NL8401778A - DEW POINT COOLER. - Google Patents

Abstract

A dew-point air cooler comprises a heat exchanger including a) a bank of vertical tubes 3 arranged to pass primary air 1 to be cooled along the outside of the tubes with exchange of heat; b) means for passing pre-cooled secondary air 2 through the interior of the tubes; c) means 4,5,7,13 for introducing water periodically into the tubes from the top and forming a film of water on the inner wall of the tubes; d) means for discharging the humidified secondary air; and e) means for passing at least a portion of the cooled primary air to a space to be cooled. <IMAGE>

Description

t * 1 * -t- VO 4892

Dew point cooler.

Dew point cooler for cooling a stream of air, the cooler comprising a heat exchanger comprising: a) a bundle of vertical pipes arranged to conduct the primary air to be cooled under heat exchange along the outside of those pipes 5; (b) means for passing pre-cooled heat-exchanged secondary air inside those pipes; c) means for introducing water into said pipes from above and to form a film of water on the inner wall of said pipes for wetting the secondary air in the pipes by evaporation and cooling the primary air outside the pipes; d) means for exhausting the humidified secondary air into the open air; e) means for exhausting at least a part of the cooled primary air 15 to the space to be cooled.

Such a dew point cooler is known per se from US patent application 4,023,949. It proposes air-conditioning (cooling) a space by cooling primary outside air in this way and leading it to the space concerned, while at the same time a flow of secondary air (which already has approximately the desired temperature) while evaporating water through the exchanger is fed to cool the primary air flow.

The same principle, but in a slightly different embodiment, is also known from United States patents 1,986,529, 2,107,280, 2,174,060, German Offenlegungsschrift 2,432,308 and Dutch patent application 77,11149.

These differ slightly from each other due to the construction of the heat exchanger used (pipe bundle or plates as exchange surface) and the way in which the evaporation of water is caused in the second flow (a mist of water droplets, a water-retaining wall surface, alternating supply of water on two halves of the exchange plates or continuous spraying of water).

In either of these methods, the secondary stream is wetted by evaporation of water and cooled below the temperature of the primary stream. The resulting temperature difference allows the secondary flow to absorb heat from the primary flow, which is thereby cooled. By this heat absorption, the secondary stream becomes unsaturated again in water vapor, so that an amount of water can evaporate again and again an amount of heat can be absorbed from the primary stream, etc.

As a result of this cooling of the first flow, the temperature of that first flow drops, while the absolute humidity of that air flow (in grams per kg of air) remains constant.

Because a temperature difference between the two flows is necessary for the transfer of sensible heat between the primary and the secondary airflow as a result of heat resistance and because this temperature difference is obtained by wetting the secondary airflow, which in a preferred form of the invention is a partial flow of is the primary flow and then has an inlet temperature equal to the outlet air temperature of the primary air stream, the achievable outlet temperature of the primary air stream is usually above the dew point of the air in the primary stream. Only ideally the outlet temperature of the primary air can be equal to its dew point. This gives rise to the name "dew point cooler".

This method of air cooling is cheaper than that of the more common compression cooling machine, because the latter uses much more energy.

In addition, some of these publications state that it is also possible to give the cooled air a smaller water content by completely or partially re-cooling the cooled primary stream coming from the heat exchanger with a compression cooling machine, so that part of the water vapor condenses. This combined cooling can be used with fruit when the outside air has a higher absolute moisture content than is desired in the cooled air. Substantial savings are then also achieved, because that compression cooling machine need only be used when it is necessary to reduce the absolute water content.

In all other cases, only the much cheaper dew point cooler is used.

Despite these clear and great advantages, such a dew point cooler has never been used in small or large sizes, as far as we know.

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A reason for this has never been stated. It is clear, however, that a dew point cooler is only eligible if the heat exchanger used is very efficient and, in view of the relatively small temperature differences between the air flows, which must necessarily occur, this will only be achieved with a large exchanging surface.

In all the cases mentioned in the literature, it has been found that a heat exchanger is proposed in which the active surface on the side of the primary flow does not differ substantially in size from the active surface on the side of the secondary flow.

Calculating the heat exchanger under these conditions reveals that under these conditions the volume of the heat exchanger will usually become unacceptably large.

A first drawback of the known method is that the theoretical treatment does start with a secondary stream which is saturated with water vapor by evaporation of water (because then the lowest secondary temperature is obtained), but in practice this condition was in many cases not reached.

According to the invention, the means for supplying water to the pipes are suitable for this supply to take place periodically and the ratio between the dry outer surface of the pipes and the wet inner surface of the pipes is greater than 3: 1. As far as known heat exchangers for two air flows with such a surface ratio have not been proposed before.

In the drawing: Fig. 1 is a vertical section through an embodiment of a dew-point cooler; Fig. 2a shows a top view of a pipe sheet, such as is preferably used according to the invention; fig.2b shows a cross-section of a part of a tube sheet according to fig.

