A method for optimizing design of a deflector for indoor ventilation

Abstract

The application discloses a kind of indoor ventilation-oriented guide vane optimization design methods, comprising the following steps: according to condition given ventilation system parameter, utilize numerical method to solve indoor ventilation optimization equation, obtain the best flow field corresponding to the lowest pollutant concentration, then obtain the streamline corresponding to flow rate ratio parameter at air supply outlet, according to the streamline shape and the height set to determine the final guide vane shape.The method of the application is suitable for any indoor ventilation system, any pollutant and any number of pollution sources, can significantly reduce the pollutant concentration of breathing zone by setting guide vane in the given system, with the advantages of simple implementation process, good performance index, good scalability, strong practicality and the like.

Description

Technical Field

[0001] This invention belongs to the field of indoor ventilation and relates to an optimized design method for a baffle plate for indoor ventilation. Background Technology

[0002] People spend most of their daily lives and work indoors, and indoor air quality directly impacts their physical and mental health. Good indoor air quality ensures physical well-being and improves work and study efficiency. Indoor air pollutants are a significant factor affecting indoor air quality; excessively high pollutant concentrations can harm human health, potentially triggering asthma or even cancer. Therefore, measures need to be taken to reduce pollutant concentrations and improve indoor air quality. Ventilation is an effective method for reducing pollutant concentrations. The process of removing pollutants through ventilation is a convective mass transfer process, and the indoor airflow field affects the diffusion of pollutants. By optimizing the indoor airflow field design, the effectiveness of ventilation in removing pollutants can be significantly improved.

[0003] Currently, most studies obtain different indoor flow fields by changing the location of ventilation vents, and then compare the pollutant distribution corresponding to different flow fields to obtain a better design. Qin et al. studied the impact of the exhaust vent location on the pollutant removal effect in a jet ventilation system for densely populated classrooms. They found that the indoor air quality was best when the exhaust vents were placed on the ceiling on the same side as the supply vents, and the pollutant removal effect was better than when multiple exhaust vents were evenly distributed on the ceiling. The results show that for large indoor spaces such as classrooms, the location of the exhaust vents should be determined according to the indoor flow field, and the key is to place the exhaust vents near areas with high pollutant concentrations (Chao Qin, Yuanping He, Jian Li, Wei-Zhen Lu. Mitigation of breathing contaminants: Exhaust location optimization for indoor space with impinging jet ventilation supply [J]. Journal of Building Engineering, 2023, 69, 106250.). For rented rooms or offices, users cannot arbitrarily adjust the ventilation system. In this case, the indoor flow field can be changed by adjusting the indoor layout to reduce the pollutant concentration. Zhuang et al. studied the effects of changing the layout of indoor furniture on indoor air quality under a given ventilation system. They compared the indoor airflow and pollutant concentration fields under three different furniture layouts. The results showed that furniture layout can affect the indoor airflow and temperature fields. Without changing the ventilation system, adjusting the furniture layout can significantly improve the air quality in the breathing zone (Ruining Zhuang, Xiangdong Li, Jiyuan Tu. CFD study of the effects offurniture layout on indoor air quality under typical office ventilationschemes [J]. Building Simulation, 2014, 7, 263-275.).

[0004] The aforementioned approach to optimizing indoor ventilation systems involves experimentally adjusting the location of ventilation openings or the indoor layout to obtain different flow fields. The pollutant removal efficiency of these different flow fields is then compared to arrive at a better result. However, this approach is empirical and lacks unified theoretical guidance. Chen et al. analyzed the relationship between the indoor flow field and the pollutant concentration field, defined a mass transfer potential capacity dissipation function, and used the extreme value of this function as the optimization objective. Using variational principles, they derived the equation that the optimal flow field should satisfy, and then obtained the optimal flow field through numerical solutions to guide improvements in the indoor flow field (QunChen, Jianxun Ren, Zengyuan Guo. Field synergy analysis and optimization of decontamination ventilation designs [J]. International Journal of Heat and Mass Transfer, 2008, 51, 873-881.). Hu et al., using the minimization of the average indoor SVOC concentration as the optimization objective, obtained the optimal indoor flow field using variational principles and numerical solutions. Based on the optimal flow field, they adjusted the air inlet and outlet positions, resulting in an optimized indoor SVOC concentration of 0.33 mg / m³. 3 Decreased to 0.23 mg / m³ 3 (Kang Hu, Qun Chen. Ventilation optimization for reduction of indoor semi-volatile organic compound concentration based on the variational principle [J]. Building and Environment, 2015, 94, 676-682.).

