The heat balance of the boiler unit establishes equality between the amount of heat entering the unit and its consumption. Based on the heat balance of the boiler unit, the fuel consumption is determined and the efficiency factor is calculated, which is the most important characteristic of the 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 for 1 kg of solid or liquid fuel or for 1 m 3 of 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 \u003d 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; ∑Q n - 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 the maintenance of 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 terms of heat loss in the heat balance equation of the boiler unit are calculated on the basis of the equalities 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 of the products of combustion of fuel and air, defined as the heat capacities of the 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 some vacuum, which is created by smoke exhausters and a 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 unit 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 ratio, flue gas temperature, moisture content of solid fuel and ballasting with non-combustible gases of 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, 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 Tx is the sulfur content of 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 sulfurous and sulfuric anhydrides, which are present in the combustion products, sulfurous and sulfuric acids are formed. As a result, the heating surfaces are subjected to intense corrosion.
Modern boiler units and kilns for firing building materials have T uh = 390 - 470 K. When burning gas and solid fuels with low humidity T uh - 390 - 400 K, wet coals
T yx \u003d 410 - 420 K, fuel oil T yx \u003d 440 - 460 K.
Fuel moisture and non-combustible gaseous impurities are gas-forming ballast, which increases the amount of combustion products resulting from fuel combustion. 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 furnace, the thermal conductivity of the material and wall thickness of the lining, the thermal performance of the boiler unit, the temperature of the outer layer of the lining and ambient air, etc. d.
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 slag removal, which is sometimes observed during flaring of pulverized fuel, T slug \u003d T ash + 100 K (T ash is the temperature of the ash in the 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.
The exchange of heat energy between an organism and its environment is called heat exchange. One of the indicators of heat transfer is body temperature, which depends on two factors: heat generation, that is, the intensity of metabolic processes in the body, and heat transfer to the environment.
Animals whose body temperature changes with the temperature of the environment are called poikilothermic, or cold-blooded. Animals with a constant body temperature are called homeothermic(warm-blooded). temperature constancy body is called isothere mia. She ensures independencemetabolic processes in tissues and organs from temperature fluctuations environment.
Human body temperature.
The temperature of individual parts of the human body is different. The lowest skin temperature is observed on the hands and feet, the highest - in the armpit, where it is usually determined. In a healthy person temperature in this area is 36-37° C. During the day, there are small rises and falls in human body temperature in accordance with the daily biorhythm:the minimum temperature is observed at 2- 4 h nights, maximum - at 16-19 hours.
T temperature muscular fabrics in the state of rest and work can fluctuate within 7 ° C. The temperature of the internal organs depends on the intensity of exchange processes. Most intense metabolic processes take place in the liver, which is the “hottest” organ of the body: the temperature in the liver tissues is 38-38.5 ° WITH. The temperature in the rectum is 37-37.5 ° C. However, it can fluctuate within 4-5 ° C, depending on the presence of feces in it, the blood filling of its mucosa and other reasons. In runners for long (marathon) distances, at the end of the competition, the temperature in the rectum can rise to 39-40 ° C.
The ability to maintain the temperature at a constant level is provided by interrelated processes - heat generation and heat release from the body to the external environment. If heat generation is equal to heat loss, then the temperature of the body remains constant. The process of generating heat in the body is called chemical thermoregulation, the process that removes heat from the body, - physical thermoregulation.
Chemical thermoregulation. Heat exchange in the body is closely related to energy. When organic matter is oxidized, energy is released. Part of the energy goes to the synthesis of ATP. This potential energy can be used by the organism in its further activity.All tissues are the source of heat in the body. Blood, flowing through tissues, heats up.
An increase in ambient temperature causes a reflex decrease in metabolism, as a result of which heat generation in the body decreases. With a decrease in ambient temperature, the intensity of metabolic processes reflexively increases and heat generation increases. To a greater extent, the increase in heat generation occurs due to an increase in muscle activity. Involuntary muscle contractions (shivering) are the main form of increased heat generation. An increase in heat generation can occur in muscle tissue and due to a reflex increase in the intensity of metabolic processes - non-contractile muscle thermogenesis.
Physical thermoregulation. This process is carried out due to the transfer of heat to the external environment by convection (heat conduction), radiation (heat radiation) and water evaporation.
Convection - direct heat transfer to objects or particles of the environment adjacent to the skin. Heat transfer is the more intense, the greater the temperature difference between the surface of the body and the surrounding air.
