An air shower is a local air flow directed at a person. In the area of effect of the air shower, conditions are created that are different from the conditions throughout the entire room. With air shower can be changed following parameters air at a person’s location: mobility, temperature, humidity and concentration of a particular hazard. Typically, the air shower's coverage area is: fixed workplaces, places where workers stay for the longest time, and rest areas. In Fig. Figure 3.19 schematically shows an air shower used to create the necessary conditions in the workplace.

Air showers are most often used in hot shops at workplaces exposed to thermal radiation.

Rice. 3.18. Onboard suction: a - simple; b - overturned; in - front blow

Rice. 3.19. Air shower: a - vertical; b - inclined; in - group

3.0 m/sec, temperature can vary from 16 to 24 °C. If an air shower is used to combat dust, the air speed should not be higher than 0.5-1.5 m/sec to prevent raising dust that has settled on the floor.

The design of the air outlet pipe (supply nozzle) has a great influence on the efficiency of the air shower. It is advisable to have this device rotatable and at the same time to provide the ability to change the angle of inclination of the flow axis by introducing rotating blades. In Fig. Figure 3.20 shows the supply nozzles designed by V.V. Baturin, made taking into account these two requirements.

Classification of ventilation and air conditioning systems

Rice. 3.20. Supply nozzles designed by V.V. Baturin: a - with upper supply; b - with lower air supply

An air shower can use outside air or air taken from the room. The latter, as a rule, undergoes appropriate processing (most often cooling). Outdoor air can also be processed to give it the necessary parameters.

Shower installations can be stationary or mobile.

Mobile units use indoor air, often treated by spraying water into the exhaust air stream.

Water evaporating adiabatically allows the air temperature to decrease. In Fig. 3.21 and 3.22 show water-air showers of this type designed by the Moscow and Sverdlovsk Institutes of Labor Protection.

In air curtains, as well as in air showers, the main property of the supply torch is used - its relative range. Air curtains are installed to prevent air from entering through technological openings or gates from one part of the building to another or from outside air into production premises. In Fig. Figure 3.23 shows diagrams of air curtains designed to prevent or sharply reduce the penetration of cold outside air into the workshop through the gates. The air supplied to the curtain can be preheated, and then the curtains are called air-thermal.

Air curtains designed to prevent the penetration of cold air should be installed at gates that are opened more than five times or for at least 40 minutes per shift, as well as at technological openings of heated buildings located laid down in areas with a design outdoor temperature for heating system design- 15 °C and below, when the possibility of installing gateways is excluded. If a decrease in indoor air temperature(technological or sanitary- hygienic reasons) is unacceptable, curtains can be designed for any duration of opening and any design temperature of the outside air. In this case, it is necessary to- economic justification for this decision.

Rice. 3.21. Water-air shower MIOT type small model:

Rice. 3.22. Mobile fan unit SIOT-3:

Rice . 3.23. Air curtains: A - operating principle; b - various air supply methods:

I - air supply from below; II - side air supply on one side; III - same on both sides

1 - water supply pipeline

from the water supply; 2 - casing; 3 - electric motor; 4 - axial fan; 5 - a drain pipe; 6 - stand 1 - axial fan; 2 - electric motor; 3 - nozzles; 4 - metal fairing; 5 - stand on wheels; 6 - pipeline for supplying water from the water supply system

In the case of a short-term (up to 10 minutes) opening of the gate, as a rule, it is allowed to reduce the air temperature in workplaces protected from air blowing through the gate by screens or partitions. The degree of reduction depends on the nature of the work performed: for light physical work - up to 14 ° C, for moderate work - up to 12 ° C, for heavy work - up to 8 ° C. If there are no permanent jobs in the gate area, it is allowed to reduce the temperature in work area this area to +5°.

Very close to air-thermal curtains in their purpose are the so-called air buffers, created by supplying warm air to the vestibules of public buildings (shops, clubs, theaters, etc.).

