Liquids – a precarious state
What is a liquid anyhow? What appears entirely clear (or even sometimes opaque) in everyday life, is a problem of definition in physics. Liquid is the state between solid and gaseous. The viscosity of a liquid varies dependent on pressure and temperature. Both these parameters also determine phase transformation to solid and gaseous. The sensitivity of liquid products makes handling them especially precarious. Brabender Technologie specializes in feeding sensitive substances and creates custom solutions.
The temperature not only determines the general physical condition of a substance, but also its viscosity. At 20 degrees Celsius, water has a viscosity of one millipascal-second (mPa•s). Grape juice has a viscosity of two to five, olive oil 100, honey 10,000 and tar 100,000. These figures change in different pressure conditions. “For this reason we do not have a wide range of standard feeders for liquids, but rather a basic feeder design, which we can customize to match each order. Each of our liquid systems is unique,” Thomas Neuhaus explains. He and his colleague Adrian Baron have been conceptually designing liquid feeder systems for Brabender Technologie for more than ten years.
“The feed unit, the weighing system and the pump, usually a piston diaphragm or gear pump, constitute the heart and lungs of a feeder system. That also depends on the viscosity of the substance,” Adrian Baron uses an anatomy analogy to illustrate his point. Measurement and control technology form the neural pathways. “The peripheral equipment surrounding these core elements are then dependent on substance, quantity and buyer preferences,” Thomas Neuhaus adds. The basic set-up includes the filling valve, which prevents drips. Flexible compensators are located in front of and behind the weighing unit with receptacle and gravimetric load cell. The pump is attached to the lower compensator.
The Coriolis system represents a special feeding case, which Brabender Technologie only incorporates upon request. This involves a flowmeter that operates to a very high level of accuracy and can be integrated directly into liquid flows and piping. It is mainly used where very large quantities of more than 1,000 liters / hour are involved and where it makes sense to use the considerably more expensive Coriolis system rather than scales. In system design terms this version features substantial differences to other solutions, since it does not include an independent weighing system. The meter is positioned directly behind the pump and determines the speed of its motor.
A question of the right pump
Piston diaphragm pumps, which function in a similar way to heart muscles, are normally used for low-viscosity substances. A piston generates a vacuum, which raises a diaphragm, allowing liquid to flow in through an inlet valve. The countermotion produces a contraction and the liquid flows out of the outlet valve. Compared to simple piston pumps, the benefit provided by these pumps is that the liquid to be conveyed does not come into contact at any stage with the drive mechanism and therefore with lubricants or residues. This is extremely important when handling foods and for hygienic processes as well as for a wide range of chemical applications.
One disadvantage of piston diaphragm pumps is their amplitude. The switch between inflow and outflow as a result of piston stroke travel causes a pulsating flow of liquid. “We use pulsation dampers in processes that are sensitive to such flows,” liquids specialist Neuhaus explains. “This involves a reservoir with a diaphragm that separates a volume of gas from the pumped liquid. A portion of the liquid is pressed during the pump stroke into the pulsation damper and discharged again during the suction phase.” The pulsation damper therefore always operates in push-pull mode vis-à-vis the pump. “If the process has to be constant or steady, we use several pump heads or, if feasible, switch pump type.”
This is when gear pumps are used. They are a type of non-pulsation continuous operation pump and are used in processes involving medium to high viscosity. They can also cope with high pressure and high temperatures, are accurate where low flow rates are involved and compact at high flow rates. Brabender Technologie works closely together with the pump manufacturers to provide the optimum pump for each application. Pump types like progressive cavity pumps, control plunger pumps or peristaltic pumps are used far less frequently.
Hot or cold – liquid or solid
Temperature is a key factor in the processing / handling of liquids; after all it determines physical condition and viscosity. Many oils, for example, become significantly more fluid the hotter they get. At certain temperatures chemicals perhaps tend to degenerate or even explode. “Because the correct temperature is so vital, we offer two options for heating up our system.” Adrian Baron illustrates these. “We can give the entire structure a double-walled configuration, meaning that the system can be heated using water or thermal oil. This version is a good option if, for example, waste heat is generated by another application or if explosive substances are processed or handled.” The alternative is electric heating provided by heating sleeves. “The benefit of this method is that each component has its own heating and therefore features its own control loop. That enables, for example, the piping leading from the pump to be heated to a greater extent than the feeder unit.” That can add up to 10 to 15 control loops. The heating system ultimately used depends on a range of factors, such as existing peripheral equipment or the user’s power supply.
Every order kicks off with a datasheet, which specifies the properties of the liquid and production set-up requirements. It determines the structure or design of the plant and the materials that can be used. Stainless steel and PTFE (Teflon®) are very durable, but individual seals and other synthetic components have to be selected to match the product being handled. “Sometimes customers do not want PTFE to be used, so then we have to be very creative. Such special requests of course have an impact on delivery lead times,” Thomas Neuhaus remarks. As a rule the total lead time for a liquid feeder unit from ordering through commissioning is between 16 and 20 weeks. Brabender Technologie manufactures over 100 of these units a year – and nearly all of them are one of a kind.
Variations include, for example, mobile feeders for laboratories – this is a special detail to protect sensitive load cells, to shield grounding plates from interference caused by ambient vibrations. There are manual filling versions for low to micro quantity performance requirements starting at around 50 grams / hour. Solutions featuring clamp connections for faster dismantling of piping components make cleaning more convenient. “We make receptacles of between 50 and 800 decimeters in volume, for example if liquids are delivered in barrels and feeder processes are best not interrupted by having to change barrels,” Thomas Neuhaus adds. Furthermore Brabender Technologie also manufactures ATEX-compliant versions for use in conjunction with flammable substances or in explosion-prone environments.
Standard remains the exception
The new standard version S-Type is the exception. “This means we offer a simple, compact, inexpensive feeder for liquids, which, although it cannot do everything, is adequate for a wide range of applications,” is how Horst Vohwinkel, General Manager of Brabender Technologie, explains the company’s model strategy. “This standard device features a more compact design with fewer components. For example the pump is located above the receptacle and sucks up the liquid, meaning a simpler inlet detail. Furthermore this design saves an enormous amount of space.” Since a changeover from one liquid to another is considerably more complex with this set-up, this device is particularly suitable for long-term production facilities that do not require any flexibility and many extras. “Here we are deliberately offering just a few options. We have a proven system for custom solutions, so this device is only suitable for simple, standard applications.”
(published in FLUX 1/2016)