In the world of flow control & measurement, we distinguish between ‘low flows’ and ‘high flows’. But what does this really mean? At Bronkhorst, we supply flow meters and controllers in the ‘low flow’ range. Do you know what ‘low flow’ means? In our blog series, we explain the difference and share our tips & tricks for low liquid flow setups.
You can read the previous parts already published here:
Part 1 “How To Handle Low Liquid Flows”: in this part, we explain what (ultra) low liquid flows are, describe the issues that can occur and provide solutions for optimal performance.
Part 2 “Tips for flowmeter selection”: in this blog, we give advice on how to optimize the stability and performance of your system and provide recommendations about dimensioning, choice of material and best practices.
As indicated in the previous part when we provided tips on how to select a suitable flow meter or controller, a highly stable inlet pressure of a flow controller is necessary to obtain a stable low liquid flow rate. While a gas as a compressible medium can have a cushioning effect in leveling the pressure of a system, a liquid is not very resilient. Although Bronkhorst flow controllers are capable of neutralizing pressure fluctuations to some extent, rapid changes in the inlet pressure can destabilize the flow.
In general, two methods are suitable for providing a stable inlet pressure to the liquid system: using a pressure vessel where gas is applied to pressurize the liquid or a pump. Whichever method you choose is partly a practical decision based on what infrastructure is available at the customer’s site.
A pressure vessel is a relatively safe option without needing electricity or moving parts, and it is a plus for some volatile liquids. However, gas bubbles dissolved in the liquid have a negative effect on flow stability.
The advantage of a pump is that in principle there is no direct contact between gas and the liquid. This means that no gas can dissolve and a pump can be operated continuously. On the other hand, their moving parts make pumps sensitive to wear and they are often more expensive than pressure vessels.
Frequently it is a balance between the volume of liquid to be dosed, the pressure and the application. In a research lab, for example, helium gas for pressurizing purposes may be available, while in a production environment, another solution needs to be found, usually pump controlled. Read Part 4 of this blog series for more details about using a pump to provide a stable inlet pressure.
When using a pressure vessel, the main challenge is to minimize the dissolution of gas in the liquid to be processed. The less gas that comes into contact with the liquid, the better it is. Henry’s law states that the amount of dissolved gas in a liquid is proportional to the pressure of the gas that is in direct contact with the liquid. This has some practical implications. The gas used to pressurize the liquid and that has been dissolved in the liquid at high pressure will be released as gas bubbles at a lower pressure further downstream in the process. This is usually an unwanted phenomenon. Moreover, the solubility of a gas in a liquid decreases as the temperature gets higher. A temperature rise of the processed liquid will therefore also result in released gas bubbles.
In this respect, it is recommended that the pressure and temperature drop over the liquid path is kept as small as possible.
If gas is used to pressurize the liquid, an important recommendation is, therefore, to prevent the gas from coming into direct contact with the liquid. For example, use a pressure vessel with a membrane that physically separates the liquid from the pressurizing gas – in a similar way as an expansion tank is used in your central heating system at home.
If it is necessary or inevitable for the pressurizing gas to come into direct contact with the liquid, there are some solutions. For example: apply gases with low solubility. Helium is usually the best choice for water-based liquids, followed by nitrogen. If possible, apply the lowest possible pressure to the liquid. This obviously depends on the working pressure of the application. In this respect, it may help to position the liquid vessel substantially higher than the flow controller – to let gravity do its work and thus require a lower gas pressure.
As a last resort, use a degasser to remove the gas from the liquid. This is a device with a porous hose to which a vacuum is applied on the outside, causing gas bubbles to be drawn out of the liquid inside the hose. Standard degassers go well with water-like media. For low liquid flow rates below 50 g/h and depending on the type of application, we recommend using a degasser if stability and maximum accuracy are key and if gas bubble disturbances need to be deleted. A degasser is considered an expensive option, but if you have a high-end application, you look for the best available solution. Metaphorically speaking, you don’t buy 25 euro tires for a very expensive racing car, or do you?
If the inlet pressure fluctuations in the pressure vessel setup are too fast for the flow controller to be able to compensate, there are some solutions. A pressure relief or a buffer tank between the pressure regulator and the liquid vessel can smooth out these fluctuations to provide a stable inlet pressure.
For use with water-based media, Bronkhorst recommends having a vessel made out of one piece of passivated steel to deal with corrosion issues of, especially welded parts. With organic solvents like methanol, toluene or acetone, corrosion of welded parts in a vessel is not usually a problem. Because liquid flow control with a pressure vessel is usually a batch process, make sure that the vessel is large enough to provide a stable flow for a sufficient amount of time between refills. To prevent splashing and liquid spills when filling, use a filling funnel at the inlet.
Copyright 2024 © All Rights Reserved Worldwide
