Friday, September 19, 2014

Efficiency losses at vacuum level control of PM vacuum system


Vacuum level is traditionally controlled at the vacuum pump system with bleed air. When the pump capacity does not match with PM requirements, these bleed air valves are open, and system efficiency is poor. 

At a blower system the bleed valves can also be open, if PM operation conditions have changed and the blower is operating at the minimum load.

This article describes a method for estimating the efficiency loss at vacuum level control of PM wet end vacuum system. Corresponding application can be found from KGU AppStore, application 3.3.

For details, see: 

1. Vacuum level control with bleed air

Paper mill vacuum systems can be divided into two categories:
   - Liquid ring pump ( LRP / Nash ) systems, and
   - Blower systems.
Traditionally the vacuum level control and operation of pump systems has been designed to be as simple as possible. Typically one suction position at the paper machine is served with one pump, and the vacuum level is controlled with an automatic "bleed" valve ( = pump systems). 

Picture 1: Principle of vacuum level control ( pump system)

Operation principle of blowers is different, and bleed losses are not so common. Diffusor control enables operation in a wide capacity range and simultaneously keep the vacuum at required level (  compare to MAN Turbo RT blowers ). 

However, sometimes requirements at the PM have changed, and also the blowers may operate at the minimum load, and bleed valves at the suction side of the blower can be open. This may happen for example when the running conditions of uhleboxes  have changed during the years, vacuum levels are lower compared to the design ( = nip dewatering ).

2. Estimation of bleed air flow at vacuum level control

Air flow through the valve can be calculated based on the valve characteristics. For example NelProf is one good tool for this estimation ( compare to picture 2).

Picture 2: Example of valve characteristic ( NelProf)

Valve type, opening angle and vacuum level are needed to calculate the air flow. We are using butterfly valve at our air flow calculation (KGU Applications), because the butterfly valve is very common at the vacuum system. Our model gives a little smaller flows compared to Nelprof, but correlation is good ( = we want to be on the safe side and we do not want to over estimate the energy saving potential at our applications).

3. Electric energy saving potential

Energy loss of the bleed air flow can be estimated based on the specific energy consumption of the vacuum pump  or blower (compare to the picture 3).  Our applications use  in feasibility estimates the red curve, which gives the best available ( =lowest ) SEC.  In practice, the efficiency of vacuum pump often lower, but again we want to be on the safe side in our energy saving estimates.     

Picture 3: Energy consumption of vacuum equipment

4. Calculation of the efficiency loss

Efficiency loss because of bleed  at vacuum level control can be estimated based on the specific energy consumption of the pump or blower ( picture 3) and bleed valve operation data. 

Calculation is done with the KGU application 3.3 ( Efficiency loss at vacuum level control).

This simple software calculates the bleed air flow, estimated energy loss (kW)  and annual energy cost saving potential ( Euro/year). Compare to the picture 4.

Picture 4: estimated bleed losses ( kW )

5. Summary and conclusions

Many paper mills have been designed during the 20th century, when energy price was low, and often investment cost and level of automation at the mill have also been minimized. 

Today, when environmental issues are more important and energy cost are higher, many new technologies can be feasibly utilized to reduce energy consumption and operation costs. Especially at PM rebuilds and grade change projects it is important to check also the efficiency of existing vacuum system.  

This is one of our first applications , which can be used to identify energy saving possibilities at paper making lines.  In our next article will talk about throttle and pressure losses at vacuum system.

We hope our simple applications will make it easier to identify feasible energy saving improvements  at paper making lines.

Kari U Kokkonen,
M.Sc,  Process technology
CEO,  KGU Engineering Ky