GB2060156A

GB2060156A - A heat exchange fluid-cooling system and apparatus

Info

Publication number: GB2060156A
Application number: GB8030133A
Authority: GB
Original assignee: Curtiss Wright Corp
Current assignee: Curtiss Wright Corp
Priority date: 1979-09-26
Filing date: 1980-09-18
Publication date: 1981-04-29
Also published as: JPS5656582A; DE3035322A1; ZA805477B

Abstract

A heat exchange system for cooling a fluid to be cooled, such as ammonia or water, has conduits (28) for conducting said fluid in indirect heat relationship to a heat absorbing coolant liquid flowing in other conduits (26). Simultaneously, air is forced or induced by a fan (18) to flow across the conduits (28) to achieve additional cooling of the fluid to be cooled. The heated absorbing liquid is conducted from its associated conduits (26) and dispersed by nozzles (56) into air, before or after the passage of the air across the conduits (28), in accordance with air temperature and heat load conditions to evaporate some of the heat absorbing liquid and thereby effect cooling of the heat absorbing liquid. The cooled heat absorbing liquid is recirculated (e.g. from a reservoir 22) to its associated conduits (26) for cooling additional quantities of fluid to be cooled flowing through its associated conduits (28). <IMAGE>

Description

SPECIFICATION A heat exchange cooling system and apparatus This invention relates to a heat exchange system and apparatus and, more particularly, cooling systems and apparatus in which a heated process fluid, as for example ammonia, water or the like, is to be cooled by utilizing ambient air.

In heretofore air-cooling systems, to increase the heat transfer capacity, the fluid conducting tubes are provided with extended surface elements or fins and water is sprayed on or cascaded over the fin-tubes. These heat exchange systems are generally known as evaporative surface condensers and such systems are exemplified in the United States Patent No.

3,472,042 and United States Patent No.

4,156,351. These evaporative surface condenser systems have not been capable of achieving optimum overall efficiency because the water on the fin-tubes reduces the flow of the air through the units and, therefore, requires additional air blower capacity than would be needed without wetting of the fin-tubes or, if additional air blower capacity is not provided, then air-flow rates are reduced which, in turn, reduces or limits cooling capability of the heat exchange system. Also, because the amount of fin-tube surface area is limited to the size of the heat exchange system, the amount of water surface available for evaporative cooling is limited. These disadvantages and limitations of existing heat exchange cooling systems of the evaporative surface condenser type are overcome by the heat exchange cooling system according to this invention.

A feature of this invention is the effecting of indirect heat transfer from the fluid to be cooled, as for example ammonia, to a heat absorbing liquid having a relatively high coefficient of heat transfer, as for example water, and then indirectly effecting heat transfer from the heat absorbing liquid to ambient air by means of evaporation of some of the water. This permits the use of relatively small size tubes or conduits for the flow of heat absorbing liquid since the heat transfer coefficient of both heat absorbing liquid and the fluid to be cooled are high as compared with the heat transfer coefficient of air.

Furthermore, with substantial heat transfer being achieved within the fin-tube heat transfer elements, water wetting of the heat transfer elements is not necessary, as is required in conventional evaporative type heat exchangers and, therefore, there is no reduction in air flow area as in such evaporative heat exchangers and, hence, no need to provide increased fan power with the attendant increase in power consumption.

According to the present invention in one aspect there is provided a heat exchange system for cooling a fluid to be cooled comprising: (a) first means for conducting the fluid to be cooled in indirect heat transfer relationship with a heat absorbing liquid of high heat transfer coefficient for absorption of heat from said fluid to be cooled; (b) second means for passing air in indirect heat transfer relationship with fluid to be cooled to remove additional heat from the fluid simultaneously with the passing of said fluid to be cooled in heat exchange relationship with said heat absorbing liquid;; (c) third means for communicating with said first means to receive the heated absorbing liquid and dispersing the latter into the air of said second means in accordance with temperature and heat load conditions to thereby effect cooling of the heat absorbing liquid by vaporization of some of the heat absorbing liquid; and (d) means for collecting and recirculating the cooled heat absorbing liquid to the first means for cooling additional quantities of fluid to be cooled.

Although hereinafter the heat absorbing liquid will be identified as water, it is to be understood that the invention is not limited thereto and that any suitable liquid having a relatively high coefficient of heat transfer may be used.

The third means may comprise spray means for spraying the water into the air provided by said second means in accordance with air temperature and heat load conditions.

