GB2160963A

GB2160963A - Dew-point cooler

Info

Publication number: GB2160963A
Application number: GB08513353A
Authority: GB
Inventors: Cornelis Doomernik
Original assignee: VAPOCHILL BV
Current assignee: VAPOCHILL BV
Priority date: 1984-06-04
Filing date: 1985-05-28
Publication date: 1986-01-02
Also published as: IT8521010A0; SE8502692D0; ES8608662A1; SE8502692L; GB8513353D0; DE3519694A1; ZA854062B; ES543806A0; IT1184561B; JPS6111580A; FR2565337A1; AU4326485A; BE902534A; NL8401778A

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

SPECIFICATION Dew-point cooler This invention relates to a dew-point cooler for cooling a stream of air, said cooler comprising a heat exchanger including a) a bank of vertical tubes arranged to pass primary air to be cooled along the outside of said tubes with exchange of heat; b) means for passing pre-cooled secondary air through the interior of said tubes; c) means for introducing water into said 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.

A similar dew-point cooler is known per se from U.S. patent 4,023,949. In it, it is proposed to air-condition (cool) a space by cooling primary atmospheric air in this manner and to conduct it to the space concerned, and at the same time passing a stream of secondary air (already having approximately the desired temperature) through the heat exchanger with evaportion of water, so that the stream of primary air is cooled.

A similar principle, but in a slightly different construction is disclosed in U.S. patents 1,986,529; 2,107,280; 2,174,060; German Offenlegungsschrift 2,432,308 and Netherlands patent application 7711149.

These are slightly different from each other in the construction of the heat exchanger used (tube bank or plates as heat exchanging surface) and in the manner in which water is caused to evaporate in the secondary stream (a mist of water droplets, a water-retaining wall surface, alternately supplying water to two halves of the heat exchanging plates, or continuously spraying water).

In each of these methods, the secondary stream is humidified and cooled to below the temperature of the primary stream by evaporating water. Owing to the resulting difference in temperature, the secondary stream is capable of receiving heat from the primary stream, which is thus cooled. As a result of this absorption of heat, the secondary stream becomes again unsaturated with water vapour, so that a fresh quantity of water can be evaporated and a fresh quantity of heat be absorbed from the primary stream, etc.

As a result of the cooling of the primary stream, the temperature of the primary stream is decreased, while the absolute humidity of that air stream (in grams per kg of air) remains constant.

Since, as a result of heat flow resistance, the transfer of tangible heat between the primary and the secondary air streams requires a difference in temperature between the two streams, and since this difference in temperature is brought about by humidifying the secondary air stream, which in a preferred embodiment of the invention is a partial stream of the primary stream and then has an entrance temperature equal to the exit temperature of the primary air stream, the exit temperature of the primary air stream that can be achieved is mostly higher than the dew point of the air in the primary stream. It is only in an ideal case that the exit temperature of the primary air can be equal to its dew point. This gives rise to the name "dew point cooler".

This method of cooling air is cheaper than that using the more common mechanical refrigerator, because the latter uses much more energy.

Some of these publications additionally mention that the cooled air can be given a lower water content by giving some or all of the cooled primary stream exiting from the heat exchanger a subsequent cooling treatment with a mechanical refrigerator, so that a portion of the water vapour is condensed. This combined cooling can be used with advantage when the atmospheric air has a higher absolute moisture content than is desirable in the cooled air. In that case, too, a considerable saving is achieved, because in fact the mechanical refrigerator only needs to be used when it is necessary to lower the absolute water content. In all other cases, the much cheaper dew-point cooler is used only.

In spite of these clear and great advantages, such a dew-point cooler has never, as far as known to the present applicants, been taken into practical use either on a small or a large scale.

A reason for this has never been given. It is clear, though, that a dew-point cooler is only eligible for use when the heat exchanger used is a highly efficient one, and, in view of the relatively small differences in temperature between the air streams, which necessarily have to occur, this will only be achieved with a large exchanging surface area.

In all the cases reported in the literature, the heat exchanger proposed turns out to have an effective surface area on the side of the primary stream which does not essentially differ in size from the effective surface area on the side of the secondary stream.

Under these circumstances, the volume of the heat exchanger will in most cases become prohibitively large.

In addition, it turns out in practice that it is extremely difficult, if not downright impossible, to produce a secondary stream that is saturated with water vapour. Such a stream has of course the lowest secondary temperature.

