What is moisture management in high-temperature processes? Differences between relative humidity, dew point, and water vapor pressure, and measurement methods

TL;DR

Key points of this article

In processes such as drying electrodes for lithium-ion batteries and degreasing and firing fine ceramics, moisture content affects process conditions and quality.
Relative humidity (%rh) can be difficult to use as an indicator for managing moisture levels in high-temperature processes because the saturated water vapor pressure changes with temperature.
By using indicators such as dew point temperature, mixing ratio, and water vapor pressure, depending on the purpose, you can understand moisture content more concretely.
If it is possible to continuously measure water vapor pressure and other parameters in a high-temperature environment, it may be useful for understanding the conditions of drying and firing process, and for evaluating changes in those conditions.

background

The challenges of "moisture management" in high-temperature processes

Numerical control is becoming increasingly common in various stages of the manufacturing process, including electrode coating, pressing, winding, and inspection.

On the other hand, process, such as drying and firing, have conditions determined by experience and know-how.

In lithium-ion batteries, a slurry containing positive and negative electrode materials is coated onto a current collector, and the electrodes are manufactured through drying, pressing, and other processes. Vacuum drying (absolute drying) after cell assembly is also process where moisture control is crucial.

Even with fine ceramics, process conditions such as drying, degreasing, and firing affect the sintering state and the quality of the material.

On-site, issues such as "Are drying conditions affecting quality?" and "We want to explain the effects of changing conditions numerically" sometimes arise.

One of the reasons for this is the need to understand moisture content in high-temperature environments.

Basic

Why is "relative humidity" difficult to use in high-temperature processes?

When we talk about humidity, most people probably think of relative humidity (%rh), which is used in weather forecasts and the like.

Relative humidity is expressed by the following formula:

Relative humidity (%rh) = Current water vapor pressure ÷ Saturated water vapor pressure at that temperature × 100

Saturated water vapor pressure increases with increasing temperature.

Therefore, even if water vapor pressure is the same, the relative humidity value decreases as the temperature increases.

Furthermore, in high-temperature regions, even if the amount of moisture changes, the change in relative humidity becomes smaller, which can make it difficult to capture changes in moisture content numerically.

In high-temperature processes, relative humidity itself isn't the problem; rather, it's crucial to choose the right indicator based on "what you want to control."

Indications

What should be controlled in high-temperature processes?

In high-temperature processes, there are methods to use indicators that allow for easier assessment of moisture content, separate from the process temperature.

index	What does it represent?	Features
Relative humidity (%rh)	Ratio of water vapor pressure to saturated water vapor pressure	The change becomes smaller at high temperatures.
Absolute humidity (g/m³)	Mass of water vapor per 1 m³ of air	Affected by temperature and pressure
Dew point temperature (℃)	The temperature at which dew condensation begins in the air.	It is used to monitor moisture levels and manage dew condensation.
Mixing ratio (g/kg)	water vapor mass per 1 kg of dry air	Independent of temperature changes
water vapor pressure (Pa)	Partial pressure of water vapor	Basic indicators of moisture levels

Dew point temperature, mixing ratio, and water vapor pressure have the characteristic of being usable as indicators for evaluating moisture state, rather than "the current furnace temperature itself."

Therefore, in drying ovens, degreasing ovens, and calcining ovens, it is important to use these indicators appropriately depending on the purpose, not just relative humidity.

Measurement

The difficulty of measuring moisture at high temperatures

In high-temperature processes, not only is "what to measure" important, but "how to measure" is also crucial.

Many common humidity sensors are designed for use in room temperature environments, which presents the following challenges in high-temperature environments.

The sensor has limitations on its heat resistance temperature.
dew condensation may occur in sampling piping.
Inline continuous measurement is difficult.
The installation conditions become an issue when retrofitting to existing equipment.

Furthermore, changes in oxygen concentration in combustion furnaces and similar environments can affect the accuracy of water vapor pressure measurements.

