The use of heaters of various types in infrared heating systems

Description of infrared heater designs

Infrared heaters are recommended to be used as a source of directed radiant heat for local local heating of workplaces and production areas in semi-open and open rooms, as well as in the open air, where the use of other types of heating is impossible or ineffective.

To assess the use of electric heaters of various types in radiant heating systems, we select from the whole variety of designs those that, in our opinion, are most applicable for directional heating and heating (see table 1). Below are the results of comparative tests of these heaters, obtained using the characteristics and methods presented on the page of our website "Infrared radiation characteristics - engineering applications".

Panel electric heaters (1) equipped with a radiator in the form of a flat aluminum plate (profile) into which a low-temperature metal heating element is mounted. Such a radiation surface, as a rule, is anodized and has an applied roughness in order to increase the emissivity (degree of emissivity). With high-quality surface preparation, the emissivity reaches values ε₁ = 0.75 -0.85. The temperature of the radiation surface is structurally limited to a maximum of T_iz1 = 250 ° C for fire safety reasons, as well as due to the pronounced warpage of the aluminum plate due to the difference in values thermal coefficient of linear expansion (TCLE) plates and heating elements.

Quartz Tubular Electric Heaters (2) have as an infrared heating element - emitter a quartz tube with a diameter of 8 to 25 mm, inside which an electric wire spiral is installed from a resistive alloy of nichrome or fechral. The spiral is installed in such a way that its turns touch the inner surface of the tube. In addition, a special shading is mechanically applied to the inner surface of the tube, which almost completely prevents the transmission of infrared rays through the tube wall, only reflecting and absorbing them. The tube itself is not tight at the ends and there is air inside it. Thus, the heated coil heats the quartz tube from the inside by means of contact thermal conductivity and thermal radiation, and the outer surface of the tube is a working source of secondary infrared radiation. The emissivity of the outer surface of the tube strongly depends on its curvature (diameter): for a tube with a diameter of 8-11 mm, the measured value ε₁ = 0.61-0.65, for a tube with a diameter of 23-25 mm measured value ε₁ = 0.80 -0.85. In the steady-state heating mode, the temperature of the outer surface of the tube (radiation surface) can be T_iz1 = 450-600 ° C, which qualifies them as high-temperature. In this case, the glow of the electric spiral through the matte tube with red light, corresponding to the temperature of the spiral 650-800 ° C, takes place.

Electric heaters with ceramic infrared heating elements (3) series NOMAKON EIUS-211, 212, etc., elements with a concave emitting surface of the IKN-101 brand, having an electric power of 500 and 650 watts, are used as the emitter. Their characteristics are presented on the pages of emitters and electric heaters of our website. Heaters work directly in the "dark" region of the radiation spectrum, i.e. practically do not glow at the temperature of the emitting surface T_iz1 = 540-600 ° C and have the highest emissivity compared to other types of radiators ε₁ = 0,96.

Carbon electric heaters (4) have a quartz infrared carbon lamp (ICL lamp and its analogs) as an emitter. The heating emitting element in the carbon lamp is made of woven carbon (carbon) filament in the shape of a spiral. The carbon coil is sealed in a transparent quartz glass vacuum flask. According to the manufacturers of carbon lamps, the source of thermal radiation is precisely heated to T_iz1 = 750 ° C carbon fiber helix. In this case, quartz glass, usually optical special grade KI (for infrared radiation) according to GOST 15130, almost completely transmits radiant heat in the wavelength range of 0.75-4.0 microns, i.e. completely in the shortwave (near IR-A 0.75-1.4 microns) and medium wave (middle IR-B 1.4-3.0 microns) regions of the infrared spectrum. It is noted that at the value of the emissivity of the heated carbonic spiral ε₁ = 0.75 - 0.77 efficiency of a carbon lamp, i.e. the coefficient of emission of electrical energy into infrared energy reaches η_em1 = 90%. In contrast to quartz tube emitters, carbon lamps glow with red light much brighter.