[0005] In summary, optimizing the indoor flow field using variational optimization equations can effectively reduce pollutant concentrations, and the resulting optimal flow field can effectively guide the design of indoor ventilation systems. However, current research targets the entire room for optimization, rather than the more critical breathing zone. Furthermore, the optimization objective is the average pollutant concentration, not the maximum concentration, which may lead to excessively high pollutant concentrations in localized areas. Simultaneously, an effective method for constructing a realistic flow field closely resembling the optimal flow field is still lacking. Therefore, it is necessary to develop a general and effective deflector optimization design method for indoor ventilation. By designing the shape of the deflector, a realistic flow field close to the optimal flow field can be constructed, thereby achieving effective improvement in indoor air quality. Summary of the Invention

[0006] In order to overcome the shortcomings and deficiencies of the existing technology, the present invention provides an optimized design method for baffles for indoor ventilation, which is applicable to indoor ventilation systems of different shapes and structures.

[0007] The present invention is achieved by at least one of the following technical solutions.

[0008] A method for optimizing the design of a deflector for indoor ventilation includes the following steps: S1. Given the indoor ventilation system parameters, including the shape and size of the indoor system, the physical parameters and boundary conditions of the ventilation air, the number, size and pollutant release rate of indoor objects, the flow rate and inlet pollutant concentration of the ventilation air, the pump power coefficient of the ventilation air, the optimization objective, the flow rate ratio parameter for controlling the shape of the baffle, and the bottom height of the baffle. S2. Solve the indoor ventilation optimization equation using numerical methods to obtain the optimal flow field corresponding to the lowest pollutant concentration; S3. At the air outlet, obtain the streamline corresponding to the flow rate ratio parameter in the optimal flow field; at the location of the streamline, set a guide vane with the same shape as the streamline; S4. Move the guide plate upwards so that the top of the guide plate is at the same height as the top of the air outlet; cut off the part of the guide plate whose height is lower than the bottom of the guide plate. The resulting guide plate is the final design guide plate.

[0009] Furthermore, in step S2, the indoor ventilation optimization equations include the continuity equation, momentum equation, concentration diffusion equation, and Lagrange multiplier equation for the breathing and non-breathing zones.

[0010] Furthermore, the control equations for the breathing zone and the non-breathing zone are as follows: The continuity equation is expressed as: ; The momentum equation is expressed as: ; The concentration diffusion equation is expressed as: ; The Lagrange multiplier equation is expressed as: ; in, , , These represent air density, effective dynamic viscosity of air, and concentration diffusion coefficient of pollutants in air, respectively. Represents the velocity vector of airflow; Represents the Hamiltonian operator. Represents the divergence of the velocity field. Represents the velocity gradient; Indicates pressure; Indicates pollutant concentration; Represents the Lagrange multipliers; The pump power coefficient indicating the ventilation air; This represents the source term of the Lagrange multiplier equation.

[0011] Furthermore, the source terms of the Lagrange multiplier equations for the breathing and non-breathing regions are as follows: The source terms in the respiratory region are: ; The source terms in the non-respiratory region are: ; in, , Indicates two reference concentrations; It represents an exponential constant greater than 0.

[0012] Furthermore, the boundary conditions for the concentration diffusion equation in the indoor ventilation optimization equation include: First type of boundary condition: air outlet, pollutant concentration is 0; Second type of boundary conditions: exhaust outlet, pollutant concentration gradient is 0; pollution source, pollutant concentration flow is constant; non-pollution source, pollutant concentration flow is 0; Coupled boundary condition: The pollutant concentration is continuous at the interface between the breathing zone and the non-breathing zone.

[0013] Furthermore, the boundary conditions for the Lagrange multiplier equations include: First type of boundary condition: air outlet, the value of the Lagrange multiplier is 0; Second type of boundary condition: exhaust vent, the gradient of the Lagrange multiplier is 0; pollution source and non-pollution source, the gradient of the Lagrange multiplier is 0; Coupled boundary conditions: The Lagrange multipliers are continuous at the interface between the breathing and non-breathing regions.