Currently the necessary conditions The air environment in the workplace is quite often created by installing special ventilated cabins. In such booths, conditions are maintained that are different from the conditions throughout the entire production facility. This is most often achieved by supplying specially prepared air into the cabins: cooled in hot shops, cooled in cold unheated rooms- heated. Ventilated cabins can be classified as local ventilation systems. Naturally, their use is possible when workplace strictly fixed, for example at the control panel. In Fig. Figure 3.24 shows a ventilated cabin for a crane control station, developed by the Leningrad Institute of Occupational Safety and Health.

General ventilation systems can be supply and exhaust (Fig. 3.5, 3.6, 3.9). When using general exchange systems, the task is to create the necessary air conditions in the entire volume of the room or in the volume of the work area. Unlike local systems, V in this case all harmful substances released in the room are distributed throughout the entire volume. Consequently, the main task that must be solved when designing the systems under consideration is to ensure that the content of one or another harmful substance in the indoor air does not exceed the maximum permissible concentration, and that the values of meteorological parameters meet the relevant requirements.

Often the room is equipped with supply and exhaust general ventilation systems (Fig. 3.10).

The general exchange method of creating specified air conditions is widely used in combination with air conditioning systems.

Rice. 3.24. Ventilated cabin

In this course, very much attention is paid to this method, since it is the main one for MO objects

Topic 2 Design of air showering of workplaces to improve microclimate parameters and air composition

When a worker is exposed to thermal radiation with an intensity of 0.14 kW/m2 or more (according to GOST 12.1.005-88), air showering is used (supply of supply air in the form of an air stream directed at the workplace). When the irradiation intensity is higher than 2.1 kW/m2, the air shower cannot provide the necessary cooling. In this case, radiation exposure should be reduced by providing thermal insulation, shielding and other measures. Or design devices for periodic cooling of workers (cabins, rest rooms, control stations).

The cooling effect of air showering depends on the temperature difference between the body of the worker and the air flow, as well as on the speed of air flow around the cooled body. To ensure specified temperatures and air velocities in the workplace, the air flow axis is directed towards the person’s chest horizontally or at an angle of 45. The distance from the edge of the shower pipe to the workplace must be at least 1 m. The minimum diameter of the pipe is taken to be 0.3 m. For fixed workplaces, the estimated width of the working platform is taken to be 1 m.

When showering fixed workplaces with treated or untreated air, cylindrical nozzles or rotary shower pipes of the PPD type (series 4.904-22) should be used.

When showering areas where workers are constantly located with treated or untreated air, you should use nozzles with an upper air supply of type PD B (series 4.904-36) or nozzles with a bottom air supply of type PD n (series 4.904-36).

When showering areas with untreated air, rotary aerators PAM-24 and VA (OV-02-134 series) should be used. The PAM-24 aerator consists of an axial fan with a diameter of 800 mm with an electric motor on one shaft. The fan rotates at an angle of up to 60 eleven times per minute. Jet range 20 m.

When showering a group of permanent workplaces, it is recommended to use air distribution devices of the VGK type (series 4.904-68). Air showering is also suitable for production processes with the release of harmful gases or vapors, if the use of local shelters and suction is not possible. In this case, to ensure acceptable concentrations of harmful substances, the air stream is directed into the breathing zone horizontally or from above at an angle of 45.

Technical data of shower pipes and distribution devices shown in .

Thus, air showering is used in the following cases:

1) With increased intensity of thermal radiation and especially in cases where it is not possible to use other methods of protection (for example, heat shields).

2) At elevated air temperatures in the work area.

3) With an increased concentration of harmful substances in the work area.

Design order air showering in case of thermal excess in production premises.

1. Determine standard air temperature values t air flow standards and speeds v standards for air showering according to and depending on the following factors:

– intensity of thermal radiation in workplaces.

2. We set the air temperature at the outlet of the cooling device t cooling and heating of air in air ducts  t when air moves from the cooling device to the shower pipe.