Halogen heaters (5) a tubular quartz halogen thermal emitter (KGT lamp and its analogs) is used as a heating element. The heating emitting element in a halogen lamp is a tungsten wire coil. The tungsten coil is sealed in a transparent quartz glass flask filled with halogen gas. According to the manufacturers of halogen lamps, the source of thermal radiation is precisely heated to a color temperature T_iz1 = 2000-2200 ° C tungsten coil. At the same time, optical quartz glass completely transmits radiation in the visible region (light range), as well as radiant heat in the wavelength range of 0.75-4.0 microns, as for carbon lamps. At the value of the emissivity of the tungsten coil at operating temperature ε₁ = 0.30 - 0.32, the efficiency of a halogen lamp, calculated for the entire region of the radiation spectrum, is not less than η_em1 = 90%. Unlike quartz tube and carbon heaters, halogen heaters glow with a bright yellow light.

Table 1. Designs of infrared electric heaters for testing

Processing of test results

During the tests, the electrical power consumption was determined P_iz1, W in nominal heating mode, temperature of the radiating surface T_iz1, ° C, as well as emissivity (emissivity) ε₁ emitting surface at radiation temperature. Radiation surface area F_iz1, cm² for emitters 1-3 were determined based on the geometric dimensions of the emitter. For emitters 4-5 (carbon and halogen lamps), the radiation surface of the spiral inside the bulb was determined by calculation based on the radial efficiency value adopted from the manufacturer η_em1 = 90%. For example, based on expressions 7 and 14 from the section "Infrared radiation characteristics - engineering applications" it is possible to write

whence the formula for calculating the radiation surface takes the form

Specific surface radiation energy E_iz1, W / m² calculated using the Stefan-Boltzmann equation

Where FROM₀ = 5.671 W / (m² ⋅ K⁴) Is the constant of the Stefan-Boltzmann equation T_iz1, ° K Is the absolute temperature of the radiating surface in degrees Kelvin.

The wavelength of the maximum spectral radiation intensity was calculated using the Wien equation

Where b₀ = 2.898 ⋅ 10^-3 m⋅K Is the constant of Wien's equation.

To plot the graphs of the distribution of the specific radiation intensity by wavelength I = f (λ) used the Planck equation

Where C₁ = 0.374⋅10^-15 Tue ², C₂ = 1.4388⋅10^-2 m⋅K - Planck's constant equations.
Calculation of the proportion of specific radiation power η_λ1, λ2 shortwave (near IR-A λ = 0.7 - 1.4 μm ), medium (medium IR-B λ = 1.4 - 3.0 μm ), and long-wave (far IR-C λ = 3.0 - 80.0 μm ) regions of the radiation spectrum were carried out by numerically integrating the Planck equation in a given wavelength range from λ₁ before λ₂:

The coefficient of emission of electrical energy into infrared energy (radial efficiency) was determined by the formula

The test results and the calculated comparative characteristics of the emitters are presented in Table 2.

Table 2. Test results and calculated comparative characteristics of emitters

To estimate the values of the specific power of infrared radiation at different distances from the emitting surface in the direction normal to the given surface E_n1 = f (L_n1), W / m² with directional radiation heating, the inverse square law was used for electromagnetic radiation

de E_ot1, W / m² - specific radiation energy reduced to the area of the heater's reflector-reflector;
L_n1, m - the current value of the distance from the radiation surface along the normal to this surface;
α₁, deg is the volumetric opening angle of the ray flux at the exit from the reflector-reflector;
R₁, m is the reduced radius of the ray flux at the exit from the reflector.

If we take the dimensions of the reflector-reflector in plan: length L_ot1, m and width B_ot1, m, then expressions for calculation E_ot1 and R₁ take the form:

For panel emitter 1, which does not have a reflector, we took E_ot1 = E_iz1, a B_ot1 equal to the width of the emitter panel. The opening angle of the beam flux at the exit from the reflector-reflector α₁ is determined by the design of the emitter and reflector to it. For panel heaters 1, many manufacturers indicate different values within α₁ = 90-120 °... For calculations, the most probable average value was taken α₁ = 105 °... For reflectors of tubular quartz 2, carbon 4 and halogen 5 heaters, the value α₁ is, as a rule, not less than 90 °. In the side projection of the heater, the tube or lamp of the emitter is a point source of radiation, which is located at a certain distance (in focus) h_ot1, m from the rear surface of the reflector with a depth H_ot1, m. In this case, the expression for calculating α₁ will take the form

For the values adopted during the tests B_ot1 = 100 mm, H_ot1 = 70 mm and h_ot1 = 20 mm calculation by formula (12) gives the value α₁ = 90 °... For heaters with ceramic radiators, the value α₁ was determined experimentally (see the section "Characteristics of infrared radiation - engineering applications") and amounted to α₁ = 68-72 °.