Heat transfer increases with air movement, for example with wind. The intensity of heat transfer largely depends on the thermal conductivity of the environment. Heat is released faster in water than in air. Clothing reduces or even stops heat conduction.
Radiation - the release of heat from the body occurs by infrared radiation from the surface of the body. Due to this, the body loses the bulk of the heat. The intensity of heat conduction and heat radiation is largely determined by the temperature of the skin. Heat transfer is regulated by a reflex change in the lumen of the skin vessels. With an increase in ambient temperature, arterioles and capillaries expand, the skin becomes warm and red. This increases the processes of heat conduction and heat radiation. When the air temperature drops, the arterioles and capillaries of the skin narrow. The skin becomes pale, the amount of blood flowing through its vessels decreases. This leads to a decrease in its temperature, heat transfer decreases, and the body retains heat.
Water evaporation from the surface of the body (2/3 of moisture), as well as in the process of respiration (1/3 of moisture). Evaporation of water from the surface of the body occurs when sweat is released. Even with the complete absence of visible sweating, it evaporates through the skin per day up to 0.5 l water - invisible perspiration. Evaporation of 1 liter of sweat in a person weighing 75 kg can lower body temperature by 10 ° C.
In a state of relative rest, an adult releases 15% of heat into the external environment through heat conduction, about 66% through heat radiation and 19% through water evaporation.
On average, a person loses per day about 0.8 l of sweat, and with it 500 kcal of heat.
When breathing, a person also allocates daily about 0.5 liters of water.
At low ambient temperature ( 15°C and below) about 90% of the daily heat transfer occurs due to heat conduction and heat radiation. Under these conditions, no visible sweating occurs.
At air temperature 18-22° With heat transfer due to thermal conductivity and heat radiation decreases, butloss increasesbody heat through evaporationmoisture from the surface of the skin.At high humidity, when the evaporation of water is difficult, overheating may occur.body and developthermal hit.
Low permeability to water vapor clothes prevents effective perspiration and may cause overheating of the human body.
hot countries, on long trips, hot workshops, a person loses a large amount fluids with sweat. This gives rise to a feeling thirst that is not quenched by taking water. This connected with the what's up with then a large amount of mineral salts is lost. If salt is added to drinking water, that feeling of thirst disappear and people's well-being will improve.
Centers for the regulation of heat transfer.
Thermoregulation is carried out reflexively. Fluctuations in ambient temperature are perceived thermoreceptors. In large numbers, thermoreceptors are located in the skin, in the oral mucosa, and in the upper respiratory tract. Thermoreceptors have been found in internal organs, veins, and also in some formations of the central nervous system.
Skin thermoreceptors are very sensitive to fluctuations in ambient temperature. They are excited when the temperature of the medium rises by 0.007 ° C and decreases by 0.012 ° C.
Nerve impulses that arise in thermoreceptors travel along afferent nerve fibers to the spinal cord. Along the conducting paths, they reach the visual tubercles, and from them they go to the hypothalamic region and to the cerebral cortex. As a result, there are sensations of heat or cold.
In the spinal cord there are centers of some thermoregulatory reflexes. Hypothalamus is the main reflex center of thermoregulation. The anterior hypothalamus controls the mechanisms of physical thermoregulation, i.e. they are heat transfer center. The posterior hypothalamus controls chemical thermoregulation and is heat generating center.
plays an important role in the regulation of body temperature cerebral cortex. The efferent nerves of the thermoregulation center are mainly sympathetic fibers.
Involved in the regulation of heat transfer hormonal mechanism especially thyroid and adrenal hormones. Thyroid hormone - thyroxine, increasing the metabolism in the body, increases heat generation. The entry of thyroxine into the blood increases when the body is cooled. Adrenal hormone - adrenalin- enhances oxidative processes, thereby increasing heat generation. In addition, under the action of adrenaline, vasoconstriction occurs, in particular the vessels of the skin, due to this, heat transfer decreases.
Body adaptation to low ambient temperature. With a decrease in ambient temperature, a reflex excitation of the hypothalamus occurs. An increase in its activity stimulates pituitary , resulting in increased secretion of thyrotropin and corticotropin, which increase the activity of the thyroid gland and adrenal glands. The hormones of these glands stimulate heat production.
In this way, on cooling the protective mechanisms of the body are activated, which increase metabolism, heat generation and reduce heat transfer.
