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A sensing system that now makes it possible to measure the energy of steam

Steam quality measurement system

Steam is widely used in industrial plants and factories to produce thermal energy. Steam quality, which is indicated by “dryness fraction,” has a great effect on energy efficiency. However, it is commonly believed that measuring steam quality accurately by a simple method is quite impossible. Azbil Corporation approached this problem by focusing on the difference between steam and water in terms of light absorption characteristics. By exposing steam flowing in pipes to near-infrared light, Azbil has developed a new technology that can calculate the dryness fraction from the observed light absorption characteristics.

Background and Needs

A simple and accurate way of measuring steam quality has never been seen before.

High temperature and high pressure steam generated in boilers has been utilized for various applications in industry, such as hot air for heating, hot water supply, electricity generation, thermal sterilization, etc. An H2O medium can easily be obtained, and among other advantages, the steam made in boilers is energy-efficient because the latent heat stored during change of phase from vapor to liquid can be used.

In a steam system, saturated vapor (gaseous phase) is condensated to saturated liquid (liquid phase) by equipment such as heat exchangers in order to efficiently extract thermal energy. If part of the saturated vapor changes into saturated liquid before it reaches the heat exchanger, for example, because of a broken pipe, the latent heat will be lost in proportion to the amount of condensation. In such cases, sensible heat is the only heat that remains to be used, drastically decreasing energy efficiency. So, one of our challenges is to ensure constant and reliable delivery of saturated vapor that has been generated in boilers to heat exchangers.

The more saturated vapor in the steam mixture, the better it is. Steam with a large amount of saturated vapor is considered to be a high quality steam. Quality, indicated by the dryness fraction, is defined as follows:

Dryness varies between 0 and 1. A dryness of 1 (100% gaseous phase in a steam pipe) is the most desirable value. Unlike relative humidity,* which is generally used to refer to the proportion of moisture in the air, dryness indicates the mass proportion of saturated vapor to total mixture.

Dryness is a very important indicator for measuring steam quality. This indicator is helpful not only for managing the system but also for maintaining equipment condition and operation process stability and improving energy conservation. In industry, however, it was believed that measuring steam quality directly in real time is extremely difficult. Only a device called a throttling calorimeter has been used to measure steam quality. However, installing one requires that the steam pipe to be branched, which affects the flow of the steam. Other disadvantages have also been pointed out. For example, it cannot measure steam quality in real time or steam that is close to the atmospheric pressure, and the measurement is not stable.

*Relative humidity refers to the proportion of the water content in the air to the maximum water content the air can contain.

Steam Energy System Applications and Benefits from the “Dryness” Indicator

Key Innovations

Focusing on the difference between steam and water light absorption characteristics, Azbil has invented a dryness measurement method using light absorption analysis.

Azbil Corporation has developed a new steam quality measurement system by focusing on the fact that water exhibits different light absorption characteristics depending on its phase, gaseous phase or liquid phase.

Generally, both gas and liquid have the characteristic of absorbing a different, particular wavelength of light. Therefore, measuring the spectrum of light passing through a gas or liquid helps to estimate the type and mass of that material. The material’s absorbance is proportional to its concentration and the length of the optical path through the material. If the optical path length is known, the concentration of the material can be calculated by checking the spectrum of light passing through it.

As to H2O, it has been known for some time that it exhibits a different absorption spectrum particularly in the near-infrared region, depending on its form, vapor or liquid. At first, the absorption spectrum was found after applying light of a certain wavelength to a saturated vapor or saturated liquid. Then, the concentration of H2O molecules can be calculated to find the mass of saturated vapor. Finally, the dryness can be calculated.

Principles and algorithm for the steam quality measurement system

What we first did was to understand in detail something which previously has not been fully understood, the absorbance spectrum of both saturated vapor and saturated liquid in the near-infrared region. Behavior of steam in a pipe was also not fully understood, so we created a testing facility to study steam flow characteristics in both vertical and horizontal pipes.

Then, based on the light absorption characteristics, Azbil chose a wavelength that is hardly absorbed by saturated vapor or saturated liquid (a reference wavelength) and a wavelength that is easily absorbed by saturated liquid. Using these two types of wavelength, the principles and algorithm for the measurement system were developed.

Verification of the measurement system at a steam pipe facility

A steam pipe facility where the dryness, mass flow rate, and pressure of steam can be controlled was used to verify the compatibility of the measurement system. Through experiments, it was proven that there is a correlation between mass flow rate ratio and dryness within the pressure ranging from 0.5 MPa to 0.7 MPa and the dryness ranging from 0.88 to 1.0. Using this ratio, the system can be verified.

Steam Pipe Facility for Testing

Results and Future Outlook

Accelerated field testing in preparation for world-first practical implementation

Using the absorbance spectrum, Azbil has developed the principles for a steam quality measurement system that can accurately measure steam quality in real-time. We have verified the compatibility of the system by using a prototype facility. Unlike a conventional throttling calorimeter, which requires pipe branching for installation, the new system can be installed in-line. Azbil believes that the system’s real-time measurement method can change the common understanding in the industry. In order to put the system into practical use, Azbil is advancing field-testing for the system. It expects to be the pioneer that introduces a product with this function into the market for the first time in the world.

If the dryness of the steam can be measured, systems that are losing steam energy can easily be identified, leading to energy conservation. In order to reduce the energy loss from steam systems around the world, Azbil is taking on a challenge the industry believes to be impossible, that of measuring dryness accurately in real time.

Why is latent heat energy-efficient?

Wet steam can be in two different states: saturated vapor (gaseous phase) and saturated liquid (liquid phase). All substances absorb or emit heat during phase transition, including the H2O used in the system. For example, 1 kg of H2O at 1 atmospheric pressure (atm) needs 2258 kJ of heat to evaporate. People spray water in gardens and streets in summer to cool down, and the temperature around them actually drops. This is because the heat is absorbed when water changes its phase. In the reverse process, vapor phase to liquid phase, 2258 kJ of heat is produced per kilogram of vapor at 1 atm.

The energy that is necessary for such a phase transition is called “latent heat.” Steam systems use a latent heat of 2258 kJ per kilogram of vapor at 1 atm.

On the other hand, the heat that is stored when a substance is in the gas or liquid form is called “sensible heat”. The amount of sensible heat required is smaller than the latent heat. Saturated liquid water at 100 °C contains as little as 418 kJ at 1 atm. Obviously, latent heat is preferred to sensible heat because there is more energy that can be used.

Latent Heat and Sensible Heat

Latent Heat and Sensible Heat

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