The heat flux Q p through the surface S st of the walls of the dryer is calculated according to the heat transfer equation:

Q p \u003d k * Δt cf * S st,

The heat transfer coefficient k is calculated using the formula for a multilayer wall:

where δ and λ are, respectively, the thickness and thermal conductivity of various layers of the lining and thermal insulation.

Find the value of the criterion Re:

Re \u003d v * l / υ \u003d 2.5 m / s * 1.65 m / 29 * 10 -6 m 2 / s \u003d 142241

Nu=0.66*Re 0.5*Pr 0.33=0.66*142241 0.5*1.17 0.33=262.2.

Heat transfer coefficient α from the drying agent to the inner surface of the walls:

α 1 \u003d Nu * λ / l \u003d 262.2 * 3.53 * 10 -2 W / (m * K) / 1.65 m \u003d 5.61 W / m 2 * K.

The total heat transfer coefficient of convection and radiation from the outer wall to the ambient air:

α 2 \u003d 9.74 + 0.07 * (t st -t c),

where t cf is the temperature of the outer wall, t st \u003d 40 0 ​​С,

t in - ambient temperature, t in \u003d 20 0 С,

α 2 \u003d 9.74 + 0.07 * (40 0 C-20 0 C) \u003d 11.14 W / m 2 * K.

According to the temperature of the gases, we select the thickness of the lining (tab. 3.1)

linings -

fireclay - 125 mm

steel - 20 mm

fireclay - 1.05 W / m * K

steel - 46.5 W/m*K

Finding the heat transfer coefficient:

We determine the surface of the wall S st:

S st \u003d π * d * l \u003d 3.14 * 1.6 m * 8 m \u003d 40.2 m 2,

Q p \u003d 2.581 W / (m 2 * K) * 89 0 C * 40.2 m 2 \u003d 9234 W.

Specific heat loss in environment determined by the formula:

where W is the mass of moisture removed from the dried material in 1 s.

q p \u003d 9234 W / 0.061 kg / s \u003d 151377.05 W * s / kg.

2.3. Calculation of the heater for air drying

The total amount of heat Q 0 is calculated by the formula:

Q 0 \u003d L * (I 1 -I 0)

Q 0 \u003d 2.46 kg / s * (159 kJ / kg + 3.35 kJ / kg) \u003d 399.381 kW

We calculate the average temperature difference using the formula of the logarithmic equation:

where Δt m \u003d t 1 -t 2n

Δt b \u003d t 1 -t 2k

t 1 - temperature of the heating steam (equal to the saturation temperature of the steam at a given pressure).

At a pressure of 5.5 atm. t 1 \u003d 154.6 0 С (st 550)

t 2n, t 2k - air temperature at the calorimeter inlet and outlet, t 2k \u003d 150 0 С; t 2n \u003d -7.7 0 C.

Δt b \u003d 154.6 0 C + 7.7 0 C \u003d 162.3 0 C,

Δt m \u003d 154.6 0 С-150 0 С \u003d 4.6 0 С,

The heat transfer surface S t of the calorimeter is determined by the heat transfer equation:

S t \u003d Q 0 / to Δt cf.,

where k is the heat transfer coefficient, which is used for finned heaters depending on the air mass velocity ρ*v. Let ρ * v \u003d 3 kg / m 2 * s; then k \u003d 30 W / m 2 * k.

We find the required number n k. sections of the heater:

n k. \u003d S t / S s,

where S c is the heat exchange surface of the section.

Let's take a finned heater:

Since the actual number of sections is chosen with a 15-20% margin, then n k. \u003d 6.23 + 6.23 * 0.15 \u003d 7.2≈8 sections.

The mass air velocity in the heater is calculated:

where L is the flow rate of absolutely dry air,

Thermal pollution refers to the phenomena in which heat is released into water bodies or into the atmospheric air. This raises the temperature much higher average norm. Thermal pollution of nature is associated with human activities and emissions greenhouse gases which are the main cause of global warming.

Sources of thermal pollution of the atmosphere

There are two groups of sources:

• natural - these are forest fires, volcanoes, dry winds, processes of decomposition of living and plant organisms;
• anthropogenic are oil and gas processing, industrial activity, thermal power engineering, nuclear power engineering, transport.