Age features of thermoregulation. In children of the first year of life, imperfect mechanisms are observed. As a result, when the ambient temperature drops below 15 ° C, hypothermia of the child's body occurs. In the first year of life, there is a decrease in heat transfer through heat conduction and heat radiation and an increase in heat production. However, until the age of 2, children remain thermolabile (body temperature rises after eating, at high ambient temperatures). In children from 3 to 10 years old, the mechanisms of thermoregulation improve, but their instability continues to persist.
In prepubertal age and during puberty (puberty), when there is an increased growth of the body and a restructuring of the neurohumoral regulation of functions, the instability of thermoregulatory mechanisms increases.
In old age, there is a decrease in the formation of heat in the body compared with mature age.
The problem of hardening of the body. In all periods of life, it is necessary to harden the body. Hardening is understood as an increase in the body's resistance to adverse environmental influences and, first of all, to cooling. Hardening is achieved by using the natural factors of nature - the sun, air and water. They act on the nerve endings and blood vessels of the human skin, increase the activity of the nervous system and enhance metabolic processes. With constant exposure to natural factors, the body gets used to them. Hardening of the body is effective under the following basic conditions: a) systematic and constant use of natural factors; b) a gradual and systematic increase in the duration and strength of their impact (hardening start with the use of warm water, gradually reduce its temperature and increase the duration of water procedures); c) hardening with the use of temperature-contrasting stimuli (warm - cold water); d) individual approach to hardening.
The use of natural hardening factors must be combined with physical education and sports. Well contributes to hardening morning exercises in the fresh air or in a room with an open window with the obligatory exposure of a significant part of the body and subsequent water procedures (pouring, shower). Hardening is the most accessible means of healing people.
Table of contents of the subject "Regulation of Metabolism and Energy. Rational Nutrition. Basic Metabolism. Body Temperature and Its Regulation.":
1. Energy costs of the body under conditions of physical activity. The coefficient of physical activity. Working increase.
2. Regulation of metabolism and energy. Metabolic regulation center. Modulators.
3. The concentration of glucose in the blood. Scheme of regulation of glucose concentration. Hypoglycemia. Hypoglycemic coma. Hunger.
4. Nutrition. Norm of nutrition. The ratio of proteins, fats and carbohydrates. energy value. Calorie content.
5. Diet of pregnant and lactating women. Baby food ration. Distribution of the daily ration. Alimentary fiber.
6. Rational nutrition as a factor in maintaining and strengthening health. Healthy lifestyle. Eating mode.
7. Body temperature and its regulation. Homeothermic. Poikilothermic. Isotherm. Heterothermic organisms.
8. Normal body temperature. homeothermal core. Poikilothermic shell. comfort temperature. Human body temperature.
9. Heat production. primary warmth. endogenous thermoregulation. secondary heat. contractile thermogenesis. non-shivering thermogenesis.
There are the following ways of heat transfer by the body to the environment: radiation, heat conduction, convection and evaporation.
Radiation- this is a method of heat transfer to the environment by the surface of the human body in the form of electromagnetic waves of the infrared range (a = 5-20 microns). The amount of heat dissipated by the body into the environment by radiation is proportional to the surface area of the radiation and the difference between the average temperatures of the skin and the environment. The radiation surface area is the total surface area of those parts of the body that are in contact with air. At an ambient temperature of 20 ° C and a relative humidity of 40-60%, the body of an adult person dissipates by radiation about 40-50% of all heat given off. Heat transfer by radiation increases with a decrease in ambient temperature and decreases with its increase. Under conditions of constant ambient temperature, radiation from the body surface increases with an increase in skin temperature and decreases with a decrease in it. If the average temperatures of the surface of the skin and the environment are equalized (the temperature difference becomes equal to zero), heat transfer by radiation becomes impossible. It is possible to reduce the heat transfer of the body by radiation by reducing the surface area of the radiation (“folding the body into a ball”). If the ambient temperature exceeds the average skin temperature, the human body, by absorbing infrared rays emitted by surrounding objects, warms up.