3. Determine the air temperature t o at the outlet of the shower pipe

t o = t cool +  t, С (2.1)

4. Determine the ratio of temperature differences

Where t o – air temperature at the outlet from the shower pipe, ˚С;

t r.z. – air temperature in the working area outside the air flow, ˚С;

t normal – standard temperature air in the workplace, ˚С;

5. We select a shower pipe for installation according to and and determine its characteristics:

– type of pipe;

– angle of inclination of the guide vanes of the pipe to the horizon  , ˚;

– temperature coefficient n;

– air flow velocity attenuation coefficient m;

– coefficient of local resistance of the shower pipe K m.s.

6. According to the conditions of the workshop (room), we accept the installation height of the shower pipe above the level of the working platform h.

The installation diagram of the shower pipe above the working platform is shown in Figure 2.1.

Figure 2.1 – Installation diagram of the shower pipe above work surface

Legend in the figure:

h– installation height of the pipe above the working platform, m;

h h – the height of a person from the floor to his chest, m;

 – the angle of inclination of the guide vanes of the pipe to the horizon;

x– distance from the shower pipe to the workplace, m;

7. Determine the distance from the shower pipe to the workplace

, (2.3)

We determine the estimated area of the outlet section of the shower pipe.

At P T< 0,6

(2.4)

9. Select the nearest standard pipe according to or and determine its cross-sectional area F y from the condition

F y  F O.

10. Check the length of the initial section of the jet by air speed

(2.5)

Length of the initial section of the jet
shows that within this area the speed of air movement is constant and equal to the flow speed at the exit from the shower pipe.

11. Determine the speed of air movement from the shower pipe:

(2.6)

12. Calculate the estimated amount of air per shower pipe

(2.7)

13. Check the length of the initial section of the jet
by temperature

(2.8)

14. Determine the air temperature at the outlet of the shower pipe

(2.9)

At  We believe that the selected pipe and the operating mode of the air conditioner will provide the necessary air flow parameters.

At < it is necessary to change the adopted design decisions and repeat the calculation of the pipe area.

15. Determine the amount of air per one shower pipe, taking into account the reserve coefficient of air flow K h.

, m 3 /s (2.10)

16. Determine the cross-sectional area of the supply air ducts to the shower pipe.

We take the diameter of the supply air ducts equal to the inlet diameter of the shower pipe according to or.

17. We accept, according to the workshop conditions, a diagram for supplying air to the shower pipe (see the previous topic of practical training).

18. Determine pressure losses in air ducts.

19. Select a fan or air conditioner to ensure the required air flow parameters.

At P t = 0.6-1.0 calculations are carried out using the formulas:

(2.11)

(2.12)

At P t > 1.0 calculations are carried out using the formulas

(2.13)

(2.14)

It should be taken into account that when P T< 1,0 применяют адиабатичесое охлаждение воздуха. При P t  1.0 artificial air cooling is required.

Design order air showering when harmful substances are released into production premises. Calculation is carried out according to the formulas

Where WITH r.z. And WITH o – concentration of harmful gas and dust vapors in the air of the working area and the air supplied from the shower pipe, mg/m3;

MPC – maximum permissible concentration of harmful substances in the air at the workplace, mg/m 3 (according to GOST 12.1.005-88).

At P To< 0,4 расчет ведут по формулам

At P k = 0.4-1.0 calculation is carried out according to the formulas

;

When radiant heat and emissions of dust and gases enter the premises at the same time, calculations are made for each hazard separately. Further calculations are made using a large pipe made from those calculated for each type of harmful substance.

Bibliography

1. Means of protection in mechanical engineering: Calculation and design: Directory / S.V. Belov et al. – M.: Mashinostroenie, 1989. – 368 p.

2. Internal sanitary installations. In 2 parts / Ed. I.G. Staroverova // Part 2. Ventilation and air conditioning: Designer’s Handbook. – M.: Stroyizdat, 1978. – 509 p.

