keith Manger, Seed C Technical Information Sheet_11 onservation DepartmentThe effective drying of orthodoxseeds prior to cold storage is the keyto successful seed banking. Dryingprolongs storage life, allows seedsto tolerate extreme temperatures,prevents germination, and reducesattack by predators and pathogens.For long-term conservation, the aimis to reduce seed moisture status to15% equilibrium relative humidity(eRH) at 15C. This is about 3-7%moisture content (on a fresh weightbasis), depending on the oil contentof the seeds.Technical Information Sheet_11Equipment specificationsFurther readingLinington, S.H. (2003). The designof seed banks, pp. 591-636. In: R.D.Smith, J.B. Dickie, S.H. Linington,H.W. Pritchard and R.J. Probert(eds), Seed Conservation: turningscience into practice. Royal BotanicGardens, Kew.A dry room is the most appropriatedrying method when you need todry large quantities of seed at thesame time (see table below). Dryrooms also provide safe storageprior to seed cleaning and a suitablelocation to package samples forstorage or to warm up frozensamples that have been removedfrom the cold room.This information sheet explains howto set up a dry room. Refer to Low-costDrying (Technical InformationSheet_08) and Linington (2003) forother drying methods.Description Model/Product SupplierInsulation Panellised Styrofoam, 100mm thick Local refrigeration companySorption dryer Dehumidifying systems facilities Refrigeration unit Local refrigeration companyStacking crates Plastic trays (ventilated)600L x 400W x 193H, ref:21035600L x 400W x 120H, ref:20016Dollies, ref:91005Allibert Handlingwww.allibert.comHygrometer to measure seed moisture status AW-DIO sensor with HygroPalm 3 displayunit or HygroClip SC04 sensor withHygroLog-D unit.Rotronic Instruments (UK) Ltd.www.rotronic.comData loggers for measuring relative humidity Tiny Tag and Tiny View data loggers Gemini Data Loggers (UK) Ltd.www.geminidataloggers.comPlease note that the above equipment is used by the Millennium Seed Bank Project and has been chosen carefully using our many years experience. The list ofsuppliers is for guidance only and does not represent an endorsement by the Royal Botanic Gardens, Kew. The manufacturers instructions must be followedwhen using any of the equipment referred to in this Information Sheet.Designing a dry roomIt is important to obtain specialistadvice when creating a dry room.However, seed curators andconservation practitioners shouldunderstand the basic principlesin order to guide the design. Thefundamental principle is to createa sealed room with low relativehumidity and a cool temperature(the MSB recommends 15% RHat 15C). Good insulation is alsoimportant to reduce energy usagein cooling and drying.The key questions when designing adry room are:What size and shape should theroom be?The maximum volume of materialthat will need to be dried at anyone time will determine the size ofroom required. A small seed bankmight require a volume of around25m, whereas an international seedbank is more likely to require a dryroom in the region of 125m.What is the likely moisture load?Apart from the seeds themselves,other sources of moisture need to betaken into account when calculatingdry room size. Together, these makeup the moisture load, expressedas kilograms per hour. Workingto a slight positive pressure, thedryer should be able to maintainthe required conditions under thepotential moisture load from: Wet seedsThe moisture status and amount ofseeds in the room at any one timewill vary, so it is difficult to estimatethe exact moisture load.Unless batches of very wet seedswill be placed in the room atfrequent intervals, there is nobenefit in designing for a worstcase scenario. The system may falloutside the optimum conditionsfor a short amount of time, but thedehumidifier will then start to workmore efficiently and will rapidlyreturn the room to the desiredmoisture status. People in the roomA person carrying out non-strenuouswork in a dry room will releaseapproximately 60g water vapour perhour. Moist air entering the roomDepends on the frequency of door-opening,the size of the air-lock, andthe external air conditions. Fresh air make-upDepends on the number of peopleusing the room. Permeation through the structureThis should be negligible. Leakage into the dehumidifierMoisture leaking into thedehumidifier is likely to be zero for awell-sealed machine.What about working conditionsfor staff in the room?A fresh intake of oxygen into theroom is required, to maintain safeworking conditions for staff. Withthe exception of very small dryrooms, air entering via the door willnot supply sufficient oxygen to theroom. Allow for a steady 10% freshair make-up via the dry-air ductwork, into the process air flow.The supply of fresh air can be shutoff when there is sufficient oxygenin the room. This is controlled via acarbon dioxide monitoring