Efficiency loss because of throttle at PM vacuum systems

Abstract

Even though variable speed drives are today more common than before, there exists many older paper machines, where vacuum pump or blower capacity does not match with PM requirements, and vacuum levels are controlled with a throttle valve. Pressures losses can also take place at the suction piping if velocity of  the air too high.  

Sometimes "hidden" energy saving potential can be found from the vacuum systems, because of the pressure loss.  If air flow (m3/s) and pressure drop (kPa) are available, the efficiency loss can be estimated.

A method for estimating the efficiency loss ( kWh/a and €/year) because of pressure drop at suction lines of vacuum system is described in detail in this article. 

1. Expansion if the air (ideal gas law)

Even though simplification of the ideal gas law ( p * V = Constant )  should be well known, it seems to be difficult notice when ( or how ) to apply it.  As an example, when vacuum level is increased from 40 kPa up to 60 kPa, the volume of air increases by 50 %. 

Picture 1: Expansion of air as function of vacuum level

Thus, if vacuum at the paper machine is 40 kPa, and pressure loss is 20 kPa, required capacity of vacuum pump or blower is 50 % higher compared to what is required. In practice, some pressure loss at the piping must anyhow be accepted ( typically about 3 kPa ).  

There exists many examples from paper mills around the world, where valves have been throttled at the suction lines, pressure losses exist at the piping and efficiency losses  because of expansion are high. The reasons vary a lot, but they can be categorized as follows:

• Variation of raw material quality ( recycled fibers ),
• Development of felts and and operation conditions of uhleboxes,
• Grade change of the PM ( high basis weight ),
• Rebuild of the machine has changed  vacuum requirements, but vacuum system not updated,
• Traditional design of automation at vacuum system ( low automation level ), 

Design figures of machines are typically also on the safe side. If losses take place, energy consumption of vacuum equipment ( pumps or blowers) is higher compared to requirements. 

2. Estimation of the cost of efficiency losses 

In order to estimate the energy savings, the characteristics of pumps or bowers should be available. It is however possible to estimate the energy saving potential also based on the specific energy consumption ( = SEC)  of vacuum equipment  ( compare to blog March, 2013 and KGU application 3.2 :  http://www.kgu.fi/downloads ). 

If we use the same figures as before,  SEC of vacuum equipment  increases when the vacuum level is increased from about 50 kW/(m3/s) up to 70 kW/(m3/s).  Five  kPa pressure loss reserved for the piping. 

Picture 2: vacuum ssytem equipment SEC ( kW/m3/s)

As a summary, if pressure loss is  20 kPa ( vacuum at PM  40 kPa), required energy at vacuum system doubles compared to ideal situation.

3. Estimation of air flow

It is important to be able to estimate the economical aspect of efficiency losses. In order to estimate the energy loss and costs, we also need to know air flow ( = not only SEC ). If pump or blower capacity ( = air flow) is known, it should be used in the estimation. Also the design air flows ( from PI diagram or machine supplier) can be used. It is important to notice, that the design figures often are on the "safe" side ( = maximum figures for design).

There exists also one more option. If vacuum level is controlled with a throttle valve, the air flow can be estimated based on the valve size and opening angle. This method can be used, if other data is not available. For details compare to picture 3, air flow through a typical butterfly valve ( Source: Nelprof valve sizing program).

Picture 3: Example of valve characteristic, Butterfly DN-400

At some cases air flow measurements may be needed to verify the air flows. For example when several suction lines are connected to the same header, and individual flows from different positions of the PM are needed, or pump and blower capacities needs to be checked.

4. Annual energy losses

Annual operation  hours and price of energy are needed to estimate the cost of the throttle loss. Typically the market price of electric energy can be used at the estimation, because often mills buy or sell electric energy depending on their own consumption of it.  

The operation conditions and annual costs of the efficiency loss are shown at the summary page of our applications. (For details, compare to KGU AppStore, App 3.4 Efficiency loss because of throttle, http://www.kgu.fi/downloads). 

Picture 4: Annual costs because of throttle loss

Our applications can store up to twelve calculated  positions in the same calculation file, and the flowchart can be displayed one by one.

5. Summary

Throttle losses are often much more difficult to identify compared to bleed losses or efficiency of equipment. That's why they often stay "hidden" and can cause significant unnecessary energy consumption and operation  costs at PM vacuum systems.

We hope our applications will help to identify efficiency losses and estimate feasibility of improvements. For details, see our home pages ( http://www.kgu.fi/ ).

In the next blog we will be talking about efficiency of  felt conditioning at the press section.