2a, which also shows a shape of a collar as used according to the invention and indicates the way in which said collars fit together; Fig. 3a shows a water distributor, such as according to a preferred form of the 8401778 v-4 invention, partly in views partly in section; Fig. 3b shows a section along the line IIIb-Illb of the water distributor, according to Fig. 3a; Fig. 3c shows a top view of the water distributor according to Fig. 3a; Fig. 4 is a Mollier diagram to explain the dew point cooling which takes place according to the invention; Figures 5a and 5b show a water supply member, such as can be used according to the invention. Figures 6a, 6b and 6c show another water supplying means, such as can be used according to the invention; fig. 7 a. Mollier diagram, illustrating a conventional cooling method; FIG. 8 is a Mollier diagram illustrating a combination of dew point and mechanical cooling; fig. 9 shows a diagram of an application of dew point coolers as> part of an air-conditioning installation according to fig. 8; fig.10 a frequency diagram of temperature and humidity conditions in the Netherlands; 20. FIG. 11 is a Mollier diagram illustrating a second combination of dew point cooling and mechanical cooling; Fig. 12 shows a diagram of the application of the dew point cooler as part of an air-conditioning installation according to Fig. 11; Fig. 13 is an exploded view of the heat exchanger used according to a preferred embodiment of the invention; 14-16 still other water supply means in combination with a water collection / distribution device, as can be used according to the invention.

30 Figure 1 is a vertical section through a dew point cooler. In it, 3 indicates a number of sections of vertical pipes.

The primary flow (1) enters these sections on the left and flows through them in a horizontal direction. The secondary (wet) stream enters the device at the top right, then flows down through the pipes of the rightmost section, as indicated by the arrows (2) and then successively through the pipes of the other sections, after which this flow is at the top left leaves the establishment again.

8401778

* V

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The water to be evaporated is fed upwards from tanks 4 (fig. 1) under the sections by pumps 5 to devices 16, from which the water is fed to the upper pipe plates 7, which are provided with an upright edge. On those pipe plates, the water is distributed over the pipes by water distributors 13 in such a way that in each pipe of a section the inner wall is covered with a layer of water along which the secondary flow of air (up or down) flows while evaporating water, whereby the necessary evaporation heat is extracted from the primary stream, which contacts the outer wall of the pipes and the lamellae plates. The non-evaporated part of the water returns to the receptacles 4.

The origin of the primary flow depends on how the dew point cooler is incorporated in the air treatment system.

In a simple embodiment, the primary stream 15 may consist of air drawn in from the outside, which is cooled in the heat exchanger and then fed from the right end of Fig. 1 to a space to be cooled.

In order to maintain the desired temperature in that space, for example, 10% per hour of the air present in that space is continuously removed and used as secondary flow 2.

During the passage of the different sections 3 water evaporates therein, at the same time this secondary flow absorbs heat from the primary flow and the result is that the temperature of that secondary flow increases during the cooling of the primary flow.

It is precisely this rise in temperature that makes it possible for more and more water to evaporate in the secondary flow, so that eventually a highly humidified flow of air escapes on the left side with a temperature higher than the temperature in the space to be cooled.

This highly humidified air is furthermore useless for the cooling process.

It is of great importance, however, that in the cooled primary stream the amount of water vapor has not changed during the passage of the heat exchanger.

35 For a good heat exchange it is necessary that the heat is first transferred from the primary (dry) stream to the (usually metal) wall. The amount of heat transferred 8401778 v * -βίε is then directly proportional to the product ct ^ F ^ in which the transfer area is and the transfer coefficient. After this heat has passed through the partition wall material, the same amount of heat must be transferred from that wall to the secondary (wet) stream and the amount of heat transferred is then directly proportional to a ^, where again F2 is the transfer area and the transfer coefficient.

From Leidenfrost, W. Analysis of Evaporative Cooling and Enhancement of Condensers Efficiency and of Coefficient of 10 Performance, Warme- und Stoffübertragung 12 (1979), biz. 5-23, it is known that in heat transfer from a wall to an air stream in the presence of an evaporating water film on that wall, the heat transfer coefficient is a multiple of the heat transfer coefficient that occurs on convective transfer on the same wall. Leidenfrost describes a minimum ratio of 3 for saturated air and higher than 5 for unsaturated air.

Apart from the difference in convective heat transfer in flow in pipes and in flow dm (lamellae) pipes, on the basis of the above, in a dew point cooler according to the invention of the type mentioned in the opening paragraph, a heat exchanger with a ratio should be used. between the dry outer surface and the wet inner surface, which is greater than 3: 1.

25 Measurements on a test setup of a dew point cooler, in which a surface ratio of 6.6 was applied, have shown that under Dutch climatic conditions the ratio c ^ / ci ^ values takes up between 8 and 13 *

The precondition for this is that the heat exchange surface on the side of the secondary air stream is always covered with a thin layer of water.

Because the ratio α „/ α. depends on the condition of

m X

the aspirated primary air, for a given construction site, the aim will be that approximately F ^ / F ^ = c ^ / ot ^. For Dutch conditions, this value is preferably between 4: 1 and 6: 1.