It is an object of the present invention to provide a dew-point cooler with which optimum cooling is obtained, using a compact construction.

To this effect, the dew-point cooler defined in the opening paragraph hereof is characterized, according to the invention, in that the means referred to under c) are adapted to supply water periodically and that, on the outside, which comes into contact with the primary air, the tubes are provided with elements, the ratio between the outer surface area and the inner surface area of the tubes provided with elements being higher than 3:1.

It has surprisingly been found that by virtue of the combination of measures according to the present invention these objects are accomplished.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a vertical cross-sectional view of one embodiment of dew-point cooler according to the present invention; Figure 2 is a diagram illustrating the use of dew-point coolers as part of an air-conditioning installation; Figure 3 is a diagram showing the use of a dew-point cooler as part of another airconditioning plant; Figure 4a shows a top plan view of a tube plate as preferably used according to the present invention; Figure 4b is a cross-sectional view of a portion of a tube plate as illustrated in Fig. 4a, which also shows a form of a collar as used according to this invention, and indicates the way in which the collars fit one into the other; Figure 5 is an exploded view of the heat exchanger used in a preferred embodiment of the present invention;; Figure 6a shows a water distributor as used in a preferred embodiment of the present invention, in part-sectional view; Figure 6b shows a cross-sectional view of the water distributor, taken on the line Illb-lllb of Fig. 6a; Figure 6c shows a top plan view of the water distributor illustrated in Fig. 6a; Figures 7a and 7b show a supply means for water as can be used according to the invention; Figures 8a, 8b and 8c show an alternative supply means for water as can be used according to this invention, and Figures 9-11 show yet other supply means for water in combination with a water collecting and distributing device as can be used according to this invention.

Fig. 1 is a vertical cross-sectional view of a dew-point cooler. In it, a plurality of sections of vertical tubes are designated at 3. The primary stream 1 enters these sections on the left and flows through them successively in horizontal direction. The secondary (wet) stream enters the device at the top right-hand corner, then flows downwardly through the tubes of the right-most section, as indicated by arrows 2, and then successively through the tubes of the other sections, whereafter this stream leaves the device at the top left-hand corner.

The water to be evaporated is raised from receptacles 4 (Fig. 1) under the sections by pumps 5 to devices 16, whence the water is passed to the uppermost tube plates 7, which are provided with an upright edge. On these tube plates the water is distributed over the tubes by water distributors 1 3 in such a manner that the inner wall of each tube of a section is covered with a layer of water along which the secondary stream of air flows (upwardly or downwardly) with evaporation of water, whereby the necessary heat of evaporation is withdrawn from the primary stream, which is in contact with the outer wall of the tubes and with the fin plates. The non-evaporated part of the water is returned to receptacles 4.

The origin of the primary stream depends on the way in which the dew-point cooler is included in the air conditioning system.

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

To maintain the desired temperature within that space, a portion of the air present in the space, for example, 10% per hour, is continuously discharged and used as a secondary stream 2. During the passage of the several sections 3, water is evaporated therein, at the same time this secondary stream absorbs heat from the primary stream, and the result is that the temperature of the secondary stream is increased as the primary stream is cooled. It is this very rise in temperature which makes it possible that more and more water is evaporated in the secondary stream, so that ultimately a greatly humidified stream of air escapes on the left-hand end with a temperature that is higher than the temperature of the space to be cooled.

This greatly humidified air is unfit for further use in the present cooling process.

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

Good heat exchange requires the heat first to be transferred from the primary (dry) stream to the (commonly metallic) wall. The amount of heat transferred is then directly proportional to the product a,F,, where F,is the transmitting surface area and a, the coefficient of transfer. When this heat has passed through the partitibn 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 a2F2, in which, again, F2 is the transmitting surface area and a2 is the coefficient of transfer.

It is known from Leidenfrost, W. Analysis of Evoporative Cooling and Enhancement of Condensers Efficiency and of Coefficient of Performance, Wärmeund Stoffubertragung 12 (1979), pp. 5-23, that in the case of heat transfer from a wall to an air stream in the presence of an evaporating water film on that wall, the coefficient of heat transfer is a multiple of the coefficient of heat transfer that occurs with convective transfer to the same wall. Leidenfrost describes a minimum ratio of 3 in the case of saturated air and of more than 5 in the case of unsaturated air.

