WO1983002151A1

WO1983002151A1 - Gilled tube construction

Info

Publication number: WO1983002151A1
Application number: PCT/SE1981/000367
Authority: WO
Inventors: Rune Andersson
Original assignee: Rune Andersson
Priority date: 1981-12-10
Filing date: 1981-12-10
Publication date: 1983-06-23
Also published as: EP0096021A1

Abstract

Heat exchangers consisting of finned tubes especially used in combustion apparatus. According to the invention the fins have a geometrical form which is defined in the formula <IMAGE> where x and y represent Cartesian coordinates and a and b represent external dimension in respectively x- and y- direction and the exponent of power n is more than 2 but less than 5.

Description

Gilled tube construction

This invention relates to an apparatus with tubes, provi¬ ded with gills and intended for a flowing medium, in par- - ticular flue gases from a combustion process, where the gills are designed to absorb an optimal amount of heat from the gases for transmission to a heat absorbing medium, e.g. water, in the tubes.

In particular, but not exclusively, the invention relates to gilled tubes for economisers in boilers.

The sections at the rear end of a boiler serve to absorb the final heat energy remained in the flue gases before they leave the unit through the stack. This heat recovery is normally accomplished by preheating the feed water and/ /or the air for the burners, and in general the heat ener- gy in the flue gases below 300°- 500°C is utilized. These sections are called economiser and air heater respective¬ ly, and the aim is of course to lower the flue gas tempe¬ rature in these sections as much as possible since this will result in an improvement of the heat economy. It can be shown that a decrease of the flue gas temperature of 10 -15 C will increase the efficiency by about 1 percent¬ age unit. The determination of the size of the heating surfaces of economisers and air heaters will thus be an economical evaluation where the improvement of the heat economy is balanced against the cost of the increased heating surface.

The lower limit for the cooling of the flue gases is de¬ termined by the temperature where there is a risk for condensation of especially sulphuric acid on the heating surfaces. What happens is that the sulphuric trioxide S0₃ that is formed when the fluel is burned reacts with the water vapour H₂0 in the gases and forms sulphuric acid H₂S0ι,. The temperature at which this is started is called the acid dew poinu, ^:and its value is dependant upon the type of fluel and its composition, especially the sulphur con¬ tent, the combustion method, excess iar and several other factors, and it is thus difficult to give an exact value of this temperature. Normally its value is in the region between 70°-170°C.

The condensation below the dew point temperature causes deposits and corrosion on the surfaces, and to avoid these problems the boiler could either be designed so that this temperature is never achieved (which results in a decrea¬ sed heat economy) or equipped with surfaces that can re¬ sist the corrosive attacks. In the former case the heating surfaces must be designed so that the exit flue gas te pe- rature exceeds the condensation level with adequate margin. It is important to notice that this condition has to be maintained at every boiler load. It is well known that the exit flue gas temperature decreases when the boiler load drops, and if the unit is to operate at various loads it is therefore necessary to have a sufficient design reserve with respect to the dew point.

When the boiler is arranged for a low gas exit temperature the heating surfaces are normally designed to resist the corrosive attacks from condensing sulphuric acid and simi- lar products. The usual method is to make the heating sur¬ faces in this region with a cover from cast iron outside the steel tubes. The purpose of the cast iron cover is to form the corrosion resistance while the steel tube consti¬ tutes the pressure resisting part. The contact between the cast iron shell and the steel tube must of course be very good to give a good heat transfer through the material.

The primary function of the heating surfaces at the rear end of the boiler is to transfer heat from the flue gases on one side of the surfaces to the feed water or the co -

OMPI WIPO bustion air on the other side of the surfaces. Within the actual; temperature region it is suitable to make the sur¬ faces with flanges or gills on the gas side since the heat transfer to a surface is lower from a gas than from a li- quid. Consequently the feed water heaters (economisers) are very often made from horisontal tubes with flanges on the outside.

From an elementary point of view the flanges (gills) of a heat exchanger surface serve the purpose of improving the heat transfer on the side where the heat transfer co¬ efficient between the flowing medium and the surface is low. This, for example, is the case with gases as compared to fluids. The low heat transfer coefficient is compensa¬ ted through the flange arrangement by the fact that the size of the heat absorbing surface is increased. The dis¬ advantage, however, with a flanged surface is that the surface temperature is different in different parts of the flange, and consequently the heat transfer at the tip of the flange will be lower than at the root since the tempe- rature difference between the surounding medium and the surface is lower at the tip than at the root. This is ex¬ pressed in the calculations by the so-called flange effi¬ ciency η, which is a measure of the relation between the average surface temperature of the flange and the tempera- ture at the root.

