The invention concerns a heat exchanger, for use in particular in connection with the process outlets at paper, pulp and board mills, which heat exchanger comprises a number of substantially parallel tubes or equivalent fitted in a duct, through which tubes or equivalent the air flow that delivers heat is fitted to pass, and the air flow that receives heat is fitted to pass through the gaps between said tubes in accordance with the cross-flow principle, and in which heat exchanger the heat faces of the tubes have been made larger at the side of the flow that receives heat by means of ribs, lamellae or equivalent.

As is known from the prior art, the principal object of the heat recovery systems is to replace primary energy in an economical way. In systems of recovery of heat, heat exchangers are used, of which the most important ones are the plate heat exchanger and the tubular heat exchanger. In the prior-art plate heat exchangers, a plate structure forms two systems of ducts perpendicular to one another, and a medium that delivers heat flows in one set of ducts and a medium that receives heat flows in the other set of ducts, said receiving medium being passed further to reuse. The tubular heat exchangers are, as a rule, provided with a supply of steam or water, and the tubes are surrounded by ribs or equivalent so as to increase the heat exchange area. In lamellar radiators the tubes are, as a rule, fitted between a plate structure, and in the ducts formed by the plate structure, as a rule, water flows, for example glycol water.

The object of the present invention is to suggest an air-air heat exchanger by whose means a more efficient transfer of heat is achieved compared with the prior-art solutions.

It is a further object of the invention to provide a compact heat exchanger which does not occupy an abundant space in the heat exchange systems.

In view of achieving the objectives stated above and those that will come out later, the heat exchanger in accordance with the invention is mainly characterized in that, in the heat exchanger, an air that is moist, saturated, or near the saturation curve is fitted to be used as the air flow that delivers heat in the heat exchanger, that the air flow that delivers heat is fitted to flow in the tubes substantially from the top towards the bottom, and the air flow that receives heat is fitted to flow substantially horizontally through the gaps between the tubes, in which connection the water that condenses from the moist air flow that delivers heat in the tubes flows down along the inner walls of the tubes and is collected in a basin placed in the duct of the heat exchanger, and that the heat exchanger comprises a spray system, by whose means water is sprayed before the heat exchanger proper to among the air flow that delivers heat in order to enhance the transfer of heat by means of an atomized water phase.

The heat exchanger in accordance with the invention preferably comprises a spray system, by whose means water is sprayed before the heat exchanger proper to among the air flow that delivers heat, the purpose being, first in the spraying stage, to transfer heat from the air flow that delivers heat to the water that is sprayed, which water further delivers heat to the air flow to be heated as it flows down along the walls of the tube, and at the same time drops are separated from the water film flowing down along the walls of the tube, which drops are later again returned back into the water film, which enhances the transfer of heat from the air flow that delivers heat.

In the air-air heat exchanger in accordance with the present invention, a moist air that is saturated or near the saturation curve is used as the medium that delivers heat, such air being, for example, the air in the process outlets at paper, pulp and board machines, in particular the exhaust air at dryer sections. In such a case, at the side of the medium flow that delivers heat the transfer of heat is considerably more efficient, because water condenses from the moist exhaust air.