Every year, about 25 billion tons of carbon monoxide, 190 million tons of sulfur oxide, 60 million tons of nitrogen oxide enter the Earth's atmosphere as a result of human activity. Half of all this waste is added as a result of the activities of the energy industry, industry and metallurgy.

Per last years the amount of exhaust gases from cars has increased.

Effects

In metropolitan cities with large industrial enterprises atmospheric air experiences the strongest thermal pollution. It receives substances that have more high temperature than the air layer of the surrounding surface. The temperature of industrial emissions is always higher than the average surface layer of air. For example, when forest fires, from the exhaust pipes of cars, from the pipes of industrial enterprises, when heating houses, streams of warm air with various impurities are released. The temperature of such a stream is approximately 50-60 ºС. This layer raises the average annual temperature in the city by six to seven degrees. "Islands of heat" are formed in and above cities, which leads to an increase in cloudiness, while increasing the amount of precipitation and increasing air humidity. When the products of combustion are added to moist air, moist smog (like London smog) is formed. Ecologists say that over the past 20 years, the average temperature of the troposphere has increased by 0.7º C.

Sources of thermal soil pollution

Sources thermal pollution soils on the territory major cities and industrial centers are:

• gas pipes of metallurgical enterprises, the temperature reaches 140-150ºС;
• heating mains, temperature about 60-160ºС;
• communication outlets, temperature 40-50º C.

The consequences of thermal influence on the soil cover

Gas pipes, heating mains and communication outlets increase the soil temperature by several degrees, which negatively affects the soil. In winter, this leads to snow melting and, as a result, freezing of the surface layers of the soil, and in summer the opposite process occurs, the top layer of soil is heated and dried. closely associated with vegetation and living microorganisms that live in it. A change in its composition negatively affects their life.

Sources of thermal pollution of hydrological objects

Thermal pollution of water bodies and coastal marine areas occurs as a result of discharge into water bodies Wastewater nuclear and thermal power plants, industrial enterprises.

Consequences of wastewater discharges

Discharge of sewage leads to an increase in water temperature in reservoirs by 6-7 ºС, the area of ​​such warm spots can reach up to 30-40 km2.

Warm layers of water form a kind of film on the surface water mass, which prevents natural water exchange, do not mix with the bottom ones), the amount of oxygen decreases, and the need of organisms for it increases, while the species number of algae increases.

The greatest degree of thermal water pollution is carried out by power plants. Water is used to cool NPP turbines and gas condensate in TPPs. The water used by power plants is heated by about 7-8 ºС, after which it is discharged into nearby water bodies.

An increase in water temperature in reservoirs adversely affects living organisms. For each of them there is a temperature optimum at which the population feels great. AT natural environment with a slow increase or decrease in temperature, living organisms gradually adapt to changes, but if the temperature rises sharply (for example, when large volume discharges from industrial enterprises), then the organisms do not have time for acclimatization. They get heat shock, as a result of which they can die. This is one of the most negative consequences thermal pollution for aquatic organisms.

But there may be other, more detrimental consequences. For example, the effect of thermal water pollution on metabolism. With an increase in temperature in organisms, the metabolic rate increases, and the need for oxygen increases. But as the water temperature rises, the oxygen content in it decreases. Its deficiency leads to the death of many species of aquatic living organisms. Almost 100% destruction of fish and invertebrates causes water temperature to rise by several degrees per year. summer time. When it changes temperature regime the behavior of fish also changes, it is disturbed natural migration, untimely spawning occurs.

Thus, an increase in water temperature can change species structure reservoirs. Many species of fish either leave these areas or die. The algae characteristic of these places are replaced by heat-loving species.

If together with warm water organic and minerals (domestic waste, washed away from the fields of mineral fertilizers), there is a sharp reproduction of algae, they begin to form a dense mass, covering each other. As a result of this, their death and decay occur, which leads to the pestilence of all living organisms of the reservoir.

Thermal pollution of reservoirs is dangerous. They generate energy with the help of turbines, the exhaust gas must be cooled from time to time. Used water is discharged into reservoirs. On large ones, the amount reaches 90 m 3. This means that a continuous warm flow enters the reservoir.

