The efficacy of radiant heat controls on workers’ heat stress around the blast furnace of a steel industry

Abstract

BACKGROUND:

Workers’ exposure to excessive heat in molten industries is mainly due to radiant heat from hot sources.

OBJECTIVE:

The aim of this study was to evaluate the efficacy of radiant heat controls on workers heat stress around a typical blast furnace.

METHODS:

Two main interventions were applied for reducing radiant heat around the blast furnace of a steel industry located in western Iran. These included using a heat absorbing system in the furnace body and installing reflective aluminum barrier in the main workstation. Heat stress indexes were measured before and after each intervention using the digital WBGT-meter.

RESULTS:

The results showed MRT and WBGT indexes decreased by 20°C and 3.9°C, respectively after using heat absorbing system and also decreased by 18.6°C and 2.5°C, respectively after installing a reflective barrier. These indexes decrease by 26.5°C and 5.2°C, respectively due to the simultaneous application of the two interventions which were statistically significant (p < 0.001). The core body temperature of workers decreased by 2.6 °C after the application of interventions which was also significant (p < 0.05).

CONCLUSIONS:

The results confirmed heat control at source can be considered as a first solution for reducing radiant heat of blast furnaces. However, the simultaneous application of interventions could noticeably reduce worker heat stress. The results provide reliable information in order to implement the effective heat controls in typical hot steel industries.

Keywords

Radiant heat controls blast furnace heat stress

1 Introduction

Heat stress is one of the most important detrimental physical factors in many workplaces. Exposure to high temperatures is common among workers employed in hot workplaces and may lead to several adverse physiologic effects [1]. Some studies have demonstrated that exposure to heat induces many complications including heat exhaustion, muscular spasm, heat stroke, heat rash, neuropsychological disturbances, reduced work performance, and elevated human errors [2 –4]. Workers are exposed to heat in industrial processes such as molten plants, foundries, glass industries.

Application of engineering controls especially in the hot sources or surfaces is the most effective approach for reducing heat in the work environment. Radiant heat control is one of the most effective engineering controls, especially in hot industries, e.g., the steel industry [5].

The radiating waves pass through the air without warming it and warm only the heat absorbent objects in their stream. The radiating heat can be reduced through decreasing temperature or emissivity of surfaces. The surface temperature can be reduced by a decreased input heat to the source or its thermal isolation. Another method for reducing exposure to the radiating heat is the use of reflecting personal protective equipments. Also, the amount of radiating heat can be reduced by putting a shield between the workstations and the hot sources [6, 7].

One of the issues regarding the interventional designs of radiating heat controls is the empirical evaluation of the efficacy and effectiveness of control designs conducted so far. In this regard, lack of sufficient data has led the industrial managers to have a negative attitude towards devoting financial sources to implementing control designs and to be skeptical about their efficacy and return of investment [8].

The efficacy of control measures is, in fact, the rate of effect of the desired intervention on physical heat stress of the work environment and the effectiveness of control measures is result of the desired intervention on heat strain of workers.

Several empirical indices have been provided for the assessment of heat stress. The most important and applicable of heat stress indices recommended by International Standard Organization (ISO) is the wet bulb global temperature (WBGT) [9]. The WBGT index can be used as a criterion for judging the efficacy of heat control measures. The heat strain indices (e.g. core body temperature and heart rate) are the direct method of assessment of heat exposure [10]. These indices can also be used to determine the effectiveness of heat control interventions for providing workers’ health in hot environments.

In the steel industry, where there is close proximity to blast furnaces, smelter workers are exposed to radiant heat. The blast furnace is one of the most important pieces of equipment in the steel industry for melting iron stones and producing steel and iron ingots. Studies showed there has been much request for change of occupations and there have been a considerable number of medical cases especially in hot seasons of the year among those employed in these units. Hajiazimi et al. showed that heat stress was higher than the standard level in most workstations in typical steel factories during summer [11]. In this study, the addition of reflective aluminum shields in the workstation and a change color of worker clothes from dark blue to light grey were used to control radiant heat in molten-cast platform. Based on the mentioned controls, radiant temperature and WBGT were decreased almost 8.25°C and 2.3°C, respectively [11].

There have been very few studies describing or evaluating the efficacy of radiant heat controls in hot environments. Hence, this study aims to design and implement radiant heat controls in a typical blast furnace and present some empirical findings on the efficiency of the employed interventions based on valid and standardized indices of radiant heat.

2 Methods

In this study, two main control designs were considered to reduce the heat stress in the blast furnace of a steel industry located in Sanandaj (western Iran). The heat sources to which 20 smelter workers are exposed include the continual radiating heat from the body of the furnace and the intermittent heat from the molten iron during its egression.