Comparative analysis of the characteristics of infrared radiation

The electric heaters for testing were selected in such a way that their electrical power consumption was of the same order of magnitude and did not differ significantly. At the same time, due to the significantly different calculated area of the emitting surface and the possibility of its effective heating (meaning the different principle of operation of the emitters described above), we have a minimum value of the consumed electric power per unit of emitting surface for panel emitter 1 and, as a consequence, a low temperature of the emitting surface ... And since the specific radiation energy according to formula (3) depends on the temperature of the emitting surface to the fourth power, even with a sufficiently high value of the emissivity, the value of the specific radiation energy of the panel will be much lower than that of other emitters. Because of this, for example, for a panel radiator 1, it is theoretically impossible to obtain high values of the radial efficiency, which determines the efficiency of the heater precisely as infrared.

In a quartz tubular emitter 2, the electrical power consumption per unit of radiation surface is quite high, higher, for example, than in a ceramic emitter 3, but the temperature of the emitting surface (outer surface of the tube) is somewhat lower than that of a ceramic one. This is due to the design features of the tubular heater, in which a tube heated to a high temperature along its perimeter is washed from bottom to top by streams of cold ambient air, i.e. the outer radiating surface of the tube is in no way protected from thermal convection losses in comparison, for example, with a ceramic radiator, the rear part of which is covered by a reflector and a heater casing. As noted above, the emissivity of the outer surface of the tube strongly depends on its curvature (diameter): with a tube diameter of 11 mm, it is significantly lower than that of a panel and ceramic radiator. The obtained low values of specific surface radiation energy and radial efficiency are typical for this type of heater. For comparison, tests were carried out on a quartz tube heater with tube dimensions (length x outer diameter) 420x23.2 mm, which in the nominal heating mode had an electrical power consumption P_iz1 = 1477 W, specific electrical power P_ud1 = 4.4 W / cm², the temperature of the emitting surface T_iz1 = 462 ° C at emissivity ε₁ = 0.85. ... In this case, a significant decrease in the temperature of the emitting surface due to heat losses is not compensated by an increase in the emissivity value from an increase in the tube diameter. The calculated value of the radial efficiency was η_em1 = 31,9%.

Comparative assessment of physiological effects on the human body

With the help of a system of directional radiation heating in the working area of the room, a thermal environment favorable for a person should be created. On the site page "Infrared heating of the human body - standards and ensuring the microclimate of industrial premises" and in [3], we presented methods that allow, on the basis of the current regulatory documents, to determine the specific power levels of thermal radiation for the zone with increased radiation intensity, for the zone of effective heating, as well as for the zone of comfortable but insufficient heating. It should be noted that the existing standards reflect only one of the quantitative characteristics of heating, namely, the value of the specific radiation power, which, in principle, can be provided by using any design of an infrared heater.

The practice of designing and operating systems of local radiation heating, as well as experience in the development, testing and certification of special devices for heating a person with directional infrared radiation [3-4] shows

that when choosing the design of the heater, additional requirements must be met to minimize the consequences associated with the so-called psychophysiological intolerance of workers to prolonged exposure to directional radiation heating. 

The main physical factors affecting the human body in this case are:

• increased selective effect of radiation on various parts of the body and, especially, on the head of a person, associated with the impossibility of providing a given optimal direction of the radiation flux;
• the presence in the radiation spectrum of a high proportion of "hard" short-wave and medium-wave radiation;
• directional light effect of "luminous" emitters.