Based upon the equation for the temperature distribution in the flange material it is possible to derive certain basical expressions for the flange efficiency. A characte¬ ristic feature of these expressions is that the thickness and the height of the flange as well as the geometrical form of the flange are basic components of the equations. Both the dimension and the form of the flange are thus of great importance for the heat transfer.

By choosing a circular flange form (seen in the direction of the axis of the tube) the highest possible flange ef¬ ficiency but the lowest value of ^"the heat surface size is obtained. The other extreme is a flange with a rectan¬ gular form. This flange form gives the smallest flange efficiency but on the contrary the heat surface size is the largest possible within the available space. With re¬ spect to the cost it is of course desirable to choose a flange form which gives the highest possible heat trans¬ fer in relation to the material consumption and it is therefore important to find a flange form that gives an optimal value with regard to this.

Another advantage with gilled tubes in the heat exchang¬ ing surfaces is that the volume will be much smaller com¬ pared to units without flanges or gills. This means that the investment will be smaller. The fact that the size of the heat exchanger surfaces are smaller also gives lower pressure drops on the gas side as well as on the water side in an economiser, and this results in significant savings in both installation and working costs for the fans and feed water pumps.

The embodiment of the invention is to give a solution with respect to the design of flanged heating elements, in par¬ ticular elements for economisers. Before this solution is presented a detailed technical summary will be given.

An economiser acts in principle as a heat exchanger with the purpose to transfer heat from a gas to a liquid. The heat quantity that is transferred can be expressed by the equation q = k-A-Δt where k = heat transfer coefficient, J/m² s C A = heating surface, m²

Δt= mean temperature difference between hot and cold side, °C. At a given temperature difference the heat transferred is thus determined by the product of the heat, transfer co¬ efficient k and the surface size A. The quantity k, which a flanged heater normally is related to the total size of the outer surface, can be divided into three components as follows: a) the heat transfer from the gas to the outer surface of the tube b) the heat transmission (conduction) through the tube material from the outer to the inner surface c) the heat transfer from the inner surface of the tube to the liquid in the tube.

The following equation can be derived for the quantity k, where the three components above are represented by the three terms on the right hand side:

+ β-

where α_f = heat transfer coefficient between the gas and the flanged outer surface, J/m² s C α. = heat transfer coefficient between the inner surface of the tube and the liquid in the tube,

J/m² s °C η = flange efficiency A_f = flanged surface size, m²

A = size of the tube surface between the flanges, m 2 A = total outer surface size (= A_f+A ) , m² A. = inner surface size, m²

A = outer surface size of an unflanged tube, m² 6 = tube thickness, λ = heat conductivity of the tube material, J/m s C a = form factor, which takes into account the curva¬ ture of the tube. The flange efficiency η, which thus is related to the heat transfer, is as mentioned before a measure of the relation between the average surface temperature of the flange and the temperature of the tube at the root of the flange, and it is an expression of the fact that the heat transmission capacity of the flange is smaller than that of the tube, measured per unit of surface area. The size of the flange efficiency is depending upon the dimensions and the geometrical form of the flange, and the scope of the invention is to achieve the best efficiency by giving a formula for the geometrical form of the flange. The op¬ timal solution can be expressed by the formula n n

where x and y are coordinates in a right angled system, a and b are proportional to the outer dimensions of the flange in the x- and y-direction and n is a coefficient whose value is between

2 = circle and co = square.

Calculations have shown that the coefficient n should have a value between 2 and 5. Higher values than 5 gives a reduced flange efficiency and higher material consump¬ tion per unit area. Enclosed drawing shows examples of various geometrical forms of flanges for finned and gilled tubes.

Claims

C l a i

Apparatus with tubes, provided with gills and intended for a flowing medium, in particular flue gases from a com¬ bustion process, where the gills are designed to absorb an optimal amount of heat from the gases for transmission to a heat absorbing medium, e.g. water, in the tubes, whereby the geometrical form of the flanges is n n a so where x and y are coordinates in a right angled system and a and b are proportional to the outer dimensions of the flange in x- resp. y-direction, c h a r a c t e r i z e d in that the value of the coefficient n is higher than 2 but smaller than 5.