device,linked to an automated fresh airintake valve in the process air duct.Seek further technical advice if thisoption is used.Below: Tasmanian Seed Conservation Centre,Australia - dry room with sorption dryer unitand -20C chiller cabinets for seed storageBelow: Dry room at The Lombardy Seed Bank,University of Pavia, Italy, with collections instacked cratesComparing seed drying facilitiesDrying capacity Cost (UK prices)Purpose-built dryroomin excess of 500L ofseed material30,000 (convertingan existing cold roominto a dry room willbe cheaper)Bespoke dryingcabinet with desiccantdehumidifier unit500L of seed material 15,000Incubator-dryer about 100L of seedmaterial2,500-4,000(incubator purchaseprice)Silica gel in a 60 litredrumup to 25L of seedmateral200In order to avoid dehydration, staffshould only work in the dry roomfor a maximum of 2 hours andshould then drink plenty of waterwhen leaving the dry room.Another problem associated withoperating a dry room is a build-upof electrostatic charge due tothe dry conditions. Use anti-staticfootwear and/or an anti-static gun,especially when dealing with tinyseeds. All room fixtures should bewell-earthed 2. If the total uncleaned peak seedvolume is sufficiently small, (lessthan 25m) one or two incubator-dryersmight be a more appropriateoption for drying.It is helpful to mark out thedimensions of the proposed dryroom on the ground. Ideally, locatethe dry room close to, or attachedto, the seed cleaning area andadjacent to the cold room(s). Anoblong-shaped room is betterthan a square one. Supplying dryair on one of the short walls andextracting it from the opposite wallwill give a good air flow to mostparts of the room (though deadspots will exist).Depending on the frequency ofaccess to the room and the humidityof the external conditions, an airlock (small lobby) may be requiredto minimise the entry of humid airinto the room when the door isopened. A 2m air lock will be largeenough for at least one personholding a crate to enter easily.Include a secondary (emergency)exit door from the dry room.What materials should be used?Ideally, install a new prefabricatedstructure, either as a stand-aloneroom or within a larger existingroom. Construct walls and ceilingwith Styrofoam panels (100mmthick). For the floor, cover amoisture-proof membrane withStyrofoam insulation, and top withmarine plywood and epoxy-resinscreed. Doors must be well insulatedand windows (including those in thedoors) should be double-glazed.If an existing brick- or wood-builtroom is to be used, install amoisture barrier (perhaps glazedtiles or plastic sheeting) on allinternal surfaces. Install thermalinsulation on the outside of thewalls (and ceiling). Both will reduceenergy usage in maintaining the dryroom conditions and the externalinsulation should help minimisecondensation in the walls.How is the air dried and cooled?The drier must have enoughcapacity to deliver about six airchanges per hour to the dryingroom. This is often the limitingfactor. To estimate the likelysuitability of a dryer, compare itsairflow capability with six times thevolume of the dry room. Confirmthis with the supplier/installer. Notethat refrigeration dryers are notdesigned to deliver this condition.The MSB recommends a rotarysorption dryer containing a silica gelor lithium chloride drying agent.If possible, install two dryers, eachcapable of coping with 66% of themoisture load.The dryer creates heat during thedrying process and should ideallybe placed outside the room (in aventilated area). Connect the drierto the room via insulated supply andextract ducts, of sufficient diameternot to impede air flow.Possible layout of dry room facilitiesThe direction of the air flow is indicated by the arrows.This process air dries the room andshould be supplied, via a chiller,high up at one end of the room andextracted low down at the otherend of the room. The drying agentpicks up moisture from the processair and is then re-dried by thesecond (reactivation) air flow. Airis drawn into the dryer, heated, andpassed over the drying agent. Thewet air produced by this processneeds to be ducted to the outsideof the building.The location of the controllinghumidistat is important. Place thehumidistat halfway along oneof the longer walls of the room.Construct a map of humidity inthe working dry room, using an RHmeter to guide where to best locatethe stacks of crates for drying.If the dryer is located within thedry room, ensure that the processair mixes rapidly with the coolingsystem air. Duct the wet reactivationair out of the room. The chiller willneed to be located in the room andbe capable of cooling the heat loadfrom the dryer, achieving a roomtemperature of 15C.Reactivation airflowProcess air flowPrefabricated dryroom constructedwith 100mmStyrofoam panelsHumidistatDryerChillerFlourescent lights


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