PRESSURE METHOD OF PRESERVATIVE

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PRESSURE METHOD OF PRESERVATIVE APPLICATION N.KANAGARAJ BSF-06-007 BSC[FORESTRY]

INTRODUCTION : 

INTRODUCTION The use of pressure for impregnation of preservatives in the wood are the best suited for uniform and effective treatment of different types of timbers

BASIC EQUIPMENT : 

BASIC EQUIPMENT Pressure cylinder with vacuum pressure systems. These may have heatin coils if creosote is being used Storage and mixing tanks for the preservative solutions

PRINCIPLE : 

PRINCIPLE In these methods, the preservative solution is injected into the wood underpressure with or without the use of initial vacuum either hot or at ambient temperatures. The nature and species of timber being treated has a bearing on the duration andmagnitude of pressure.

MAIN ADVANTAGES : 

MAIN ADVANTAGES a) There is a quick and continuous availability of treated timber. b) The quality and rate of treatment can be monitored and any changes made, if required c) Pressure can be cut off at the appropriate time

VARIOUS PRESSURE METHODS : 

VARIOUS PRESSURE METHODS Full cell process Empty cell processes Alternate pressure method Boulton process Lowry process Rueping process

FULL CELL PROCESS OR BETHELL PROCESS : 

FULL CELL PROCESS OR BETHELL PROCESS is adopted when it is desired to have high levels of absorption of the preservative. involves filling the cells and saturating the cell walls with the preservative. The timber to be treated is placed in the pressure chamber. - charge. The door of the chamber is tightly closed and a vacuum of atleast 56 cm of mercury is created and maintained for about 30 minutes. aim -to force out as much air from the cells as is possible.

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Thereafter, the preservative is introduced into the cylinder with the vacuum working. The vacuum pump is stopped when the preservative has completely filled the cylinder. subjected to an antiseptic pressure whose intensity depends on Ø Species Ø Size of the material being treated Ø Refractoriness of the material

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Due to this pressure, the preservative is introduced into the timber. It is maintained till absorption of the desired extent is obtained after which the preservative is withdrawn from the cylinder. The dripping preservative is removed from the timber by applying a vacuum of 38 to 56 cm of mercury for a period of about 15 minutes. This process is fairly effective if the correct technique has been adopted and the right preservative used in the treatment

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Stages of Treating for the Full Cell Process

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Once the correct retention of chemical has been achieved, excess is then pumped out.

MODIFIED BETHELL CYCLES : 

MODIFIED BETHELL CYCLES A series of modified Bethell cycles is employed by the industry to provide better control of the treatment process, reduce post treatment drip and facilitate faster post treatment drying where applicable.

Empty Cell Process : 

Empty Cell Process The two basic procedures are the Lowry Process and the Rueping Process both require that the timber be placed in an impregnation cylinder.

LOWRY PROCESS : 

LOWRY PROCESS used for some reasonably permeable species or to limit the liquid absorption and omits the initial vacuum of the Full Cell Process. The cylinder is flooded with preservative, the pressure raised and held until refusal is reached. After releasing the pressure, the preservative is returned to the holding tanks and a final vaccum is drawn on the timber to remove excess preservative.

RUEPING PROCESS : 

RUEPING PROCESS is used when it is desired to minimize the retention of preservative liquid and air pressure is applied to the timber prior to flooding with preservative. A normal preservative pressure period follows and when the final vacuum is applied, the expansion of the air in the timber forces out excess preservative liquid.

ALTERNATING PRESSURE METHOD : 

ALTERNATING PRESSURE METHOD This system combines carefully controlled pre-steaming of freshly cut wood with a special treatment process. The steaming process causes some moisture in the sapwood to be lost everywhere. The treatment process is similar to the Lowry Process, except that the pressure is applied and released many times. This provides a pumping action in the wood, allowing the required penetration and retention of preservative to be achieved

ADVANTAGES : 

ADVANTAGES i) The timber need not be predried. It is possible to treat even freshly sawntimber at moisture per cent ranging from 100 to 200, by applying a cyclic pressure schedule. ii) It is convenient, cheaper and faster than other pressure methods.

Double Vacuum Process : 

Double Vacuum Process simple variation -Full Cell process -used for the controlled application of light organic solvent preservatives The timber is enclosed in a sealed container and a partial vacuum drawn. The unit is then flooded with preservative and the fluid pressure may or may not be increased to a predetermined level depending on timber permeability.. A final vacuum is applied to remove excess preservative. The timber is then removed and the organic carrier evaporates after a period of 48 hours, leaving the timber dry.

Double Vacuum Process : 

Double Vacuum Process

High Pressure Treatment : 

High Pressure Treatment This method uses the normal Full Cell treatment process but instead of using the conventional fluid pressure of about 1,4 MPa, the impregnation cylinder is designed to achieve pressures up to about 7 MPa. Oil type preservatives are used at temperatures up to 95 deg. C and penetration and retention can be improved in sawn hardwoods. The commercial use of high pressure treatment is limited and requires a degree of care and experience to avoid collapse and distortion of the timber.

BOULTON PROCESS : 

BOULTON PROCESS is used to condition green or partially green timber in the impregnation cylinder prior to the Full Cell treatment process. It is applied to naturally round timbers to avoid an initial air seasoning period or to green, sawn, difficult-to-treat hardwoods. It may also be used to improve preservative uptake in certain relatively impermeable species. The process consists of introducing hot (95 deg. C) oil type preservative into the cylinder so that a void is left in the cylinder above the liquid surface. While the temperature is maintained, a vacuum of not less than 60 kPa is drawn on the liquid surface for periods up to 10-24 hours.

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