3

In accordance with the invention, the following arrangement is used in order to provide and efficient and compact heat exchanger. In a heat exchanger in accordance with the invention, the exhaust air flows from the top towards the bottom in tubes of circular section, and the replacement air flows horizontally through the gaps between the tubes in accordance with the cross-flow principle. At the replacement air side the heat transfer face has been made larger by means of the principle known from the prior-art rib-tube radiators or lamellar radiators, in which connection it is possible to utilize the more efficient transfer of heat at the exhaust air side. The condensate formed in the tubes flows down along the tubes and is collected in the basin at the bottom. The condensate collected in the basin at the bottom is sprayed back into the top portion of the heat exchanger, in which connection a water flow of a certain magnitude flows constantly through the tubes. It is the objective of this arrangement that, by the effect of the exhaust air flow, drops are separated all the time from the water film that flows down, which drops collide again against the tube walls. Out of the exhaust air, heat is transferred efficiently into the drops and further into the tube walls as the drops are returned into the water film that flows down. Also, a signifi¬ cant extent of transfer of heat takes place between the water that is sprayed and the exhaust air even at the spraying stage before the heat exchanger proper. When this heated water flows down along the walls of the tube, by its means energy is trans- ferred directly from the water into the tube wall. Further, the water film flowing down along the tube walls is somewhat uneven by the effect of the shear forces arising from the difference in speed between the air flow and the water film, which further enhances the convective transfer of heat, compared with a smooth tube. In the arrangement in accordance with the invention no washing devices are needed, because the water spray system replaces them. For example, in the heat exchangers in accordance with the invention fitted in connection with the outlets of the wire parts in paper machines, no separate water separators are needed, because the passing of water along with the exhaust air is exclusively desirable. Preferably, in an arrangement in accordance with the invention, there are drop traps after the heat exchanger.

A heat exchanger in accordance with the invention can be applied, e.g., to removal of humidity from the exhaust air of a hood or a wire part, or it can be used for heating the ventilation air by means of the air flows mentioned above.

In the following, the invention will be described in more detail with reference to the figures in the accompanying drawing, the invention being, however, not supposed to be strictly confined to the details of said illustrations.

Figure 1 is a schematic illustration of principle of a heat exchanger in accordance with the invention.

Figure 2 is a schematic illustration of the principle of the flows of air and water in an individual ribbed tube in a heat exchanger in accordance with the invention.

Figure 3 shows an application in which the heat exchanger is used for separation of moisture out of the exhaust air and for removal of mist.

Figure 4 is a schematic illustration of a second exemplifying embodiment of a heat exchanger in accordance with the invention.

Figure 5 is a schematic illustration of a further exemplifying embodiment of a heat exchanger in accordance with the invention.

Figure 6 is a schematic illustration of a further exemplifying embodiment of a heat exchanger in accordance with the invention.

Figure 7 is a schematic illustration of another further exemplifying embodiment of a heat exchanger in accordance with the invention.

Figure 8 is a schematic illustration of the principle of a heat exchanger in view of an examination of the transfer of heat by means of calculation.

As is shown in Fig. 1 , through the heat exchanger 10, exhaust air 23 and replace¬ ment air 21 are passed in accordance with the cross-flow principle. The heat exchanger 10 is composed of a number of ribbed tubes 11, through which tubes 11 the exhaust air 23 is passed and between which tubes the replacement air 21 is passed, i.e. the air flows 23,21 have been passed in accordance with the cross-flow principle. To among the exhaust air 23, moisture, for example condensate, is sprayed by means of spray devices 17, after which the exhaust air 23 and the condensate pass through the heat exchanger 10 substantially from the top towards the bottom. The water condensed in the tubes 11 flows into the basin 13 at the bottom of the duct 12, from which basin the water is removed along the exhaust duct 14, or it is used again in the spray devices 17 of the spray system, in which case it is passed into the duct 18, in which there is a pump 30 or an equivalent actuator for passing the flow of water into the spray devices 17. The exhaust air 23, which contains drops and which has passed through the heat exchanger 10, is passed through a drop trap 15 into the exhaust duct 16 as a flow 25. The heated replace¬ ment air which has passed through the heat exchanger 10 is denoted with the reference arrow 22. As comes out from Fig. 1, the exhaust air 23 is introduced into the top portion of the heat exchanger 10, in which condensate taken from the basin 13 at the bottom is sprayed into the exhaust air. After this the exhaust air 23 and the condensate flow through the heat exchanger 10, in which connection the exhaust air and the condensate heat the replacement air 21, whereby heated replacement air is obtained. In the lower portion of the heat exchanger 10, the condensate that flows as a film on the walls of the tubes 11 falls mostly onto the bottom of the basin 13 at the bottom. The drops that were grasped by the exhaust air 24 along with it are separ- ated in the drop trap 15.