Rice. 13.4. Types of heat transfer. The ways of heat transfer by the body to the external environment can be conditionally divided into “wet” heat transfer associated with the evaporation of sweat and moisture from the skin and mucous membranes, and “dry” heat transfer, which is not associated with fluid loss.Heat conduction- a method of heat transfer that takes place during contact, contact of the human body with other physical bodies. The amount of heat given off by the body to the environment in this way is proportional to the difference in the average temperatures of the contacting bodies, the area of the contacting surfaces, the time of thermal contact and the thermal conductivity of the contacting body. Dry air, adipose tissue are characterized by low thermal conductivity and are heat insulators. The use of clothing made from fabrics containing a large number of small, immobile "bubbles" of air between the fibers (for example, woolen fabrics) enables the human body to reduce heat dissipation by conduction. Humid air saturated with water vapor, water are characterized by high thermal conductivity. Therefore, a person's stay in an environment with high humidity at low temperature is accompanied by an increase in body heat loss. Wet clothing also loses its insulating properties.
Convection- a method of heat transfer of the body, carried out by transferring heat by moving particles of air (water). Heat dissipation by convection requires air flow around the surface of the body with a temperature lower than that of the skin. At the same time, the layer of air in contact with the skin heats up, reduces its density, rises and is replaced by colder and denser air. Under conditions when the air temperature is 20 ° C and relative humidity is 40-60%, the body of an adult dissipates about 25-30% of heat into the environment through heat conduction and convection (basic convection). With an increase in the speed of movement of air flows (wind, ventilation), the intensity of heat transfer (forced convection) also increases significantly.
The release of heat from the body through heat conduction, convection and izlu cheniya, called together "dry" heat dissipation, becomes ineffective when the average temperatures of the body surface and the environment equalize.
Heat transfer by evaporation- this is a way for the body to dissipate heat into the environment due to its costs for the evaporation of sweat or moisture from the surface of the skin and moisture from the mucous membranes of the respiratory tract ("wet" heat transfer). In humans, sweat is constantly secreted by the sweat glands of the skin (“perceptible”, or glandular, loss of water), the mucous membranes of the respiratory tract are moistened (“imperceptible” loss of water) (Fig. 13.4). At the same time, the “perceptible” loss of water by the body has a more significant effect on the total amount of heat given off by evaporation than the “imperceptible” one.
At an ambient temperature of about 20 ° C, the evaporation of moisture is about 36 g / h. Since 0.58 kcal of thermal energy is spent on the evaporation of 1 g of water in a person, it is easy to calculate that, by evaporation, the body of an adult gives off under these conditions to the environment about 20% of the total heat dissipated An increase in external temperature, physical work, prolonged stay in heat-insulating clothing increase sweating and it can increase up to 500-2000 g / h. If the external temperature exceeds the average skin temperature, then the body cannot give off to the external environment heat by radiation, convection and heat conduction.The body in these conditions begins to absorb heat from the outside, and the only way to dissipate heat is to increase the evaporation of moisture from the surface of the body.Such evaporation is possible as long as the ambient air humidity remains less than 100%. high humidity and low air velocity, when Sweat, not having time to evaporate, merge and drain from the surface of the body, heat transfer by evaporation becomes less effective.
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 more heated 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.
For reduction of heat consumption strict accounting for heat losses in process equipment and heat networks. Heat losses depend on the type of equipment and pipelines, their proper operation and the type of insulation.
Heat loss (W) is calculated by the formula
Depending on the type of equipment and pipeline, the total thermal resistance is:
for an insulated pipeline with one layer of insulation:
for an insulated pipeline with two layers of insulation:
for technological apparatuses with multilayer flat or cylindrical walls with a diameter of more than 2 m:
for technological apparatuses with multilayer flat or cylindrical walls with a diameter of less than 2 m:
carrier to the inner wall of the pipeline or apparatus and from the outer surface of the wall into the environment, W / (m 2 - K); X tr, ?. st, Xj - thermal conductivity, respectively, of the material of the pipeline, insulation, walls of the apparatus, /-th layer of the wall, W / (m. K); 5 ST. — apparatus wall thickness, m.
The heat transfer coefficient is determined by the formula
or according to the empirical equation
The transfer of heat from the walls of the pipeline or apparatus to the environment is characterized by the coefficient a n [W / (m 2 K)], which is determined by criterion or empirical equations:
according to criteria equations:
The heat transfer coefficients a b and a n are calculated according to criterion or empirical equations. If the hot coolant is hot water or condensing steam, then a in > a n, i.e. R B< R H , и величиной R B можно пренебречь. Если горячим теплоносителем является воздух или перегретый пар, то а в [Вт/(м 2 - К)] рассчитывают по критериальным уравнениям:
by empirical equations:
Thermal insulation of devices and pipelines is made of materials with low thermal conductivity. Well-chosen thermal insulation can reduce heat loss to the surrounding space by 70% or more. In addition, it increases the productivity of thermal installations, improves working conditions.