Apart from the difference in convective heat transfer in flows within tubes and in flows around (finned) tubes, it would be necessary, on the ground of the above, to use a heat exchanger in a dew-point cooler according to the present invention, of the type referred to in the opening paragraph hereof, in which the ratio between the dry outer surface area and the wet inner surface area is more than 3:1.

One condition of this is that the heat exchange surface on the side of the secondary stream of air should always be covered with a thin layer of water.

As the ratio of a2/al depends on the condition of the primary air taken in, the aim will be for F,/F2 to approximate a2/a,. The value for the ratio between outer surface area and inner surface area of the tubes is preferably between 5:1 and 1 0:1.

As the value of the outer surface area can be used the effective outer surface area that can be calculated using published formulae for constructions of tubes provided with elements.

All of the above publications about dew-point coolers agree in describing an F,/F2 ratio that is little or no different from 1:1. This, then, is probably the reason that none of these proposals have led to practical applications.

When a heat exchanger is designed in which F,/F2 is about 6, it is found that, for an equal capacity, the volume only needs to be 1/3 to 1/4 of the volume of an exchanger in which F, = F2.

To produce the necessary difference in exchanging surface area it is possible, in analogy with known methods of construction , to start with tubes on which fins are mounted, whereafter these finned tubes are combined to larger units, for example, fin batteries. Known exchangers of this type are commonly constructed for considerable differences in pressure between the two exchanging gas streams. In a dew-point cooler, however, this difference in pressure is at most a few mbar and this makes a very light construction possible.

The usual method of constructing fin batteries is as follows: In metal plates, for example, aluminum plates with a thickness of 0.2-1.0 mm, for example 0.25 mm, hoies are punched, which mostly are also provided with a collar. The fin plates are then shifted onto core tubes, and these tubes are mechanically or hydraulically expanded to clamp the fins onto the tubes. These finned tubes are then mounted in conventional tube plates.

This construction can also be used in the present case.

Preferably, however, the heat exchanger is built up from a plurality of plates having an array of openings therein, each opening having a collar fitting within an opening of an adjacent plate, the plates being stacked to form a packet in which the collars jointly constitute a plurality of tubes interconnected by the remaining portion of the plates, which serve as fins.

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

For constructional reasons, the shape of the holes is mostly round. A diameter of 30 mm and a collar height of 6 mm have been found to be very effective.

When the collar is appropriately formed, each collar may be shifted into the subjacent tubes for about half its height, as shown in Figs. 4a and 4b. The core tubes are then unnecessary, and the combination of tubes and fins can be made at considerably lower cost and in more compact design. Hereinafter the word tube should be construed as including a "tube" made of such collars shifted one partly into another. When the collars can be shifted one into the other with a slight pressure only, a simple, yet firm construction can be obtained, in which the various tube sections are in sufficiently sealing surface-to surface contact.

The tubes, no matter how they are designed must permit being provided on the inside with a thin layer of water, and this layer should, if possible, cover the entire tube wall and should be of as nearly uniform thickness as possible. Furthermore, all tubes should naturally be wetted to the same degree. The water supply system should not, therefore, be sensitive to misalignment or to disturbance of the water level on top of the uppermost tube plate.

The above U.S. patent 2,107,280 proposes using plate-type exchangers which on the wet side are covered with a fabric. It mentions that better results are obtained if the water is supplied not continuously, but periodically. A construction as described in that patent has the disadvantage that the plate surface is covered with a wet fabric which, although it can be kept uniformly wet, yet forms a greater resistance to heat flow than does a water film directly on the metal.

In a preferred embodiment of the dew-point cooler according to the present invention, means are provided for using a portion of the cooled primary air as secondary air.

It is also possible to use means for humidifying the secondary stream (air) before it is supplied to the secondary side of the tubes. Suitable means provide for direct evaporation of water in the stream. Examples thereof are a wet mat or a curtain of water.

More particularly the means for humidifying are present in the "reversing point", that is to say, in the secondary stream after the air cooled has been split into a primary and a secondary stream.

It has been found, and this is a further feature of the invention, that the distribution of the water can be effectively carried out by placing a water distributor on top of each vertical tube (Figs. 6a-6b-6c), consisting of two short, concentric tube portions (30, 32), the inner 30 of which extends into or around the tube, where it is sealingly clamped down, while the outer one is supported on the topmost tube plate and provided at its bottom edge with one or more apertures 33 extending up to the annular slit 31 between the two tube portions, the inner 30 being provided with a plurality of apertures 34 spaced about its circumference, which connect the annular slit 30 with the interior of the tube and are spaced some distance above the dew plate. These apertures 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. Preferalby a funnel-shaped portion 35 is provided at the top of the inner tube. These water distributors according to the invention have the advantage of a very low pressure drop.