Ensuring optimal directionality of the beam flux is determined by the possibility of installing the heater, as on the ceiling (suspended) with the reflector directed vertically downward (angle of inclination to the horizon β₁ = 90 °, and the presence of a special mount for frontal installation of the heater on the wall with the ability to change the angle of inclination of the reflector from β₁ = 0 ° (reflector mounted horizontally) to β₁ = 60 ° and more relative to the horizontal axis. In addition, with a large value of the angle of opening of the radial flux α₁ Already at a small distance to the heater, the size of the "hot spot" of a given specific heating power becomes large and, as it were, covers the person completely with the head, creating a feeling of discomfort on the more sensitive skin of the face and head. Smaller value α₁ combined with a wide range of variation β₁ allows heaters to work directionally from a greater distance and form a "hot spot" in the work area below the worker's head.

Panel heaters 1 are usually ceiling mounted β₁ = 90 ° at the maximum value of the angle of aperture of the radiant flux α₁... Moreover, their ability to create an optimal heat flow is most limited. Quartz tubular 2, carbon 4 and halogen 5 wall-mounted heaters usually have a limitation of the angle of inclination to the horizon no more than β₁ = 45 °, which is associated with the possible overheating of the case by the air heated to a high temperature, emanating from the emitter. Moreover, they have average values α₁... Ceramic heaters 3 in the design EIUS-211, EIUS-212, etc. have universal ceiling and wall mounting β₁ = 0-90 ° at the minimum value α₁.

Physiological effects of "hard" short-wave and medium-wave radiation on the human body is determined by its penetrating ability into the layers of skin and tissue. Near infrared IR-A [5] at a wavelength λ = 0.95μm the penetrating power reaches its maximum and is 60-70 mm [5] (see Figure 5). In the middle range of IR-B, the penetration capacity decreases to 20-30 mm, and in the far IR-C it is no more than 0.3-0.5 mm. Taking into account the physiological characteristics of a person, modern medicine characterizes these areas of the spectrum as follows: wavelength 0.75-1.4 microns - radiation that penetrates deep into the human skin (IR-A range); wavelength 1.4-3 microns - radiation absorbed by the epidermis and the connective tissue layer of the skin (IR-B range); wavelength over 3 microns - radiation absorbed on the skin surface (IR-C range). In this case, the greatest penetration is observed in the range from 0.75 to 3 microns and this range is called the "window of therapeutic transparency" [6]. According to [7], the radiation intensity from TV screens, video monitors, oscilloscopes, measuring and other devices and means of displaying information with visual control should not exceed 0.05 W / m2 in the near-IR range (0.76-1.050 microns) and 4 W / m² in the far (over 1.050 microns) IR range. In this case, the permissible level of intensity of the integral flux of infrared radiation from electrical appliances of soft heat and emitters on the surface of the human body should not exceed 100 W / m²... Thus, the unsafeness of short-wave radiation is confirmed.

It should be noted that the physiological effect of infrared radiation associated with its penetration deep into the human body has not been sufficiently studied even at the level of short-term therapeutic procedures. From this point of view, to ensure complete safety of the worker during prolonged exposure to radiation heating, it is necessary to limit as much as possible the range of "therapeutic transparency" in the intensity spectrum of the radiation flux from the heater. The studies carried out during the certification of the developed by us changing tables for newborns with directional infrared heating [4] showed that “soft” long-wave IR-C radiation with a share of short-wave IR-A radiation in terms of specific power not more than η_A = 0,5% and with a fraction of medium-wave radiation IR-B no more η_B = 20%... Experiments on irradiation of a sensitive part of the body - the back surface of a person's hand with “soft” radiation from a ceramic heater and “hard” radiation from a halogen heater have shown that a person begins to feel “soft” radiation directly at a higher specific power of the radiation flux (by about 20-30%) than "Tough". This is due to the fact that "soft radiation" penetrates much less deep into the skin and irritates the subcutaneous receptors less than "hard" radiation. Thus, the human tolerance of heating with "soft" normalized radiation will always be higher. According to the results of the presented tests, long-wave heaters fully correspond to the safety criterion of physiological impact on a person with prolonged directional heating: panel 1, quartz tubular 2 and ceramic 3 (see table 2). The shares of short-wave and medium-wave radiation of carbon 4 and halogen 5 heaters do not meet this criterion.

Directional light exposure of emitters with local radiation heating, it manifests itself in the case of the frontal location of the heater directly in front of the employee, when the pronounced glow of the emitters and the reflector-reflector illuminated by them constantly fall into the field of view of the employee, irritating and distracting from the production process. When heated at night, light exposure creates an induced illumination of the room, which interferes with calm falling asleep and the normal course of the sleep process.