3. SNiP 2.04.05-86. Heating, ventilation and air conditioning / Gosstroy USSR. – M.: CITP Gosstroy USSR, 1987. – 64 p.

4. Handbook of labor protection at industrial enterprises / K.N. Tkachuk et al. – K.: Tekhnika, 1991. – 286 p.

Task No. 1 for the practical lesson "Design of air showering"

Air showering is organized in the production area. It is necessary to determine the required air exchange for one shower pipe (m 3 /h). The initial data is given in Table 2.1.

Task No. 2 for the practical lesson "Design of air showering"

Air ventilation of workplaces has been organized in the production area. Determine the pressure that the fan must develop to ensure the required air flow parameters. The initial data is given in table 2.2.

Table 2.1 – Initial data for task No. 1 (t r.z. =32˚C)

Options

Pipe type

Tilt angle, α

Coefficient,n n

Coefficient,m n

Coef. losses K P m.s.

Sectional area of the pipe, m 2

Permissible air speed at the workplace, m/s

Permissible air temperature, ˚С

Distance from the nozzle to the workplace, m

Installation height of the pipe above the working surface, m

Table 2.2 – Initial data for task No. 2

Options

Pipe type

PD V -3

PD V -5

PD n -4

PD n -3

PD V -4

PPD-5

PD V -3

PD V -5

PD n -5

PPD-8

PPD-6

PPD-10

PPD-8

PD V -4

Coef. losses K P m.s.

l 1 , m

l 2 , m

l 3 , m

l 4 , m

l 5 , m

l 6 , m

Ud. friction losses, Pa/m

Air density, kg/m3

Allocation per pipe, m 3 /s

Filter losses, Pa

D under, m

An air shower is a stream of air directed at a confined work area or directly at a worker.

The use of air showers is especially effective when a worker is exposed to heat. In such cases, an air shower is installed at the place where a person spends the longest time, and if short breaks for rest are provided during work, then at the place of rest.

The upper parts of the body should be blown with air, as they are most sensitive to the effects of thermal radiation.

The speed and temperature of the air in the workplace when using air showers are prescribed depending on the intensity of a person’s thermal irradiation, the duration of his continuous stay under irradiation and the ambient temperature.

Fan unit type VA-1

1 - electric motor;
2 - shell;
3 — mesh;
4 — axial fan;
5 - confuser;
6 — fairing;
7 - pneumatic nozzle;
8 - guide vanes

Air showering should be provided in permanent workplaces with an irradiation intensity of 350 W/m2 or more. In this case, an air flow can be directed at a person at a speed o = 0.5...3.5 m/s and a temperature of 18-24 ° C, depending on the period of the year and the intensity of physical activity.

Constructive implementation of air showers.

The air coming out of the shower pipe must wash the head and body of a person at a uniform speed and have the same temperature.

The axis of the air flow can be directed to the person’s chest horizontally or from above at an angle of 45° while ensuring the specified temperatures and air speeds in the workplace, as well as to the face (breathing zone) horizontally or from above at an angle of 45° while ensuring acceptable concentrations of harmful emissions.

The distance from the shower pipe to the workplace must be at least 1 m when minimum diameter pipe 0.3 m. The width of the working platform is assumed to be 1 m.

Design of VA-1 units

According to their design, showering units are divided into stationary and mobile.

The fan unit type VA-1 consists of a cast iron frame on which is mounted an axial fan No. 5 type MC with an electric motor, a shell with a collector and mesh, a confuser with guide vanes and a fairing, a pneumatic nozzle type FP-1 or FP-2 and pipelines with fittings And flexible hoses for water supply and compressed air. The unit is manufactured with the fan rotated around the axis of the frame up to 60° and the barrel raised vertically by 200-600 mm.

In addition to fan units of type BA, a rotating unit PAM.-24 is used in the form axial fan with a diameter of 800 mm with an electric motor on one shaft. The unit's productivity is 24,000 m 3 /h with a jet range of 20 m. The unit is equipped with a pneumatic nozzle for spraying water in the air flow.

