Abstract
In automotive industry plants, which use injection molding machines for rubber processing, tar contaminates air to such an extent that air fails to enter standard heat recovery systems. Accumulated tar clogs ventilation heat recovery exchangers in just a few days. In the plant in which the research was conducted, tar contamination causes blockage of ventilation ducts. The effect of this phenomenon was that every half year channels had to be replaced with new ones, since the economic analysis has shown that cleaning them is not cost-efficient. Air temperature inside such plants is often, even in winter, higher than 30°C. The air, without any means of heat recovery, is discharged outside the buildings. The analyzed plant uses three types of media for production: hot water, cold water at 14°C (produced in a water chiller), and compressed air, generated in a unit with a rated power consumption of 180 kW. The aim of the study is to determine the energy efficiency improvement of this type of manufacturing plant. The main problem to solve is to provide an air purification process so that air can be used in heat recovery devices. The next problem to solve is to recover heat at such a temperature level that it would be possible to produce cold for technological purposes without air purification. Experimental studies have shown that air purification is feasible. By using one microjet head, a total of 75% of tar particles was removed from the air; by using 4 heads, a purification efficiency of 93% was obtained. This method of air purification causes air temperature to decrease from 35°C to 20°C, which significantly reduces the potential for heat recovery. The next step of the research was designing a cassette-plate heat exchanger to exchange heat without air purification. The economic analysis of such a solution revealed that replacing the heat exchanger with a new one even once a year was not cost-efficient. Another issue examined in the context of energy efficiency was the use of waste heat from the air compressor. Before any changes, the heat was picked up by a chilled water system. The idea was to use the heat for cold generation. Temperature of oil and air in the compressor exceeds 65°C, which makes it a perfect heat source for an adsorption refrigeration device. This solution reduced the cooling demand by 147 kW, thus reducing power consumption by 36.75 kW. This study shows that even in factories where air is heavily polluted with tar, there are huge potentials for energy recovery using existing technical solutions. It is important to note that problems of this kind should always be approached individually.
1 Introduction
Most production plants struggle with a heat overproduction problem, especially with regard to low-temperature heat [1]. Such heat is generated mainly in technological processes performed in production plants [2]. The largest amount of heat comes from plastic processing machinery, machine tools, welding processes, soldering, or air compression. Although in winter this effect can be used for space heating, in summer it is a serious problem and makes the temperature in production halls too high. Also, it often happens that even in winter the heat production exceeds the demand. The result is that machinery has to be cooled for the whole year, which is associated with high operating costs and increasing CO2 emissions. In many cases, the main problem of low energy efficiency is the lack of connection between heat, cold, or other media generation [3].
In most cases, this problem can be solved using standard recuperation [4], but there are industries in which air pollution on the shop floor is large enough to make recuperative heat exchangers block the air flow in just a few days. One of such industries is rubber parts production for automotive branches. As a consequence of rubber injection molding, large amounts of tar enter into the air. Tar clogs heat exchangers quite quickly and effectively builds up in exhaust ducts. These types of plants are usually characterized by low energy efficiency. According to the latest EU regulations, it is worth considering whether such plants can manage to boost their energy efficiency [5, 6]. Improving energy efficiency is not only a direction of industrial development but even a requirement [7, 8]. As such, it supports developing technologies that have not yet been successfully implemented in industrial applications. These include solutions for fuel cells [9], thermoelectric generators [10], ejector devices [11], or adsorption refrigeration devices [12, 13]. Nevertheless, the solutions have faced many barriers during implementation [14].
