RU2219425C1 - Method of heat insulation of pipe lines - Google Patents

Abstract

FIELD: thermal protection of pipe; steam and hot water supply pipe lines; construction engineering. SUBSTANCE: proposed method consists in weakening radiant heat transfer and avoidance of convection between pipe line and surrounding medium by means of air cavities formed between pipe line and protective envelope; pipe line is provided with shields made from heat-conducting material; clearance δ between shields is determined depending on Rayleigh number Ra_* calculated beforehand and lying within Ra_*<10⁴; used as protective envelope is steel pipe whose inner surface is coated with heat-reflecting compound, aluminum powder, for example and its outer surface is coated with non-heat-conducting material, for example foam polyurethane. EFFECT: reduced heat losses; increased thermal resistance of insulation; avoidance of free convection. 1 dwg

Description

 The method of thermal insulation of pipelines relates to thermal protection and can be used in various sectors of the economy for energy saving, mainly for pipelines for supplying steam and hot water to consumers, as well as in construction.

 The length of pipeline transport is limited by hydraulic and heat losses, the value of which affects the decrease in the efficiency of district heating, the cost of generating heat energy, and can lead to a loss in the quality of the coolant, which does not allow its further use in the process.

 For hot water, the decisive factor in the loss of quality is the increase in hydraulic losses, for steam, on the contrary, due to its low density, the increase in heat losses exceeds the increase in hydraulic losses. Heat losses, lowering the temperature of steam, change its state - from overheated, it passes into saturated, from dry saturated into a mixture of steam and water.

 For this reason, and also because of the unrestrained increase in fuel prices, the task of reducing heat losses is becoming of primary importance in heat supply.

 The only way to reduce heat loss is to increase the thermal insulation resistance. The thermal resistance of insulating products is created by the thermal conductivity of the material used and the thickness of the insulating layer. The maximum thickness is regulated by the "Instructions for the design of thermal insulation of equipment and pipelines of industrial enterprises" SN 542-81.

 The well-known "Method of installing thermal insulation on the pipeline" according to A.S. 1606798 F 16 L 59/05 dated 03.16.88, which consists in placing rigid heat-insulating panels on the pipeline in the form of segments, metal coatings, fastening the latter with fasteners and placing elastic elements discretely around the circumference of the pipeline.

 According to the patent of the Russian Federation 2095679, also known as "Thermo-waterproofing element and method of thermal waterproofing of metal pipelines" F 16 L 59/00 from 06/01/95, in which a layer of insulating material in the form of cylinders and half-cylinders having a coating on the inner cylindrical surface, heat-reflecting mirror coating is laid on the pipe to obtain different values of the coefficients of thermal conductivity.

 The closest in technical essence is SU 1427145 A1, class. F 16 L 59/06, 09/30/1988, in which a method of thermal insulation of a pipeline, in which closed air cavities are formed, is disclosed. To do this, support rings are installed on the pipeline, then shells of reflective heat-conducting material (beam-reflecting screens) are placed one above the other between the support rings.

 The objective of the proposed technical solution is to reduce heat loss, increase thermal insulation resistance, weaken radiant heat transfer and eliminate free convection, ease of implementation, the ability to use when repairing existing heating mains, improving the ecology of work.

The problem is solved by the method of thermal insulation of the pipeline by attenuating radiant heat transfer and eliminating convection between the pipeline and the environment using closed air cavities located between the pipeline and the protective sheath, formed between the support rings mounted on the pipeline and attached to them one above the other with a gap of screens from reflective heat-conducting material, while the gap δ between the screens of reflective heat-conducting material is determined depending t calculated previously Rayleigh number Ra _*, is in the range Ra _* <April ^10, as well as the containment is used the steel pipe whose inner surface is coated with heat-reflecting structure such as aluminum powder, and the outer - nonconductor material such as polyurethane foam.

 The proposed method of thermal insulation of pipelines is based on the principle of using the properties of air enclosed in closed air cavities created by screen shells and end asbestos seals.

 In closed air cavities of the estimated thickness of the multilayer air insulation, the air is stationary, which allows its low thermal conductivity to be used to form a conductive heat flow. Free convection is not present in cavities of such thickness. Radiant heat transfer is attenuated by screens located one above the other from a material with high reflectivity.

 Using the physical properties of still air as an insulator allows to increase the thermal resistance of an insulating structure by a factor of two in comparison with typical products.

 The increased thermal resistance of the new insulation can be demanded when solving the following engineering problems: expanding the boundaries of steam supply without reducing its quality, which in certain cases can be a decisive argument on the feasibility of building new sources of heat supply: reducing heat losses during steam transportation through existing networks. The use of the new insulation allows to reduce heat losses by half compared with standard designs, and four times compared with standard losses: maintaining the homogeneity (uniform composition) of the vapor medium throughout the transport, which is necessary for accurate measurement of steam flow.

 When transporting hot water, the new insulation made by the proposed method can be used both to reduce heat loss and to reduce material costs (while maintaining standard losses).

