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Design Theory of Composite Fume Purification Unit Authors: Qiu Huiqing, Ren Jinsheng, School of Mechanical Engineering, Tongji University Abstract: To improve the fume removal efficiency of small high-pressure fume purifiers, a composite fume purification unit that utilizes an electric field to even out the air flow in the intake area and a simple pre-mounted vortex dust collector has been developed. Given that large industrial electrostatic dust collectors are generally used in applications with high air flow rates, high voltages, large pole distances, long electric field lengths, and large ventilation cross-sections, small high-pressure oil fume purifiers have distinct characteristics compared to them. Therefore, the selection and calculation of the structural parameters, airflow parameters, electrical parameters, and performance parameters for small high-pressure oil fume purifiers should follow design principles different from those of traditional large industrial electrostatic dust collectors. The design theory of a composite oil fume purifier that combines volute cyclone and high-voltage electrostatic technology is introduced. Keywords: volute cyclone, high-voltage static electricity, oil fume purification 1 Introduction The emission of oil fumes from kitchens in the urban catering industry has always been a significant source of air pollution in cities, and these oil fumes posing a nuisance to residents have become a focal point in many environmental complaints in urban areas. **The Environmental Protection Administration promptly introduced mandatory environmental protection standards, stipulating that fumes from the food service industry must not be released into the atmosphere naturally but must be purified, and it set limits on the maximum concentration of such fumes as well as the minimum removal efficiency required of fume purification equipment. Recently, the author developed a composite oil fume purifier that combines volute cyclone technology with high-pressure static pressure. After being used by users, tested by **authorized testing institutions, and evaluated by environmental protection agencies, this purifier has achieved an oil fume removal rate of 90.7%. Small high-voltage electrostatic oil fume purifiers differ from large high-voltage electrostatic industrial dust removal equipment; this article attempts to introduce the theory behind the design of composite oil fume purifiers through a comparison between the two. 2 Structure and Working Principle: Large industrial electrostatic precipitators are typically characterized by a high air handling capacity and high dust collection efficiency. They use high voltages (above 100 kV, sometimes even up to 600 kV), wide pole distances (300–600 mm). The corona electrodes are usually in the form of spikes or stars, while the collection electrodes are often plate or tube-shaped. The length of the electric field is several meters or more. Due to the large volume of air to be processed, and in order to maintain the wind speed limits within the electric field, it is necessary to increase the cross-sectional area; as a result, their volume is very large. Small high-voltage electrostatic oil fume purifiers are generally used for removing oil fumes from restaurants in urban areas, where installation space is limited and large sizes are not allowed. To reduce costs and ensure safe use, it is necessary to lower the voltage; therefore, low-to-medium voltages (20–30 kV), small pole distances (40–60 mm), and an electric field length of only 400–600 mm are commonly used. An oil fume removal efficiency of 80%–90% is sufficient to meet **environmental standards. The corona electrode is generally a thin circular tube with a diameter of 2–3 mm or a platinum-rhodium alloy wire. The dust collection electrodes can be of plate type, honeycomb type, square shape, or circular tube shape; among these, the honeycomb type has the largest surface area for dust collection, which is why it is the most widely used. The composite oil fume purifier developed by the author, which combines a volute cyclone with high-voltage electrostatic technology, makes clever use of the air intake and distribution section at the bottom of conventional high-voltage electrostatic oil fume purifiers. A volute cyclone dust collector is designed to separate larger oil fume particles through cyclonic action, thereby rendering the oil fume particles entering the electric field smaller and more uniform in size. This not only improves the dust collection efficiency of the electric field but also prevents electric field breakdown caused by large particles at small pole distances. The aerodynamic resistance of a simple volute cyclone is very low; therefore, the original axial flow fan used for exhausting kitchen fumes can generally still be used. The overall structure of the machine is shown in Figure 1. The simple volute utilizes the cyclone separator found in conventional dust collectors; the dust-laden airflow enters the volute at high speed in a tangential direction. The oil and smoke particles in the rotating airflow are thrown against the wall of the device due to centrifugal inertial force, where they then fall and get collected as dust. Figure 1: Structure of the composite oil fume purifier. The mechanism of high-voltage electrostatic