This spraying is accomplished either before or after the air flows in indirect heat transfer with the fluid to be cooled to effect vaporization of some of the sprayed water. This vaporization of some water cools the non-evaporated water which is then recirculated to the first means for absorption of heat from further quantities of fluid to be cooled. While water lost by evaporation is small to effect the desired degree of water cooling, water make-up means is provided for replacing the water lost by vaporization. If the spraying of water is done before the air passes in indirect heat exchange with the fluid to be cooled, spraying is accomplished in a manner so as to avoid entrainment of droplets of water in the air.

It is also contemplated that the first means may include control means for preventing flow of water in indirect heat exchange with the fluid to be cooled when air temperature and/or heat load is sufficiently low to effect cooling of the fluid to be cooled by air alone.

In a narrower aspect of this invention, the first means comprises first and second elongate hollow members or conduits forming independent flow paths for the fluid to be cooled and cooling water, constructed and arranged to define a heat conductive flow path between the fluid to be cooled and cooling water to effect transfer of heat from the fluid to be cooled to the water. By this arrangement of heat transfer between the cooling water and the fluid to be cooled, a substantially greater heat transfer can be achieved than by cooling the fluid to be cooled indirectly solely by ambient air.Also by effecting heat absorption within the first means, the need under all operating conditions to apply water to the exterior of the conduits of the first means, is eliminated and, thus, the need for additional power to force or induce air flow through the heat exchanger to compensate for the increased pressure drop through heat exchanger caused by the wetting of the conduits is also eliminated.

Additionally, the heat exchange air-cooling system at least minimizes corrosion and fouling of the heat exchange air-cooling system by eliminating the repetitive wetting and drying of the heat exchange surfaces which occurs in conventional evaporative condensers.

According to the present invention in another aspect there is provided a heat exchanger comprising: (a) plurality of spaced first elongate hollow members; (b) each of said first elongate hollow members having extended surface elements projecting from at least part of its exterior surface; (c) a plurality of second elongate hollow members, one for each of said first elongate hollow members and disposed within an associated first elongate hollow member to define therebetween a first fluid flow passage; (d) each of said second elongate hollow members defining a second fluid flow passage; (e) a first inlet header means connected to receive said fluid to be cooled from a source thereof and communicating with said first fluid flow passages to conduct fluid to be cooled to the latter.

(f) a second inlet header means connected to receive a heat absorption liquid from a source of cool heat absorption liquid and to pass the same into each of said plurality of second flow passages so that said cool heat absorption liquid passes in indirect heat exchange relationship with said fluid to be cooled to cool the latter; (g) first outlet header means connected to receive from each of said first fluid flow passages cooled fluid to be cooled and connected to deliver said cooled fluid to be cooled to a place of use or storage; (h) second outlet header means connected to each of said second fluid flow passages; (i) means for directing air across said first elongate hollow members to absorb heat from the latter; and (j) spray means connected to said second outlet header means to receive the heated heat absorption liquid and for spraying liquid into said air flow prior to its passing across said first elongate hollow member so that the heat absorption liquid is cooled by evaporation of some heat absorption liquid and recirculated to said second inlet header means.

An embodiment of the present invention will now be described by way of an example, with reference to the accompanying drawings, in which: Figure 1 is a schematic iliustration of the heat exchange system according to the present invention, and Figure 2 is a fragmentary view in perspective of a heat exchange element which may form part of the heat exchange system according to the present invention.

The reference number 10 generally indicates the heat exchange system according to the invention. The heat exchange system 10, as schematically shown in Fig. 1, comprises essentially at least one bank 12 of heat transfer elements connected to a source 14 of fluid to be cooled, as for example ammonia, to receive such fluid to be cooled and connected to a source of heat absorbing liquid, as for example water and hereinafter referred to as "cooling water" to pass the fluid to be cooled and cooling water in indirect heat transfer relative to each other to effect cooling of the fluid to be cooled and heating of the cooling water.A spray distributor assembly 1 6 is connected to receive heated cooling water from the bank 12 while a fan or blower 18 or other suitable means, for inducing or forcing flow of air across the bank 12 of heat transfer elements is provided. A pump 20 is also provided for circulating the cooling water, from the source thereof, such as a reservoir or sump 22, to the bank 12 of heat transfer elements.

The bank 12 of heat transfer elements, comprises, as shown in Fig. 1, a plurality of first conduits 24 and a plurality of second conduits 26.

Each of the second conduits 26 may, as shown, be disposed coaxially within a first conduit 24 and is of smaller cross-sectional dimensions so that a flow passage 28 is defined between the outer surface of the second conduits 26 and the inner surface of the first conduits 24. A dual-header assembly 30 is dispqsed at one end of the first and second conduits 24 and 26 while a dual-header assembly 32 is disposed at the opposite ends of the conduits 24 and 26. The dual-header assembly 30 is divided by a wall 34 to form an outlet chamber 36 and an inlet chamber 38.

