6/24/2015 Ch07

http://www.unesco.org/webworld/ramp/html/r8707e/r8707e07.htm 1/2

Contents PreviousNext

5.Alternatemethodsofdrying5.1Vacuumdrying

Thedifferencebetweenvacuumfreezedryingandvacuumdryingis,inreality,amatterofpumps.Inafreezedry system the pump has the capacity to pull a vacuum of 4mmHg and less at temperaturesbelow0C(32F)waterasasolid(ice)can,undercertainconditionsofpressureandtemperature,peestowater as avapor (sublimation).On theotherhand, invacuumdrying thepumphas the capacityofpullingavacuumof5mmHgandaboveattemperaturesgreaterthanOCwaterasaliquidpassestowaterasavapor(evaporation).AgraphicrepresentationofthisphenomenoncanbeseenifanimaginarylineisdrawninFigure1torepresentthelevelsofpressuresandtemperaturescited.

Aswehaveseen,theboilingpointofwaterasaliquidcanbelowerediftheairpressureisdecreased.Thereducedboilingpointatlowpressurefindsconsiderablepracticalapplicationinthefieldofvacuumevaporation (evaporation under low pressure). This process is of primary importance in the augerindustry. Boiling off thewater from the syrup at normal atmospheric pressurewould char the sugar.However,thepressureiskeptsolowthatthewatermayberemovedatratherlowtemperatures.

Intheaerospaceindustry,thermalvacuumchambersareusedtotestthebehaviorofspacecraftsuchasweather satellites under simulated space conditions. Such chambers, rather large in capacity, are veryeffectivefordryinglargequantitiesofwetted(notfrozen)archivalmaterials.Inonesuchchamberthereisapurgedairsystem,amethodwherevacuumpressureat45mmHgisappliedforaboutoneandahalfhours. Then the chamber is purged with dry air of less than 1 percent humidity the air passes overheatersatatemperatureof60C(140F).Thisoperationcontinuesforabouttwoandahalfdaysthenthevacuum is applied again. The cycle lasts for about five days. Steam injection is used to eliminateevaporationfromthewettedmaterials(15).Othermethodsofeliminatingortrappingareavailable,forexample,throughtheuseofrefrigeratedcondensers.

5.2Deepfreezedrying

Thereisconsiderableconfusionovertheuseofterminologytodescribethemethodsavailablefordryingwetted archival and library materials. Therefore, at this point, it may be useful to review the termsnormallyused.

Vacuum freezedrying is theprocess for drying frozenmaterials by sublimation at lowpressure (highvacuum)andattemperaturesbelowthefreezingpoint.Possiblyalessconfusingtermtodescribevacuumfreezedryingisthesingleword"lyophilization."BothtermsareinterchangeableintheEnglishlanguagealthough the former is more in use. However, in other languages, French and Spanish, for example,lyophilizationisthepreferredwordtechnicaldictionariesgiveitaclear,concisemeaning:rapidfreezingofamaterialataverylowtemperaturefollowedbyrapiddehydrationbysublimationinahighvacuum.

Vacuumdrying is the process used to drywetted (nonfrozen)materials by evaporation in a vacuumchamberatrelativelyhigherpressures(lowvacuum)andattemperaturesabovethefreezingpoint.

Deep freeze drying requires only a freezer. It is a nonvacuum processwhich has been described byseveral names: simply freezedrying, ambient freezedrying, cold storage drying, nonvacuum drying.Thedeepfreezemethoddriesasolidbydehydration.Thesamelawsofphysicsthatgovernsublimationapplyherealso.Theprocess isquite familiar to thosewhohaveplacedunwrappedfood in thefreezer

6/24/2015 Ch07

http://www.unesco.org/webworld/ramp/html/r8707e/r8707e07.htm 2/2

compartmentof adomestic refrigerator.Over aperiodof time the foodbecomesdehydrated it froze,acquired a vapor pressurehigher than thepoint of coldest temperature (freezingunit), itswater vapormoleculesweredrawntothefreezerunitandtrappedthere.

Deep freeze as a method of drying is relatively wellknown and has bees used by a number ofconservatorsinthepast.However,thetechniquehasnotcaughtonperhapsbecauseofthetimerequiredtodehydratethefrozenmaterials.Butinarecentlypublishedarticle,Smith(16)describestheuseofacommercialupright freezerwhichwasmodified todry200 to300wettedbooks in two to fourweeks(anditcanalsobeusedtoexterminateinsectsinbooks).Themodelisselfcontained,hasaverticalglassdoor freezer, andwas chosen for availability ofmaintenance centers internationally, and its ability tocope adequately with minor disasters in libraries. The modification include the establishment of atemperaturedifferentialbetweenbookcompartmentandtheevaporator(freezingunit),theeliminationofunnecessary heat sources, installation of temperature controls, indicating thermocouple, signal devicesandtimers.

5.3Naturalfreezedrying

Naturalfreezedryingismentionedbrieflyasamatterofinterestandtoavoidconfusionwiththeothermethods inuse fordrying. It is a technique for a specificpurpose and is limited to thoseparts of theworldwherewinterclimatesprevailet temperaturesbelowfreezing.Notehowsimilar it is to theothermethodsalreadydescribed.

Gratten and McCawley (17) write of the method they call it "exterior freezedrying'. in whichwaterlogged wood is freezedried without the need of a vacuum chamber. They take advantage ofCanadianwinter:thelowtemperature,thedailyhoursofsunshinetooffsetthelatentheatofsublimation,and the wind conditions to help remove the watersaturated air from the frozen object. A tentlikehousing with polyethylene walls protects the object undergoing treatment from undesired exteriorconditions.

Conceivably, frozen documents and books could be dried under the same conditions as waterloggedwood.Theprocesswouldbequiteslow,butitwouldstillbeaformoffreezedrying"aunature!."

Contents PreviousNext