The thermal insulation of the pipeline consists mainly of a single layer, top-coated for strength with a layer of sheet metal (roofing steel, aluminum, etc.), dry plaster from cement mortars, etc. If a cover layer of metal is used, its thermal resistance can be neglected. If the cover layer is plaster, then its thermal conductivity differs slightly from the thermal conductivity of thermal insulation. In this case, the thickness of the cover layer is, mm: for pipes with a diameter of less than 100 mm - 10; for pipes with a diameter of 100-1000 mm - 15; for pipes with a large diameter - 20.
The thickness of the thermal insulation and the cover layer should not exceed the limiting thickness, depending on the mass loads on the pipeline and its overall dimensions. In table. 23 shows the values of the maximum thickness of the insulation of steam pipelines, recommended by the standards for the design of thermal insulation.
Thermal insulation of technological devices can be single layer or multilayer. Heat loss through thermal
insulation depends on the type of material. Heat losses in pipelines are calculated for 1 and 100 m of pipeline length, in process equipment - for 1 m 2 of the apparatus surface.
A layer of contaminants on the inner walls of pipelines creates additional thermal resistance to the transfer of heat into the surrounding space. Thermal resistances R (m. K / W) during the movement of some coolants have the following values:
Pipelines supplying technological solutions to apparatuses and hot heat carriers to heat exchangers have fittings in which part of the flow heat is lost. Local heat loss (W / m) is determined by the formula
The coefficients of local resistance of fittings of pipelines have the following values:
When compiling the table. 24 calculation of specific heat losses was carried out for steel seamless pipelines (pressure< 3,93 МПа). При расчете тепловых потерь исходили из следующих данных: тем-
the air temperature in the room was taken equal to 20 °C; its speed during free convection is 0.2 m/s; steam pressure - 1x10 5 Pa; water temperature - 50 and 70 ° C; thermal insulation is made in one layer of asbestos cord, = 0.15 W / (m. K); heat transfer coefficient а„ \u003d 15 W / (m 2 - K).
Example 1. Calculation of specific heat losses in a steam pipeline.
Example 2. Calculation of specific heat losses in an uninsulated pipeline.
Given conditions
The pipeline is steel with a diameter of 108 mm. Nominal diameter d y = 100 mm. Steam temperature 110°C, ambient temperature 18°C. Thermal conductivity of steel X = 45 W / (m. K).
The data obtained indicate that the use of thermal insulation reduces heat losses per 1 m of pipeline length by 2.2 times.
Specific heat losses, W/m 2 , in technological apparatuses of leather and felting production are:
Example 3. Calculation of specific heat losses in technological devices.
1. The Giant drum is made of larch.
2. Dryer firm "Hirako Kinzoku".
3. Longboat for dyeing berets. Made of stainless steel [k = 17.5 W/(m-K)]; there is no thermal insulation. The overall dimensions of the longboat are 1.5 x 1.4 x 1.4 m. The wall thickness is 8 ST = 4 mm. Process temperature t = = 90 °C; air in the workshop / av = 20 °С. Air velocity in the workshop v = 0.2 m/s.
The heat transfer coefficient a can be calculated as follows: a = 9.74 + 0.07 At. At / cp \u003d 20 ° C, a is 10-17 W / (m 2. K).
If the surface of the coolant of the apparatus is open, the specific heat losses from this surface (W / m 2) are calculated by the formula
The industrial service "Capricorn" (Great Britain) proposes to use the "Alplas" system to reduce heat losses from open surfaces of coolants. The system is based on the use of hollow polypropylene floating balls that almost completely cover the surface of the liquid. Experiments have shown that at a water temperature in an open tank of 90 ° C, heat losses when using a layer of balls are reduced by 69.5%, two layers - by 75.5%.
Example 4. Calculation of specific heat losses through the walls of the drying plant.
The walls of the dryer can be made from various materials. Consider the following wall structures:
1. Two layers of steel with a thickness of 5 ST = 3 mm with insulation located between them in the form of an asbestos plate with a thickness of 5 And = 3 cm and thermal conductivity X and = 0.08 W / (m. K).