As the value of the outer surface, one can use the effective outer surface, which can be calculated using 8401778 -7- β i published formulas for such constructions.

All of the aforementioned publications on dew point coolers agree therein that they all describe a ratio that differs little or no different from 1: 1.

5 This is probably why none of those proposals have led to practical application.

When a heat exchanger is designed, in which F = approximately 6, it appears that for equal capacity the volume need only be 1/3 to 1/4 of the volume of an exchanger 10 in which = F ^.

In order to obtain the necessary difference in exchanging surface, tubes can be started, analogous to known constructions, on which ribs or fins are applied, after which those tubes with fins or fins are combined into larger units. The known exchangers of this type are usually constructed for significant pressure differences between the two exchanging gas flows. With a dew point cooler, however, this pressure difference is at most a few cm. water pressure and that makes a very light construction possible.

20 The usual construction method for lamella batteries is as follows:

Holes are punched into metal plates, for example aluminum plates with a thickness of 0.2 - 1.0 mm, for example 0.25 mm, which are usually also provided with a collar.

The fins or rib plates are then slid onto core pipes and these pipes are expanded mechanically or hydraulically so that the fins are clamped on the pipes. These corrugated pipes are then mounted in conventional pipe plates.

This construction can also be used now.

Preferably, however, the heat exchanger is built up from a number of plates, having a series of openings, each opening being provided with a collar, which fits into an opening of an adjacent plate, which plates are stacked into a package, in which the slid collars together form a series of pipes 35, which pipes are interconnected by the rest of the plates, which then serve as slats.

3401778 j 4 -8-

The openings in the collars can advantageously have a diameter of 20 - 40 mm and the collar height can be 4 - 7 mm.

The holes are usually round for constructional reasons. A diameter of 30 mm and a collar height of 6 mm has proved very effective.

By proper shaping of the collar, it is possible for each collar to be slid into the underlying pipe about half of its height, as shown in Figures 2a and 2b. This makes the core pipes superfluous and the combination of pipes and fins can be built considerably cheaper and more compact. In the following, a "pipe" consisting of such partially interlocked collars will also be considered a pipe. When the collars can only be pushed together with a light pressure, a simple and yet sturdy construction can be obtained, in which the different pipe sections lie sufficiently sealing on each other.

The pipes, however constructed, must be provided with a thin layer of water on the inside and this layer must cover the entire pipe wall if possible and be as thick as possible everywhere. Furthermore, all pipes must of course be wetted to the same extent. The water supply system must therefore not be susceptible to skewing or to disturbance of the water level on top of the top tube plate.

The aforementioned U.S. Pat. No. 2,107,280 suggests using plate exchangers coated on the wet side with a fabric. It mentions that better results are obtained if the water is not supplied continuously, but periodically. One or the other half of the entire wall is alternately moistened and the water can then evaporate in contact with the free-flowing air. After some time, the water has dripped down and / or evaporated, after which a new supply is required. It is therefore necessary to supply a controlled amount of water in a rather short time, then stop the supply and repeat this after a controlled time. A construction according to this patent has the drawback that the plate surface is covered with a wet fabric, which can be kept uniformly wet, but which nevertheless forms a greater resistance to the heat flow than a water film directly on the metal.

8401778 * 5 '% -9-

For constructional reasons, with a dew point cooler it is often desirable that in some of the vertical pipes, the air flows upward and down through others. However, the water flowing along the wall always flows down, so that in some pipes water and air flow in the same direction and in others in counterflow. This makes it difficult to control the water supply, especially when the air speed is variable.

It has been found, and that is a further feature of the invention, that this distribution of the water can be efficiently carried out by placing a water distributor (fig. 3a - 3b - 3c) on top of each vertical pipe, which consists of two short, concentric pipe sections, (30, 32), the inner (30) of which extends into or around the pipe and is tightly clamped therein, while the outer one rests on the upper pipe plate and is provided with one or more 15 openings on the lower edge (33 ), which extend to the annular gap (31) between the two tube sections, the inner (30) having a plurality of circumferentially divided openings (34) connecting the annular gap (30) to the inside of the tube and which are located some distance above the tube sheet. These openings (34) distribute the water supplied along the circumference of the tube. Gas flowing through the tube can then escape or enter through the inner tube portion. A funnel-shaped portion (35) is preferably arranged at the top of the inner tube.

As shown in Fig. 1, as with the known 25 'dew point coolers, one or more receptacles (4) for evaporating water are found under the pipe bundle (12), where the non-evaporated water collects and from which water is supplied to the top tube plate (7) on which the water distributors (13) rest. This pipe plate has a raised edge.

Although Fig. 1 shows a concrete embodiment with 4 sections of pipes, it will be clear that a different number of sections can also be used and that the primary flow (1) and the secondary flow (2) can pass through different sections as required. If the tube bundle consists of several sections (3, 3), which are successively traversed by the secondary flow (2), it is useful that each section has its own receptacle (4) because those sections correspond to different temperatures for the secondary flow.