Stationary shower installations supply both untreated and treated (heated, cooled and humidified) outside air to the shower pipes. Mobile units supply room air to the workplace. Water may be sprayed into the air flow they supply. In this case, droplets of water falling on clothing and exposed parts of the human body evaporate and cause additional cooling.

Fixed workplaces can be showered with shower pipes various types. The HIP pipes have a compressed outlet section, a swivel joint for changing the direction of air flow in the vertical plane, and a rotating device for changing the direction of flow in the horizontal plane within 360°.

Regulation of the direction of the air flow in the PD nozzles is carried out in the vertical plane by turning the guide vanes, and in the horizontal plane using a rotary device. PD pipes can be used both with and without nozzles for pneumatic water spraying. The pipes should be installed at a height of 1.8-1.9 m from the floor (to the bottom edge).

Air showering is the most effective measure for creating the required meteorological conditions (temperature, humidity and air speed) at permanent workplaces. The use of air showers is especially effective when there is significant heat radiation or in open production processes, if technological equipment emitting harmful substances, has no shelter or local exhaust ventilation. Air showering is a stream of air directed at a limited workplace or directly at a worker.

Air mobility in the workplace during air showering reaches from 1 to 3.5 m/s. Douching is carried out with special nozzles, and the jet is directed to the irradiated areas of the body: head, chest. The size of the blown area is m. Dusting can be carried out with external untreated air, adiabatically cooled air or isohumidity cooling. In some cases, it is possible to use recirculated air, but there should be little thermal radiation and no harmful emissions.

The cooling effect of air showering depends on the temperature difference between the body of the worker and the air flow, as well as on the speed of air flow around the cooled body. When the jet coming out of the hole mixes with the surrounding air, the speed, temperature difference and concentration of impurities in the cross section of the free jet change. The jet should be directed so that, if possible, it prevents it from sucking in hot or gas-contaminated air. For example, if there is a fixed workplace near an open furnace opening, you should not place a showering device near the opening with the jet directed towards the worker, since in this case it is impossible to avoid the suction of hot gases, as a result of which superheated air will flow to the worker. When calculating air shower systems, one should take design parameters A for warm and calculated parameters B for cold periods of the year. To calculate year-round air showering, the warm period is taken as the calculation period, and for the cold period only the supply air temperature is determined.

Systems supplying air to air shower nozzles are designed separately from systems for other purposes. The distance from the air outlet to the workplace should be at least 1 m. Calculation procedure

1. Set the air parameters at the workplace, mark the installation location of the pipe, the distance from the pipe to the workplace, and also set the type of shower pipe. 2. We determine the air speed at the outlet of the nozzle depending on the normalized air mobility in the room, where is the normalized air mobility, is the distance from the nozzle to the workplace, m, is the coefficient of change in speed, is the cross-section of the selected nozzle. 3. Define minimum temperature at the outlet of the pipe, where is the normalized temperature, and is the coefficient of temperature change. 4. Determine the air flow required to supply to the nozzle.

1700 W/m2. Air temperature in the working area = 25 0C. According to table. 4.23 average temperature=19 0С, air mobility in the workplace

2.3 m/s. Distance from the shower pipe to the working pipe X = 1.8 m.

During the adiabatic cooling process, the air temperature at the outlet of the nozzle chamber is 18.5 0C.

We accept the PDN-4 shower pipe

Dimensions 630 mm h1=1540 mm l1=1260 mm

Estimated area 0.23 m2

Coefficient m=4.5 n=3.1 =3.2 =00-200

Determine the thermal cross-sectional area of the pipe:

Table value =0.23 m2

Find the air speed at the outlet of the pipe:

We set the air flow supplied by the shower pipe:

During the cold season and in transitional conditions, the temperature and air speed in the workplace should be within the following limits:

18...19 0С =2.0...2.5 m/s =16 0С

We leave unchanged those adopted for the warm period, determine the air temperature at the outlet of the shower pipe at =16 0C and =19 0C using the formula:

Ventilation of crane operator's cabins

Ventilation system for crane operator cabins with outside air supply. Ventilation should provide a back-up of 10-15 Pa.

