Process arrangement in the short circulation of a paper machine

Abstract

A process arrangement in the short circulation of a paper machine comprises a feed pump (10), a headbox (11), means (12) for passing a pulp suspension jet (J) in a closed space from the headbox (11) into a forming gap defined by two wires (13, 14), dewatering elements (15, 16) placed against the wires (13, 14) a dilution duct (17) for passing wire water from the dewatering elements (15, 16) to stock dilution, and an outlet duct (18) for passing excess wire water out of the short circulation. The dilution duct (17) and the outlet duct (18) are in flow communication, and the outlet duct (18) has means (19, 22) for keeping the pressure in the dilution duct (17) even. For controlling the pressure in the short circulation of a closed former, excess wire water is passed out through a tank (19) provided with an overflow or other control.

Description

CROSS REFERENCES TO RELATED APPLICATIONS [0001] This is a U.S. national stage application of International application No. PCT / FI03 / 00327, filed Apr. 25, 2003, the disclosure of which is incorporated by reference herein, which claims priority on Finnish Application No. 20020826, Filed May 2, 2002. STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT [0002] Not applicable. BACKGROUND OF THE INVENTION [0003] The invention relates to a process arrangement in the short circulation of a paper machine comprising a feed pump, a headbox, means for passing a pulp suspension jet in a closed space from the headbox into a forming gap defined by two wires, dewatering elements placed against the wires, a dilution duct for passing wire water from the dewatering elements to stock dilution, and an outlet duct for passing excess wire water out of the short circulation, which dilution duct and outlet duct are in flow communication with each other. [0004] The inv...

AI Technical Summary

Benefits of technology

[0008] An object of the invention is to make it possible to maintain a higher back pressure than heretofore in the short circulation of a closed former, which enables a substantial reduction in the energy needed for pumping in the short circulation.

[0010] No vacuum need be arranged for the dewatering elements of the closed former because the pressure of the pulp suspension fed from the headbox is sufficient for removal of water through the dewatering elements. As there is no direct connection with the ambient pressure, the overpressure of the wire water can be utilized effectively in the short circulation, which reduces the energy needed in pumping. An outlet connection from the short circulation to the atmospheric pressure is arranged through an outlet duct and a tank provided with surface level control and placed at a level that is substantially higher than the level of the former. Alternatively, the difference in levels can be replaced partly or totally, for example, with valves or the like.

[0012] The arrangement in accordance with the invention reduces considerably the number of devices needed in the short circulation, which means that the arrangement in question is very economical in its investment and operation costs. From the point of view of stock cleaning, the situation is also excellent since the stock can be screened at high consistency before it enters the short circulation, thus allowing low flow rates and a low pressure to be used in screening. In production efficiency, advantages are achieved because of very quick grade change.

Problems solved by technology

However, thick stock may bring air with it to the short circulation, which air is apt to remain circulating in the process, so that the short circulation may, in the course of time, become saturated with air.

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