Luquid ring pumps ( later LRP) are widely used at paper mill vacuum systems. They are known to be very reliable, but sometimes capacity losses ( efficiency losses) can take place because of wearing of the pump, traditional vacuum level control, lack of seal water or too high temperature of the seal water.
Air flow measurements were carried out at a paper mill wet end vacuum system, and results were compared to pump characteristics and theoretical estimation of the capacity loss due to the high seal water temperature ( seal water cooling tower at the mill had been bypassed).
Measurements corralate well with the theoretical calculation, and electric energy saving potential could be identified ( if the seal water temperature is first controlled down to the normal level).
1. Open seal water system
So called open seal water system is often used at the PM vacuum systems. This open system is very common at Scandinavia and northern parts of America, where the fresh water consumption is not limited.
Often the seal water temperature is controlled with cold fresh water make-up, and typical setpoint is about 30 C ( about 90 F). If water separation is in a good condition, the seal water is not contaminated and can be directed back to environment without further threatment.
Picture 1: Open seal water system
2. Closed seal water system
In many areas cold fresh water resourses are limited, and the seal water system needs to be closed. About 10 % make-up water can be recommended also at closed systems.
Good desing temperature of seal water is about 30 C, but at warm climate it is not always possible, and higher operation temperatures has to be accepted ( up to 35 C ... 40 C ).
Water separation is needed also at closed systems to avoid contamination of the seal water loop and plugging problems of the cooling tower. This is often the reason, why the cooling towers are not operating porperly, and have even been bypassed.
Picture 2: Closed seal water system
3. Effect of the seal water temperature to vacuum pump capacity
Seal water temperature has a clear effect to the capacity of LRP ( compare for example to the link: http://www.dekkervacuum.com/static4/service.asp and picture 3 ).
Picture 3: Vacuum pump capacity and operation temperature
4. Process temperature
The temperature of the incoming air to to vacuum pump is typically about 37 C ( from 30 C up to 40 C). Higher air temperatures ( over 40 C) can be measured only at the beginning of forming section, where air flows are small and air has contact to the warm white water.
Sometimes the PM headbox temperature is over 50 C, but the air flow through the web and felts however cool the process, and also the paper web temperature at press section is normally under 40 C.
Thus for exampe 37 C ( 100 F) is typically a good design temperature for the incoming air to the vacuum pumps. A model was developed to estimate the effect of seal water temperature to the pump capacity. Calculation is based on change the water vapour pressure at different temperatures, and it seems to correlate well with picture 3.
Picture 4: Water vapour pressure
5. Air flow measurements at the paper mill
Benchmarking of the PM of the vacuum system indicated high SEC ( Specific electric Energy Consumption), in average about 100 kWh/tn. Compare also to the blog, Oct 2012. Vacuum system study was carried out at the mill, and air flows from the PM and temperatures were measured.
This mill has a closed seal water system ( compare to the picture 2), and for some reason the cooling tower was bypassed. Because of the high process temperature and warm climate, incoming air temperature to the vacuum pumps was at high (about 40 C) and pump surface temperatures about 50 C. The seal water temperatures were over 55 C ( up to about 60 C).
Measured air flows were compared to the vacuum pump design figures, and are presented at the picture 5. Red and yellow curves are calculated with the model, and red dots are the measured air flows from different positions of the paper machine.
Picture 5. Pump capacity compared the desing.
Air flow measurements were done with pitot pipe, thus accuracy in mill conditionds can not be very good. However, the measurements and the capacity estimation cleary seem to correlate with each other (picture 5).
High seal water temperature decreased the pump capacity at high vacuum positions (vacuum over 50 kPa) in average about 20 %.
7. Energy saving potential
In this example mill the NRL load of all vacuum pumps is about 4,0 MW ( installed motor load about 5 MW).
If the seal water temperature is decreased by 10 ... 15 C, capacity of the pumps increase and the pump speeds could be correspondingly decreased to save energy. In practice rotation speed optimization is needed at high vacuum positions only. Alternatively vacuum levels at the PM will increase ( = if pump speeds or vacuum level controls are not updated).
Already 10 % capacity improvement in average equals to 400 kW savings which equals annually about 200.000 € savings ( energy price 60 €/MWh).
However, the first action would be get the seal water system and water temperatures under control. Only after it, further optimization can be considered.
8. Summary and conclusions
Water vapour pressure increases exponentially when the temperature increases, thus the vacuum pump seal water temperature should always be under 40 C (favourably at 30 C).
High seal water temperature ( over 40 C) is harmfull especially at high vacuum pumps ( = vacuum level over 50 kPa).
Water separation must always be adequate, to avoid contamination of the seal water.
References and links
1. Effect of seal water temperature to the vacuum pump capacity
2. Estimation of pump capacity on-line as a function of seal water temperature
www.kgu.fi ( these pages are under consruction)