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Key Points Revision:

Role of furnish.

Diminishing gains of dryness with increase in vacuum.

Diminishing gains of dryness with increase in dwell time.

Web dryness as a function of basis weight.

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Air Flow Increases With:

Increase in vacuum

Increase in open area or dwell time.

Increase in web dryness.

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General Thumb Rules:

In the suction boxes of wire part, the vacuum must be in increasing order.
More vacuum: higher dryness
More dwell time: higher dryness

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Can I Reduce Energy Required for Vacuum?

Dryness achieved (x) and energy required (y) follow a pattern like this graph. (Please omit the values)

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Strategies:

Apply vacuum in increasing order.
Last suction box operates at maximum vacuum.
Reduction of its open area reduces air flow requirements, hence vacuum power consumption will reduce.

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Strategies:

We generally apply less than 7” vacuum.
For such a lower level, water ring vacuum pump is not really required.
Consider twin/three lobe blower or similar equipment.

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In-Plant Experimentation:

Start with vacuum in increasing order.
Typical values could be-

Check dryness after last box versus peak vacuum, and the effect of peak vacuum in last box on dryness.

Reduction Factor	Box 6	Box 5	Box 4	Box 3	Box 2	Box 1
60%	150	90	54	32	19	11mmHg
70%	150	105	73	51	35	24mmHg

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Effect of Peak Vacuum:

Quite possibly, you may get a peak vacuum level after which increase in dryness is almost negligible.
Initially, the same exercise may be done by throttling the valves.
Consider using a VFD to get desired peak vacuum.

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Effect of Peak Vacuum:

Remember-

Reduction of peak vacuum will also help in getting increased wire life.
Furthermore, drag load on wire will also decrease in addition to the possibility of energy saving in vacuum pump.

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Dwell Time:

For most furnishes and basis weight range of 50-100gsm, 10-15ms is sufficient.
Many mills operate at 50-150ms.
Bagalley Box normally works at 2-3ms.
A couple of graphs on coming slides indicate effect of dwell time.

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Effect of Dwell Time

For 48gsm newsprint

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Effect of Dwell Time

127gsm (experimental data)

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Dwell Time:

As clear from earlier graphs-

Even a single slot suitable for 15-20ms can increase web solids from 5-6% to practically maximum achievable.
Multiple slots, with increasing vacuum help maintain fiber network with lesser open area in higher vacuum zone.

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Roots Blower:

Many mills have installed roots blower and are saving energy.
Some faced problems of pulsation and hammering. Proper layout and piping design is a must to get results from roots blower.

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Typical Savings:

In a small paper mill (40 TPD), wire part vacuum was reduced from 72m3/min(110kW motor) to just 28m3/min(55kW motor) after dwell time was reduced from 150ms to 100ms.
A significant improvement in wire life (nearly double) was observed after modification.
Further reduction in dwell time is under planning.

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How to Achieve this?

Just check your table, and calculate total open area.
Find out total dwell time. Explore possibilities to reduce if it is too high, by shutting vacuum in one or two boxes.
Recheck web dryness after the same. If everything is fine, replace boxes with lesser open area ones.
Or, discuss with some expert.

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Conclusion:

There exists tremendous scope for energy conservation in paper machine wire part.
Proper designing of wire table and optimization of vacuum play important role in energy conservation and wire life improvement.

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THANK YOU

D K Singhal

deveshksinghal@gmail.com