2756 



Chapter 23 



GUN CHAMBER 



OPERATING FLOOR 



HYDRAULIC 



PISTON FOR OPENING 



AND CLOSING PORT 



DISCHARGE PIPE 



HYDRAULIC VALVE 



GROUND FLOOR 



OPERATING LEVER FOR 

 CHIP INLET VALVE 



HIGH PRESSURE STEAM VALVE 



INLET FROM HIGH 

 PRESSURE BOILER 



OPERATING LEVER FOR 

 HYDRAULIC VALVE 



SUPPLY FROM PUMP 



OVER FLOW TO TANK 



TO CYLINDER 



;y:j:^^:K(;■^v^.^:■:•V::^v^:■^:.:/;^^n->.vvr^■:v^-,.^/..:^^ 



Figure 23-9. — Cross section of Masonite gun. (Drawing after Mason 1927.) 



release of internal pressure; at the same time they are forced by the expanding 

 steam through the slotted bottom port plate where they are shredded into fiber 

 bundles. Steam and fibers are separated in a cyclone. The entire cycle from chip 

 charging to fiber discharge requires about 60 seconds. 



Effect on wood. — In the Masonite process the foregoing pressure program, 

 while typical, can be varied considerably. Part of the wood substance becomes 

 soluble, and the lignin bond is chemically and physically weakened, allowing 

 fibers to separate on decompression; fibers also darken from thermal degrada- 

 tion. The dissolution of part of the wood substance is due to hydrolysis of the 

 hemicellullose under the catalytic action of acetic acid. The hemicellulose 

 breaks down to water-soluble sugars (hexoses and pentoses) which are removed 

 from the pulp by washing. The degree of hydrolysis and the extent of wood 

 losses can be controlled by modifying the gun cycle. The catalyzing effect of 

 acetic acid, which is believed to be generated by cleavage of acetyl groups of 

 hemicellulose at steam pressures between 300 and 400 psi, is reflected as a 

 sudden rise in the hot water extractability (fig. 23-10). 



