Energy efficiency of PM vacuum system equipment

 

1. General

There are many reasons, why the electric energy consumption of the PM vacuum system 
varies from one machine to another:

     1. Paper machine geometry

      The amount of suction rolls and suction boxes vary from one machine to another, 
      and the required vacuum level differ depending on the machine geometry.

     2. Vacuum system efficiency
     
     The design of the vacuum system process and vacuum level control  may 
     cause efficiency losses at the system ( bleed air or expansion losses). System
     efficiency losses take place, when the capacity of vacuum pumps or blowers do 
     not match  with the vacuum level or air flow from the machine.

     3. Efficiency of  vacuum system equipment

     The efficiency of equipment itself varies also. At the following is presented one
     example to evaluate the efficiency of vacuum system equipment ( = fans,  pumps 
     and blowers).

2. Specific energy consumption

The required electric energy consumption of fans, pumps and blowers can be presented
as a function of the vacuum level ( compare to the picture 1).

The attached graph has been developed based on the best available technology at 
different vacuum levels, and can not be achieved with one single piece of equipment.
at the whole operation area ( low and high vacuum lelvels ).



   
       Picture 1: Specific energy consumption


The picture 1 presents a simple method for estimating the efficiency of equipment itself, 
and it does not take into account the system efficiency losses.
     - Specific energy consumption is estimated based on the suction conditions of the
        blower or pump ( air flow and vacuum level at the intake ).  

Based on picture 1, a "rule of thump" has also been developed, to estimate the energy
consumption.

      P ( kW) = Q ( m3/s) * p ( kPa) * K                                                    ( 1 )

      Where K = constant, 1,3 ... 1.5

3. Elecric energy consumption and efficiency

Opinions of different vacuum systems are often presented, where one or another system 
is found to be better compared to the other ones. However, if the comparison is done 
based on the electric energy consumption, the result depends on many variables.
The process desgn and control strategy plays also an important role in it.

Overall feasibility of vacuum system improvement ( or new line investment)  depends also
on investment cost, required space ( mill lay-out) and heat recovery. possibilities. Some general quidelines can however also be presented based on electric energy consumption.

   1. At low vacuum positions ( < 30 kPa ) of the PM, a simple fan should always be
       considered first. The efficiency of liquid ring pump is always poor at a low vacuum, 
       and the invesment cost of a simple fan is smallest.

   2. At medium vacuum area ( 40 kPa ... 55 kPa) the the process system design plays
       an important role.  Conrollability must be good when the requirements of the PM vary
       depending on the produced grade, PM speed and felt age. 

       The efficiency of pumps is also acceptable, if the rotation speed is not too high.
       
       Multistage and singlestage turboblowers normally operate at at good efficiency at 
       this vacuum level. Important is to be able to control vacuum and  air flow according 
       to PM needs ( diffusor and variable speed drive needed to avoid system efficiency 
       losses = bleed air and expansion).

   3. High vacuum ( > 60 kPa) can be achieved only with multistage blowers or liquid ring
       pumps. At well designed systems the diference at SEC is reasoably small.

      Sometimes pumps at the older mills have been selected to operate at high speed, 
      and the efficiency can be very poor.

      Multistage blower normally operates at a good efficiency level. However, because of 
      over dimensioning , or variating operation conditions, expansion may  take place 
      ( = vacuum too high at the suction ) and reduce the overall efficiency of the system.
      On the other hand, the multistage blower offers the possibility  for the heat recovery
      ( exhaust air at high temperature).

4. Summary

In general (especially at energy improvement projects) it can not be decided without 
a feasibility study, which system is the best one. 

Even though fans and blowers often have better  efficiency ( low vacuum area), quite 
often 80 % of the energy savings can be achieved with 20 % investment ( 20 / 80 rule),
when the operation of the existing system is optimised and controls are updated 
( instead of replacing them with new pieces equipment).

At existing systems the first action is always to make the benchmarking to find out 
weather the system energy consumption high or low compared to the other machines
( compare to the earlier article, block fall 2012, Benchmarkng ).

This article offers an alternative to find out the reasons for the high energy consumption
at the vacuum system. Comparison of equipment efficiency often requires a more detail
pre-engineering ( air flow and vacuum level measurements at the machine)


                                                                                                         Kari U Kokkonen 
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