In terms of light exposure, panel heaters 1 are out of competition, because the glow of the emitter is completely absent. The glow of ceramic emitters 3 under normal artificial or natural lighting in the room is practically not noticeable and begins to manifest itself somewhat only with a significant darkening of the room and in complete darkness. In this case, we can talk about a low level of light exposure. The average level of light exposure of quartz tubular 2 and carbon 4 emitters is determined by the operating temperature of the coil, equal to 650-800 ° C. The highest comparative level of light exposure has a halogen emitter 5 with an operating temperature of the coil 2000-2200 ° C.

Criteria for choosing heaters for directional radiant heating

The above test results show that any types of infrared heaters can be compared, at least, by 16 parameters that determine their design features and operating modes of infrared emitters directly, spectral characteristics of the generated radiation and radiation efficiency, features of the directional distribution of radiant heat in space. , as well as safety conditions for physiological effects on the human body.

In order to figure out where and how to use infrared heaters of various types more efficiently, you need to define the basic concepts:

• Infrared heating - this is the process of using heating devices - heaters for the purpose of rapid local directional heating of a person, animals, surrounding objects and materials in closed, semi-open and open rooms, as well as in the open air;
• Infrared heating Is the process of using heating devices - heaters for the purpose of maintaining the desired air temperature in a closed room for a long time.

Thus, the infrared heating devices presented above can be used both as heaters, and as heaters, and as additional heating systems.

In the process of infrared heating of a room, if there are no people constantly there, the total amount of electrical energy consumed by the heater and then generated in the volume of the room in the form of thermal radiation and convection heat of hot air heated by heat transfer from the radiators and the heater body is important. 

This is the total amount of energy and is intended to compensate for external heat losses through walls and windows to maintain the desired room temperature. The selection criteria for the characteristics presented in Table 2 in this case are somewhat leveled, since, for example, the parameters of the emitters, the characteristics of the radiation and even the radial efficiency do not affect the total amount of released energy, which depends only on the consumed electric power of the heater.

In the process of infrared heating, the heater must provide a directed radial flux for warming up the worker in such a way that at a given distance from the heater the required standardized specific radiation power is provided, as well as the size and location of the heating area, which determine the conditions for a comfortable thermal environment in this area of the room, or outdoors. 

In this case, the comparative characteristics of Table 2 provide comprehensive information on the disadvantages and advantages of each type of heater. In the table, comparative values of characteristics are highlighted in red, which, in our opinion, do not satisfy the requirements of directional radiation heating.

Panel heater 1 has a minimum value of the consumed electric power per unit of the radiating surface and, as a consequence, low values of the temperature of the radiating surface and the specific surface energy of radiation. The low radial efficiency in combination with the high angle of the radial flux opening does not allow obtaining sufficient specific power of the radiant heating at a distance of more than 1 m from the heater. When creating a zone of comfortable heating, electricity consumption will be almost 2 times higher than that of the most efficient heater.

Quartz tube heater 2 because of the low value of the emissivity, the tube cannot provide an acceptable radial efficiency and specific radiation energy reduced to the area of the reflector. In addition, with a close proximity to the subject of heating due to the additional light exposure of the emitters, there is a possibility of intolerance to prolonged radiation exposure to the worker.

Application problem carbon 4 and halogen 5 Heaters for directional radiation heating of a person consists in the increased temperature of the emitters and, for this reason, the presence of increased levels of short-wave and medium-wave radiation. In combination with the increased light exposure of the emitters, in this case, we have a high probability of intolerance to prolonged radiation exposure to the worker.

Characteristics heater with ceramic radiators 3 according to the infrared heating mode, they are exactly within the permissible values of short-wave and medium-wave radiation. At the same time, the values of radial efficiency and specific radiation energy per unit area of the reflector are provided, comparable to the values for carbon and halogen heaters. The indisputable advantage of the heater with ceramic emitters of the design EIUS-211, EIUS-212 is the ability to provide the maximum range of variation of the angle of inclination of the reflector to the horizon with the minimum value of the angle of opening of the beam flux and the practical absence of the light effect of the emitters with frontal heating of a person.
Thus, the heater makes it possible to most effectively create a sufficient specific power of the radiant heating at a distance of more than 1 m from the radiating surface with minimal power consumption.