8401778 -10- 4 «

The level in those reservoirs can be kept constant in the usual manner. The primary stream (1) is then passed outside those pipes between the lamella plates.

Various devices can be used to feed the water periodically, i.e. during the correct period of time and in the correct quantity and with the correct interval, on the upper tube plate.

The most convenient is a pump, which is switched on and off in a suitable known manner. However, such circuits 10 * are sensitive to disturbances. In the aforementioned

US patent 2,107,280 has already described a device with a double tilting trough, to which an amount of water is supplied via a continuously operating pump, while each time after filling of a tilting trough its content is poured out on the fabric covered cooling plates, after which the other tilting trough is filled. This system works well, but has the drawback that it has moving parts so that it is subject to wear and will not work reliably over very long periods of time.

Therefore, a method was sought which has the same result, but which has no moving parts and is therefore not subject to wear and does not require maintenance.

The principle of such a system is given in fig. 5. There the reservoir R is filled by a continuous water supply, for instance by a pump, which supplies water from the receptacle (4) under the pipes concerned. As soon as the water in this reservoir has reached level (1), the siphon H initially starts to function only as an overflow channel. However, the siphon is provided with a spring dam D, which causes the water flowing over the bottom of the siphon H to spray against the top of the siphon and thereby shut off the air volume above the spring dam. Due to the relatively high speed of the water, the pressure in that jet at the location of the spring dam becomes small and as a result the air is drawn away from the top of the siphon. The siphon thereby rapidly fills with water, so that the situation is shown in Fig. 5b. From that moment on, the siphon quickly fills up completely, after which the water discharge through the siphon increases rapidly to its maximum. This discharge continues 8401778 c t -11- until reservoir R is emptied to level (2), so that air can re-enter at the left end of the levers. The siphoning then stops.

With this device it is possible to supply a quantity of water at regular intervals, which is determined by the size of the reservoir R.

Tests showed that the siphon time was still too long and that the correct ratio between siphon time and cycle time could not be achieved.

10 After all, as said, there is a spring dam D in the hedge

present. This does cause the air to be sucked away, so that the siphon starts quickly and easily, but the same spring dam also forms an obstacle in the outlet after starting, so that the amount of water flowing out is strongly hindered in its course.

For that reason, according to a preferred embodiment of the invention, a second larger siphon A is mounted next to or above the (small) siphon B (see fig. 6a and 6b and 14), the outlet of which is in water, or of which the spout a water seal is provided, as in fig. 6. The larger siphon 20 has a highest point, which is slightly higher than the highest point of the small siphon, so that the small siphon B is self-starting, but the large one is not.

A spring dam is therefore not included in the larger siphon. The small siphon starts in the manner already described, but now also sucks in addition to itself via one or more connecting channels, for example the connecting channels V, shown in fig. 6c, the larger siphon mounted next to or above, so that that large siphon also starts quickly is becoming. Due to the large run-out diameter without an obstacle as a barrier, the discharge capacity of siphon A is such that a short run-out time is achieved.

In summary, the essence of the described combination of a large (main) siphon A with a small (auxiliary) siphon B is that the small siphon serves primarily to start itself and then the large siphon. The water drainage through the small siphon is incidental. In contrast, the large siphon is constructed to allow large water drainage in a limited time. The large siphon is not self-starting and need not be.

It is important that this water drainage takes place in a short time, because in the (wet) pipes of the heat exchanger periodically so much water has to be poured out that the entire wall in all pipes is simultaneously moistened so that the desired water film is formed there.

“84 0 1 77 8 .........................................__ .. ....................................

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During this humidification, normal flow of air under heat exchange is not possible in those pipes for a short time; only then can normal heat exchange be continued. This periodic water supply not only maintains the water film, but at the same time any dust and deposits on the pipe wall are washed away.

In order to distribute the water as evenly as possible over the pipes, this water is not poured directly into the pipes, but for instance on an upper pipe plate 7, which is provided with an upright edge (see fig. 1). Hence the water flows between the different water distributors 13 and rises in the gap between the inner and the outer tube parts of those water distributors. The resistance in the gap ensures that each pipe can only absorb a limited amount of water, which is distributed through the openings 34 (see fig. 3) along the circumference of each pipe. After the pipes have each absorbed the desired amount of water, the level of the water on the top tube plate has fallen below the edge of the openings 34, so that no further water is admitted into the pipes for the next water supply.

In practice, the size of the small siphon will be chosen so that it can start its operation with the water supply applied, and the large siphon so that the reservoir is emptied in less than 10 seconds and preferably in about 5 seconds.

Due to the high outflow velocity of the water from the large siphon, in practice at one point (depending on the location of the siphon outlet above the tube plate) the water is pushed up, from which it makes its way over the tube plate. The consequence of the impoundment is that the nearest rows of pipes receive an overdose of water, while pipes located further away receive too little.

It has proved effective to collect the water from the siphon in a tray (10 in Fig. 15), which is equipped with a narrow spout 11, which opens under water along an edge of the tube plate 12.