The research in the manufacturing plant was divided into several stages. In the first one, the flow of air in the production hall, the way the fresh air flows in, and the way the polluted air flows out were determined. The next step was to establish potential energy that could be used in heat recovery processes. Velocity and temperature of air in two production halls were measured at 2, 4, and 6 m above the floor. In the next stage of the research, the exhaust air temperature was examined using a bypass provided in the ventilation system directly behind a working machine. The bypass line was used not only for temperature measurement but also for an air purification experiment. Since standard methods of air purification using filters have proved inefficient, it was decided to examine the use of water micro-jets to precipitate solid tar particles from the air stream. The next step was an experimental test of possibilities of recovering heat from an air compressor for the production of cold. For this purpose, an adsorption refrigeration unit, which can convert low-temperature heat (of up to 55°C) into cold, was used.
2 Description of the production plant
The analyzed manufacturing facility is located in northern Poland and consists of two production halls, a warehouse and an office area. It produces components for the automotive industry and, for production purposes, it requires heat, cold, and pressurized air. In the factory, there are oil boilers with a total capacity of 3 MW to produce heat for rubber injection molding and to provide space heating of the halls in winter. The plant operates more than 80 rubber injection molding machines. In the factory, there is an air compressor with a rated power consumption of 180 kW. The compressor is lubricated and cooled with oil. Oil and compressed air are cooled in heat exchangers with chilled water fed from a chiller. The oil temperature at the inlet of the heat exchanger can reach a maximum of 85°C. The average temperature is 75°C. Compressed air is used for technological purposes. “Cold” is produced by a vapour compressor chiller with a cooling capacity of 515 kW. The working fluid in the unit is R134a. Chilled water temperatures are 14°C and 20°C respectively. The cold is used for cooling the air compressor, for technological purposes in injection molding machines, and, in summer, for cooling the air in supply ducts. Air removed from the production hall has a temperature of 35°C. The reason for such a high temperature of the extracted air is that the exhaust ducts are connected directly to the injection molding machines. First, the air removed from the halls cools the injection molding machines and then is expelled outside. Since the air is heavily polluted withtar, no heat recovery system is mounted. Such attempts were made in the past, but standard heat exchangers become totally clogged in just a few days.
2.1 Distribution of air in the production halls
In the first stage of the research, the flow of air in the production hall No. 1 was examined. For this purpose, velocities and temperatures were measured in the plant. Figures 1 to 6 show velocities and temperatures of air at 2, 4, and 6 m above the floor.
Air velocity in Hall1 at a height of 2 m, m/s.
Air temperature in Hall1at a height of 2 m, °C.
Air velocity in Hall1 at a height of 4 m, m/s.
Air temperature in Hall1 at a height of 4 m, °C.
Air velocity in Hall1 at a height of 6 m, m/s.
Air temperature in Hall1 at a height of 6 m.
According to the measurements, the highest air velocity was at the gates connecting the production hall with the warehouse. Exhaust fans in one production hall have total capacity of 60,000 m3/h, while the total capacity of supply fans is only 50,000 m3/h. The result is that the production hall is under a vacuum. For this reason, the air is sucked out of the warehouse. This advantage of the solution is that pollutants from the production halls do not get into the warehouse. The experimental studies show that the air temperature rises as the place of measurement moves away from the inlet of air, with temperatures of 21°C to 26°C on the opposite end of the hall. Near the supply air duct, no high velocity of air was observed, which resulted from the duct design as a perforated steel sheet; hence, the air flows into the hall with the same velocity along the entire length of the channel.
3 Experimental research
In order to determine the heat recovery potential, two experiments were performed. The first one involved the purification of air and consisted in determining the potential of heat recovery before and after purification. The second experiment focused on the use of an adsorption refrigeration unit.
3.1 Air purification
Exhaust air from the production hall is extracted via the channel mounted directly on the rubber injection molding machine. This way the air can be used for additional cooling of the injection molding machine. In order to determine the temperature of the air downstream of the machine, a ventilation passage was made, which directs the air first into a measuring system. Figures 7 and 8 present the temperature and relative humidity, respectively, of the air immediately downstream from the injection molding machine. The temperature was measured with type K thermocouples, while relative humidity with Honeywell HIH-4000 series humidity sensors.