 The advantages of the proposed method of thermal insulation of the pipeline include: environmental friendliness of the products and the facilitation of the work of insulators due to the lack of loose and skin-irritating mineral wool; neutral insulation with respect to the metal of the pipe even if it is damaged; when mineral wool gets wet, which happens especially often in underground networks, an acidic environment is formed with a pH <7.0, leading to the destruction of the pipe metal; with our method, even when the trays are flooded, the pipe is in a guaranteed dry condition; increased stability of the thermal insulation created by the proposed method against electromagnetic corrosion caused by stray currents due to the presence of screens and the absence of their contacts with the pipe; the use of dielectrics (rubber gaskets) under the structural supports and in flange connectors increases the independence of the design from stray currents.

 The set of proposed essential features is new and allows to reduce heat loss, increase thermal insulation resistance, weaken radiant heat transfer and eliminate free convection, simplify the implementation of works, and use existing heating mains when repairing.

 The drawing shows a pipeline insulated by the claimed method, where 1 is a steam or hot water pipeline, 2 is an asbestos seal, 3 is a closed air cavity, 4 is a support ring of asbestos cement, 5 is a protective sheath of two half-cylinders of foam concrete, 6 are mounting rings from knitting wire, 7 - screens made of rigid foil or polished aluminum, 8 - rubber sealing cuffs, 9 - bandage.

A method of thermal insulation of a pipeline by attenuating radiant heat transfer and eliminating convection between the pipeline and the environment is carried out in the following sequence. After applying an anti-corrosion coating to the steam or hot water pipe 1, heat-reflecting heat-conducting material 6, for example, aluminum foil, is applied to reduce the emissivity; on it at the place of installation of the support ring 4 lay an asbestos seal 2. On the seal 2 impose a support ring 4 of asbestos cement. Between the support rings 4 are placed to weaken the radiant heat exchange one above the other with the calculated air gap according to the following formula, screens 7 of reflective heat-conducting material, for example, of rigid foil or polished aluminum. The thickness of the gap to eliminate convection heat transfer is selected depending on the value of the Rayleigh number, which in turn determines the convection coefficient. With a Rayleigh number <10 ^4, the convection coefficient is 1.0, i.e. convective heat transfer is absent. Between the support rings 4 and the screens 7, closed air cavities are formed in which there is no convective heat transfer. Each screen 7 is attached to the support ring 4 and secured with fixing rings 6 of knitting wire. On the support rings 4, the half-cylinders of the containment shell 5 are laid and tightened with a bandage 9. At the junctions of the half-cylinders 5, rubber sealing cuffs 8 are placed to prevent air convection.

The thermal resistance of thermal insulation structures is created by the thermal conductivity of the material used for insulation and the thickness of the insulating layer. The thermal conductivity of mineral wool (the main material used in the insulation of steam pipes), determined by the formula
λ = λ ₀ + β • t _avg ,
lies in the range of 0.064-0.079 W / m ^o C for average heat carrier temperatures of 150-250 ^o C.

The maximum achievable values of thermal resistance of mineral wool insulation are given in the table. It also shows the resistance of a new, created by the proposed method to the introduction of multilayer air insulation. The insulation thickness is assumed to be the same for both insulation. The calculations were carried out for a single steam line with an overhead installation with a steam temperature of 150-250 ^o C.

 In the case of using multilayer air insulation within the limits of the current normative heat losses, its use becomes rational for both steam and hot water: significant cost savings are achieved, since the design of the insulation itself is greatly simplified - the number of screens is reduced to 1-3 pcs.

где

δ - толщина прослойки;

- отношение длины пути конвективного тока к его проекции на вертикальную ось.According to research data for interlayers of various shapes, the convection coefficient is in the following dependence on the Rayleigh number:
Ra _* <10 ⁴ • ε _k ≈ 1.0;

Where

δ is the thickness of the layer;

- the ratio of the path length of the convective current to its projection on the vertical axis.

For horizontal cylindrical layers
I = (n • R ₁ + δ) :( D-δ);
where R ₁ is the radius of the inner wall;
D is the outer diameter of the cavity.

.n ₁ = 3: n ₂ = 0
Given that n ₂ = 0,

.

=1, а значит Ra_*=Ra.For vertical cylindrical layers

= 1, and so Ra _* = Ra.

 The calculation results are summarized in tables 1 and 2.

Claims (1)

A method of thermal insulation of a pipeline by attenuating radiant heat transfer and eliminating convection between the pipeline and the environment using closed air cavities located between the pipeline and the protective sheath formed between the support rings mounted on the pipeline and attached to them one above the other with a gap of screens of reflective heat-conducting material, characterized the fact that the gap δ between the screens of reflective heat-conducting material is determined depending on the preliminary calculation of the given Rayleigh number Ra _* , which is within the limits of Ra _* <10 ⁴ , and a steel pipe is used as a protective sheath, the inner surface of which is covered with a heat-reflecting compound, for example, aluminum powder, and the outer one with a non-conductive material, for example polyurethane foam.

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