Damage from pollution of aquatic ecosystems

All the consequences of thermal pollution of water bodies cause catastrophic harm to living organisms and change the habitat of the person himself. Pollution damages:

• aesthetic (violated appearance landscapes);
• economic (liquidation of the consequences of pollution, the disappearance of many species of fish);
• ecological (species of aquatic vegetation and living organisms are destroyed).

The volumes of warm water discharged by power plants are constantly growing, therefore, the temperature of water bodies will also increase. In many rivers, according to environmentalists, it will increase by 3-4 °C. This process is already underway. For example, in some rivers in America, water overheating is about 10-15 ° C, in England - 7-10 ° C, in France - 5 ° C.

Thermal pollution of the environment

Thermal pollution (thermal physical pollution) is the shape that results from an increase in ambient temperature. Its causes are industrial and military emissions of heated air, large fires.

Thermal pollution of the environment is associated with the work of enterprises of the chemical, pulp and paper, metallurgical, woodworking industries, thermal power plants and nuclear power plants, which require large volumes of water to cool equipment.

Transport is a powerful pollutant of the environment. About 80% of all annual emissions come from cars. Many harmful substances dispersed over considerable distances from the source of pollution.

When gas is burned at thermal power plants, in addition to chemical exposure to the atmosphere, and thermal pollution occurs. In addition, approximately within a radius of 4 km from the torch, many plants are in a depressed state, and within a radius of 100 meters, the vegetation cover is dying.

About 80 million tons of various industrial and domestic wastes are generated annually in Russia, which are a source of pollution soil cover, vegetation, underground and surface water, atmospheric air. In addition, they are a source of radiation and thermal pollution of natural objects.

Land waters are polluted with a variety of chemical wastes that get there when mineral fertilizers and pesticides are washed off the soil, with sewage and industrial effluents. Thermal and bacterial pollution occurs in reservoirs, many species of plants and animals die.

Any release of heat into the natural environment leads to a change in the temperature of its components, especially strong influence testing the lower layers of the atmosphere, soil and hydrosphere objects.

According to ecologists, thermal emissions into the environment are not yet able to affect the balance of the planet, but they have an impact on a specific territory. significant influence. For example, the air temperature in major cities usually somewhat higher than outside the city, the thermal regime of rivers or lakes changes when wastewater from thermal power plants is discharged into them. The species composition of the inhabitants of these spaces is changing. Each species has its own temperature range in which the species is able to adapt. For example, trout can survive in warm water but unable to reproduce.

Thus, thermal discharges also affect the biosphere, although this is not on a planetary scale, but it is also noticeable for humans.

Temperature pollution of the soil cover is fraught with the fact that there is a close interaction with animals, vegetation and microbial organisms. With an increase in soil temperature, the vegetation cover changes to more heat-loving species, many microorganisms die, unable to adapt to new conditions.

thermal pollution groundwater occurs as a result of runoff entering aquifers. This negatively affects the quality of water, its chemical composition, thermal mode.

Thermal pollution of the environment worsens the conditions of life and human activity. In cities with elevated temperature in combination with high humidity, people experience frequent headaches, general malaise, jumps blood pressure. High humidity leads to corrosion of metals, damage sewer outlets, heat pipelines, gas pipes and so much more.

Consequences of environmental pollution

It is possible to specify all the consequences of thermal pollution of the environment and highlight the main problems that need to be addressed:

1. Heat islands are formed in large cities.

2. Smog is formed, air humidity increases and permanent cloudiness forms in megacities.

3. Problems arise in rivers, lakes and coastal areas of the seas and oceans. Due to the rise in temperature, the ecological balance many species of fish and aquatic plants are dying.

4. Chemical and physical properties water. It becomes unusable even after cleaning.

5. Living organisms of water bodies are dying or are in a depressed state.

6. Increasing groundwater temperatures.

7. The structure of the soil and its composition are disturbed, the vegetation and microorganisms living in it are suppressed or destroyed.

