In many applications, such as aging processes, validation tests, or plant growth research, a specific flow rate of humid air is essential to create the desired environmental conditions in a test chamber.
There are different solutions to achieve this goal, including controlled evaporation and mixing systems. The advantages of these systems over more traditional bubbler systems will be illustrated below.
How a Bubbler System Works
Small concentrations of humid air can be generated using a bubbler system. This conventional method requires optimal pressure and temperature control. A bubbler level measurement system includes a source of compressed air, an air flow restrictor, a sensing tube, and a pressure regulator, which converts back pressure to provide an output to a controller that calculates the liquid level.
The quality of the humid air depends on the theoretical calculation of the degree of saturation of the air passing through the liquid and the accuracy of the pressure and temperature regulator. This approach makes it difficult to achieve a specific moisture content in the air.
Bronkhorst CEM System Configuration
In addition to the bubbler method, Bronkhorst has developed a CEM system, based on Controlled Evaporation and Mixing, for humid air applications. This CEM system represents an innovation in vapor delivery, using a liquid flow controller (LIQUI-FLOW or mini CORI-FLOW), a gas flow controller and a temperature-controlled mixing and evaporation device.
Advantages of the CEM System over the Bubbler System
Compared to the conventional bubbler system, the CEM system offers a more direct and simple approach. In theory, any concentration can be achieved in a few seconds with high accuracy and repeatability. The relative humidity can be adjusted between 5% and 95%. The moisture content is precisely controlled by the liquid flow controller, while the air flow rate is regulated by the gas flow controller. The mixing valve on the top of the CEM allows proper atomization of the water in the air flow. Due to the low-pressure ratio of the water mist in the air, the water can evaporate at a low temperature in the spiral heating tube at the outlet of the mixing valve.
Insights into CEM System Configuration
The configuration of a CEM system essentially comprises:
- A mass flow controller for gases for the measurement and control of the carrier gas flow rate (e.g. EL-FLOW Select series).
- A mass flow meter for liquids for measuring the flow rate of the liquid source (e.g. LIQUI-FLOW series, mini CORI-FLOW series).
- A temperature-controlled mixing and evaporation device (CEM) for controlling the liquid flow rate and mixing with the carrier gas flow rate, resulting in total evaporation; complete with temperature-controlled heat exchanger to add heat to the mixture.
Bronkhorst CEM systems are available as complete solutions, including control electronics, offering total flexibility in creating a vaporization solution in almost any situation.
For more information on CEM technology, visit the “Vapor Flow Control” section of the Bronkhorst online catalog, where you will find all the products and applications for vapor control.
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