Fig. 2 is a schematic illustration of a single ribbed tube 11. The walls of the tube 11 are denoted with the reference numeral 19 and the ribs, which have been formed onto the wall of the tube 11 in order to increase the heat exchange area, with the reference numeral 20. When the exhaust air 23 and the condensate or equivalent introduced into it by means of the spray device 17 are passed through the ribbed

tube 11 , a water film 31 is formed on the walls 19 of the tube 11, and part of the moisture passes as drops 32 in the middle of the ribbed tube 11.

Fig. 3 illustrates the use of the heat exchanger 10 for separation of moisture from the exhaust air 23 and for removal of mist, when cold outdoor air 21 is passed through the heat exchanger 10, in which connection heated outdoor air 22 is obtained. The moist exhaust air 23 obtained from the process outlets is passed through the tubes 11 in the heat exchanger 10 through the drop trap 15 so that it is combined with the heated outdoor air 22. In Fig. 3 the cold outdoor air is denoted with the reference arrow 21 and the heated outdoor air with the reference arrow 22. The exhaust air 23 is passed through the heat exchanger 10, and condensate or equivalent moisture is mixed into the air by means of spray devices 17, in which connection the cold outdoor air can be heated in the heat exchanger 10. The exhaust air 24 which contains drops and which has been passed through the heat exchanger 10 is passed through the drop trap 15 to be combined with the heated outdoor air 22. In the heat exchanger 10 a large proportion of the moisture in the exhaust air condenses into water. This and the mixing of the heated outdoor air 22 with the exhaust air flow 24 reduce the formation of mist and the problems that arise from the mist when the mixture 25 of exhaust air and heated outdoor air is passed outdoors. The water formed in the basin 13 at the bottom of the heat exchanger duct 12 is recirculated along the duct 18 further to the spray device 17, and any excess amount is removed through the exhaust duct 14.

The exemplifying embodiment shown in Fig. 4 corresponds to the exemplifying embodiment shown in Fig. 1, but in this exemplifying embodiment, in view of increasing the heat exchange area, the ribbed tubes 11 in the heat exchanger 10 have been fitted at an angle in relation to the directions of the air flows, and the angle in relation to the vertical direction is α = 0...45 ⁰ , preferably 0...15 ⁰ .

As is shown in Figs. 1 to 4, in an air-air heat exchanger, a moist air that is saturated or near the saturation curve is used as the medium that delivers heat, such air consisting, for example, of the air from the process outlets at paper, pulp or board

machines, in particular of the exhaust air from dryer sections. In such a case, at the side of the medium flow that delivers heat, the transfer of heat is considerably more efficient, because water condenses from the moist exhaust air. In the heat exchanger 10 the exhaust air 23 flows from the top downwards in tubes 11 of preferably circular section, and the replacement air 21 flows horizontally through the gaps between the tubes 11 in accordance with the cross-flow principle. At the replacement air side the heat transfer face has been increased from the prior-art by means of the principle known from the prior-art ribbed-tube radiators or lamellar radiators, i.e. ribs 20 have been formed on the tube 11. In the tubes 11 the condensate 31 flows down and is collected in the basin 13 at the bottom. Condensate collected in the basin 13 at the bottom is sprayed by means of spray devices 17 back into the top portion of the heat exchanger 10, in which connection a water flow of a certain magnitude flows constantly through the tubes 11. It is the objective of this arrange¬ ment that, by the effect of the exhaust air flow 23,24, drops 32 are separated all the time from the water film 31 that flows down, which drops collide again against the tube 11 walls 19. Out of the exhaust air 23, heat is transferred efficiently into the drops 32 and further into the tube 11 walls 19 as the drops 32 are returned into the water film 31 that flows down. Also, a significant extent of transfer of heat takes place between the water that is sprayed by means of the spray devices 17 and the exhaust air 23 even before the heat exchanger proper. When this heated water 31 flows down along the walls 19 of the tube 11, by its means energy is transferred directly from the water 31 into the tube 11 wall 19. Further, the water film 31 flowing down along the tube 11 walls 19 is somewhat uneven by the effect of the shear forces arising from the difference in speed between the air flow 23 and the water film 31 , which further enhances the convective transfer of heat, compared with a smooth tube.