The cabin ventilation system with outside air supply is carried out according to the diagram shown in Fig. 1. The structure contains a manifold located along the path of movement of the crane, an intake device moving in the slot of the manifold and rigidly connected to the crane operator’s cabin. A rubber band or hydraulic seal is used as a sealing device for the manifold gap.

Rice. 1 - Ventilation of the crane cabin with air supply through the collector: 1 - collector, 2 - fan, 3 - crane cabin, 4 - muffler, 5 - rubber sealing tube

Local exhaust ventilation

Local suction from equipment emitting vapors, gases, bad odors

Calculation of the umbrella - canopy over the loading hole of the heating furnace

An umbrella - a canopy over the loading opening of the furnace is designed to catch the flow of gases emerging from the opening under the influence of overpressure in the oven. The dimensions of the suction opening of the umbrella must correspond to the dimensions of the suction jet, taking into account its curvature under the influence of gravitational forces (Fig. 2.)

Rice. 2

Let's determine the volume of air removed and the dimensions of the umbrella - visor thermal oven, having a loading hole with dimensions h?b=0.5?0.5 m. The gas temperature is maintained in the furnace tg=1150 0С, air temperature in the working area =25 0С

1. Let's define average speed, with which gases are knocked out of the furnace opening, having previously calculated:

where - flow coefficient 0.65

Excess pressure in the furnace, Pa

h0 - half the height of the loading opening, m

and - density, respectively, of the air in the working area and the gases leaving the furnace, kg/m3

2. Volume of gases leaving the working opening of the furnace, m3/s

where is the area of the furnace working opening, m2

2.78(0.5?0.5)=0.69 m3/s

0.690.25=0.17 kg/s

3. Calculate Archimedes' criterion

where is the equivalent area diameter of the working opening, m

and - temperature, respectively, of gases in the furnace and air in the working area, K

Archimedes' criterion at m

4. The distance at which the axis of the gas flow, curved under the pressure of gravitational forces, reaches the plane of the suction opening of the zone, m

where m, n are the coefficients of change in speed and temperature at the ratio of the height of the loading opening h to its width and in the range of 0.5...1, which are applied equal to 5 and 4.2, respectively. Let's determine the distance x at h0=0.25 m=5 n=4.2

5. Diameter of gas flow at a distance x at

0.565+0.440.653=0.852 m

6. Find the reach and width of the umbrella

B=b+(150...200)=b+0.2=0.5+0.2=0.7 m

7. Determine the flow rate of the sucked mixture of gases and air:

8. Air consumption drawn from the room:

0.727-0.69=0.037 m3/s

0.0371.18=0.044 kg/s

9. Temperature of gas mixture and mixture, 0C

Which is unacceptably high for natural (< 300 0С) и для механической (< 80 0С). Принимаем =300 0C, когда расход подсасываемого воздуха м/с, увеличивается до значения:

Total volume:

Let's determine the height chimney to remove the found mass of air. Let's take the pipe diameter dTP=500 mm

square cross section pipes:

0.7850.52=0.196 m2

Air speed in the pipe m/s

We preliminarily set the pipe height htr = 6 m. At the pipe head we install a deflector with a diameter ddef = 500 mm, deflector height hdef = 1.7 ddef = 1.70.5 = 0.85 m

Deflector local resistance coefficient

Umbrella local resistance coefficient

The pressure loss in the exhaust pipe together with the deflector, taking into account contamination of the walls, is determined by the formula:

Let's check the approximate height exhaust pipe from the equality:

Outside air temperature tн=21.2 0С, then:

Umbrella height:

Let's substitute the calculated values into the formula:

5.73 m close to previously applicable