Air temperature in the exhaust duct.
Relative humidity of the air in the exhaust duct.
The results of the experiment presented in Figures 7 and 8 show that the variations in air temperature and relative humidity during operation are low. Injection cycles are visible, but they do not cause a significant change in temperature and humidity. A temperature change was observed in the range of 2 K and a relative humidity change in the range of 2%.
The next stage of the research was determining the possibility of air purification by the use of water microjets. For this purpose, an experimental apparatus (Fig. 9) was built to show the effect of cleaning the air by one, two, three, or four microjet batteries. Tests of filtration effectiveness were carried out in accordance with EN 779:2012.
Microjet air purification unit used for the experiment.
Figure 10 (Mollier I, x diagram for moist air) presents the results of changes in the exhaust air parameters during the cleaning process for one, two, three, and four microjet heads, respectively.
Air parameters during the air purification process.
The results show that the process is not purely isenthalpic, which follows from the theory of humidification. In addition, as a consequence of air humidification, the air temperature felt down. This phenomenon is a result of water evaporation. Another issue considered was recuperation of heat from the exhaust air. In this case it was important that the filtering process does not decrease the air temperature. As shown in Figure 10, with one microjet head applied the air temperature decreases by 7 K, while with 4 microjet heads the air temperature decreases by 15 K.
Filtration eflciency.
| Working microjet | Filtration eflciency |
|---|---|
| heads | [%] |
| 1 head | 75 |
| 2 heads | 81 |
| 3 heads | 87 |
| 4 heads | 93 |
3.2 The use of an adsorption refrigeration unit
In order to reduce energy consumption, it was decided to check how much energy can be saved using a sorption refrigeration unit. There are two types of sorption refrigeration devices [15, 16]:absorption and adsorption units [13, 17] More devices are used for the absorption process and they are more widely known, but in this case it is only possible to use an adsorption refrigerating appliance, because only this type can work with heat at a temperature lower than 75°C [12]. The adsorption refrigeration unit will be driven by heat obtained from the air compressor cooling system. The heat transfer fluid will be water at a temperature of 60 to 75°C. The adsorption refrigeration unit works with three circuits (Fig. 11). In this case, the heat source circuit will be a cooling system for the air compressor; the second one is the chilled water circuit for the production of cold; and the third one is the outside heat sink in the form of a dry cooler.
The adsorption refrigeration unit and cooperating circuits.
As part of the experimental work, an adsorption refrigeration unit with a nominal capacity of 10 kW was installed and the effects of its operation were examined. During the test, temperatures were measured at the points shown in Figure 11. In each of the three circuits, the flow of water was continuously measured. A five-sectioned heater with a total heating power of 25 kW was used as a heat source. The dry cooler as well as the cold receiver were finned coil heat exchangers with variable fan speed control.
The results were compared with those of a standard vapour compressor refrigeration unit with the same nominal capacity. The results of the comparison of these two systems are presented in Figures 12 and 13. In both graphs the results are shown as a function of the ambient air temperature, because it turned out to be most important parameter affecting the operation of the adsorption refrigeration device. The coefficient of performance COP was determined according to the equation
The cooling capacity
where
where
Influence of the outdoor air temperature on the cooling capacity of the refrigeration equipment.
Influence of the outdoor air temperature on the coefficient of performance COP.
According to Figure 12, no cooling power is generated when the ambient temperature exceeds 30°C. The coefficient of performance COP drastically drops from 0.4 at 15°C to 0.0 at 30°C. The results indicate that the use of the adsorption refrigeration equipment is limited to a certain part of the year. To expand the scope of application of adsorption refrigeration appliances, an evaporative heat exchanger should be used instead of the dry cooler.
4 Proposed improvements in the factory
Some proposals for improvements in the manufacturing plant are given in this section. It should be noted that they are to be considered separately, since in combination they affect each other.