The exemplifying embodiment of the invention shown in Fig. 5 is substantially similar to the exemplifying embodiments of the invention shown in Figs. 1 and 3, but in this exemplifying embodiment it has been taken into account that, by means of the heat exchangers shown in Figs. 1 and 3...4, it is possible to produce heated replacement air 22 exclusively when warm and moist exhaust air 23 is fed from a

paper machine into the heat exchanger 10. At times of breaks and standstills, the supply of exhaust air 23 to the heat exchanger 10 can be discontinued, because the temperature and the moisture of the exhaust air 23 are considerably lower than in a production situation. If heated replacement air 22 must also be produced during breaks and standstills, a spare heating system 29 is needed.

In the exemplifying embodiment shown in Fig. 5, the additional heating has been accomplished by heating the water flow in the duct 18, i.e. the water flow is heated that is sprayed by means of the spray devices 17 into the top portion of the heat exchanger 10. The heating can be accomplished, for example, by means of a steam heat exchanger 29, which heats the water flow taken from the basin 13 at the bottom and passing in the duct 18, or, in the simplest version, by ejecting steam directly into this water flow. When the supply of exhaust air 23 is stopped completely, the heating of the replacement air 21 relies exclusively on the water circulation, in which case the temperature of the water is regulated based on the requirement of heating by means of regulation members 36 fitted in connection with the steam heat exchanger 29. In order to avoid outflow of heat, i.e. to avoid discharge of moist air and losses of energy, the exhaust air duct 26,16 passing into the heat exchanger 10 and/or departing from the heat exchanger 10 can be closed by means of a latticework 27,28 when the supply of exhaust air 23 has been stopped.

The exemplifying embodiment of the invention shown in Fig. 6 is, regarding its main principles, similar to the exemplifying embodiment shown in Fig. 1 , but in this embodiment the top portion of the heat exchanger duct 12 has been divided into two parts by means of a wall 33 so that the spraying of water by means of the spray devices 17 takes place exclusively in the part placed next to the direction of inlet of the replacement air 21. This embodiment is advantageous, because, under certain conditions, it is then possible to achieve a higher ultimate temperature of the replacement air.

Regarding its main principles, Fig. 7 is similar to the exemplifying embodiments shown in the preceding figures, but in this exemplifying embodiment of the invention

a scrubber 35, i.e. a spray water tower, is fitted in the heat exchanger duct 12 after the heat exchanger 10, in which scrubber the water to be sprayed into the top portion of the heat exchanger is pre-heated. The duct 18 is branched into a scrubber duct 34, in which spray devices 35 are fitted. In this embodiment of the invention the temperature of the water pumped out of the basin at the bottom is raised by making use of the energy of the exhaust air to be blown out.

The heat exchanger in accordance with the invention is suitable for use as a heat exchanger, for example, for replacement air for a paper machine or for any other application of recovery of heat. The heat exchanger in accordance with the invention can be used highly advantageously in process outlets at paper, pulp and board machines, in particular in the process outlets of a dryer section.

In the arrangement in accordance with the invention, very finely divided mist is sprayed, and a separate atomizing space is employed above the heat exchanger 10, or the spraying of water can also be carried out in the air duct passing into the heat exchanger. In the arrangement in accordance with the invention, an increased heat transfer face has been provided when the vertical or inclined metal tubes have been provided with ribs, which form a threading or spiral around the tubes. It is also possible to employ the technique known from lamellar radiators in order to increase the heat area at the replacement air side. Since a large quantity of water is employed in the arrangement of the present invention in the heat exchanger, it is easy to keep the heat exchanger clean, in which case it does not require so much cleaning, which also provides the advantage that the exhaust side is not blocked readily.