Along with the characteristics that determine the physical process of infrared heating (see table 2), when choosing the design of the heater, one should also take into account its performance in production conditions, the most significant of which are:

• Climatic performance;
• service life of the emitter and heater;
• resistance to temperature extremes, high humidity and condensation, the effects of deposited dust and soot, the influence of corrosive and chemically active substances in the air of the working area;
• the ability to install on objects moving with acceleration and deceleration, resistance to vibration loads.

It should be noted that quartz tubes and lamps are very sensitive to surface contamination - you should not even touch KGT lamps with your hands. The appearance of dust contamination of the surface of the quartz bulb or its sootiness leads to overheating of the emitting coil and rapid lamp failure. Accordingly, quartz tubes and bulbs do not tolerate the presence of condensation on the surface when turned on. Therefore, the vast majority of panel, quartz tubular, carbon fiber and halogen heaters have a climatic version of UHL4 in accordance with GOST 15150: operation in closed heated rooms at an air temperature from +1 to + 40 ° C, with ventilation and no sand, dust and moisture condensation from the outside air. ... In ceramic emitters, the coating of the emitting surface is made of an engobed ceramic glaze based on zirconium oxide, which, unlike quartz tubes and lamps, is absolutely insensitive to dust and organic contaminants that are burned out and removed from the surface with a brush. The glaze is not sensitive to high humidity and condensation on the surface when switched on, the presence of chemical compounds in the air of the working area. With the use of ceramic emitters, it is possible to manufacture heaters with a climatic version of UHL2 (operation in the open air under a canopy) and UHL5 (operation in rooms with high humidity - mines, basements with frequent moisture condensation on the walls and ceiling).

One of the operating conditions for quartz tube emitters, carbon and halogen lamps is their fixed horizontal position when turned on and heated. When the lamp is turned, for example, to a vertical position, the distribution of the heated spiral inside the bulb can be disturbed due to the reduced mechanical properties of the spiral material. The same can happen when the lamp moves with acceleration, a sharp turn in the horizontal plane, or when vibration loads occur. In ceramic radiators, the heating coil is filled with ceramic mass and for this reason all the above-mentioned movement factors do not affect the heater's performance.

Thus, the test results presented in the work, the comparative assessment of the characteristics of the emitters, as well as the analysis of the selection criteria for various types of heaters show that heaters with ceramic infrared emitters are the most promising for directional radiation heating in industrial conditions.

Literature:
1. Rabko A.E. Infrared ceramic emitters and electric heaters NOMAKON / A.E. Rabko, I. L. Kozlovsky, P. P. Pershukevich, M.V. Belkov // Electronics info. - 2011. - No. 5. - S.26-29.
2. Rabko A.E. Industrial infrared heating by IR electric heaters NOMAKON / A.E. Rabko, I.L. Kozlovsky, Yu.N. Zhilinsky, A.G. Patsevich // Electronics info. - 2012. - No. 4. - S. 89-92.
3. Rabko A.E. Heating of premises with infrared electric heaters NOMAKON / A.E. Rabko, I.L. Kozlovsky, V.A. Orsich // Electronics info. - 2013. - No. 9. - S. 45-48.
4. Demchenko A.I. Multifunctional infrared system for providing a thermoneutral environment for newborns / A.I. Demchenko, A.V. Bezyazychnaya, V.V. Bezyazychnaya, I.L. Kozlovsky, A.E. Rabko, A.G. Patsevich // Sat. Proceedings of the 22nd International Conference "Microwave Engineering and Telecommunication Technologies". - 2012, Sevastopol. - S.989-990.
5. Ponomarenko G.N., Turkovsky I.I. Biophysical Foundations of Physiotherapy. - M .: JSC "Medicine Publishing House". - 2006 .-- 176 S .: ill.
6. Henderson R. Wavelength considerations. Instituts für Umform- und Hochleistungs, 2007.
7. SanPiN No. 9-29-95 (RF No. 2.1.8.042-96) Sanitary norms of permissible levels of physical factors when using consumer goods in domestic conditions.