dust removal is briefly described as follows: A corona electrode is supplied with high-voltage electricity, thereby creating a non-uniform electric field. Due to the very small curvature radius of the corona electrode, the electric field strength on its surface is extremely high, causing the air at the surface to be ionized and free electrons to be generated. As these electrons move toward the collection electrode, they collide with the oil and smoke particles in the dust-containing airflow, ionizing them as well; all of these particles then move toward the collection electrode at a certain velocity. Once neutralized, they fall downward, thereby enabling dust collection. The ionization of a large number of neutral molecules is known as an electron avalanche. This process produces blue flashes and a hissing sound; it is also known as corona discharge. The electric field strength at the occurrence of corona discharge is called the critical electric field strength, and the voltage at this point is called the critical voltage. If the voltage is increased further thereafter, a spark breakdown will occur between the two poles, which is known as the breakdown voltage. The electrostatic oil fume purifier should operate between the corona voltage and the breakdown voltage. 3 Selection and calculation of main technical parameters 3.1 Processing air volume Series of products can be developed based on the processing air volume specified by the Environmental Protection Administration for different numbers of stoves. This article takes the Sanzao unit with an air volume of 6,000 m3/h as an example. 3.2 Cyclone wind speed: The cyclone wind speed entering the volute is generally preferably in the range of 10–18 m/s; for this model, a value of 17 m/s is adopted. Based on this, the size of the air inlet can be determined. 3.3 Electric field wind speed: The electric field wind speed is generally taken as 0.4–1.5 m/s, based on which the effective ventilation cross-sectional area of the electric field can be determined. 3.4 Critical electric field strength can be calculated using the empirical formula for large-scale industrial electrostatic dust collectors: Where Ec is the critical electric field strength, in V/cm ； δ——relative density of the gas ； r —— corona electrode half, cm. 3.5 The critical voltage can be calculated using the following formula: where R is the radius of the collection electrode, in cm. 3.6 Dust propulsion velocity The dust propulsion velocity refers to the speed at which charged and ionized particles move toward the collection electrode, and it is calculated using the following formula: Where ue is the dust propulsion velocity, in m/s ； E — average electric field strength, V/m ； dp —— dust particle size, m ； ——Gas viscosity, Pa·s. 3.7 Electric field length: To ensure that the oil and smoke particles settle on the dust collection electrode as the dusty airflow passes through the electric field, the length of the electric field must satisfy the following formula: Where L is the electric field length, in meters ； u——electric field wind speed, m/s. 3.8 Dust Collection Efficiency The dust collection efficiency of this model should be the sum of the dust collection efficiency provided by the pre-installed volute cyclone and that provided by the rear-mounted high-voltage electrostatic system. The dust collection efficiency of simple volute cyclones can be referenced from cyclone dust collectors. Generally, when the cyclone wind speed is 12–18 m/s, the dust particle size dp is >10 µm, and the dust concentration is below 50 δ/m3, the dust collection efficiency η1 of the volute cyclone is 80%–85%. The dust collection efficiency η2 for high-voltage electrostatic dust removal can be calculated using the following formula: where A is the area of the dust collection electrode, in m² ； θ —— Dust-containing gas flow rate, m3/s. The overall dust removal efficiency is given by: η＝η1＋(1－η1)·η2. 4 Conclusion 4.1 The preliminary use of a simple volute-type cyclone for dust removal allows for the removal of larger oil and smoke particles in advance; this not only prevents electric field breakdown but also ensures that the oil and smoke particles entering the electric field are distributed evenly, thereby **improving the efficiency of high-voltage oil and smoke purification devices. 4.2 Based on the different characteristics of small-scale oil fume purifiers and large-scale electrostatic dust collectors, design principles and simple and practical formulas for calculating the main technical parameters of a composite oil fume purifier that combines volute cyclone and high-voltage electrostatic technology are proposed. 4.3 The prototype developed was tested by the Dust Removal Equipment Testing Center of the Ministry of Environmental Protection; its oil fume removal efficiency reached 90.7%, with an oil fume emission concentration of only 0.94 mg/m3. This shows that the parameter selection calculations based on the design theory proposed by the author are feasible. 5 References 1 Cen Kefa. Theory and Technology of Gas-Solid Separation. Hangzhou: Zhejiang University Press, 1999. 2 Tong Zhiquan. Purification and Utilization of Industrial Waste Gases. Beijing: Chemical Industry Press, 2001. The first author is Qiu Huiqing, male, born in 1948; he graduated from Shanghai Maritime College in 1982 and is an associate professor.