The optimal ratio of the mass flows of water and of the air that delivers heat depends on the temperature and moisture of the air flow that delivers heat and on the initial temperature and the desired final temperature and the mass flow of the air flow that receives heat, i.e. there is an optimal ratio of ιh _watcr /m _{eχhaust air} , and the magnitude of this ratio depends on a number of factors.

In the following, the principles of the transfer of heat in an air-air heat exchanger will be illustrated with reference to Fig. 8.

1. Energy balances

A heat exchanger is assumed, in which replacement air is heated by means of exhaust air. The magnitude of the heat capacity transferred in the heat exchanger is equal to the change in the energy content in the exhaust air or to the change in the energy content in the replacement air. If no changes in phase take place in any of the air flows, the transferred heat capacity can be indicated as follows:

Q ^{= m} p ^c p ( ^T p,in ' ^T p,out) ^{= m} k ^c k ^(T k,out - T _{k in} )

wherein m = mass flow of exhaust air (kg/s) c _p = heat capacity of exhaust air (J/kg/°C)

T _{p in} = exhaust air temperature before heat exchanger (°C)

T _{p oul} = exhaust air temperature after heat exchanger (°C)

and corresponding denotations for replacement air with the subscript k.

The same matter can also be presented by means of changes in enthalpy in the air flows, which formula is also applicable in cases in which changes in phase occur:

Q = ιiι _p (h _{p in} - h _{p out} ) = ιtι _k (h _{k out} - h _{k in} )

wherein h = enthalpy of the air flow (J/kg). In connection with air flows, mass flows m _ki of dry air (kgk.i./s) ( = kilograms of dry air per second) and the enthalpy i (J/kgk.i) based on same are used, in which connection the energy balance is

Q ^{= m} ki,p (yin ^" ip.out) ^{= m} ki,k ^k,out ^" ^.in)

2. Transfer of heat

The flow of heat from the exhaust air to the replacement air complies with the following formula:

dQ = α _tot dA (T _p - T _k )

wherein dQ = heat flow (W) ^α _t o _t ^{= overa} U heat transfer coefficient (W/m ² /°C) dA = area of heat face (m ² )

T _p = temperature of exhaust air (°C)

T _k = temperature of replacement air (°C).

Since the temperatures of the air flows are changed as the flows pass through the heat exchanger, the formula is correct exclusively when a little area element in the heat exchanger dA is examined in which the difference in temperature between die air flows does not change essentially.

When a whole heat exchanger is examined, the above formula must be integrated either numerically or, in simpler cases, also analytically. Then, for example, for a conventional counterflow heat exchanger, the following formula is obtained:

Q = α _tot A Δ Tι _n

wherein ΔT _ln = logarithmic difference in temperature.

3. Transfer of heat when condensation takes place at the side that delivers heat

Above, a what is called dry transfer of heat was examined, in which no changes in phase take place at any of the heat faces. As the exhaust air from a paper machine is moist, a significant proportion of the transfer of heat often takes place when the moisture contained in the exhaust air condenses on a cold heat face. The condensing

can be calculated, based on analogy between heat transfer and mass transfer, as follows:

a. dm,„ -J- dA (x _n - xJ

wherein α„ = heat transfer coefficient at exhaust air side (W/m ² /°C)

C„ = specific heat capacity of exhaust air (J/kgk.i./°C) ( = Jper kilogram of dry air per °C) moisture of exhaust air (kgH ₂ O/kgk.i.) (= kilograms o f

H ₂ O per kilogram of dry air) ^x w saturation moisture corresponding to temperature of the wall in the heat exchanger (kgH ₂ O/kgk.i.)

