Regular heating processes seem relatively straightforward. The sample just needs to be placed next to the heat source and its temperature will rise. This may be a simple, reliable solution but it is not always efficient. Both speed and flexibility are called for by current requirements regarding drying processes. Such requirements can only be met through the design of a device with its operation based on knowledge confirmed by numerous tests.
Two series of Radwag moisture analyzers have been developed in this way: Moisture analyzers of MA.R, MA.3Y series. Infrared radiation is used by both for the purpose of sample heating.
Infrared Radiation Theory in Brief
Not all emitted infrared energy will be absorbed by the sample, some may be partly reflected or transmitted elsewhere. As such, the sample is not heated directly and may be completely lost in the drying process. The length of infrared radiator's wave, the type of surface onto which IR energy is emitted and several other crucial variables all significantly influence the amount of absorbed, reflected or transmitted energy.
When IR energy touches the surface it gets transferred into heat due to conductivity. Materials with high thermal conductivity, such as metals, quickly and evenly transmit the heat onto the entire volume. The thermal conductivity is low in materials such as plastic and wood. The surface of such materials may reach a high temperature long before the internal temperature is recorded as being high. For this reason, a crust can form on some of the samples' surface.
Aside from glass and some plastics, most of the materials are infrared-impermeable, meaning that the energy is either absorbed or reflected. Glass and see-through plastic films can reflect and transmit a major part of the radiation and, as such, it seems unreasonable to dry these materials exclusively through radiation.
Methods for Heating Samples in the Drying Chamber
An infrared radiator is the standard heat source of MA.3Y and MA.R series and operates in a feedback loop with a temperature sensor, ensuring thermal stability throughout the analysis. One of the elements designed by Radwag that facilitates a short analysis duration and drying series repeatability is the method of dynamic control of temperature in the drying chamber.
As an alternative to an IR emitter, a halogen lamp (HAL) or resistance heating wire can also be installed inside the drying chamber. The method of heat transmission to the sample's surface is the main difference between these heating modules. Waves ranging from 0.76 mm to 1000 mm are emitted by IR and HAL lamps to transmit heat. While both lamps are infrared radiators, they emit waves of different lengths. To distinguish them from one another they have been given different names.
Figure 1. Electromagnetic waves spectrum with range specified for moisture analyzers. Image Credit: Radwag Balances & Scales
In terms of wavelengths, the general classification of radiators is as follows:
- IRS (infrared short) - short-wave radiation, where IR wavelength is 1.2 µm,
- IRM (infrared medium) - medium-wave radiation, where IR wavelength is 3 µm,
- IRL (infrared long) - long-wave radiation, where IR wavelength is 5 µm.
The wavebands actually cross each other, which is quite conventional for this kind of classification. In some scenarios, a certain band may be considered to generate short waves. Yet, in others, the same band is considered to generate medium-length waves. The crucial information concerning the drying processes is the way in which heat gets to the sample. The process of radiation and convection are two phenomena to mention here.
RADIATION is the transmission of heat from one body to another and the amount of transmitted heat depends on:
- Temperature difference between the emitting element (radiator) and the receiver (sample)
- Radiated wavelength
A thermal imaging camera can be used to demonstrate that almost every single body emits radiation. The same also applies to lamps placed in the drying chamber. The hotter the lamp temperature, the more heat is sent to the sample.
Figure 2. Infrared lamp radiation. Image Credit: Radwag Balances & Scales
Figure 3. Radiation spectrum of the hot sample. Image Credit: Radwag Balances & Scales
Image 2 demonstrates how each hot body is a radiation emitter. As such, both the radiator and a major surface of the top panel heat the sample causing moisture content to be released fast.
Heat is transmitted as a result of movement of air particles circulating in the drying chamber. This is known as convection. The warm, thinner air moves upwards and the cool, denser air moves down. Energy is transferred through this circulation.
The effectiveness of the heating solution used is dependent on the heater type, the sample's absorption properties (for example, ability to take up radiation), and the level of reflectivity, (for example, ability to reflect radiation). This raises the question as to what the optimal choice might be.
Longer wavelengths are emitted by infrared radiators (IR) in comparison to HAL radiators. The longer the wave, the lower the value of coefficient of reflexion. This is dependent on the type of sample's surface. More energy passes through the material due to this relationship and thermal energy spreads evenly over the sample’s whole volume.
HAL radiators emit shorter wavelengths than infrared and, therefore, produce a brighter light. The shorter the wavelength, the more radiation is reflected. In terms of the whole transferred energy balance, the contribution of radiation may be estimated at 50%. A comparison of these two radiators is presented in the table below.
Table 1. Convection and radiation conditioned by the heat source. Source: Radwag Balances & Scales
With resistance heating wire, the sample is heated through radiation. Convection will also initially contribute slightly to the heating process. This contribution is greater later on, but it is still almost insignificant. When considering the physics of the process, this heat source is the most appropriate for reference methods requiring the use of the induction heater. Radiators, such as the resistance heating wire, are not commonly applied in moisture analyzers. However, they do guarantee satisfactory effectiveness in the course of the drying process. A delay in the drying temperature and thermal inertia are two major disadvantages. These may cause issues when flexibility, in terms of drying temperatures, and fast operation for many samples are required. The pictures below demonstrate the differences between all the heat sources mentioned.
Table 2. Heat absorption conditioned by the heat source. Source: Radwag Balances & Scales
||Resistance heating wire
|mainly convection surface heating
||convection and radiation deep layers heating
||mainly radiation volumetric heating of the sample
powders, semi -liquid samples, liquid samples
most semi -liquid samples, liquid samples, crushed solids
thick consistence samples and solids
Please note, samples may differ in terms of absorption capabilities. Smooth and even surfaces reflect more radiation meaning less energy is absorbed and the sample takes longer to heat. The sample's color and the reaction to temperature rising (the effect of crust formation) both dictate the effect of absorption. There are no clear distinguishing factors that would indicate which solution is preferable.
The photos present infrared radiators that are used in moisture analyzers. Judging by the intensity of light one can easily distinguish IR emitter from the radiator of HAL type.
Image Credit: Radwag Balances & Scales
Drying chamber's heating system
Before the moisture analyzer start-up, the operator should undergo training regarding safety and functionality issues. This approach is beneficial for the distributor and user. The results of the first drying tests are of no real value when not confirmed by a respective methodology. While attempting to determine the best methods, it is advisable to consult either the manufacturer or the authorized distributor.
It is possible to assess accuracy of drying carried out using the moisture analyzer, however one condition must be fulfilled, namely, the calculated value must be referred to the real (known) moisture content. If such information is unknown, then it must be obtained via test carried out in accordance with recommended procedures.
In case of a product for which there are no standards, the test should be carried out with reference to characteristic features of the sample (type, color, absorption capability, homogeneity etc.). For such objects, initially, drying temperature of 105 °C is adapted, however, there are cases when the tests must be repeated. The reason for that may be, for example, surface burning of the sample.
It can be easily noted that there are certain differences between infrared radiators, but when it comes to accuracy of the drying process there are two crucial factors: the drying temperature and the methodology (sampling, sample preparing process, etc.). This means that similar moisture content results can be obtained regardless of used heat source type (IR, HAL or resistance heating wire).
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