Similarly, the dual-header assembly 32 is divided by a wall 40 to form an inlet chamber 42 and an outlet chamber 44.

The opposite ends of first conduits 24 are supported in walls sheets 33 and 41 and communicate with the outlet chamber 36 and the inlet chamber 42, while the opposite end portions of the second conduits 26 are supported in walls 34 and 40 so as to communicate with the inlet chamber 38 and the outlet chamber 44. This arrangement of header chambers provides for flow of fluid to be cooled and cooling water in counterflow relationship to each other. Suitable means, as for example, spacer lugs or ribs 46, as shown in Fig. 2, are provided to maintain the first and second conduits 24 and 26 in spaced relation to each other. Additionally, as is conventional and well-known in the heat exchange art, means-may be provided in flow passages 28 for preventing or breaking up the formation of boundary layers in the fluids adjacent the conduit surfaces to thereby promote heat transfer between the fluid and the conduit through which it is flowing. It is also to be understood that while the first conduits 24 and second conduits 26 are shown concentrically disposed relative to each other, the invention is not limited thereto and it is within the scope of this invention to provide conduits 24 and 26 which are arranged in alternate parallel relationship to each other and heat conductively connected together to provide indirect heat transfer between the fluids.

The inlet chamber 42 of the inlet header 32 is connected, via a pipe 46, to receive a fluid to be cooled, such as vaporized ammonia, from source 14 so that the fluid to be cooled flows through flow passages 28 and into the outlet chamber 36 of the outlet header 30. The inlet chamber 38 of the inlet header 30 is connected, via a conduit 48, to receive cooling water from the reservoir 22 to provide for flow of cooling water through conduits 26 and in indirect heat exchange with the fluid to be cooled flowing through passages 28.

Outlet chamber 36 is connected by way of a conduit 50 to a place (not shown) of use or storage of the cooled fluid to be cooled, while outlet chamber 44 is connected by a conduit 52 to the spray distributor assembly 1 6.

The heated cooling water which is delivered to the spray distributor assembly 1 6 is sprayed into the air stream produced by the fan 18, or other suitable means, to primarily cool the water by the absorption of heat by vaporization of some of the cooling water. The spray nozzles 56 are constructed, arranged and directed, when disposed for spraying cooling water into the air upstream of bank 12, so as to minimize the possibility of entrainment of the liquid droplets in the air stream and to avoid the attendant wetting of the heat transfer elements of bank 12 and the resulting increased pressure drop of the air through the bank 12. The cooled cooling water is collected in the reservoir 22 from where it is pumped by pump 20 through the conduit 48.

Make-up cooling water is supplied to the reservoir 22 by way of a feed conduit 60. The flow of cooling water through the system is controlled in accordance with ambient air and heat load conditions by a valve 62 in conduit 52 or by some other flow control means, or by operation of the pump 20. This enables the system 10 to be operated when ambient air temperatures and/or heat loads are sufficiently low to effect cooling, solely as an air cooling system and alternately, when air cooling alone is insufficient to provide the desired cooling, then as a combination evaporation and air system.

It is contemplated by this invention that any heat absorbing liquid other than water shall have a relatively high coefficient of heat transfer so that optimum heat absorption or transfer occurs between the fluids in bank 12.

As is conventional in the art, the heat transfer elements comprising first conduit 24 of the bank or banks 12 may be provided with extended surface elements or fins 64. The conduits 24 and 26 may be constructed as shown in detail in Fig. 2 or may be elongated tubes cylindrical in cross section, arranged concentrically relative to each other without departing from the scope and spirit of this invention. Also, alternatively, tubes 26 may be eliminated and conduits 24 connected at opposite ends to inlet and outlet headers constructed and arranged to provide flow of cooling water and fluid to be cooled through alternate flow passages 28 of conduits 24. As shown in Figs. 1 and 2, the conduits 24 are formed in a heat transfer element having a polygonal configuration in cross-section, preferably having (as shown) a rectangular shape in cross-section as disclosed in U.S. Patent No.

3,202,212 and having fins 64 which are integrally formed by skiving. The tubes 26, as previously mentioned, may be supported in spaced relation to the surfaces of ports or passages 28 by ribs 46 or by other suitable means.