Thermal pollution. Prevention and measures to prevent it

The main measure to prevent thermal pollution of the environment is the gradual abandonment of the use of fuel, a complete transition to alternative renewable energy: solar, wind and hydropower.

To protect the water areas from thermal pollution in the turbine cooling system, it is necessary to construct reservoirs - coolers, from which the water after cooling can again be used in the cooling system.

AT recent decades engineers are trying to eliminate the steam turbine in thermal power plants, using the magnetohydrodynamic method of converting thermal energy into electrical energy. This significantly reduces the thermal pollution of the surrounding area and water bodies.

Biologists seek to identify the limits of the stability of the biosphere as a whole and certain types living organisms, as well as the limits of equilibrium of biological systems.

Ecologists, in turn, study the degree of influence economic activity people on natural processes in the environment and seek to find ways to prevent negative impacts.

Protecting the environment from thermal pollution

It is customary to divide thermal pollution into planetary and local. On a planetary scale, pollution is not very large and amounts to only 0.018% of the incoming on the planet solar radiation, that is, within one percent. But, thermal pollution has a strong impact on nature at the local level. To regulate this influence in most industrialized countries, limits (limits) of thermal pollution have been introduced.

As a rule, the limit is set for the regime of water bodies, since it is the seas, lakes and rivers that suffer to a large extent from thermal pollution and receive its main part.

In European countries, water bodies should not warm up by more than 3 ° C from their natural temperature.

In the United States, in rivers, water heating should not be whiter than 3 ° C, in lakes - 1.6 ° C, in the waters of the seas and oceans - 0.8 ° C.

In Russia, the water temperature in reservoirs should not rise by more than 3 °C compared to the average temperature of the hottest month. In reservoirs inhabited by salmon and other cold-loving fish species, the temperature cannot be increased by more than 5 °C, not more than 20 °C in summer, and 5 °C in winter.

The scale of thermal pollution near large industrial centers is quite significant. So, for example, from industrial center with a population of 2 million people, with a nuclear power plant and an oil refinery, thermal pollution spreads 120 km away and 1 km in height.

Ecologists suggest using thermal waste for household needs, for example:

• for irrigation of agricultural land;
• in the greenhouse industry;
• to maintain the northern waters in an ice-free state;
• for the distillation of heavy products oil industry and fuel oil;
• for breeding heat-loving fish species;
• for the construction of artificial ponds, heated in winter, for wild waterfowl.

On a planetary scale, thermal pollution natural environment indirectly affects global warming climate. Emissions from industrial enterprises do not directly affect the temperature increase, but lead to its increase as a result of the greenhouse effect.

For solutions environmental issues and prevent them in the future, mankind must solve a number of global problems and direct all efforts to reduce air pollution, thermal pollution of the planet.

The heat balance of the boiler unit establishes equality between the amount of heat entering the unit and its consumption. Based heat balance of the boiler unit determine the fuel consumption and calculate the coefficient useful action, which is the most important characteristic energy efficiency of the boiler.

In the boiler unit, the chemically bound energy of the fuel during the combustion process is converted into the physical heat of the combustible combustion products. This heat is used to generate and superheat steam or heat water. Due to the inevitable losses during heat transfer and energy conversion, the product (steam, water, etc.) absorbs only part of the heat. The other part is made up of losses that depend on the efficiency of the organization of energy conversion processes (fuel combustion) and heat transfer to the product being produced.

The thermal balance of the boiler unit is to establish equality between the amount of heat received in the unit and the sum of the heat used and heat losses. The heat balance of the boiler unit is compiled per 1 kg of solid or liquid fuel or for 1 m 3 gas. The equation in which the heat balance of the boiler unit for the steady state thermal state of the unit is written in the following form:

Q p / p = Q 1 + ∑Q n

Q p / p = Q 1 + Q 2 + Q 3 + Q 4 + Q 5 + Q 6 (19.3)

Where Q p / p is the heat that is available; Q 1 - used heat; ∑Qn - total losses; Q 2 - heat loss with outgoing gases; Q 3 - heat loss from chemical underburning; Q 4 - heat loss from mechanical incompleteness of combustion; Q 5 - heat loss to the environment; Q 6 - heat loss with the physical heat of slag.

