Heat exchanger

A heat exchanger for the condensation of vapors, comprises a coil pipe having an upper inlet end and a lower outlet end and a plurality of lengths of pipe arranged in zigzag fashion between the upper and lower ends. Liquid is sprayed from above onto the coil pipe, and air is moved upwardly along the coil pipe. The lengths of pipe are inclined downwardly at acute angles to the horizontal from the inlet end to the outlet end. These acute angles progressively increase downwardly in such a way that the angles between pairs of adjacent lengths of pipe progressively increase downwardly for a plurality of those pairs. The inclination to the horizontal of the lowermost length of pipe is not more than 30.degree. and that of the uppermost not less than 3-5.degree.. The increment of angle increase, from pipe length to pipe length, is likewise 3-5.degree.. Vertical strips of plate are disposed between the lengths of pipe, to increase the evaporation surface.

The invention relates to a heat exchanger, the heat exchanging surfaces of 
which consist of a coil pipe with a changing angle of inclination and of 
secondary surfaces which are functionally connected to the same. With heat 
exchangers built up of pipes, in the inside of the pipes a condensing 
medium, f.i. water vapour flows, while on the outer surface of the pipes 
another medium, e.g. a liquid or ambient air flows. With these heat 
exchangers, in order to save water, nowadays mostly air is used for 
cooling purposes. However, the heat transfer coefficient between the air 
and the pipe is smaller by an order of magnitude, than the heat transfer 
coefficient of condensation inside the pipe, accordingly a small amount of 
water is sprayed onto the outer surface of the pipe, whereas an air flow 
is induced between the pipes. A part of the water evaporates and exerts a 
cooling effect on the pipe surface. The remaining part of the water flows 
to the space beneath the heat exchanger, whence it is recirculated via the 
pump to the space over the coil pipe. In such a manner the cooling process 
requires considerably less water; between the pipe and the air an 
evaporating and convective phenomenon may be observed, so we are 
confronted with a combined heat transfering process. 
In the known constructions used for this process, either one pipe row is 
arranged or several, approximately horizontally arranged parallel pipes 
are connected in series forming a coil pipe. The condensing medium, e.g. 
ammonia vapour is led into the upper row of the coil pipe. In the pipe 
rows lying beneath each other the medium gradually condenses and the 
condensate formed flows towards the lowest pipe. The coil pipe is arranged 
in a casing, the ventilators having been arranged on the top or on the 
bottom thereof putting the cooling air into motion. 
The common drawback of the known solutions lies in that the condensate 
accumulates in a continuously increasing quantity in the pipes lying 
beneath each other and completely fills the cross-section of the lowest 
pipe, accordingly, here condensation cannot take place. 
A further drawback of the known solutions lies in that compared to the 
utmost advantageous heat transfer coefficient within the pipes, there is a 
considerable difference between the heat transfer coefficient of the outer 
convection and the evaporation, respectively, as a consequence, relatively 
large heat surfaces have to be used. 
In order to be able to eliminate the drawbacks mentioned above, either the 
outer heat transfer coefficient has to be increased by increasing the 
velocity of air and the output of the ventilator, or the temperature 
difference between the pipe wall and the spray water has to be increased 
by spraying colder water onto the pipe surface. 
The solution according to the invention is based on these phenomena; here 
the inclination of the pipes changes in compliance with the prevailing 
conditions of condensation, i.e. the angle of inclination is increased, as 
the pipes tend downwards. 
Furthermore, means for the improvement of heat transfer are provided by the 
supplementary surfaces. This latter solution is based on the recognition, 
that spray water need not be evaporated exclusively on the surface of the 
pipes with a large wall-thickness because of the internal pressures and 
thus representing expensive components, but instead the water may be 
cooled in an easier and cheaper manner on the supplementary surfaces 
connected to the pipes in an advantageous manner from the point of view of 
fluid mechanics. 
In the invention the supplementary surfaces are formed in such a manner, 
that they do not restrict the path of the air flowing upwards, at the same 
time water should be collected from the pipes and sprayed onto the 
surfaces. For this reason the supplementary surfaces are formed with a low 
resistance; that means, that the dimension lying perpendicularly to the 
stream is as small as possible, expendiently less than the one tenth of 
the pipe diameter. For the magnitude of the supplementary surfaces an 
optimal proportion between the surface of the pipe /f.sub.p / and the 
supplementary surface /f.sub.s / is f.sub.s /f.sub.p =2. 
In order to be able to reduce air flow resistance, expediently the pitch of 
the supplementary surfaces is co-ordinated with the diameter of the pipes, 
i.e. the O pitch should be chosen as a multiple of the quarter of the pipe 
diameter, D/4. 
In the arrangement according to the invention the optimal inclination of 
the pipes can be obtained in the following manner: among the pipes lying 
beneath each other the angle of inclination of the lowest pipe lies in the 
range between 0.degree. and 30.degree. in dependence of the cross-section 
of the pipe, while the angle of inclination of the following pipe is 
progressively upwardly reduced by 3.degree. to 5.degree., accordingly, 
supposing, that the angle of inclination at the lowest pipe amounts to 
30.degree., that of the second from below it equals 25.degree., the third 
20.degree., the fourth 15.degree. and so forth, up to 5.degree..

