The air inside industrial buildings is much more polluted than in apartments and private houses. The types and amount of harmful emissions depends on many factors - the production industry, the type of raw materials, the technological equipment used, and so on. It is rather difficult to calculate and design the ventilation of industrial premises, which removes all the harmfulness. We will try to present in an accessible language the calculation methods prescribed in the regulatory documents.
Design algorithm
The organization of air exchange inside a public building or in production is carried out in several stages:
- The collection of initial data - the characteristics of the structure, the number of workers and the severity of labor, the varieties and the amount of hazard generated, the localization of the places of allocation. It is very useful to understand the essence of the process.
- The choice of the ventilation system of the workshop or office, the development of schemes. Three basic requirements are put forward for design decisions - efficiency, compliance with SNiP (SanPin) standards and economic feasibility.
- Calculation of air exchange - determination of the volume of supply and exhaust air for each room.
- Aerodynamic calculation of air ducts (if any), selection and placement of ventilation equipment. Clarification of the schemes for supplying and removing polluted air.
- Installation of ventilation according to the project, start-up, further operation and maintenance.
Note. For a better understanding of the process, the work list is greatly simplified. At all stages of documentation development, various approvals, clarifications, and additional examinations are required. Design engineer constantly works in conjunction with the company's technologists.
We are interested in points No. 2 and 3 - the choice of the optimal air exchange scheme and determination of air flow. Aerodynamics, installation of ventilation ducts and equipment are extensive topics of other publications.
Types of ventilation systems
To properly organize the updating of the air environment of the room, you need to choose the best ventilation method or a combination of several options. The classification scheme of existing ventilation systems arranged in production is shown in simplified form below in the structural diagram.
Explain each type of air exchange in more detail:
- Unorganized natural ventilation includes ventilation and infiltration - the penetration of air through door narthexes and other gaps. Organized supply - aeration - is made from windows through exhaust deflectors and anti-aircraft lights.
- Auxiliary roof and ceiling fans increase the intensity of exchange during the natural movement of air masses.
- A mechanical system involves the forced distribution and extraction of air by fans through air ducts. This also includes emergency ventilation and various local exhausts - umbrellas, panels, shelters, fume hoods.
- Air conditioning - bringing the air of the workshop or office to the required condition. Before being fed into the working area, the air is cleaned by filters, humidified / dried, heated or cooled.
Reference. According to the normative documentation, the lower part of the volume of the workshop 2 meters high from the floor, where people are constantly located, belongs to the serviced (working) zone.
Often, mechanical ventilation is combined with air heating - in winter, the street stream heats up to the optimum temperature, water radiators are not installed. Contaminated hot air is sent to the recuperator, where it gives 50-70% of the heat to the inflow.
A combination of the listed options allows achieving maximum operational efficiency at a moderate price of equipment. Example: in the welding workshop it is allowed to design natural aeration provided that each station is equipped with a forced local exhaust.
Selection tips
Direct instructions on the development of air exchange schemes give sanitary and industry standards, nothing needs to be invented or invented. The documents are developed separately for public buildings and various industries - metallurgical, chemical, catering and so on.
Example. When developing ventilation of a hot welding workshop, we find the document “Sanitary rules for welding, surfacing and cutting of metals”, we read section 3, paragraphs 41-60. It sets out all the requirements for local and general ventilation, depending on the number of employees and the consumption of materials.
Supply and exhaust ventilation of industrial premises is selected depending on the purpose, economic feasibility and in accordance with applicable standards:
- In office buildings, it is customary to do natural air exchange - aeration, ventilation. With increased crowding, it is planned to install auxiliary fans or organize air exchange with mechanical motivation.
- In machine-building, repair and rolling workshops of large sizes, arranging forced ventilation will be too expensive. The generally accepted scheme: natural exhaust through anti-aircraft lights or deflectors, the inflow is organized from openable transoms. Moreover, in winter, the upper windows open (height - 4 m), in the summer - lower.
- When toxic, hazardous and unhealthy vapors are released, aeration and ventilation are not allowed.
- At workplaces near heated equipment, it is easier and more correct to strangle people with fresh air than to constantly update the entire volume of the workshop.
- In small industries with a small number of pollution sources, it is better to install local suction in the form of umbrellas or panels, and general ventilation should be provided.
- In industrial buildings with a large number of jobs and sources of hazardous emissions, powerful forced air exchange must be done. To fence 50 or more local hoods is impractical, unless such events are dictated by the rules.
- In the laboratories and workrooms of chemical plants, all ventilation is mechanical, and recycling is prohibited.
Note. Recirculation - the return of part of the selected air back to the shop in order to save heat (in summer - cold) spent on heating. After filtration, this part is mixed with fresh street stream in various ratios.
Since it is unrealistic to consider all varieties of production within the framework of one publication, we have outlined general principles for planning air exchange. A more detailed description is presented in the relevant technical literature, for example, a textbook by O. D. Volkov “Designing ventilation of an industrial building”. The second reliable source is the ABOK Engineers Forum (http://forum.abok.ru).
Methods for calculating air exchange
The purpose of the calculations is to determine the flow rate of the supply air. If point hoods are used in production, then the amount of air mixture removed by umbrellas is added to the received inflow volume.
For reference. Exhaust devices have very little effect on the movement of flows within the building. Supply air jets help to tell them the right direction.