The heat capacity produced by the condensing is

dQ cond ^dm cond ^Δh vap

wherein Δh _vap = vaporization enthalpy of water vapour (J/kg) .

The difference between dry and moist transfers of heat is compared when the temperature of the heat transfer face is T _w :

dQ a p dA ( ^v T n - T w )' dry

dQ (x - _x ) moist

From these two formulae it is possible to calculate a more illustrative presentation of how condensation enhances the transfer of heat compared with dry heat transfer.

dQ k α _p dA (T _p T _w )

wherein the coefficient k depends mainly on the moisture of the exhaust air. For saturated exhaust air, the following values are obtained for the coefficient k, depending on the difference in temperature T _p - T _w :

T - T = 10 20 30 °C

30 3.8 3.2 2.8

40 5.7 4.8 4.0

50 8.7 7.2 6.1

60 13.5 11.1 9.3

70 21.4 17.4 14.5

80 36.1 28.4 23.2

Since the wet temperatures of the exhaust air at different stages in the recovery of heat are typically of an order of 40...60 °C, in practice the condensation at the exhaust air side increases the transfer of heat to 5...15-fold. In such a case, it is obvious that dry transfer of heat at the replacement air side becomes a factor that limits the transfer of heat.

In such a case, by making the heat face at the replacement air larger, a significant increase in the efficiency is obtained. With the ribbed-tube radiator technique the heat face at the replacement air side is typically 8...12 times as large as the heat face at the exhaust air side. With the lamellar-radiator technique the heat face at the replacement air side is typically 20...30-fold.

4. Transfer of heat from a water film flowing down in a tube to the tube wall

As regards the transfer of heat between the water film and the tube wall, it can be estimated that this transfer is about two decades higher than the dry transfer of heat from an air flow.

5. Calculation of the overall heat transfer coefficient

5.1 Both heat faces are equally large

In a conventional plate heat exchanger or tubular heat exchanger, the heat faces are equally large at both sides. In such a case, the overall heat transfer coefficient between the exhaust air and the replacement air can be stated simply:

^a to, =

-L + ^s + ^l a _p λ a _k

wherein α _p = heat transfer coefficient at exhaust air side

(W/m ² /°C) α _k = heat transfer coefficient at replacement air side (W/m ² /°C) s = thickness of plate or tube wall (m) λ = thermal conductivity of plate or tube wall (W/m/°C)

The resistance to transfer of heat arising from the plate or from the tube wall can often be overlooked, in which case the formula is simplified to the form

tot

— + _L

5.2 Heat face at the replacement air side has been enlarged

The heat face is taken in accordance with the exhaust air side, A = A The heat face at the replacement air side is denoted with A _k . Then, the overall heat transfer coefficient is

1

"tot

wherein η = efficiency of heat face s = thickness of the wall of core tube (m) λ = thermal conductivity of core tube (W/m/°C)

The efficiency of a heat face is typically of an order of 0.8...0.95.

Since the temperature of the ribs readily remains slightly lower than the surface temperature of the core tube, a rib efficiency ιj _rib has been defined. In such a case, the formulae can be derived based on the assumption that the temperature of the ribs is the same as that of the core tube, but the transfer of heat is multiplied by the rib efficiency. Since transfer of heat into the replacement air takes place both from the ribs and from the bare portions of the core tube, the efficiency of heat face, which is present in the above formula, has been defined additionally, which efficiency is calculated from the rib efficiency τj _rib , from the total heat face A _k at the replacement air side, and from the heat face A _rib of the ribs at the replacement air side:

^l rib

⁽ i - n„» ⁾

Above, the invention has been described with reference to some preferred exemplify¬ ing embodiments of same only, the invention being, however, not supposed to be strictly confined to the details of said embodiments. Many variations and modifica- tions are possible within the scope of the inventive idea defined in the following patent claims.