If each term on the right side of equation (19.3) is divided by Q p / p and multiplied by 100%, we get the second form of the equation, in which the heat balance of the boiler unit:

q 1 + q 2 + q 3 + q 4 + q 5 + q 6 = 100% (19.4)

In equation (19.4), the value q 1 represents the efficiency of the installation "gross". It does not take into account the energy costs for servicing the boiler plant: the drive of smoke exhausters, fans, feed pumps and other costs. The "net" efficiency factor is less than the "gross" efficiency factor, since it takes into account the energy costs for the installation's own needs.

The left incoming part of the heat balance equation (19.3) is the sum of the following quantities:

Q p / p \u003d Q p / n + Q v.vn + Q steam + Q physical (19.5)

where Q B.BH is the heat introduced into the boiler unit with air per 1 kg of fuel. This heat is taken into account when the air is heated outside the boiler unit (for example, in steam or electric heaters installed before the air heater); if the air is heated only in the air heater, then this heat is not taken into account, since it returns to the furnace of the unit; Q steam - heat introduced into the furnace with blast (nozzle) steam per 1 kg of fuel; Q physical t - physical heat of 1 kg or 1 m 3 of fuel.

The heat introduced with air is calculated by the equality

Q V.BH \u003d β V 0 C p (T g.vz - T h.vz)

where β is the ratio of the amount of air at the inlet to the air heater to the theoretically necessary; c p is the average volumetric isobaric heat capacity of air; at air temperatures up to 600 K, it can be considered with p \u003d 1.33 kJ / (m 3 K); T g.vz - temperature of heated air, K; T x.vz - the temperature of cold air, usually taken equal to 300 K.

The heat introduced with steam for spraying fuel oil (nozzle steam) is found by the formula:

Q pairs \u003d W f (i f - r)

where W f - consumption of injector steam, equal to 0.3 - 0.4 kg/kg; i f - enthalpy of nozzle steam, kJ/kg; r is the heat of vaporization, kJ/kg.

Physical heat of 1 kg of fuel:

Q physical t - with t (T t - 273),

where c t is the heat capacity of the fuel, kJ/(kgK); T t - fuel temperature, K.

The value of Q physical. t is usually insignificant and rarely taken into account in calculations. The exceptions are fuel oil and low-calorie combustible gas, for which the value of Q physical.t is significant and must be taken into account.

If there is no preheating of air and fuel and steam is not used for fuel atomization, then Q p / p = Q p / n. The heat loss terms in the heat balance equation of the boiler unit are calculated on the basis of the equations given below.

1. Heat loss with exhaust gases Q 2 (q 2) is defined as the difference between the enthalpy of gases at the outlet of the boiler unit and the air entering the boiler unit (air heater), i.e.

where V r is the volume of combustion products of 1 kg of fuel, determined by the formula (18.46), m 3 / kg; c р.r, с р.в - average volumetric isobaric heat capacities combustion products of fuel and air, defined as heat capacities gas mixture(§ 1.3) using tables (see Appendix 1); T uh, T x.vz - temperatures of flue gases and cold air; a - coefficient taking into account losses from mechanical underburning of fuel.

Boiler units and industrial furnaces operate, as a rule, under a certain vacuum, which is created smoke exhausters and chimney. As a result, through the lack of density in the fences, as well as through inspection hatches, etc. a certain amount of air is sucked from the atmosphere, the volume of which must be taken into account when calculating I ux.

The enthalpy of all air entering the unit (including suction cups) is determined by the coefficient of excess air at the outlet of the installation α ux = α t + ∆α.

The total air suction in boiler installations should not exceed ∆α = 0.2 ÷ 0.3.

Of all the heat losses, Q 2 is the most significant. The value of Q 2 increases with an increase in the excess air coefficient, the temperature of the flue gases, the moisture content of solid fuel and the ballasting with non-combustible gases gaseous fuel. Reducing air suction and improving the quality of combustion lead to some reduction in heat loss Q 2 . The main determining factor influencing the loss of heat by the exhaust gases is their temperature. To reduce T uh, the area of ​​heat-using heating surfaces - air heaters and economizers - is increased.