According to SNiP, the calculation of ventilation of a production room is done according to the following indicators:
- excess heat emanating from heated equipment and products;
- water vapor saturating the shop air;
- harmful (toxic) emissions in the form of gases, dust and aerosols;
- the number of employees.
An important point. In utility rooms and various household rooms, the regulatory framework also provides for the calculation of the multiplicity of exchange.You can familiarize yourself with the methodology and use the online calculator on this page.
Ideally, the inflow rate is considered for all indicators. The largest of the results obtained is accepted for the subsequent development of the system. One caveat: if 2 types of hazardous gases are released that interact with each other, the inflow is calculated for each of them, and the results are summarized.
We consider the flow of heat
Before undertaking the calculations, you need to carry out preparatory work to collect the source data:
- find out the area of all hot surfaces;
- find out the heating temperature;
- calculate the amount of heat released;
- determine the temperature of the air in the working area and beyond (above 2 m above the floors).
In practice, the problem is solved in conjunction with the enterprise’s process engineer, who provides information on production equipment, product characteristics and the intricacies of the manufacturing process. Knowing these parameters, perform the calculation according to the formula:
Explanation of symbols:
· L - the desired volume of air supplied by air-handling units or penetrating through transoms, m³ / h;
- Lwz - the amount of air taken from the service area by point suction, m³ / h;
- Q - the amount of heat, W;
- c is the heat capacity of the air mixture, taken equal to 1.006 kJ / (kg ° C);
- Tin is the temperature of the mixture supplied to the workshop;
- Tl, Twz - air temperature above the working area and within it.
The calculation seems cumbersome, but if there is data, it runs without problems. Example: indoor heat flow Q is 20,000 W, exhaust panels remove 2,000 m³ / h (Lwz), the outside temperature is + 20 ° C, and the inside is + 30 and 25, respectively. We consider: L = 2000 + [3.6 x 20000 - 1.006 x 2000 (25 - 20) / 1.006 (30 - 20)] = 8157 m³ / h.
Excess water vapor
The following formula practically repeats the previous one, only the heat parameters are replaced by humidity symbols:
- W - the amount of water vapor coming from sources per unit of time, grams / hour;
- Din - moisture content in the influx, g / kg;
- Dwz, Dl - moisture content of the air of the working area and the upper part of the room, respectively;
- the remaining notation is as in the previous formula.
The complexity of the method lies in obtaining the source data. When the facility is built and production is running, moisture readings are not difficult to determine. Another issue is to calculate the vapor emissions inside the workshop at the design stage. The development should be carried out by 2 specialists - a process engineer and a designer of ventilation systems.
Dust and hazardous emissions
In this case, it is important to study the intricacies of the process well. The task is to compile a list of hazards, determine their concentration and calculate the flow rate of clean air supplied. Settlement formula:
- Mpo - mass of harmful substance or dust emitted per unit time, mg / hour;
- Qin is the content of this substance in street air, mg / m³;
- Qwz - maximum permissible concentration (MPC) of harmfulness in the volume of the served zone, mg / m³;
- Ql - concentration of aerosol or dust in the remaining part of the workshop;
- the decoding of the notation L and Lwz is given in the first formula.
The ventilation operation algorithm is as follows. The estimated amount of inflow is sent to the room, diluting the internal air and reducing the concentration of pollutants. The local share of harmful and volatile substances is drawn in by local umbrellas located above the sources, a mixture of gases is removed by a mechanical hood.
Number of working people
The methodology is used to calculate the inflow to office and other public buildings where there are no industrial pollutants. You need to find out the number of permanent jobs (indicated by the Latin letter N) and use the formula:
Parameter m shows the volume of air clean mixture allocated to 1 workplace. In ventilated offices, the value of m is taken to be 30 m³ / h, fully closed - 60 m³ / h.
Comment.Only permanent jobs where employees stay at least 2 hours a day are taken into account. The number of visitors does not matter.
Calculation of a local hood umbrella
The task of local suction is to take away harmful gas and dust at the extraction stage, directly from the source. To achieve maximum efficiency, you need to choose the right size of the umbrella depending on the dimensions of the source and the height of the suspension. The calculation method is more convenient to consider with reference to the drawing of the suction.
Decipher the lettering in the diagram:
- A, B - the desired dimensions of the umbrella in the plan;
- h is the distance from the lower edge of the retractor to the surface of the source of emission;
- a, b - the size of the overlapping equipment;
- D is the diameter of the ventilation duct;
- H - suspension height, taken no more than 1.8 ... 2 m;
- α (alpha) - the opening angle of the umbrella, ideally does not exceed 60 °.
First of all, we calculate the dimensions of the suction in terms of simple formulas:
Further, by the selection method, we determine the opening angle and proceed to the calculation of the intake air flow:
- F is the area of the wide part of the umbrella, calculated as A x B;
- ʋ - air flow velocity in the gauge section, for non-toxic gases and dust we take 0.15 ... 0.25 m / s.
Note. If it is necessary to suck out toxic hazards, norms require increasing the speed of the exhaust flow to 0.75 ... 1.05 m / s.
Knowing the amount of air drawn, it is not difficult to choose a channel fan of the required capacity. The cross section and diameter of the exhaust duct is determined by the inverse formula:
Conclusion
Designing ventilation networks is the task of experienced engineers. Therefore, our publication is for guidance only, explanations and calculation algorithms are somewhat simplified. If you want to thoroughly understand the issues of ventilation of premises in the workplace, we recommend that you study the relevant technical literature, there is no other way. Finally - a methodology for calculating air heating in the video.