Vapor flow control

In our daily life, we are surrounded by vapors. We inhale it, and we can sense its presence through sight or touch. Another profound example is water vapor as part of the air that surrounds us in our homes. Noticeable if the air encounters a colder surface, such as a window. The contained water vapor may condense on the windowpane and becomes visible and touchable as a liquid.  

Also in industrial processes and laboratories vapors do have a useful role. Time to explain: what are vapors, where are they used, and how can they be delivered in a controlled way to these processes? 



What is a vapor?

A vapor is very similar to a gas - which is a fundamental state of matter, just as solids and liquids are. Gases and vapors consist of separate molecules that move as free particles. However, there is an essential difference between a gas and a vapor. If a compound is a liquid at room temperature (around 20 °C) and normal pressure (1 atmosphere), then we call the ‘gaseous’ form of that compound a vapor. That is why we call the ‘gaseous’ form of water a vapor, whereas the gaseous form of oxygen is a gas, as oxygen is still a gas at ambient conditions. 

How to generate a vapor?

When raising the temperature or lowering the pressure, a liquid can evaporate and convert into a vapor. On a molecular level, at any temperature at the liquid surface there are always molecules with enough velocity to leave the liquid. So, above a liquid there is always a vapor of the same liquid. Evaporation occurs at any temperature, and not just at the liquid’s boiling temperature. This boiling temperature is just a definition: it is a point at which the vapor pressure of the liquid equals the ambient pressure. 

Playing with temperature and pressure are two ways to control the vapor pressure and because of that the amount of vapor or - in a dynamic situation - the vapor flow. A third way to control the vapor pressure - actually reducing it - is by diluting the vapor, for example by adding an inert gas such as nitrogen to the vapor. 

Why use vapors?

There are circumstances in which you may want to add a vapor to a process in a controlled manner. For example, consider fuel cells - PEMFC – of which the electrolytes need to be in a hydrated state (humidified) to maintain a high conductivity and, hence, optimal performance. Read more about this in our customer story alternatively, you might want to supply accurate water vapor concentrations for the calibration and certification of humidity sensors, in order for these devices to display the correct humidity values. 

Another type of vapor delivery is in the controlled supply of metalorganic vapors to a reactor. These vapor compounds act as precursors in a chemical vapor deposition reaction to deposit a thin layer on an object, for example to deposit semiconducting thin films. Accurate vapor supply is necessary here, to precisely control the layer growth - even on complex shaped objects - and to avoid spilling expensive metalorganic precursors.  

Traditional vapor flow control using bubblers

A traditional way to deliver a vapor to a client’s process is by using a bubbler system. Here, a gas flow is bubbled through a heated vessel filled with a liquid compound. This carrier gas flow becomes entirely or partly saturated with the compound vapor, and this vapor flow is further guided by the carrier gas to the client’s process. Although this is quite a simple setup which can be used versatilely, there are a few drawbacks. Small changes in process conditions may give large variations in vapor flow, rendering it a relatively inaccurate delivery technique with a poor long-term stability. Since the vapor pressure largely relies on the vessel temperature, a slight change in temperature will result in a rather large deviation in vapor pressure and thus vapor flow. In addition, the total pressure and the carrier gas flow rate need to be stable to give a stable vapor flow. This vapor flow solution strongly relies on temperature and pressure. 

Vapor process
Carrier gas through the liquid
Vapor flow solution
Vapor flow solution

Improved vapor flow solution via Controlled Evaporation & Mixing

One way to overcome the above hurdles is by making use of a CEM (Controlled Evaporation & Mixing) system for vapor delivery. In this vapor system, a gas mass flow controller (such as an EL-FLOW Select model) provides an accurately controlled carrier gas flow rate, whereas a liquid flow meter (such as a mini CORI-FLOW or LIQUI-FLOW) measures the flow of the liquid to be evaporated, for example drawn from a room temperature pressurized liquid vessel. The liquid control valve, functioning also also as a 3-way actuator, is tasked with blending tiny liquid droplets with the carrier gas. 

The mix fluid enters a heated, temperature-controlled mixing and evaporation chamber where the liquid fully evaporates immediately, and a homogenous vapor/gas mixture is generated.

A complete CEM system can be monitored with an external readout/control unit, including power supply or with a PC by using Bronkhorst software like Bronkhorst Flowsuite for operation of the CEM-system components. A CEM system is a straightforward vapor delivery module that outperforms a bubbler in many ways. The process is independent of pressure and temperature because gas and liquid flows are controlled using mass flow controllers, ensuring precise control of the molar ratio due to the precision and repeatability of the mass flow instruments. This accurate mass flow control provides significant stability of vapor output from the evaporator.
The direct injection of liquid flow into the carrier gas stream enables better dispersion of liquid molecules in the gas and significantly reduces response time.

The evaporation chamber is designed with an internal geometric shape that generates turbulence, thereby enhancing the uniformity of the gas-liquid mixture. Additionally, it serves to heat the fine liquid droplets introduced into the gas stream, facilitating their evaporation.
The mass flow rate of liquid can be easily determined using the online Fluidat on the Net application. Additionally, the relative humidity (%RH) and molar concentration can also be calculated, thus facilitating the utilization of Bronkhorst's CEM evaporation system.

For vapor flow applications: avoid condensation

A vapor can be converted back into a liquid relatively easily by increasing the pressure or decreasing the temperature - something that will never work with a dry gas. One significant challenge in working with vapors is preventing condensation, as it can result in droplets that may disrupt or harm the ongoing process. 

So, without altering the system pressure, keep the temperature beyond the CEM always higher than the temperature inside the CEM and higher than the dew point temperature. Or alternatively, without changing the temperature, keep the pressure beyond the CEM always lower than the pressure inside the CEM.  

Examples of vapor flow applications

Vapor delivery using CEM systems occurs typically in applications such as surface treatment, food & beverage industry, pharmaceutical applications, material study and testing and environnemental research.

  • As an example, a CEM system was used in a sterilization process for aseptic packages. Read the customer story.
  • Moreover, CEM is used to provide an atmosphere with a controlled relative humidity for fuel cell test benches in the automotive industry. Read more about this application.

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