After the main siphon (fig. 14) has started, it initially fills the tray 10 (fig. 15) up to the overflow edge 13. After reaching this edge, the water will overflow and the entire length of the tube plate will be divided. Because the spout 11 is very narrow, while the instantaneous water supply from the main siphon is very large, the water level in the trough rises to greater height, accompanied by an increasing thrust on the overflowing water. As a result, the air present in the head of the overflow is discharged downwards and the overflow will function as a full-fledged siphon. As a result, apart from an even distribution of the outflowing water over the length of the tube sheet, the intermittent effect of the main siphon is further enhanced.

The final result of the described combination of water reservoir with main and auxiliary siphon on the one hand and the water collector with spout-shaped spout on the other, is a perfect wetting of all the pipes involved in 3 to 4 seconds.

The sizing of an auxiliary siphon determines which. minimum water flow is required to initiate the siphon action. The reservoir content is then fixed together with the desired cycle time.

The siphon effect is also triggered for a water collection tray above a certain (instantaneous) water flow rate. In practice, in the dimensions of the water collection tray, such as these are used in dew point coolers, this flow rate is of such a size that it is impossible: to use only a water collection tray as a stable water dosing device with the correct cycle time, outflow time and correct 20 outflow quantity.

For larger installations with a proportionately larger amount of water to be dosed, in the previously described combination of a reservoir with coupled main and auxiliary siphon and the water collector, the coupled main and auxiliary siphon can be replaced by a (vertical-25 ie) overflow pipe, which reaches the highest necessary level in the reservoir and (for example through a bell jar) is covered in such a way that water can enter the overflow pipe under the edge of that cover (see fig. 16). The overflow pipe must form a water seal in the water collector. The reservoir is filled with a continuous water steam.

From the moment that the water flows out of the reservoir through the overflow pipe, the water collection tank is filled with the same water flow and the water level rises both in the collection tank and in the outlet of the overflow pipe.

As soon as the sump has started its siphoning action, the water level in the sump drops and at the same time the level in the outlet of the overflow pipe. As a result of this drop in level, the air present in the overflow pipe and canopy moves downwards. Because the pressure in this volume of air remains constant, the water in the roof is raised higher, so that the overflow pipe will function as a siphon. The remainder of air in the overflow pipe is also discharged, so that the reservoir 5 is emptied with full siphon capacity, until air enters the overflow pipe at the bottom edge of the stopper.

The best results with the combination described above are achieved if the dimensions of the water collector are such that the water flow discharged by the siphon of the receptacle is equal to the water flow supplied by the siphon overflow pipe, or is so little larger that both heddles terminate their action at about the same time.

Fig. 4 shows a Mollier wet air diagram vertically plotting the dry bulb temperature, that is, the actual temperature measured by a dry bulb thermometer. Plotted horizontally is the water content expressed in kg H20 per kg dry air, as well as the partial water vapor pressure expressed in Pa.

Curved lines are drawn for some values of relative humidity. The bottom curved line (100 * relative humidity) is the saturation line, also called dew point line, which contains the points G, H, D and C.

Fig. 4 shows the course of the cooling for the same equipment and predominantly the same conditions for continuous water supply on the one hand and periodic water supply on the other. The equipment used for these 25 experiments had an area ratio of 6.6.

O

In both cases the air was assumed to be 26.5 ° C and a relative humidity (RH) of 62% (point A). This air was used as the primary flow. After passing through the heat exchanger, a partial flow of that air was used as the second flow while evaporating water.

When this water was supplied continuously, it was found that the first stream was cooled to point B (22.8 ° C, RH 78%). When humidifying the cooled air thus used as a secondary stream, the temperature of the primary air initially showed, as shown by the line AB, while the temperature of the secondary air rose, as shown by the dotted line BC.

However, when the water was supplied periodically, the temperature of the primary stream dropped to point E (20.9 ° C and 88% RH). The temperature of the secondary current was now found to change along the dotted line EF.

It thus appears that with periodic water supply 5 a better cooling is obtained than with continuous water supply. The average thickness of the water film is decisive. From fig. 4 further shows that with this equipment it is even possible to lower the temperature of the primary flow below the value D, which is the extreme limit, which can be achieved by adiabatic evaporation of water in the primary air flow.

Bottom right in Fig. 4, a line is shown indicating how many kcal / kg is required to convert dry air from 0 ° C into air of a different temperature and humidity.

Thus, the energy required to effect conversion of the gas is given by the projection of the road (eg AB) on that scale. The projection of ΑΞ on the scale shows the net energy extracted from the first stream and the projection of BC on the same scale shows the amount of energy absorbed by the second stream, all per kg of dry air. It is then easy to read that with discontinuous humidification the cooling is much more efficient.

Since in these tests the secondary stream was a partial stream of the first stream, the energy absorbed by 1 kg of the secondary stream will of course always comprise more than the energy delivered by 1 kg of the primary stream.

Thus, in the two tests described, points F

and C are always to the right of point D. (If the secondary flow has a different volume or origin, this may not always be the case).