[0014] Furthermore, in the Lagrange multiplier equation for the respiratory region, the reference concentration... and Must meet: .

[0015] Furthermore, in the Lagrange multiplier equations of the respiratory region, when the exponent... γ When the index is set to 1, the optimization objective is to minimize the average concentration of pollutants in the breathing zone; when the index is set to 1... When the value is greater than 5, the optimization goal is to minimize the highest concentration of pollutants in the breathing zone.

[0016] Furthermore, the flow ratio parameter for controlling the shape of the deflector should be greater than 0 and less than 1.

[0017] Furthermore, in order not to affect the activities of people indoors, the bottom of the deflector should be no lower than the highest point of the breathing zone.

[0018] Compared with existing technologies, the beneficial effects of the present invention are as follows: (1) Simple implementation process: The design of the guide vane shape in this invention mainly involves two stages. The first stage is to obtain the optimal flow field corresponding to the lowest pollutant concentration by numerically solving the indoor ventilation optimization equation under given indoor ventilation system parameters; the second stage is to set the guide vane at the air outlet after obtaining the optimal flow field and adjust the shape of the guide vane to obtain an actual flow field that is as close as possible to the optimal flow field. The implementation process of this invention is simple and does not involve complicated steps; (2) Good performance indicators: The present invention sets the shape of the guide plate according to the optimal flow field streamline of pollutants, so that the actual flow field is close to the theoretical optimal flow field. Therefore, the actual flow field after setting the guide plate has a better pollutant removal effect, which can significantly improve indoor air quality and has the advantage of good performance indicators. (3) Good scalability: The method involved in this invention can be extended to the solution of similar problems, including indoor spaces with irregular shapes and complex layouts, various other types of pollutants, and various complex sources of pollutant emission, etc., and has the advantage of good scalability; (4) High practicality: The research results of this invention do not require any modification to the existing ventilation system. Only a guide plate needs to be added at the air outlet to reduce the concentration of indoor pollutants and improve indoor air quality without significantly increasing costs. It has high practicality. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating an optimized design method for a baffle plate for indoor ventilation, as described in an embodiment. Figure 2 This is a schematic diagram of any room described in the embodiment; Figure 3 This is a schematic diagram of the deflector setup and adjustment in an embodiment; Figure 4 This is a schematic diagram of the room size parameters in Example 1; Figure 5 This refers to the pollutant concentration field in front of the deflector in Example 1; Figure 6 This is the pollutant concentration field after the guide plate is set in Example 1. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0021] like Figure 2As shown, an indoor ventilation system includes a breathing zone, a non-breathing zone, pollution sources, air supply vents, and exhaust vents. In any indoor ventilation system, there are one or more pollution sources. These pollution sources can vary in size and heat intensity. When optimizing the design of the airflow guide plate for indoor ventilation, this invention is needed to design the guide plate according to the optimization objective, so that the pollutant concentration in the breathing zone is minimized after the guide plate is installed. The process of setting and adjusting the airflow guide plate described in this invention is as follows: Figure 3 As shown.

[0022] This embodiment of a deflector optimization design method for indoor ventilation includes the following steps: S1. Given the parameters of the indoor ventilation system, including the shape and size of the indoor system, the physical parameters and boundary conditions of the ventilation air, the number, size, and pollutant release rate of indoor objects, the flow rate and inlet pollutant concentration of the ventilation air, the pump power coefficient of the ventilation air, the optimization objective, and the flow rate ratio parameters for controlling the shape of the baffle. α q Height of the bottom of the deflector H d wait.

[0023] S2. Solve the indoor ventilation optimization equation using numerical methods to obtain the optimal flow field corresponding to the lowest pollutant concentration.

[0024] The indoor ventilation optimization equations include the continuity equation, momentum equation, concentration diffusion equation, and Lagrange multiplier equation for the breathing zone, as well as the continuity equation, momentum equation, concentration diffusion equation, and Lagrange multiplier equation for the non-breathing zone.

