The Light Green Machine Institute

29 Jun 16: Experience in a vacuum
Neil McCubbin offered this after last week's column on vacuum pumps:

I am sure there are lots of unnecessary, or at least oversized, vacuum pumps on paper machines.
I used to do a lot of detailed mass and energy balances on paper machines, mostly looking for ways to reduce water consumption, fiber loss and energy consumption.
I noticed a huge range in vacuum pump capacities for similar service in different machines.   I was never able to find an expert (mostly the design engineers relied on pump vendors) who could provide a rational design basis.
In the case of the (normally large) pumps designed to remove air from stock (deculators etc.)  I wondered about the energy lost by evaporating water.  The balances showed that newsprint machines were effectively being run with sufficient steam addition to the wire pit to cause the quantity of water vapor evaporated to load up the pump.  Bigger pump, more steam, more heat to sewer.

Most operators were horrified at the idea of reducing vacuum pump capacity.

The "experts"  (mostly vendors), seemed uninterested in Btu and kilowatts. 
In one case that allowed simple empirical verification of the situation, the machine had two 400 HP pumps in series.  The operators said that occasionally one kicked out, but the machine ran OK without it.  Management wanted to run both, as insurance against lost production if a single pump kicked out.

The combined cost of steam to keep the stock hot, due to the excessive evaporation, and the motor power was over $250,000/year at the time, about $2.50/ton product.  This one energy wasting pump was cutting into the profit margin in a significant way, as well as having wasted capital by installing it.  All that was needed was one push of a button to save the money, but it did not happen.

Other vacuum pumps on the then very modern machine were also unusually large.
This machine, and its twin, were two of the earliest modern (as opposed to old and decrepit) newsprint machines to shut down as the newsprint market declined.  They had been expensive, highly publicized, machines when built a few years before I worked on them.  No doubt the giant vacuum pumps with all the associated necessary building space, energy and water supply etc. contributed to the high capital costs.

Neil McCubbin

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