A very interesting application of the dew point cooler 30 according to the invention can be seen from the following calculation example:

In large air-conditioning systems, the space charge is covered by recirculating the room air, cooling this air in one or more central air handling units, while fresh air, which is required for refreshment, or in separate treatment units, is brought to the correct condition, after which it is air is fed into the main recirculation system or mixed directly with the recirculation air.

8401778 V% -17-

If this refreshing air treatment box (s) is carried out with a cooling device according to the invention fz.g. dew point cooler) in combination with a mechanical aftercooling, it is then possible to carry out the recirculation air treatment units without mechanical cooling and to cover the main space load entirely with the aid of a dew point cooler according to the invention.

The invention therefore also relates to an air-conditioning device for a room, which device is provided with a first part where the temperature and the water content of the outside air are brought to a desired value and the air thus treated is fed into the room and provided with a the second part, where air contained in the space is circulated and the temperature is brought back to the desired value with the return of the major part of the circulating air and with the removal of a smaller part thereof, the device being characterized in that in the first part of the outside air is cooled in a dew point cooler and, if necessary, then by mechanical cooling the temperature and / or the moisture content of that air is reduced to the desired value and in the second part the cooling is carried out with a dew point cooler, 2Q as indicated above .

The invention also relates to an air-conditioning device for a room, which device comprises a part in which the temperature and water content of a mixture of air and outside air contained in that room are brought to a desired value, with return of the major part of the mixture and with the discharge of a smaller part thereof, the information being characterized in that part of the cooling is carried out in that part with a dew point cooler of the type described above, after which, if necessary, the temperature and the water content of the air 30 are further reduced in known manner.

Insight into the energy savings to be achieved with these two embodiments is obtained by means of the calculation example below, in which the two types of air-conditioning devices mentioned are compared with an device that only uses mechanical cooling.

We assume an office space to be conditioned with a heat production (space load) of 100 kW, a recirculation air 8401778 3 * & '3τ -18- amount of 30,000 m / h and a minimum refresh air volume of 15% of the recirculation air.

Based on comfort diagrams from literature (Recknagel, Taschenbuch fur Heizung und Klimatechnik), Oldenburg 5 Verlag, Munich 1974), a room condition of 25 ° C / 45% is desired in the office. We assume 30 ° C / 50% for the condition of the outside air (= freshening air).

For the sake of convenience, we still assume that no water vapor is produced in the office space (absolute moisture content x = constant).

The amount of recirculation air is 30,000 m3 / h = 36,000 kg / h; the mass flow is thus 10 kg / sec. Heat production = 100 kW (= space charge).

O

Enthalpy increase = 10 kJ / kg; this corresponds to a temperature rise of 10 ° C. It follows that the inlet temperature of the room air is 15 ° C

must be.

Method 1; Mechanical cooling only

For the sake of simplicity, it is assumed that the fresh air and recirculated air are mixed before they pass through the cooling system.

The mixture thus consists of 85% room air with condition 25 ° C / 45% (point A in Figure 7) and 15% fresh air with condition 20 30 ° C / 50% (point B in Figure 7). Find the condition M of the mixture

Figure 7 on line AB at 15/100 of the length of AB

away from A; the enthalpy of the mixture is: hM = 51 kJ / kg.

The blowing condition I is 15 ° C / 85% with enthalpy h ^. = 38 kJ / kg.

Enthalpy reduction on cooling = h „~ h„ = 13 kJ / kg. The mass flow

M I

25 of the mixture at 10 kg / s. The required cooling capacity then becomes

q, = m x (h „- h_) = 130 kW. lm MI

Method 2: Dew point cooling on main load and combined dew point / mechanical cooling on fresh air

Experiments with a test set-up of the dew point cooler show that it is possible to cool an air stream to a temperature that is less than 1.5 ° C away from the associated wet bulb temperature (point E and H in Figure 4).

It follows from this fact that in this example the space load can be covered entirely by dew point cooling. The enthalpy decrease 35 is 10 kJ / kg (see Figure 8). It is also known that by correctly setting the mass flow ratio between primary and secondary air, the difference between primary air inlet temperature and saturated secondary air outlet temperature is also 1.5 ° C. can be kept. The enthalpy increase of the secondary air is then equal to 70 - 38 = 32 kJ / kg (see Figure 8), so that the mass flow ratio must be 3/2, or the secondary air flow in the main dew point cooler is equal to 1 x 36,000 - 10,800 kg / h.

3.2

This quantity must be supplied to the system as fresh air. This fresh (outside) air with original condition 30 ° C / 50% 10 is also cooled to condition C (= 21 ° C / 85%) by means of dew point cooling and then mechanically cooled to condition I (= 15 ° C / 85 %).

The enthalpy decrease during this mechanical cooling is hc - h ^ = 56 - 38 * 18 kJ / kg, the mass flow of this exchange air m ^, * 10 10,800 kg / h = 3 kg / s. The cooling capacity q ~ β m_ (h - h) = 3 x 18 =

4 »V w X

54 kW. Expressed as a percentage of the 54 cooling capacity required according to method 1, this is x 100% * 41.5%. The flow chart for the method 2 use is shown in figure 9.