As shown in Fig. 1, as in known dew-point coolers, one or more receptacles 4 for water to be evaporated are provided under the tube bank 12, in which non-evaporated water is collected, and from which water is supplied to the top tube plate 7 on which the water distributors 1 3 are supported. This tube plate is provided with an upright edge.

Although Fig. 1 shows a concrete embodiment with 4 sections of tubes, it will be clear that it is also possible to use any other number of sections, and that the primary stream 1 and the secondary stream 2 can pass through such different sections to suit requirements. When the tube bank consists of a plurality of sections 3,3, successively traversed by the secondary stream 2, it is useful for each section to have its own receptacle 4, because the sections correspond to different temperatures for the secondary stream. The level in the reservoirs can be kept constant in a conventional manner. The primary stream 1 is then passed between the plates outside the tubes.

In order for the water to be periodically supplied to the top tube plate, i.e. during the correct period, in the correct quantity, and with the correct intervals, various devices can be used.

The simplest possibility is a pump which in known manner is switched on and off. Such switching systems, however, are subject to failure. The above-cited U.S. patent 2,107,280 describes an apparatus with a double tilting trough. This system operates satisfactorily, but has the drawback of having movable parts, so that it is subject to wear and tear, and will not operate reliably over very long periods of time.

According to the present invention the preferred system is a self-starting siphon system.

The principle of such a system is given in Fig. 7. The reservoir R is filled by continuous water supply, for example, by means of a pump which supplies water from receptacle 4 under the tubes concerned. As soon as the water in the reservoir has reached level 1, the siphon H initially begins to function as an overflow passage only. However, the siphon is provided with a bouncing dam D, which causes the water flowing over the bottom of siphon H to spout against the top of the siphon, thereby shutting off the air volume above the bouncing dam. Owing to the relatively high speed of the water, the pressure in the jet at the bouncing dam becomes low, and as a consequence the air is withdrawn from the top of the siphon. As a result the siphon is rapidly filled with water to reach the situation shown in Fig. 7b. From that moment, the siphon is rapidly filled completely, whereafter the discharge of water through the siphon increases rapidly to its maximum. This discharge lasts until the reservoir R is emptied to level 2, so that air can again enter at the left end of the siphon, and the supply of water to the tubes is stopped.

In this way a quantity of water determined by the size of reservoir R can be supplied at regular intervals.

Experiments have shown that in some cases the siphoning time was too long, so that the correct ratio between siphoning time and cycle period could not be achieved.

In fact, as stated before, a bouncing dam D is present in the siphon. This does cause the air to be removed, so that the siphon starts fast and easily, but at the same time dam D may form an obstacle in the outlet after the siphon has been started, so that it interferes with the flow of the effluent amount of water.

For that reason, in a further preferred embodiment of the invention, next to or above the (small) siphon B described (see Figs. 8a and 8b and 9), a second larger siphon A is mounted, whose outlet is in water, or in which a water lock is provided in the outlet, as shown in Fig. 8.

The larger siphon has a vertex that is slightly higher than the vertex of the small siphon, so that the small siphon B is self-starting, but the large one is not.

The larger siphon does not have a bouncing dam. The small siphon starts in the manner described above, but now, in addition to itself, also evacuates the larger siphon mounted next to, or above, it, via one or more connecting passages, for example, connecting passages V shown in Fig. 8c, so that the larger siphon is also rapidly started. Owing to the large outlet diameter without a bouncing dam as an obstacle, the discharge capacity of siphon A is such that a short discharge period is reached.

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 essentially serves for starting first itself and then the large siphon. The discharge of water throught the small siphon is of secondary importance. The large siphon, on the other hand, is designed to make possible a discharge of a large amount of water in a limited period of time.

The discharge of water in a short period of time is of importance, for it is necessary to pour such a large quantity of water periodically into the (wet) tubes of the heat exchanger that in all tubes the entire inner wall is wetted at the same time to form the desired water film.

During the moistening period, the normal throughflow of the air in the tubes with exchange of heat with a secondary stream is not possible for a short period of time; it is only thereafter that normal heat exchange can be continued. Owing to the periodical supply of water, not only is the water film maintained, but at the same time any dust and depositions are removed from the tube wall.

