Chemistry of pulp and paper making . s to do the same amountof work, and in winter the water used in it has to be heated, whichadds to the expense. This type of barker is suitable only wherevery clean pulp is not essential or where bleached pulp is to bemade as it does not remove all the thin inner bark. It is bestapplied to wood which has been in the water two or three monthsas the bark is then more readily removed. Of the drum barkersthe intermittent give better results than the continuous. With any system of barking the logs should be inspected onthe conveyor as they go to the chipper, and
Chemistry of pulp and paper making . s to do the same amountof work, and in winter the water used in it has to be heated, whichadds to the expense. This type of barker is suitable only wherevery clean pulp is not essential or where bleached pulp is to bemade as it does not remove all the thin inner bark. It is bestapplied to wood which has been in the water two or three monthsas the bark is then more readily removed. Of the drum barkersthe intermittent give better results than the continuous. With any system of barking the logs should be inspected onthe conveyor as they go to the chipper, and any with bark remain-ing in cracks or around knots put at one side to be cleaned by WOOD AND ITS PREPARATION 159 hand. Some operators clean very little by hand but put thedefective wood at one side to be cooked separately into secondgrade stock. This necessitates a very thorough cleaning up ofall apparatus afterwards. The barked wood next goes to the chippers which should berun slowly enough to produce even chips. The length of chip. Fio. 21. Pulp Wood Chipper depends on the method of cooking as well as the kind of hemlock they should be | to f inch long while spruce ischipped about as follows: For Mitscherlich unbleachable ij to if inches long. For quick cook, newsprint stock f to i inch long. For easy bleaching f to f inch coarse chips are generally further reduced in size by somesort of crusher and are then screened to remove coarse pieces,which are rechipped, and sawdust, which goes to waste. The saw-dust from chipping should not amount to more than 3 per cent. The knots may be removed from the chips in several ways; onevery successful process is to blow them against an inclined wire l6o THE SULPHITE PROCESS screen through which the dust and fine material passes while thegood chips shde down the screen and are collected on a proper adjustment of the air blast the knots, being heavierthan the good chips, fall short of the screen and are collecte
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