The following should be noted here: 20 Data from the KNMI in De Bilt (Figure 10) shows that the outdoor air condition, which is based on the calculation example above, occurs in the Netherlands on average only 1 hour per year - although figure 10 only applies to outside conditions between 8 am and 6 pm, but it will be clear that extreme conditions, as mentioned here, can only occur during the day.

The power required for mechanical post-cooling according to method 2 depends on the absolute moisture content x of the outside air. Figure 10 shows that the same absolute moisture content occurs in 27 hours per year as in the example mentioned. In 22 hours 30 of this, we can reach condition C (21 ° C / 85%) with the aid of dew point cooling. This means that the calculated 54 kW cooling capacity is only required for 22 hours per year and that, with the exception of 14 exceedance hours (see figure 10), less energy is always sufficient.

35 It therefore makes sense to introduce the term "cooling degree hours" here. This is the product of the number of hours in which a certain amount of air has to be cooled in order to determine a certain condition and the temperature range in degrees, over which 8401778 "- - 4 -20- which is cooling down. It is a measure of the amount of energy required for this cooling. For clarification; if we had to cool 1 kg of air with condition C in figure 10 for 1 hour to condition I, this would involve the same amount of energy as if we cooled the same amount of air from condition K to I.

The number of degree hours is the same for both cooling processes and amounts to 1 x (33 - 15) = 18 cooling degree hours.

With the aid of figure 10, the above can be described. 10, it is calculated that for conditioning the office space in our example according to method 2, the number of cooling degree hours is 3,720 per year.

From the aforementioned publication of Recknagel we find for cooling method 1: 8.175 cooling degree hours per year. In method 1, the 15 number of cooling degree hours operates at 15% of 36,000 kg / h = 5,400 kg / h, while the remaining 30,600 kg / h must be cooled at 10 ° C for a period of approximately 3.5 months during the cooling season during office hours (8 am-6pm). This total number of cooling hours (approx. 1,000) corresponds well with the number found for air temperature in fig.

• O

20 rats higher than 15 C. This number is 1,015, so that the number of cooling degree hours is equal to 10,150. With method 2, the calculated number of cooling degree hours is only effective at 10,800 kg / h.

The cooling energy to be used per year in the two methods is therefore in the following manner: 25 £ 2 = 3,720 x 10,800 = 0 11 8,175 x 5,400 + IQ. 150 x 30,600

This means an energy saving of 89% with the currently proposed cooling method.

Note: In this example it has not been taken into account that with outdoor conditions, which are not as extreme as 30 ° C / 50% and also occur more frequently, the cooling requirement in a room is less. In the tables given by Recknagel these influences are included.

In summary, it can be stated that with the cooling method proposed here 2: 1. the cooling capacity to be installed can be reduced to approx. 40% of that with fully mechanical cooling 8401778 -21, 2. the number of full load hours on this reduced cooling installation in the Netherlands. maximum is only 40 hours per year, 3. energy savings of almost 90% are achieved.

5 Method 3: Combined dew point / mechanical cooling on mixture of recirculation and fresh air

Of course it is also possible to run a mixture of the room air and the exchange air first through a dew point cooler and then mechanically cool it. The dew point cooling principle implies that the mixture must now consist of approximately 70% room air and 30% fresh outside air.

The condition M of the mixture is found in Figure 11 on line AB 30/100 of length AB away from point A; the enthalpy of the mixture is hM = 53 kJ / kg. Cooling to a temperature 1.5 ° C above the associated wet bulb temperature means cooling to 17 ° C (point C in Figure 11 with an enthalpy h * s * 44 kJ / kg). The enthalpy decrease is then 9 kJ / kg.

Based on the final temperature of 25 C to be achieved

(this is 1.5 ° C below the mixture temperature} the enthalpy increase of the secondary airflow is 76 - 44 = 32 kJ / kg.

The mass flow ratio between primary and secondary airflow must be m / m_ - 32/9, so that now = * 0.28 m m2 2 m (the mixing ratio 70% - 30% thus appears to be correct). The difference between m and now is the amount of recirculated air and therefore equal to m & 25 36,000 kg / h, so that m - m „» (1 - 0.28) m = 36,000, from which it follows: m2 mms * 50,000 kg / h and m_ = 14,000 kg / h (see fig. 12). m 2

Due to the dew point cooling, the mixture of room air and fresh air reaches the condition of 17 ° C / 85%. In order to obtain the blow-in condition 15 ° C / 85%, this amount of air is mechanically cooled afterwards (36,000 kg / h or 10 kg / s). The enthalpy decrease is hc - h ^ = 44 - 38 = 6 kJ / kg.

The mechemical cooling capacity becomes: q3 * 10. 6 s * 60 kW

Expressed as a percentage of the cooling capacity in method 1 this is: 35. 100% = s 46%.