In order that the water may be distributed over the tubes as uniformly as possible, the water is not poured direct into the tubes, but for example onto a top tube plate 7 provided with an upright edge (see Fig. 1). From the tube plate, the water flows between the various water distributors 1 3 and rises in the gap between the inner and outer tube portions of the water distributors. Owing to the flow resistance in the gap, each tube can take a limited amount of water only, which is distributed along the circumference of each tube through the apertures 34 (see Fig. 6). When the tubes have each taken the desired amount of water, the level of the water on the tube plate has decreased to below the edge of the apertures 34, so that no further water is admitted to the tubes until the next amount of water is supplied.

In practice, the size of the small siphon will be selected so that it will be actuated by the water supply applied, and the large siphon so tht the reservoir is emptied in less than 10 seconds and preferably in about 5 seconds.

Owing to the high outlet velocity of the water from the large siphon, in practice there will be a surge of water in one point (depending on the position of the siphon outlet above the tube plate), whence the water will find its way over the tube plate. The result of the surge is that the rows of tubes located in the immediate vicinity will receive an overdose of water, while more remote rows of tubes will receive too little. It has been found to be effective to collect the water from the siphon in a trough (10 in Fig. 10) fitted with a narrow outlet 11 opening under water along an edge of the tube plate 1 2.

When the main siphon (Fig. 9) has been started, it will, in the first instance, fill trough 10 (Fig. 10) up to the overflow edge 1 3. When this edge has been reached, the water starts flowing over it, and is distributed throughout the entire length of the tube plate. As the outlet 11 is very narrow, whereas the instantaneous supply of water from the main siphon is very large, the water will rise to a higher level in the trough, which is accompanied by an increasing thrust on the overflowing water. This causes the air present in the head of the overflow to be discharged downwardly, and makes the overflow operate as a full-fledged siphon. The result is that, in addition to a uniform distribution of the outflowing water over the length of the tube plate, the intermittent effect of the main siphon is reinforced.

The ultimate result of the described combination of water reservoir with main and auxiliary siphons, on the one hand, and the water collecting trough with slit-shaped outlet, on the other, is that all the tubes involved are perfectly moistened in 3 to 4 seconds.

The dimensions of an auxiliary siphon determine what minimum rate of flow of water is required to actuate the siphon. The volume of the reservoir is then determined in conjunction with the desired cycle period.

With regard to the water collecting trough, too, siphon action is actuated above a given (instantaneous) rate of flow. In practice, with dimensions of the water collecting trough as used in dew-point coolers, this rate of flow is of such magnitude that it is impossible just to use a water collecting trough as a stable water dosing device with the proper cycle period, outflow period and correct effluent quantity.

For larger plants with a proportionally larger dosage of water, the coupled main and auxiliary siphons in the above-described combination of a reservoir with coupled main and auxiliary siphons and the water collecting trough, can be replaced by a (vertical) overflow pipe which extends to the highest necessary level in the reservoir and is hooded (for example by a bell jar) in such a manner that water can enter the overflow pipe under the edge of the hood (see Fig.

11). The overflow pipe must form a water lock in the water collecting trough. The reservoir is filled with a continuous stream of water. From the moment when the water flows away from the reservoir through the overflow pipe, the water collecting trough is filled with the same stream of water, and the water level rises both in the collecting trough and in the outlet of the overflow pipe.

As soon as the collecting trough and in the outlet of the overflow pipe.

As soon as the collecting trough has started its siphon action, the water level in the collecting trough is lowered, and at the same time the level in the outlet of the overflow pipe. As a result of this decrease in level, the air present in the overflow pipe and the hood is displaced downwardly. As the pressure in this volume of air is constant, the water is "pulled" up into the hood, as a result of which the overflow pipe will begin to function as a siphon. The remainder of air in the overflow pipe is then also discharged, so that the reservoir is emptied with full siphon capacity until air enters the overflow pipe at the lower edge of the pipe.

The best results are obtained with the combination described above when the dimensions of the water collecting trough are such that the stream of water discharged by the siphon of the collecting trough is either equal to the stream of water supplied by the siphoning overflow pipe, or a little greater so that the two siphons terminate their operation approximately at the same time.

In large air conditioning systems, the space load is covered by recirculating the air in the space, which air is cooled in one or more central air processors, while the required fresh outside air is either conditioned in separate processors and then passed into the main recirculation system, or directly mixed with the air being recycled.

