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Blog series: How to handle low liquid flows? Part 1

January 21, 2020 Ron Tietge
Laboratory tubes

Part 1: What are low liquid flows?

What do micro reactors, catalyst research and odorant dosing have in common? Well, they all require the handling of low liquid flows. In the world of flow control & measurement, we distinguish between ‘low flows’ and ‘high flows’. But what does this really mean? Bronkhorst High-Tech is a renowned supplier of flow meters and flow controllers in the ‘low flow’ range. So, time to explain what we mean when we refer to ‘low liquid flow’.

We have therefore prepared a blog series with recommendations for a low liquid flow setup. Besides low flow definition and tips for flow meter selection, these blogs also give advice on system lay-outs, connection material and liquid supply systems. Because flow setups and process conditions are rarely the same for different customers; there is no one fix for all available. Providing the best advice requires insight into the customer application.

What are (ultra) low liquid flows?

The definition of 'low' is arbitrary and depends on the area of business. In bulk industry, flows of much less than 500 kg/h are considered low flows, whereas in research this term is attributed to flows that are smaller than 100 g/h. The current blogs focus on handling - measuring as well as controlling - liquid flow rates up to 100 g/h. We also focus on ultra low flows - which we define being in the range < 5 g/h.

To give you an idea, consider a water droplet. With a typical diameter of half a centimetre, 100 grams per hour is equivalent to about 2000 water droplets per hour - quite low. And 100 drops are an equivalent of 5 grams - to be dosed in our hour.

Accurate instruments for measuring and controlling low liquid flows have proven their use in a wide array of applications. For example: 

In all these cases, the measuring or dosing of the correct amount of liquid - not too much and not too little - is critical for the good performance of the process concerned. 
 

Liquid low flow: Lubricant dosing in airplane manufacturing
Lubricant dosing in airplane manufacturing
Low liquid flow with gas
Commercial gas
Mass flow versus Volume flow, what are low liquid flows

Mass flow versus Volume flow

In the previous paragraph, the flow is expressed in units of mass, such as grams/hour or milligrams/second. However, many users think and work in units of volume. This is fine, at least when we are talking about the same reference conditions. Check our blog ‘Do you know why mass flow reference conditions matter?’ to learn more about reference conditions.

Learn more about mass flow reference

What is so typical about low flows?

How is a low liquid flow of less than 100 g/h different from 'normal' or high flows? Well, (ultra) low flow applications involve some phenomena which are not observed in or are not relevant to larger flows. Due to the (very) small amount of liquid that is being moved, (ultra) low flows are so sensitive that even the tiniest disturbances in process or ambient conditions can have a massive effect on flow stability. The influence of external conditions on flow stability is therefore key here - as well as the means to control these external conditions. For example, even small leaks of gases or liquids into or out of the process have a considerable influence on the intended liquid flow. Furthermore, any obstruction caused by solid particles or contaminations in the small liquid flow lines will obviously reduce the flow in an undesired way. For low liquid flow dosing in particular, unstable pressures will lead to unstable flows. Variations in pre-pressure, pulsation due to excessive pump stroke volumes compared to the flow rate, and dissolution of gas (pressurised air) when pressurising the liquid to be dosed will all result in an unstable flow. 

Knowledge of the application as well as the physical transport phenomena of the process are essential to deal with this complex matter of low flow handling. Optimising flow stability and performance of fluid systems requires in-depth knowledge of fluid characteristics and system components in a wide range of circumstances. Every component used in a fluid system can affect the behaviour of a fluid or interact with other components, especially when it comes to low flows.

Solutions for optimal performance

In the Bronkhorst range of products, thermal-based μ-FLOW and LIQUI-FLOW mass flow meters and controllers, as well as Coriolis-based mini CORI-FLOW ML120 and mini CORI-FLOW M12 device, are particularly suitable for (ultra) low liquid flow applications. Where a mass flow meter consists of a sensor that only measures the flow rate of the medium, a mass flow controller combines such a sensor with a control valve to control the medium flow rate. Check out the ‘mass flow controller theory’. 

Flow controllers are typically used to generate a stable flow. However, optimal performance requires a good deal more than just an excellent flow controller. For example, make sure that there are no leaks in the setup and use small volume tubing. Moreover, in pressurised containers, avoid using gas that dissolves in liquid, or use means to remove this gas. In the next part of our blog series, we will discuss these and other matters in more detail by focusing on practical tips on how to select the right low flow device.

Would you like more information about low flow?

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