The value of Tx affects not only the efficiency of the unit, but also the capital costs required to install air heaters or economizers. With a decrease in Tx, the efficiency increases and fuel consumption and fuel costs decrease. However, this increases the areas of heat-using surfaces (with a small temperature difference, the heat exchange surface area must be increased; see § 16.1), as a result of which the cost of the installation and operating costs increase. Therefore, for newly designed boiler units or other heat-consuming installations, the value of T uh is determined from a technical and economic calculation, which takes into account the influence of T uh not only on efficiency, but also on the amount of capital costs and operating costs.

Another important factor, influencing the choice of Т ux, is the sulfur content in the fuel. At low temperatures (less than the flue gas dew point temperature), water vapor may condense on the pipes of the heating surfaces. When interacting with sulfur and sulfuric anhydrides, which are present in the combustion products, sulfurous and sulfuric acid. As a result, the heating surfaces are subjected to intense corrosion.

Modern boiler units and kilns building materials have T y x = 390 - 470 K. When burning gas and solid fuels with low humidity T y x - 390 - 400 K, wet coals

T yx \u003d 410 - 420 K, fuel oil T yx \u003d 440 - 460 K.

Moisture fuel and non-combustible gaseous impurities are gas-forming ballast, which increases the amount of combustion products resulting from the combustion of fuel. This increases the loss Q 2 .

When using formula (19.6), it should be borne in mind that the volumes of combustion products are calculated without taking into account the mechanical underburning of the fuel. The actual amount of combustion products, taking into account the mechanical incompleteness of combustion, will be less. This circumstance is taken into account by introducing a correction factor a \u003d 1 - p 4 /100 into formula (19.6).

2. Loss of heat from chemical underburning Q 3 (q 3). The gases at the outlet of the furnace may contain products of incomplete combustion of fuel CO, H 2 , CH 4 , the heat of combustion of which is not used in the furnace volume and further along the path of the boiler unit. The total heat of combustion of these gases determines the chemical underburning. The causes of chemical underburning can be:

• lack of an oxidizing agent (α<; 1);
• poor mixing of the fuel with the oxidizer (α ≥ 1);
• a large excess of air;
• low or excessively high specific energy release in the combustion chamber q v , kW/m 3 .

The lack of air leads to the fact that part of the combustible elements of the gaseous products of incomplete combustion of the fuel may not burn at all due to the lack of an oxidizing agent.

Poor mixing of fuel with air is the cause of either a local lack of oxygen in the combustion zone, or, conversely, a large excess of it. A large excess of air causes a decrease in the combustion temperature, which reduces the rates of combustion reactions and makes the combustion process unstable.

The low specific heat release in the furnace (q v = BQ p / n / V t, where B is the fuel consumption; V T is the volume of the furnace) is the cause of strong heat dissipation in the furnace volume and leads to a decrease in temperature. High qv values ​​also cause chemical underburning. This is explained by the fact that a certain time is required to complete the combustion reaction, and with a significantly overestimated value of qv, the time spent by the air-fuel mixture in the furnace volume (i.e., in the zone of the highest temperatures) is insufficient and leads to the appearance of combustible components in the gaseous combustion products. In the furnaces of modern boiler units, the permissible value of qv reaches 170 - 350 kW / m 3 (see § 19.2).

For newly designed boiler units, the values ​​of qv are selected according to the normative data, depending on the type of fuel burned, the method of combustion and the design of the combustion device. During balance tests of operating boiler units, the Q 3 value is calculated according to gas analysis data.

When burning solid or liquid fuels, the value of Q 3, kJ / kg, can be determined by the formula (19.7)

3. Loss of heat from mechanical incomplete combustion of fuel Q 4 (g 4). During the combustion of solid fuels, the residues (ash, slag) may contain a certain amount of unburned combustible substances (mainly carbon). As a result, the chemically bound energy of the fuel is partially lost.