When these fresh-air processor(s) is (are) fitted with a cooler according to the present invention (so-called dew-point cooler) in combination with mechancal after-cooling, it is possible for the recycle-air processors to be designed without mechanical cooling and to cover the main space load fully by means of a dew-point cooler according to the present invention.

The present invention accordingly also relates to an air conditioning unit for a space, which unit comprises a first section in which the temperature and the water content of the outside air are adjusted to a desired value, and the air so treated is passed into said space, and a second section in which air present in the space is circulated, and the temperature re-adjusted to the desired value with the major part of the circulating air being returned, and a minor part thereof being discharged, said unit being characterized in that, in said first part, outside air is cooled in a dew-point cooler according to the present invention, and, if necessary, thereafter the temperature and/or the moisture content of said air is decreased to the desired values by mechanical cooling. and in the second part the cooling operation is carried out with a dew-point cooler according to the invention as indicated above. Such a system is illustrated in Fig. 2.

The invention also relates to an air conditioning unit for a space, which unit comprises a section in which the temperature and the water content of a mixture of air present in said space and outside air are adjusted to desired values with the major part of the mixture being returned and a minor part thereof being discharged, said unit being characterized in that part of the cooling process in said section is carried out using a dew-point cooler of the type defined above, whereafter, if necessary, the temperature and the water content of the air are reduced further in known manner.

An insight into the saving of energy to be obtained with these two embodiments is given in the following example, in which the two types of air conditioning units referred to are compared with an apparatus using mechanical cooling only.

EXAMPLE We start from an office space to be air-conditioned with a heat production (space load) of 100 kW, a quantity of recycle air of 30,000 m3/h and a minimum quantity of fresh air of 15% of the recycle air.

On the ground of comfort diagrams from the literature (Recknagel, Taschenbuch fur Heizung and Klimatechnik), Oldenburg Verlag, Munich 1974) the desired space condition in the office is 25 C/45%. The condition of the outside air (= fresh air) is assumed to be 30'C/50%.

It is assumed that no water vapouris produced in the office space (absolute moisture content X = constant). The amount of recycle air is 30,000 m3/h = 36,000 kg/h; the mass stream is therefore 10 kg/ sec.

Heat production is 100 kW (= space load). Increase in enthalpy = 10 kJ/kg; this corresponds to 10"C temperature increase. It follows that the inlet temperature of the space air must be 15"C.

Method 1: mechanical cooling only (comparative) The fresh air and the recycle air are mixed before traversing the cooling unit. Accordingly, the mixture consists as to 85% of space air with condition 25 C/45% and as to 15% of fresh outside air with condition 30 C/50%. The enthalpy of the mixture is hM = 51 kJ/kg. The inlet condition is 15 C/85% with enthalpy h = 38kJ/kg. Decrease in enthalpy by cooling is 1 3 kJ/kg. The mass stream of the mixture mm = 10 kg/s. The required cooling capacity then is q, =mm (Ah) = 130 kW.

Method 2: dew-point cooling of main load and combined dew-point/mechanical cooling of fresh air (Fig. 2) On the ground of experiments with a test arrangement of the dew-point cooler, it was found to be possible to cool a stream of air to a temperature less than 1.5"C from the associated wet bulb temperature.

It follows from this data that, in this example, the space load can be fully covered by dewpoint cooling. The decrease in enthalpy is 10 kJ/kg. It is also known that by suitably adjusting the mass stream ratio between primary and secondary air the difference between entry temperature of the primary air and exit temperature of the saturated secondary air can also be kept at 1.5"C. The increase in enthalpy of the secondary air is then 70-38 = 32 kJ/kg, so that the mass stream ratio must be 3.2, or the secondary air stream in the main dewpoint cooler is X 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 the original condition of 30 C/50% is also cooled by means of dew-point cooling to condition 21 C/85% and subsequently cooled mechanically to condition 15 C/85%.

The decrease in enthalpy during this mechanical cooling treatment is hc - h1 = 56 - 38 = 1 8 kJ/kg, the mass stream of this fresh air m = = 10,8000 kg/h = 3 kg/sec. The cooling capacity q2 = mv (Ah) = 3 X 1 8 = 54 kW. Expressed in percents of the cooling capacity required according to method 1, this is 54 x 100% =41.5%.

130 Statistical data for the situation in the Netherlands show that, per year, 27 hours the same absolute moisture content occurs as in the above example. In 22 hours thereof, the condition 21 C/85% can be reached by means of dew-point cooling. This means that the calculated 54 kW of cooling capacity is required for 22 hours per year only and that, except for 1 4 excess hours, for the rest it is always sufficient to use less energy.