In the same manner as in method 2, 1,852 cooling degree hours are calculated in this example, which operate on 36,000 kg / h air. The required annual mechanical cooling energy ratio is 8401778 ** j -4 -22- to that in method 1 if ^ 3 = 1,852 x 36,000_ Q, 8,175 x 5,400 + 10,150 x 30,600 '

The energy savings in this method are also considerable (over 80%). Compared to method 2, the energy savings in method 3 are less, but on the other hand, method 3 provides a considerable saving in investment costs, because the dew point cooler to be installed separately for the exchange air is eliminated.

Finally, it should be noted that when using dew-point coolers, higher transport energy must be taken into account, primarily because in addition to the operating air, an amount of air must also be drawn in, which is used as secondary flow after cooling , and secondly, because the pressure losses in the heat exchanger now described are greater than those in conventional cooling installations.

The construction of the cooling device according to the invention is extremely suitable to serve as a recuperator in winter time. The freshly supplied air flows around the pipes, while the finished room air is passed through them. Thus, in some cases, condensation water can flow down the inner circumference of the pipes and be collected in the existing receptacle.

In addition, the humidifier allows regular rinsing of pipes to remove adhered dust.

In recuperation mode, the mass ratio between the two air flows is in principle equal to 1. Due to the surface enlargement using slats on the dry side of the heat exchanger, the heat transfer coefficient on the surface of the pipes is mainly determined by the heat transfer coefficient at the wet side (= inner surface) and in numerical value 30 it is approximately equal to it. Together with the large pipe share of the heat exchanger, this leads to a large total heat transfer, so that the efficiency of the exchanger can rise to 80 to 90%.

8401778

Claims (10)

1. Dew point cooler to cool a stream of air, the cooler comprising a heat exchanger comprising: a) a bundle of vertical pipes arranged to heat-exchange the primary air to be cooled along the outside of those pipes, b) means for pre-cooled heat exchanged secondary air inside through those pipes, c) means for introducing water into those pipes from above and forming a film of water on the inner wall of those pipes to evaporate the secondary air into the pipes humidifying and cooling the primary air outside the pipes, d) means for exhausting the humidified secondary air to the outside air, e) means for exhausting at least a part of the cooled primary air to the space to be cooled, with characterized in that the means referred to in c) are suitable for periodically supplying water to the pipes and that the ratio between the dry outer surface of the pipes and the wet inner water area is greater than 3: 1. Dew point cooler according to claim 1, characterized in that a heat exchanger is used, in which the ratio between the dry outer surface and the wet inner surface is between 4: 1 and 6: 1. Dew point cooler according to claim 1, characterized in that the heat exchanger comprises a number of horizontal plates, provided with a series of openings, each opening being provided with a collar which fits into an opening of an adjacent plate, which plates are stacked into a package, in which the collars together form a series of pipes, interconnected by the rest of the plates, which serve as slats. Dew point cooler according to claim 3, characterized in that the openings within the collars have a diameter of 20-40 mm and a collar height of 4-7 mm. 5. Dew point cooler according to claims 1-4, characterized in that each pipe is provided at the top with a water distributor, consisting of two concentric pipe pieces, the inner of which extends into or around the pipe and since it is clamped in a sealing manner, while the outer one rests on the upper tube plate and at the lower edge is provided with one or more openings, which extend to the annular gap between the two short tubes and the inner one is provided with several circumferentially divided openings, which connect the annular gap to the inside of the tube and which are located some distance above the tube sheet. Dew point cooler according to claim 5, which is provided with a periodically operating water supply, characterized in that said water supply consists of a water reservoir, which is filled at almost continuous speed and of a self-starting siphon to periodically expand the contents of the reservoir. deposit. Dew point cooler according to claim 5, characterized in that the reservoir is provided with two levers, the first of which is self-starting and which draws the air out of the second, larger siphon upon start-up and thereby activates said second siphon. 8. Dew point cooler according to claim 6, characterized in that an elongated collection jaw is arranged under the outlet of the siphon, which is provided on one side with a siphon, which inlet and outlet also extend along one side of the upper tube plate, transverse to the flow direction of the secondary air over that tube plate. 9. Climate control device for a room, which in direction is provided with a first part, where the temperature and water content of the outside air are brought to a desired value and the air thus treated is fed into the room to be cooled, and a second part, where air in the room is circulated and the temperature is brought back to the desired value with the return of most of that circulating air and with the removal of a smaller part thereof, the first part of outside air is cooled in a dew point cooler and if necessary, afterwards by mechanical cooling the temperature and / or the moisture content of this pre-cooled air is lowered to the desired value and in the second part the cooling is carried out with a dew point cooler according to claims 1-8. 10. Climate control device for a room, which device 8401778 -25- is provided with a part in which the temperature and the water content of a mixture of air and outside air present in the room are brought to a desired value, while most of the mixture is recycled. and discharging a smaller part thereof, characterized in that in that part part of the cooling is carried out with a dew point cooler according to claims 1-8, after which, if necessary, the temperature and the water content of the air are further reduced at known manner. 8401778