4.1 Regenerative heat exchangers without air purification
As far as refrigeration and air conditioning systems are concerned, heat exchangers designed with microchannels [18, 19] have gained popularity. In the case of the plant considered, the approach to be taken is exactly opposite. The use of the other type of heat exchangers was suggested. The proposed heat exchanger is temporarily insensitive to dirt. The structure of the heat exchanger, being a cassette-type heat exchanger, is shown in Figure 14. A shell-and-tube construction was also considered, but its effectiveness with regard to air proved to be highly inadequate [20].
Cassette-type heat exchanger.
Table 2 lists the results of calculations for the proposed heat exchanger for one exhaust fan (15,000 m3/h) using the finite difference method and the dependence of the heat transfer coefficient k according to the equation:
where αout(Δ Tout) is the heat transfer coefficient between the sheet and the exhaust air from the hall as a function of the temperature difference between the wall and the air Δ Tout ⋅ α(ΔT) is the coefficient of heat transfer between the sheet and the fresh air as a function of the temperature difference between the wall and the air ΔT. δsheet is the thickness of the sheet, λsheet the thermal conductivity of the sheet material, and Rdeposit(t) the thermal resistance of the tar sludge building up as a function of time t.
The calculation results for the plate heat exchanger.
| The distance between plates | The coeflcient of heat transfer from the exhaust air side | Heat transfer coeflcient from the air supply side | The heat transfer coeflcient - k | Heat exchanger capacity Q | The eflciency of the heat exchanger heat |
|---|---|---|---|---|---|
| [cm] | [W/m2/K] | [W/m2/K] | [W/m2/K] | [kW] | [%] |
| 5 | 11.95 | 12.60 | 6.13 | 28.60 | 15.4 |
| 4 | 12.51 | 13.16 | 6.41 | 35.13 | 19.1 |
| 3 | 13.38 | 14.02 | 6.85 | 46.02 | 24.9 |
| 2 | 14.92 | 15.53 | 7.61 | 64.33 | 34.9 |
As the technology of heat exchanger construction advances, new materials have been developed in addition to titanium and grapheme [21]. However, in the case of the proposed heat exchanger, the membrane material is of secondary importance. The reason is that the resistance to heat transfer is associated mainly with the heat transfer coefficient between air and the membrane.
4.2 Cassette-type heat exchangers with microjet air purification
According to the measurements described in Section 3, the application with two heads can remove particulates from the air with the efficiency of 81% and this leads to a temperature decrease of only 8 K. Therefore, the second solution proposed is a combination of this method of air purification and the use of a cassette-type heat exchanger. Table 3 presents the results for the exhaust air temperature reduced to 27°C and a smaller distance between the sheets inside the heat exchangers. The distance can be smaller because the amount of sludge in the heat exchanger will be smaller. At a distance of 1 cm, a larger heat flux was achieved in the recovery unit than in the case of an untreated air and the distance of 2 cm between the plates.
The calculation results for the plate heat exchanger.
| The distance between plates | The coeflcient of heat transfer from the exhaust air side | Heat transfer coeflcient from the air supply side | The heat transfer coeflcient - k | Heat exchanger capacity Q | The eflciency of the heat exchanger heat |
|---|---|---|---|---|---|
| [cm] | [W/m2/K] | [W/m2/K] | [W/m2/K] | [kW] | [%] |
| 2.0 | 15.08 | 15.57 | 7.61 | 50.21 | 34.8 |
| 1.0 | 18.16 | 18.57 | 9.18 | 78.43 | 55.2 |
| 0.5 | 22.45 | 22.74 | 11.29 | 103.80 | 73.3 |
4.3 The use of superheat from the refrigeration unit
The water chiller is operated through the whole year and at this stage, superheat is not used (Fig. 15). The superheat is the heat ata temperature higher than it is in the condenser. For this reason, it may become economically attractive [18].
Superheat in the refrigeration cycle.