To address these challenges, hygrometers capable of continuously measuring water vapor pressure in-line in high-temperature environments have been put into practical use.

Another method involves simultaneously measuring and correcting for oxygen concentration, thereby suppressing the effects of changes in oxygen concentration due to combustion and other factors during measurement.

If water vapor pressure can be measured, the dew point temperature, mixing ratio, relative humidity, and other parameters can be computation from that data.

Therefore, the data can be used in accordance with the control indicators required in the actual process.

Applications

moisture management by industry

Lithium-ion battery

Manufacturing of electrodes for lithium-ion batteries

In electrode drying, the evaporation rate of the solvent and the drying conditions affect the electrode structure and quality.

Continuously monitoring moisture content inside the furnace makes it easier to use the data for examining drying conditions, identifying uneven drying, and quantitatively evaluating changes in conditions.

Furthermore, moisture control is an important management item even during vacuum drying (absolute drying) after cell assembly.

Fine Ceramics

Degreasing and firing of fine ceramics

In the degreasing and firing process, the atmospheric conditions affect the sintering state and the quality of the component.

By accumulating objective data such as water vapor pressure, it may be possible to standardize conditions, models know-how, and improve repeatability of quality improvements.

What they all have in common is the idea of understanding "what is happening inside the furnace" using numerical data.

Measuring moisture levels makes it easier to consider quality improvements, energy savings, and productivity enhancements based on data.

Before Adoption

Three points to consider before implementing

point	Organizing
1. What should be measured?	Measurement points where you want to check moisture content, such as the inlet and outlet of the drying oven and the exhaust line of the degreasing oven.
2. How to integrate it	Whether it's a new installation or an add-on to existing equipment, installation location, piping conditions, etc.
3. What will it be used for?	Condition monitoring, quality evaluation, comparison before and after improvement, traceability, etc.

When evaluating the return on investment, it is crucial to first obtain measurement data and understand the current situation.

Clearly defining "which locations to measure and for what purpose" makes it easier to consider the necessary measurement methods.

Summary

Transforming invisible moisture into measurable data

Improving the quality of high-temperature processes does not mean rejecting experience and intuition.

The goal is to measure moisture levels, which were previously difficult to observe, and use that data to reinforce experience and know-how.

In high-temperature environments, relative humidity alone may not adequately capture moisture content.

Therefore, by using indicators such as water vapor pressure, dew point temperature, and mixing ratio according to the purpose, and combining them with methods that can be measured even in high-temperature environments, it becomes easier to understand the drying and firing process with data.

When considering moisture control in high-temperature processes, it's a good idea to start by clarifying "where," "what," and "why" you will be measuring.

FAQ

Which should I use, relative humidity or dew point temperature?

It depends on the purpose. In a room temperature environment, relative humidity is easy to work with, while in high-temperature processes, water vapor pressure, dew point temperature, and mixing ratio can be used as control indicators.

Can hygrometers be installed in high-temperature drying and firing furnaces?

Whether or not installation is feasible depends on factors such as furnace temperature, atmosphere, measurement location, and structure of existing equipment. Since there are measurement methods designed for high-temperature environments, it is important to consider the equipment conditions before making a decision.

Where should I start?

First, we recommend clarifying where in your company's process you want to monitor moisture levels, what you want to evaluate and improve, and then defining the measurement points.

What can we learn by measuring water vapor pressure?

water vapor pressure is an indicator of the partial pressure of water vapor. From the measured water vapor pressure, you can also computation the dew point temperature, mixing ratio, relative humidity, and other parameters depending on your purpose.

We can advise you on measurement points and implementation methods for measuring moisture content in high-temperature environments such as drying ovens, degreasing ovens, and firing ovens.

Contact us

What is moisture control in high-temperature processes? Differences between relative humidity, dew point, and water vapor pressure, and how to measure them.