Heat loss from mechanical incomplete combustion includes heat losses due to:

• failure of small particles of fuel through the gaps in the grate Q CR (q PR);
• removal of some part of unburned fuel with slag and ash Q shl (q shl);
• entrainment of small fuel particles by flue gases Q un (q un)

Q 4 - Q pr + Q un + Q sl

The heat loss q yn takes on large values ​​during flaring of pulverized fuel, as well as during the combustion of non-caking coals in a layer on fixed or movable grates. The value of q un for layered furnaces depends on the apparent specific energy release (heat stress) of the combustion mirror q R, kW / m 2, i.e. on the amount of released thermal energy, referred to 1 m 2 of the burning layer of fuel.

The permissible value of q R BQ p / n / R (B - fuel consumption; R - combustion mirror area) depends on the type of solid fuel burned, the design of the furnace, the excess air coefficient, etc. In layered furnaces of modern boiler units, the value of q R has values ​​in the range of 800 - 1100 kW / m 2. When calculating boiler units, the values ​​q R, q 4 \u003d q np + q sl + q un are taken according to regulatory materials. During balance tests, the loss of heat from mechanical underburning is calculated according to the results of laboratory technical analysis of dry solid residues for their carbon content. Usually for furnaces with manual fuel loading q 4 = 5 ÷ 10%, and for mechanical and semi-mechanical furnaces q 4 = 1 ÷ 10%. When burning pulverized fuel in a flare in boiler units of medium and high power q 4 = 0.5 ÷ 5%.

4. The loss of heat to the environment Q 5 (q 5) depends on a large number of factors and mainly on the size and design of the boiler and furnaces, thermal conductivity of the material and wall thickness of the lining, thermal performance of the boiler unit, temperature of the outer layer of the lining and ambient air, etc.

Heat loss to the environment at nominal capacity is determined according to the normative data depending on the power of the boiler unit and the presence of additional heating surfaces (economizer). For steam boilers with a capacity of up to 2.78 kg / s steam q 5 - 2 - 4%, up to 16.7 kg / s - q 5 - 1 - 2%, more than 16.7 kg / s - q 5 \u003d 1 - 0.5% .

Heat losses to the environment are distributed through various gas ducts of the boiler unit (furnace, superheater, economizer, etc.) in proportion to the heat given off by gases in these gas ducts. These losses are taken into account by introducing the heat conservation coefficient φ \u003d 1 q 5 / (q 5 + ȵ k.a) where ȵ k.a is the efficiency of the boiler unit.

5. The loss of heat with the physical heat of ash and slag removed from furnaces Q 6 (q 6) is insignificant, and it should be taken into account only for layered and chamber combustion of multi-ash fuels (such as brown coal, shale), for which it is 1 - 1, 5%.

Heat loss with hot ash and slag q 6,%, calculated by the formula

where a shl - the proportion of fuel ash in the slag; С sl - heat capacity of slag; T sl - slag temperature.

In case of flaring of pulverized fuel, a shl = 1 - a un (a un is the proportion of fuel ash carried away from the furnace with gases).

For layered furnaces a sl shl = a sl + a pr (a pr is the proportion of fuel ash in the "dip"). With dry slag removal, the slag temperature is assumed to be Tsh = 870 K.

With liquid ash removal, which is sometimes observed during flare combustion of pulverized fuel T shl \u003d T ash + 100 K (T ash is the temperature of ash in a liquid melting state). In the case of layered combustion of oil shale, the ash content Ar is corrected for the carbon dioxide content of carbonates, equal to 0.3 (СО 2), i.е. the ash content is taken equal to A P + 0.3 (CO 2) p / k. If the removed slag is in a liquid state, then the value of q 6 reaches 3%.

In furnaces and dryers used in the building materials industry, in addition to the considered heat losses, it is also necessary to take into account the heating losses of transport devices (for example, trolleys) on which the material is subjected to heat treatment. These losses can reach up to 4% or more.