2.1 Control designs

Control designs included reducing the temperature of hot surfaces by (a) designing and installing a heat absorbing system in the body of the blast furnace and (b) controlling emission of heat radiation by installing a reflective barrier in the main workstation. As shown in Fig. 1 for cooling down the body of the furnace, a cooling system in the form of a cooling tower was used in which the water in a reservoir was cooled by cooling tower first, and then pumped into a circulatory circuit designed as tubing in the body surfaces of the furnace. Cold water for the tubing system in the furnace body is secured by cooling towers used in most steel industries. This chiller system has warm water input tanks and cold water output tanks towards the furnace and performs the cooling process in a circulatory operation. So, there is no need for water supply provision and extra costs. Secondly, a reflective barrier in form of a partial enclosure was used for controlling radiant heat in the main workstation near the furnace. The strong steel framed structure with a width of 3 m, height of 2 m, and thickness of 3 mm was installed in the workstation around the molten slag route as shown in Fig. 2. Multilayer aluminum foil with an emissivity of 0.03 as well as a reflectance of 97% was applied over the steel framed structures. Aluminum foil has a layer of fiber glass as a backing material. Heat reflective glass is specifically designed to be placed in typical control room where high radiant heat conditions exist. For monitoring the process in this workstation, a reflective glass with a width of 1 m, height of 5 m, and thickness of 3 mm which has a reflectance of 90% was installed within the radiant shield barrier. This special glass will effectively reflect long wave length infrared radiation, while providing a high grade of transparency. In addition, other complementary methods as changing the color of workers uniforms from dark blue to grey for reducing radiant heat absorption were applied.

2.2 Evaluation of control interventions

To evaluate the efficacy of control interventions, wet bulb glob temperature (WBGT) index as a main heat stress index was measured before and after each intervention using the digital WBGT-meter (Casella Model in the studied workstation [12]. WBGT index was calculated as follows. $WBGT = 0.7 t_{nw} + 0.2 t_{g} + 0.1 t_{a}$ (1)where t_nw is natural wet temperature, t_g is glob temperature, t_a is dry temperature.

These measurements were obtained during the continual operation of the furnace and egression of molten iron. Furthermore, the environmental distribution of mean radiant temperature (MRT) in the furnace lot was plotted and analyzed before and after using the cooling system via SURFER software. SURFER is a contouring and 3D surface mapping program that runs under Microsoft windows. This software interpolates regular or irregularly-spaced XYZ data on to a grid, and then employs this to create contour maps and surface plots. It can be used to create color contour plots of output such as temperature.

To evaluate the effectiveness of interventions for reducing the workers’ heat strain, the direct physiologic strain indices included core body temperature was selected. This was measured before and after employing interventions based on the standard method ISO 9886 in 20 male workers around the furnace at the end of each work shift at 12 noon in the middle of July [10]. The mean core body temperature was measured using the thermometer, model DL7105. The smelter worker’s metabolism rate was determined using ISO 8996 standard method [13]. In regard to worker activities, metabolism rates for furnace workers were determined within 130– 200 w/m². Based on WBGT index, occupational exposure limit for eight-hour work shift with these metabolism rates is about 28°C [12].

In this study, the data was analyzed using SPSS software. The paired t-test were used for comparing the performance of control interventions. The paired t-test is generally used when measurements are taken from the same groups before and after some interventions. Statistical significance level was considered at the p value less than 0.05.

3 Results and discussion

The results of environmental parameters in the workstation before and after control interventions are shown in in Table 1. In order to evaluate the efficacy of interventions, the results show the MRT and WBGT indices decreased by 20°C and 3.9°C, respectively in the studied workstation after application of heat absorbents system in the body of the furnace.

Distribution of mean radiant temperatures around the furnace before and after implementing heat absorbing system was shown in Figs. 3 and 4.

Moreover, the MRT and WBGT indices decreased by 18.6°C and 3.6°C, after installing the heat reflecting barrier near the workstation. It is noted that, in this situation, the heating absorbing system was shut down.

Finally, the simultaneous application of the two interventions resulted in a decrease in MRT and WBGT indices by 26.5°C and 5.2°C, respectively. The results confirmed that decreases in the WBGT and MRT due to the simultaneous use of the heat absorbent system and reflecting barrier in the workstations were statistically significant (p < 0.001). Because of the hot nature of the studied process, the efficacy of the two interventions was acceptable. However, to achieve more ideal conditions, other control solutions can be considered in future plans.

As shown in Table 2, the core body temperature of the studied workers decreased by 2.6°C after the application of two interventions. Regarding the effectiveness of interventions on workers heat strain, these reductions were statistically significant (p < 0.05).

The results confirmed that the glob temperature were higher than the dry temperature around of the studied furnace due to radiant heat emissions. Considerable differences between glob and dry temperature were also reported in workstation near the radiant heat sources in scientific literature. Srivastava et al. showed significant difference between environmental temperatures due to high rate of radiating heat in workstations of typical glass factory in India [14].

In the studied workstation, WBGT index was approximately about 37°C, which exceeded the occupational exposure limit for eight-hour work shift with metabolism rate between 130 to 200 w/m² [15]. On the other hands, the results of physiological strains in 20 smelter workers showed that the core body temperature was higher than recommended limits [15].

American conference of governmental industrial hygienists (ACGIH) stated that core body temperature above 38°C is not acceptable. So, we have to accept the furnace as a huge radiant heat production source. Hence, the use of heat absorbent system as cold water tubing in the furnace body and resorption of its radiating heat in all components specifically those in the workstations seem to be efficient and applicable. In next step, reflective aluminum barrier as a complement strategy for protecting workers for radiant heat form residual heat from furnace body was employed in the workstation.

