So you need a vacuum system for your paper making machine.- Take a deep breath and read on. Blowers versus liquid ring vacuum pumps, the short list:
- If you need less than 3”Hg vacuum (90 kPa) you want a fan.
- If you need more than 18” Hg (40kPa) you will need a liquid ring vacuum pump.
- If you want a machine that will run for a very long time with little or no attention, you will want a liquid ring pump.
- If you want to run without measuring or controlling anything, you want a liquid ring pump.
- If you want to save energy, you will want a turbo blower.
- If you want to use a blower, you will have to measure, understand and control the blower speed to the process parameters.
The approximate energy comparison of the best turbo blower versus the best very large liquid ring pump is on this chart; note that the liquid ring pump is running at a constant speed, the turbo blower would have to be variable speed to meet this curve:
Blowers vs. Liquid Ring Vacuum Pumps- How they Work
Any air pressure, lower than local atmospheric, is called a vacuum. This is the definition. A machine that can remove air from a chamber, faster than it can leak in, is called a vacuum pump. The vacuum pump gathers air at a low pressure and discharges it to a higher pressure (the atmosphere). It is by definition a compressor.
A paper machine carefully removes water from the paper sheet. Some of this water removal is done by passing air through the sheet, thus moving the water from the sheet to the wire. Air is moved by creating a pressure differential across the sheet. This is normally done by putting the sheet on a wire and then putting a box under the wire and then evacuating the air from the box. The resistance of the air movement through the sheet and wire causes the pressure drop from the machine room to the box.
Two different devices are commonly used to create a vacuum, the liquid ring pump and the turbo blower. Within practical bounds of operation, they work as follows:
The liquid ring pump is a positive displacement isothermal compressor. In thermo dynamic terms, isothermal compression is the most efficient. It is also a constant volume device if it is run at a constant speed. Unfortunately, the liquid ring pump uses water as the piston, and moving the water in the largest most efficient liquid ring pumps consumes one third of the power put into the pump. In smaller, more commonly used pumps, this number is closer to half of the power.
The turbo blower is a constant pressure adiabatic compressor. In thermodynamic terms it is less efficient than the isothermal compressor. However, the enormous energy drain of moving the water in the liquid ring pump makes the turbo compressor more efficient. Also, because the compression is adiabatic (without exchange of heat), the air coming out of the blower is typically 392°F (200°C), there is energy there that can be reclaimed.
In a liquid ring system, the vacuum in the box is created by the resistance to the air flow through the sheet and carrier because the pump is constant volume. As a felt fills and compacts the resistance to flow increases and the air velocity through the felt increases thus increasing the velocity of the air through the felt. This is an automatic compensation for felt filling as the increased air velocity will increase the water removal.
In a turbo blower system the pressure in the box is created by the blower, and the air flow is dictated by this pressure. As the felt fills the resistance to air flow increase however the pressure remains constant and the air flow decreases. The power used by the blower also decreases as it is dependent upon the air flow. In modern systems, the water removal by the vacuum system is measured and the blower speed increased to optimize the water removal. In this sense it mimics the liquid ring without controls.
The liquid ring pump is robust, runs slowly and can handle an enormous amount of excess water and junk running through it. The turbo blower is finicky to control, runs very fast and cannot tolerate any contamination. Failure of a liquid ring pump is usually very slow and predictable. Failure of a turbo blower is usually rapid and catastrophic.
Two of twelve medium-sized liquid ring vacuum pumps on a fine paper machine.
Paper Machine Vacuum System Surveys
Measurement of the water removed and careful experimentation with your product versus water removal is a very good idea, as it will remove variability from your process (remember 6 sigma?). Either the turbo blower or the liquid ring pump can and should be speed controlled for optimum energy performance.
Most paper machine have too many vacuum pumps and also have uncontrolled vacuum systems. The very large energy gains reported by going from a liquid ring system to a turbo blower system are partially caused by elimination of vacuum capacity from the system. In many cases this vacuum elimination can be done without replacing the pumps. A vacuum survey, experimentation, performance measurement and controlling the speeds of the liquid ring pumps will often get good enough results to avoid the higher capital of a complete system rebuild.
For a new paper machine, a hybrid system with fans, blowers and liquid ring pumps is desirable.
The vacuum system must be measured and controlled. I worked on one such system in Europe where after the machine was built with 4 liquid ring pumps with 3000 kW connected and after two years two of the pumps were shut down wit the other two running faster, for a savings of 1000 kW over the original design. Without the experimentation, measurement and controls this would not have happened.
The design of the vacuum system and the troubleshooting of an existing system is complicated but not impossible. Before you undertake this activity, you should study and consult with independent people who have done this before.
So take a deep breath and understand that your lungs work like a liquid ring pump. If you could breathe 250 times per minute you would move about 1800 m³/hr. This is what a liquid ring pump with your lung capacity would do. Call me for the calculation.
About the Author
Andy Smiltneek is an independent consultant and president of Growth Solutions Consultants, LLC., in the U.S. Andy has lived and worked in the U.S. and China and has retired 4 times. He will consult anywhere in the world. Visit www.growthsolutionconsultants.com.
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