Thus, the "gross" efficiency can be defined as

ȵ k.a = g 1 - 100 - ∑q losses (19.9)

We denote the heat perceived by the product (steam, water) as Qk.a, kW, then we have:

for steam boilers

Q 1 \u003d Q k.a \u003d D (i n.n - i p.n) + pD / 100 (i - i p.v) (19.10)

for hot water boilers

Q 1 \u003d Q k.a \u003d M in with r.v (T out - T in) (19.11)

Where D is the boiler capacity, kg/s; i p.p - enthalpy of superheated steam (if the boiler produces saturated steam, then instead of i p.v one should put (i pn) kJ / kg; i p.v - enthalpy of feed water, kJ / kg; p - amount of water removed from the boiler unit in order to maintain the permissible salt content in the boiler water (the so-called continuous blowdown of the boiler),%; i - enthalpy of boiler water, kJ / kg; M in - water flow through the boiler unit, kg / s; c r.v - heat capacity of water , kJ/(kgK); Tout - hot water temperature at the boiler outlet; Tin - water temperature at the boiler inlet.

Fuel consumption B, kg / s or m 3 / s, is determined by the formula

B \u003d Q k.a / (Q r / n ȵ k.a) (19.12)

The volume of combustion products (see § 18.5) is determined without taking into account losses from mechanical underburning. Therefore, further calculation of the boiler unit (heat exchange in the furnace, determination of the area of ​​heating surfaces in gas ducts, air heater and economizer) is carried out according to the estimated amount of fuel Вр:

(19.13)

When burning gas and fuel oil B p \u003d B.

Heat exchange of the human body with the environment.

From the analysis of expression (1) it follows that in the process of decomposition of complex hydrocarbons (food) a certain amount of biological energy is formed. Part of this energy, as a result of the irreversibility of the processes occurring in the human body, is converted into heat, which must be removed to the environment.

The removal of heat from the human body in the general case occurs due to convection, thermal (radiation) radiation and evaporation.

Convection - (from the Latin transfer, delivery) - occurs due to the movement of microscopic particles of the medium (gas, liquid) and is accompanied by the transfer of heat from a hotter body to a less heated body. There are natural (free) convection caused by the inhomogeneity of the medium (for example, a temperature change in gas density) and forced. As a result of convective heat transfer, heat is transferred from the open surfaces of the human body to the ambient air. Heat transfer by convection for the human body is usually small and amounts to approximately 15% of the total amount of heat released. With a decrease in the ambient air temperature and an increase in its speed, this process is greatly intensified and can reach up to 30%.

Thermal radiation (radiation) - this is the dissipation of heat into the environment from the heated surface of the human body, it has an electromagnetic nature. The share of this radiation, as a rule, does not exceed 10%.

Evaporation - this is the main way of heat removal from the human body at elevated ambient temperatures. This is due to the fact that in the process of heating the human body, peripheral blood vessels expand, which in turn increases the rate of blood circulation in the body and, consequently, increases the amount of heat transferred to its surface. At the same time, the sweat glands of the skin open (the area of ​​the skin of a person, depending on its anthropological size, can vary from 1.5 to 2.5 m 2), which leads to intensive evaporation of moisture (sweating). The combination of these factors contributes to the effective cooling of the human body.

With a decrease in air temperature on the surface of the human body, thickening of the skin (goose bumps) and narrowing of peripheral blood vessels and sweat glands occur. As a result, the thermal conductivity of the skin decreases, and the rate of blood circulation in the peripheral areas decreases significantly. As a result, the amount of heat removed from the human body due to evaporation is significantly reduced.

It has been established that a person can work highly productively and feel comfortable only at certain combinations of temperature, humidity and air velocity.

The Russian scientist I. Flavitsky in 1844 showed that a person's well-being depends on changes in temperature, humidity and air velocity. He found that for a given combination of microclimate parameters (temperature, relative humidity and air velocity), one can find such a value for the temperature of still and fully saturated air that creates a similar thermal sensation. In practice, to search for this ratio, the so-called method of effective temperatures (ET) and effective equivalent temperatures (EET) is widely used. The assessment of the degree of influence of various combinations of temperature, humidity and air velocity on the human body is carried out according to the nomogram shown in Figure 3.

On the left axis of ordinates, the temperature values ​​are plotted according to the dry thermometer, and on the right - according to the wet thermometer. The family of curves intersecting at one point corresponds to lines of constant air velocity. The slanted lines define the values ​​of effective-equivalent temperatures. At zero air velocity, the value of the equivalent effective temperatures coincides with the value of the effective temperature.