For refrigerant R134a, the highest temperature of fluid in a pipe downstream the compressor can be 62°C, which makes it a good heat source for hot water or space heating production. If the chiller operates throughout the year with a nominal cooling capacity of 515 kW, the super heater could achieve the heating capacity of 85 kW.
4.4 The use of an adsorption refrigeration apparatus
Experimental studies show that when dry coolers are used in combination with an adsorption refrigeration device, the adsorption units fail to produce cold during warm summer periods. A much better solution is to use evaporative coolers. They are able to decrease the temperature of cooling water to a lower level than the air temperature. In principle, evaporative cooling operation tends to obtain dew point temperature. This solution will enable the adsorption refrigeration unit to work even in the days when the air temperature exceeds 30°C.
5 Final proposal of changes
The selected proposals for changes are the result of the economic analysis based on the analysis of simple payback times. It turned out that the best solution is, first, to use cassette-type heat exchangers without air purification. The investment cost for a single heat exchanger is 50,500.00 PLN net and savings for one exchanger amount to 51,700.00 PLN/year. The payback period is less than one year. The operating costs include the replacement of the exchanger once a year.
The second improvement in the plant is a combination of processes according to Figure 16. This solution can be called a kind of poly generation [22].
Diagram of the proposed improvements.
Table 4 lists investment costs of the proposed solution. The main item (and the most expensive one) is the adsorption refrigerating equipment.
Summary of investment costs.
| Name | Price |
|---|---|
| Adsorption refrigeration unit (160 kW) | 799,590 PLN |
| Evaporative recooler | 45,000 PLN |
| Plate heat exchanger in chilled water circuits | 10,000 PLN |
| Heat exchanger HX1 | 12,000 PLN |
| Water pump for HX1 loop | 1,000 PLN |
| Materials and installation | 15,000 PLN |
| Total | 882,590 PLN |
Due to the fact that the refrigeration chiller will not operate at full capacity, since the adsorption device can partly replace it, and no cooling will be required for the air compressor, the water chiller capacity will decrease by 147 kW. This will give profits at a level of 160,965.00 PLN per year. The capacity of the super heater will be proportionally smaller and decrease to 60.74 kW (first calculated as 85 kW). The use of the super heater will result in saving a total of 32,5341 of oil, i.e. 71,249 PLN/year.
The annual profits are:
The total profit is 232,214.00 PLN/year, which gives a simple payback time
6 Conclusions
Through an individual approach to technological issues at the factory, changes were proposed which can quickly help achieve results in terms of energy efficiency and profits in the current operation. If cassette-type heat exchangers with a 2-cm distance between the plates are used, each heat exchanger obtains 64.33 kW with the average exchanger efficiency of 34.9%. Twelve such heat exchangers will be placed in the production halls, which will make the total consumption of heat produced by oil boilers in winter fall by 772 kW, while the cost of improvements is PLN 606,000.00. That will give the simple payback time of less than one year. Another aspect of the improvement is connecting the processes of cold, heat, and compressed air production. This solution causes the generation of cooling capacity in the chiller to fall by about 147 kW, and provides an extra heat generation of 60.74 kW from the super heater. The solution offers the profit of 232,214.00 PLN/year and a simple payback time of 3.8 years. The study found that in industrial plants where the air is contaminated with tar, cleaning is not cost-efficient, but improving energy efficiency is unquestionably feasible.
Acknowledgement
This contribution is the result of the project ’Studies to evaluate the possibility of temperature stabilization of the production process of vulcanization and its surroundings while reducing energy consumption’ under the Regional Operational Programme for Kujawsko-Pomorskie Voivodeship, implemented within the framework of Measure 5.4 Strengthening the Regional Capacity for Research and Technological Development of the Regional Operational Programme for Kujawsko-Pomorskie Voivodeship 2014-2020.
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© 2017 Andrzej Grzebielec et al.
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