[0025] Specifically, the control equations for the respiratory zone are as follows: The continuity equation is expressed as: ; The momentum equation is expressed as: ; The concentration diffusion equation is expressed as: ; The Lagrange multiplier equation is expressed as: ; in, , , These represent air density, effective dynamic viscosity of air, and concentration diffusion coefficient of pollutants in air, respectively. Represents the velocity vector of airflow; Represents the Hamiltonian operator. Represents the divergence of the velocity field. Represents the velocity gradient; Indicates pressure; Indicates the concentration of pollutants in the breathing zone; Lagrange multipliers representing the respiratory region; The pump power coefficient indicating the ventilation air; , Indicates two reference concentrations, , The values ​​of all values ​​are greater than 0, and > ; It represents an exponential constant greater than 0.

[0026] The control equations for the non-respiratory zone are shown below: The continuity equation is expressed as: ; The momentum equation is expressed as: ; The concentration diffusion equation is expressed as: ; The Lagrange multiplier equation is expressed as: ; in, , , These represent air density, effective dynamic viscosity of air, and concentration diffusion coefficient of pollutants in air, respectively. Represents the velocity vector of airflow; Represents the Hamiltonian operator. Represents the divergence of the velocity field. Represents the velocity gradient; Indicates pressure; Indicates the concentration of pollutants in the non-respiratory zone; Lagrange multipliers representing non-breathing regions; The pump power coefficient indicating the ventilation air; , Indicates two reference concentrations, , The values ​​of all values ​​are greater than 0, and > ; It represents an exponential constant greater than 0.

[0027] The boundary conditions for the pollutant concentration equation in the indoor ventilation optimization equation are as follows: The boundary conditions for the air outlet are expressed as follows: ; The boundary conditions for the exhaust vent are expressed as follows: ; The boundary conditions of a pollution source are expressed as follows: ; The boundary conditions for non-pollution sources are expressed as follows: ; The boundary conditions at the interface between the breathing zone and the non-breathing zone are expressed as follows: ; in Indicates concentration. Indicates the concentration of pollutants in the breathing zone. Indicates the concentration of pollutants in the non-respiratory zone; Represents the unit normal vector of the boundary surface; This represents the dispersion coefficient of pollutants in the air. Represents a constant.

[0028] The boundary conditions for the Lagrange multiplier equation in the indoor ventilation optimization equation are as follows: The boundary conditions for the air outlet are expressed as follows: ; The boundary conditions for the exhaust vent are expressed as follows: ; The boundary conditions of a pollution source are expressed as follows: ; The boundary conditions for non-pollution sources are expressed as follows: ; The boundary conditions at the interface between the breathing zone and the non-breathing zone are expressed as follows: ; in It represents the Lagrange multiplier.

[0029] In the Lagrange multiplier equations of the breathing region, when the exponent γ When the index is set to 1, the optimization objective is to minimize the average pollutant concentration in the breathing zone. When the index... γ When the value is greater than 5, the optimization goal is to minimize the highest concentration of pollutants in the breathing zone.

[0030] S3. Several streamlines exist in the flow field. Different flow rate ratios correspond to different streamlines. Based on a given flow rate ratio parameter, the streamline corresponding to that flow rate ratio can be obtained. At the air outlet, the optimal flow field corresponds to the flow rate ratio parameter. α q The corresponding streamlines are denoted as S α ; in streamline S α Set a shape and streamline at the location. S α The same type of deflector is denoted as D α ; Flow ratio parameters that control the shape of the guide vane α q The value should be greater than 0 and less than 1.

[0031] S4, Place the deflector plate D α Move it upwards so that its top is at the same height as the top of the air outlet; cut off the guide vane so that its height is lower than the bottom of the guide vane. H dThe part obtained is the guide vane of the final design.