FIG. 1 shows an embodiment of the heat exchanger according to the 
invention. Condensation of one of the media taking part in the heat 
exchange takes place in the continuous coil pipe 1. Along the outer 
surface of the coil pipe the air stream--induced by the ventilators 2a, 
2b--flows upwards, and water--sprayed by means of the sprayer 3 onto the 
pipes--flows downwards. The water sprayed onto the pipes and flowing 
therefrom is collected in the drip pan 4, from here the water is 
recirculated to the sprayer 3 via the pump 5. The construction is housed 
in a casing 6. The supplementary surfaces 7 according to the invention are 
arranged between the heat exchanging pipes. From the figures it becomes 
obvious, that the angle of inclination of the heat exchanging pipes 
increases progressively downwardly. 
In FIG. 2 the change of the angles of inclination of the coil pipe 1 has 
been illustrated. The inclination of the lowest row 11 of pipes is the 
largest, e.g. the angle of inclination (sz.sub.1) amounts to 30.degree., 
the angle of inclination (sz.sub.2) of the next row 12 equals 25.degree., 
the angles of inclination of the following rows 13, 14, 15, 16 equal 
20.degree., 15.degree., 10.degree., 5.degree., while the angle of 
inclination of the final rows 17, 18, remains constant, e.g. 5.degree.. 
In FIG. 3 the coil pipe according to the invention is to be seen, similarly 
to the previous embodiment there are the ventilator 2, the sprayer 3, the 
drip pan 4, the casing 6, simultaneously the cross-section of the 
supplementary surfaces is also shown. It can be seen, that the 
supplementary surfaces form an organic unit with the coil pipe in respect 
to fluid mechanics, they do not restrict the path of the air streaming 
upwards, simultaneously they ensure the accumulation of the water having 
been sprayed thereon and lead it forward to the next row of pipes. 
In FIG. 4 the arrangement incorporating two parallel coil pipe rows 
displaced in relation to each other, mlay be seen, showing two possible 
embodiments of the supplementary surfaces 7a, 7b. The common 
characteristics lie in, that in both cases the surfaces are arranged 
directly below the pipes. The supplementary surface 7a may be arranged 
between two adjacent pipes of the coil pipe displaced in relation to each 
other, while the supplementary surface 7b fills out the space between two 
pipes arranged below each other. 
In FIG. 5 another possible arrangement of the supplementary surfaces may be 
seen; the surfaces 7c extend in a horizontal direction and do not contact 
directly the pipes, not even their lower flanges contact the pipes lying 
underneath. The supplementary surfaces are fixed by means of the wedges 9 
of the required dimension, which are arranged between the fastening laths 
8 and the pipes. In such a manner the supplementary surfaces can be formed 
with identical heights despite the fact that according to the invention 
the angles of inclination of the pipes--in particular at the bottom--are 
different and as a consequence, the gap between them also changes. 
In FIG. 6 an embodiment of the supplementary surfaces is to be seen, 
wherein the surfaces 7d may be arranged not only below the lower edge of 
the pipes, but also below the outer edges thereof. Such a solution should 
be used in cases, when a large quantity of water is sprayed onto the pipes 
and it may happen that the water film is dislodged at the outer rims of 
the pipes. 
Finally, in FIG. 7 the possible versions of the cross-sections of the 
supplementary surfaces 7 may be seen. As to the surface 7e--which is in 
direct contact with the pipe--the upper arch 71 and the lower arch 72 are 
identical with the radius of the pipe. 
In case of the supplementary surface 7f the upper part of said surface is 
parallel with the tangent of the pipe lying above it. 
The side of the supplementary surface 7g is provided with the complementary 
surfaces 73 for collecting the water. The supplementary surfaces 7h merely 
touch the pipes lying below and above said surfaces.