It is meaningful, therefore, to introduce the term "cooling degree hours" here. This is defined as the product of the number of hours in which a certain amount of air must be cooled to reach a given condition and the temperature range in centrigrades through which such cooling occurs.

It is a measure for the amount of energy required for such cooling.

By means of the above statistical data, it can be calculated in the above manner that conditioning the office space in our example by method 2 requires 3,720 cooling degree hours per year.

Recknagel, in his above publication, specifies for cooling method 1 8,175 cooling degree hours per year. In method 1, the number of cooling degree hours is applicable to 15% of 36,000 kg/h = 5,400 kg/h, while the remaining 30,600 kg/h must be cooled through 10"C for a period of about 3.5 months in the cooling season during office hours (8-18 hours). This total number of cooling numbers (about 1,000) corresponds well to the number found in Fig.

10 for air temperatures higher than 15"C. This number is 1,015, so that the number of cooling degree hours is 10,1 50. In method 2, the calculated number of cooling degree hours applies to 10,800 kg/h only.

Accordingly, the cooling energy Q, and Q2 to be used in the two methods per year are related to each other as follows: Q 3,720X10,800 ~~~~~~~~~~~~~~~~~~~~~~~~ - 0.11 Q1 8,175 X 5,400 + 10,150 X 30,600 This means a saving in energy of 89% in the cooling method proposed according to this invention. Note: This example does not take into account that with ouside conditions not so extreme as 30 C/50%, which occur more frequently at that, the cooling requirements in the space will be less. In the tables given by Recknagel, these influences have been taken into account.

In summary it can be stated that, in the cooling method 2 here proposed: 1. the cooling capacity to be installed can be reduced to about 40% of that in the case of fully mechanical cooling, 2. the number of full-load hours of this reduced cooling plant is, in the Netherlands, at the worst, as low as 40 hours per year, 3. a saving in energy of almost 90% is achieved.

Method 3: Combined dew-point/mechanical cooling of mixture of recycle and fresh air (Fig.

3) Naturally, it is also possible first to have a mixture of the space air and the fresh air traverse a dew-point cooler and then cool it mechanically. The dew-point cooling principle involves that the mixture must now consist as to about 70% of space air and as to about 30% of fresh outside air. The enthalpy of the mixture is h = 53 kJ/kg. Cooling to a temperature that is 1.5"C above the associated wet bulb temperature means cooling to 17"C (with an enthalpy h = 44 kJ/kg,) The decrease in enthalpy is then 9 kJ/kg.

On the basis of the final temperature of 25"C to be reached (i.e. 1.5"C below the temperature of the mixture) the increase in enthalpy of the secondary air stream is 76-44 = 32 kJ/kg.

The mass flow ratio between primary and secondary air streams must be mm/m2 = 32/9, so that m2 = 0.28 mm (accordingly, the mixing ratio of 70%-30% turns out to be correct). The difference between mm and m2 is the amount of recycle air and hence equal to 36,000 kg/h, so that mm-m2 = (1-0.28) mm = 36,000, from which follows: mr,, = 50,000 kg/h and m2 = 14,000 kg/h (see Fig. 12).

As a result of the dew-point cooling treatment the mixture of space air and fresh air reaches the condition 17 C/85%. To produce the inlet condition of 15 C/85%, this quantity of air (36,000 kg/h or 10 kg/sec) is subsequently cooled mechanically. The decrease in enthalpy therein is 44-38 = 6 kJ/kg. The mechanical cooling capacity becomes: q3= 10.6=60kW.

Expressed in percents of cooling capacity in method 1, this is: 60 .100% = 46% .

130 In the same way as in method 2, in this example 1,852 cooling degree hours are calculated, which apply to 36,000 kg/h air. The required mechanical cooling energy on an annual basis Q3 is related to that in method 1 as Q3 1,852 x 36,000 = = 0. 19 Q1 8,175 x 5,400 + 10,150 x 30,600 In this method, too, therefore, the saving in energy is considerable (more than 80%). Relative to method 2, in method 3 the saving in energy is somewhat less spectacular, it is true, but on the other hand method 3 gives a considerable saving in capital outlay, as there is no separate freshair dew-point cooler.