[0032] Example 1 This embodiment considers a two-dimensional room, the location and dimensions of each part of the room are as follows: Figure 4 As shown: The room is 4 m long and 3 m high. A piece of furniture is placed in the lower left corner of the room. The furniture is 0.5 m wide and 1.8 m high. The air supply vent is in the upper left corner of the room, and the air exhaust vent is in the lower right corner of the room. The width of both the air supply and exhaust vents is 0.2 m. The density and dynamic viscosity of the ventilated air are 1.225 kg / m³. 3 1.79 × 10 5 The sources of pollution in the room were the floor and furniture, with the surface concentration of the floor and furniture consistently at 0.27 mg / (m²). 2 Formaldehyde is released at a rate of ·h), while no formaldehyde is released from the other surfaces; the diffusion coefficient of formaldehyde in air is 1.9 × 10⁻⁶. -5 m 2 / s; the supply air outlet boundary condition is a velocity inlet with an air velocity of 0.2 m / s and a pollutant concentration of 0; the exhaust air outlet boundary condition is a pressure outlet with a gauge pressure of 0; the breathing zone is 0.3 ~ 1.8 m above the ground. The system pollutant concentration field is as follows: Figure 5 As shown, the highest formaldehyde concentration in the breathing zone was 1.58 × 10⁻⁶. -7 kg / m 3 .

[0033] The optimization method of this invention aims to minimize the highest formaldehyde concentration in the breathing zone, determining the shape of the baffle plate to reduce the highest formaldehyde concentration in the breathing zone as much as possible. Figure 1 The workflow diagram of this invention, and the specific steps of the optimization process are as follows: S1. Based on the optimization problem, given the shape and size of the indoor system, the physical properties and boundary conditions of the ventilation air, the number and size of indoor objects and their pollutant release rates, the flow rate of the ventilation air, and the inlet pollutant concentration. In this embodiment, the pump power coefficient of the ventilation air is taken as 9.6 × 10⁻⁶. -16 The optimization objective is to minimize the highest formaldehyde concentration in the breathing zone. γ Value: 20; Flow rate ratio parameter α q The value is 0.5; the height of the bottom of the deflector plate. H d It is 1.8 m.

[0034] S2. Solve the indoor ventilation optimization equation using numerical methods to obtain the optimal flow field corresponding to the lowest pollutant concentration.

[0035] S3. At the air outlet with the optimal flow field, obtain the streamline corresponding to a flow rate ratio parameter of 0.7, denoted as...S 0.7 ; in streamline S 0.7 Set a shape and streamline at the location. S 0.7 The same type of deflector is denoted as D 0.7 ; S4, Place the deflector plate D 0.7 Move it upwards so that its top is at the same height as the top of the air outlet; cut off the portion of the guide vane whose height is less than 1.8 m, and the resulting guide vane is the final design guide vane. D 0.7,1.8 .

[0036] The pollutant concentration field of the optimized indoor ventilation system is as follows: Figure 6 As shown, the highest formaldehyde concentration in the breathing zone was 1.24 × 10⁻⁶. -7 kg / m 3 Compared to the highest concentration of the system before the baffle was installed (1.58 × 10⁻⁶), -7 kg / m 3 The efficiency was reduced by 22%. This embodiment verifies the effectiveness of the present invention for optimizing the design of deflectors in indoor ventilation systems.

[0037] Example 2 This embodiment considers, for example Figure 4 The example shown illustrates the optimized design of a deflector plate for an indoor ventilation system, focusing on optimizing the shape of the deflector plate. The flow rate ratio parameter is also considered. α q The value is 0.5; the other parameters are the same as in Example 1. Using the optimization method of this invention, with the goal of minimizing the highest formaldehyde concentration in the breathing zone, the indoor ventilation system is optimized with a baffle design to reduce the highest formaldehyde concentration in the breathing zone as much as possible. After optimization, the highest formaldehyde concentration in the breathing zone of the indoor ventilation system is 1.33 × 10⁻⁶. -7 kg / m 3 Compared to the system without the deflector, the maximum concentration was reduced by 16%. This embodiment verifies the effectiveness of the present invention for optimizing the design of deflectors in indoor ventilation systems.

[0038] Example 3 This embodiment considers, for example Figure 4 The example shown illustrates the optimized design of a deflector for an indoor ventilation system, focusing on the shape of the deflector. Specifically, the height of the bottom of the deflector... H dThe value was taken as 2.5 m; the other parameters were the same as in Example 1. Using the optimization method of this invention, with the goal of minimizing the highest formaldehyde concentration in the breathing zone, the indoor ventilation system underwent a baffle optimization design to reduce the highest formaldehyde concentration in the breathing zone as much as possible. After optimization, the highest formaldehyde concentration in the breathing zone of the indoor ventilation system was 1.47 × 10⁻⁶ m. -7 kg / m 3 Compared to the system without the deflector, the maximum concentration was reduced by 7%. This embodiment verifies the effectiveness of the present invention for optimizing the design of deflectors in indoor ventilation systems.