The results confirmed that heat stress in the workstation after employing two interventions was decreased significantly. Table 2 showed after the the simultaneous application of the two interventions, core body temperatures were equal to 36.80 ± 0.16°C which were lower than the recommended limits of ACGIH (T_Core < 38°C).

Based on results in Table 1, the heat absorbing system and reflective barrier have approximately same contribution for reducing radiant heat in the studied workstation. However, the heat absorbing system could be considered as the main intervention due to heat loss in wider areas around the furnace. Note in Figs. 3 and 4 that the heat distribution map of the furnace surrounding after employing heat absorbing system showed significant decrease in radiant heat around the furnace.

On the other hand, reflective barrier despite its efficiency may be having interference with worker movements for performing activities. Generally, employing two interventions simultaneously could reduce mean radiant temperature up to 7°C more than employing the two interventions, separately. Hajiazimi et al. also used an aluminum reflector barrier to control radiant heat in molten-cast platform of a foundry. In this way, the mean radiant temperature and WBGT index decreased by 8.24°C and 2.31°C, respectively [16]. The results of our study confirmed that employing engineering control at hot sources included heat absorbing system is efficient and applicable, noticeably [17]. However, in regard to occupational exposure limit based on WBGT (equal to 28°C) for the studied workstation, another complementary method for reducing heat is proposed as moisturizing and water-jetting of the furnace surrounding during molten iron evacuation operation. In regard to the lack of empirical data about the performance of heat control methods, the results of this study could provide reliable and comprehensible information about performance of different radiant heat controls for industrial managers and occupational health professionals and facilitated the identification and implementation of effective heat controls in typical hot industries.

4 Conclusions

Excessive heat in molten industries is due to radiating heat and in most cases there is no possibility of elimination of heat-producing sources. In the present study, the heat stress and strain was reduced significantly using radiant heat control interventions. The use of a heat absorbing system in the furnace body was more effective and applicable than the use of the reflector barriers in the workstation. In this regards, heat control at source can be considered as a first solution for reducing radiant heat of blast furnaces. However, the simultaneous application of interventions could noticeably reduce worker heat stress. These results provide reliable information in order to implement the effective heat controls in typical hot industries.

Footnotes

Acknowledgments

The authors would like to thank managers of steel industry for cooperation and providing financial support for this project.

References

Rodahl

. Occupational health conditions in extreme environments. Ann occup Hyg 2003;47(3):241–52.

Pilcher

, Nadler

, Busch

. Effects of hot and cold temperature exposure on performance: A meta-analytic review. Ergon 2002;45(10):682–98.

Pinto

, Xavier

AAP

, Regiane

TDA

. Analysis of the thermal comfort model in an environment of metal mechanical branch. Work 2012;41:1606–1611.

Brake

, Bates

. Fluid losses and hydration status of industrial workers under thermal stress working extended shifts. Occup Environ Med 2003;60:90–96.

Ismail

, Rani

MRA

, Makhbul

ZKM

and Deros

. Assessment of thermal comfort and optimization of environmental factors at automotive Industry. Eur J Sci Res 2009;31(3):409–423.

AIHA. Heating and cooling for man in industry. American Industrial Hygiene Association, First edition,Technology & Engineering, Dey 11, 1353 AP; 1970.

Parsons

. Human Thermal Environments, Second edition, Taylor & Francis; 2003.

Malchaire

, Gebhardt

, Piette

. Strategy for evaluation and prevention of risk due to work in thermal environment. Ann Occupational Hyg 1999;43(5):367–376.

Tanaka

Masatoshi

. Review article of heat stress standard for hot work environments in Japan. Ind Health 2007;45:85–90.

10.

ISO 9886. Ergonomics—Evaluation of thermal strain by physiological measurements. International Standards Organization, Geneva; 2004.

11.

Hajiazimi

, Khavanin

, Aghajani

. Heat stress measurement according to WBGT index in smelters. J Mil Med 2011;13(2):59–64.

12.

ISO 7243. Hot environments –Estimation of the heat stress on working man, based on the WBGT index. International Standards Organization, Geneva, 2003.

13.

ISO 8996. Ergonomics of the thermal environment – determination of metabolic heat production. International Standards Organization, Geneva; 2004.

14.

Srivastava

, Kumar

, Joseph

and Kumar

. Heat exposure study in the workplace in a glass manufacturing unit in India. Ann Occup Hyg 2000;44(6):449–453.

15.

ACGIH. Threshold limit values for chemical substances and physical agents & biliological exposure indices, American Conference of Governmental Industrial Hygienists, Cincinnati; 2012.

16.

Hajiazimi

, Khavanin

, Solymanian

, Valipour

and Dehghan

. Heat stress control in the foundry platform of a steel plant of Iran. J Health Syst Res 2011;7(6):866–874.

17.

RIMA. Understanding and using reflective insulation, radiant barriers, and radiation control coatings. Reflective Insulation Manufacturers Association International, Second Edition; 2002.