It is finally observed that when dew-point coolers are used, it should be taken into account that the energy required for transport is somewhat higher, first because, in addition to the useful air an amount of air must be sucked in which after cooling is used as the secondary stream, and second because pressure losses in the heat exchanger described are higher than those in conventional cooling plants.

The construction of the cooling device according to this invention is extremely suitable for use as a recuperator in winter time. The fresh air supplied flows around the tubes, while the spent space air is passed through them. In this way, if required, water of condensation can flow downwardly along the inner circumference of the tubes and be collected in the collecting trough provided.

In addition, the humidifying device makes it possible for the tubes to be regularly washed to remove adhering dust.

In operation for recuperation, the mass ratio between the two air streams in principle equals 1. Owing to the increase in surface area by means of fins on the dry side of the heat exchanger, the coefficient of heat transmission, calculated on the surface area of the tubes, is mainly determined by the coefficient of heat transfer on the wet side (= inner surface) and numerically is approximately equal to it. Together with the large tube share of the heat exchanger, this leads to a high total heat transfer, so that the efficiency of the exchanger can be as high as 80 to 90%.

Claims

1. A dew-point cooler for cooling a stream of air, said cooler comprising a heat exchanger - including a) a bank of vertical tubes arranged to pass primary air to be cooled along the outside of said tubes with exchange of heat; b) means for passing pre-cooled secondary air through the interior of said tubes; c) means for introducing water into said 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, characterized in that the means referred to under c) are adapted to supply water periodically and that, on the ouside, which comes into contact with the primary air, the tubes are provided with elements, the ratio between the outer surface area and the inner surface area of the tubes provided with elements being higher than 3:1.

2. A dew-point cooler according to claim 1, wherein the ratio between the outer surface area and the inner surface area of the tubes ranges between 5:1 and 10:1.

3. A dew-point cooler according to claim 1 or 2, characterized in that the heat exchanger comprises a plurality of horizontal plates having an array of openings therein, each opening having a collar fitting within an opening of an adjacent plate, the plates being stacked to form a packet, in which the collars jointly constitute a plurality of tubes interconnected by the remaining portion of the plates, which serve as fins.

4. A dew-point cooler according to claims 1-3, having a periodically operating water supply, characterized in that said water supply includes a water reservoir, which, in operation, is filled virtually continuously, and a self-starting siphon for periodically discharging the contents of the reservoir.

5. A dew-point cooler according to claim 4, characterized in that the reservoir is provided with two siphons, the first of which is of the self-starting type, and when starting sucks away the air from the second, larger siphon, thereby actuating said second siphon.

6. A dew-point cooler according to claim 4, characterized by the provision of an elongated receptacle under the outlet of the siphon, said receptable being provided on one side with a siphon whose inlet and outlet extend along one side surface of the top tube plate transversely to the direction of flow of the secondary air over said tube plate.

7. A water distributor suitable for a dew-point cooler as claimed in any of claims 1-6, characterized in that it comprises two concentric tube members, the inner one of which can extend into or around a tube and can be sealingly attached thereto, and the outer one of which can be supported on a tube plate and is provided at its lower edge with one or more openings extending into the annular gap between the two tubes, the inner one having an aperture therein, or a plurality of apertures spaced about its circumference, connecting said annular gap with the interior of the tube.

8. A dew-point cooler as claimed in any of claims 1-6, and further comprising a water distributor as claimed in claim 7.

9. An air conditioning plant for a space, said plant comprising a first section in which the temperature and the water content of outside air are adjusted to a desired value and the air so treated is passed into the space to be cooled, and a second section in which air present in said space is circulated and its temperature is re-adjusted to the desired value with the major part of said circulating air being returned and a minor part thereof being discharged, characterized in that, in said first section, outside air is cooled in a dew-point cooler as claimed in any of claims 1-6 or 8 and, if necessary, the temperature and/or the moisture content of this pre-cooled air is thereafter decreased to the desired value by mechanical cooling, and in the second section the cooling process is carried out by means of a dew-point cooler as claimed in any of claims 1-6 or 8.

1 0. An air conditioning plant for a space, said plant including a section in which the temperature and the water content of a mixture of air present in said space and outside air are adjusted to a desired value with the major part of said mixture being returned and a minor part thereof being discharged, characterized in that, in said section, at least part of the cooling process is carried out by means of a dew-point cooler as claimed in any of claims 1-6 and 8, and thereafter, if necessary, the temperature and the water content of the air are reduced further in known manner.

11. A dew-point cooler substantially as described herein with reference to the accompanying drawings and in the example.