[0039] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the embodiments described above. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for optimizing the design of a baffle plate for indoor ventilation, characterized in that, Includes the following steps: S1. Given the indoor ventilation system parameters, including the shape and size of the indoor system, the physical parameters and boundary conditions of the ventilation air, the number, size and pollutant release rate of indoor objects, the flow rate and inlet pollutant concentration of the ventilation air, the pump power coefficient of the ventilation air, the optimization objective, the flow rate ratio parameter for controlling the shape of the baffle, and the bottom height of the baffle. S2. The indoor ventilation optimization equation is solved using numerical methods to obtain the optimal flow field corresponding to the lowest pollutant concentration. The indoor ventilation optimization equation includes the continuity equation, momentum equation, concentration diffusion equation, and Lagrange multiplier equation for the breathing zone and non-breathing zone. The control equations for the breathing zone and the non-breathing zone are shown below: The continuity equation is expressed as: ; The momentum equation is expressed as: ; The concentration diffusion equation is expressed as: ; The Lagrange multiplier equation is expressed as: ; in, , , These represent air density, effective dynamic viscosity of air, and concentration diffusion coefficient of pollutants in air, respectively. Represents the velocity vector of airflow; Represents the Hamiltonian operator. Represents the divergence of the velocity field. Represents the velocity gradient; Indicates pressure; Indicates pollutant concentration; Represents the Lagrange multipliers; The pump power coefficient indicating the ventilation air; Represents the source terms of the Lagrange multiplier equation; The source terms of the Lagrange multiplier equations in the breathing and non-breathing regions are as follows: The source terms in the respiratory region are: ; The source terms in the non-respiratory region are: ; in, , Indicates two reference concentrations, and > >0; It represents an exponential constant greater than 0; S3. At the air outlet, obtain the streamline corresponding to the flow rate ratio parameter in the optimal flow field; at the location of the streamline, set a guide vane with the same shape as the streamline; S4. Move the guide plate upwards so that the top of the guide plate is at the same height as the top of the air outlet; cut off the part of the guide plate whose height is lower than the bottom of the guide plate, and the resulting guide plate is the final design guide plate.

2. The method for optimizing the design of a baffle plate for indoor ventilation according to claim 1, characterized in that, The boundary conditions for the concentration diffusion equation in the indoor ventilation optimization equation include: First type of boundary condition: air outlet, pollutant concentration is 0; Second type of boundary conditions: exhaust outlet, pollutant concentration gradient is 0; pollution source, pollutant concentration flow is constant; non-pollution source, pollutant concentration flow is 0; Coupled boundary condition: The pollutant concentration is continuous at the interface between the breathing zone and the non-breathing zone.

3. The method for optimizing the design of a baffle plate for indoor ventilation according to claim 1, characterized in that, The boundary conditions for the Lagrange multiplier equations include: First type of boundary condition: air outlet, the value of the Lagrange multiplier is 0; Second type of boundary condition: exhaust vent, the gradient of the Lagrange multiplier is 0; pollution source and non-pollution source, the gradient of the Lagrange multiplier is 0; Coupled boundary conditions: The Lagrange multipliers are continuous at the interface between the breathing and non-breathing regions.

4. The method for optimizing the design of a baffle plate for indoor ventilation according to claim 1, characterized in that, In the Lagrange multiplier equations of the breathing region, when the exponent γ When the index is set to 1, the optimization objective is to minimize the average concentration of pollutants in the breathing zone; when the index is set to 1... γ When the value is greater than 5, the optimization goal is to minimize the highest concentration of pollutants in the breathing zone.

5. The method for optimizing the design of a baffle plate for indoor ventilation according to claim 1, characterized in that, The flow ratio parameter for controlling the shape of the deflector should be greater than 0 and less than 1.

6. The method for optimizing the design of a baffle plate for indoor ventilation according to claim 1, characterized in that, To avoid affecting indoor activities, the bottom of the deflector should be no lower than the highest point of the breathing zone.

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