The system 10, herein described, selectively functions as an air-cooling system or as a combination wet and air-cooling system depending upon the ambient air temperature and/or the heat load. For purposes of the hereinafter described function of system 10, it will be assumed that the fluid to be cooled is ammonia vapour and the heat absorbing liquid is cooling water.

The system 10 is functionally efficient as an aircooling system when ambient air temperature is relatively low, and/or when the heat load is relatively low. Under higher ambient temperatures and/or higher heat loads, the water is circulated through the system so that water passes through tubes 26 counter-current to and in indirect heat exchange relationship with the ammonia flowing through passages 28. This heat transfer is relatively great since the heat transfer coefficients of both fluids are high as has been already explained in detail.

In the dry operation of the system 10, the aircooling capability is reduced in proportion to the reduction of the log mean temperature difference between the air and ammonia due to the increasing air temperature. However, in the wet condition or phase of operation, this reduction in cooling capacity is compensated for by the heat transfer between the ammonia and water in banks 12. This is because of (i) the higher heat transfer coefficient of water as compared with that of air, (ii) the higher specific heat of water as compared to that of air, and (iii) the capability for higher log mean temperature difference between the water and ammonia compared with the difference between air and ammonia. The latter will occur because the water inlet temperature to tubes 26 can be decreased below the ambient air temperature to approach the wet bulb temperature.Assuming a 95 F (40.4 C) ambient air temperature and a relative humidity of 30%, the wet bulb temperature is only 710F(21.50C).

Since the wet bulb temperature limits the temperature to which the water can be cooled by evaporative means, then water temperatures below 950F !40.40C) are clearly obtainable in system 10. The resulting water temperature is then determined by the amount of water evaporation, and the amount of evaporation is primarily a function of the spray characteristics of nozzle 56 and the residence time of the falling droplets in the air. For water temperatures higher than 950F (40.4cC), the required water cooling can be accomplished by increasing the rate of water pumping. Since much lower water temperatures are achievable in system 10 because of evaporation by water spraying, the cooling load is more economically obtained at lower water rates of flow.Of course, optimum heat transfer is determined by the rate of water evaporation, heat loads or cooling requirements, the cost of water and cost of pumping the water.

The increase in the relative humidity and wet bulb temperature of the air flowing past banks 12, when the system 10 is operating with water spray, does not significantly adversely affect the heat transfer efficiency of system 10. This occurs because the increased water vapour in the air, as a result of water spray, will increase the specific heat and lower the weight flow of air and water vapour mixture by only a very small amount. To illustrate this fact, it will be assumed that inlet air temperature and relative humidity, respectively, changes from 950F (40.40C) and 30% to 950F (40.40C) and a 50% after water spraying, the weight of the air and water vapour is decreased by a factor of 0.9957 while the specific heat of the mixture is increased by a factor of 1.0056.

Therefore, the cooling capacity which is proportional to the product of these factors is changed by a factor of only 1.0011 or essentially unchanged. Similarly, the air pressure drop is proportional to the square of the weight of the air flow divided by the density of the mixture, or equivalent to the weight flow. For example, the weight flow and hence the air pressure drop is reduced by a factor of 0.9957. Thus, the pressure drop past the banks 12 remains essentially unchanged after water spraying as contemplated by this invention.

It is believed now readily apparent that the present invention provides a heat exchange aircooling system having increased cooling capacity over conventional air-cooling systems of comparable size. It is a system in which air-cooling effectiveness is increased utilizing a heat absorbing liquid to absorb heat from the fluid to be cooled and spraying that heated heat absorbing liquid into the air, either before or after the air passes the heat transfer elements, to cool the water by evaporative means. It is a system wherein the cooling capacity is increased over conventional air-cooling systems without increasing size, air flow rates and thus air power consumption, and without increasing fouling or corrosion of the heat transfer elements. It also eliminates the need for a separate cooling means for the heated heat absorbing liquid as is required in evaporative surface condensers where it is desired to recirculate such liquid. It is further a system wherein the inlet dry bulb temperature in the heat transfer elements may be reduced to thereby increase the difference in the air temperature and the temperature of the fluid to be cooled flowing in the heat transfer elements to increase the heat transfer rate and hence the cooling capacity.

Although but one embodiment of the invention has been illustrated and described in detail, it is to be understood that the invention is not limited thereto. Various changes can be made in the arrangement of parts without departing from the scope of the invention as defined in the appended claims as the same will now be understood by those skilled in the art.

Claims

1. A heat exchange system for cooling a fluid to be cooled comprising: (a) first means for conducting the fluid to be cooled in indirect heat transfer relationship with a heat absorbing liquid of high heat transfer coefficient for absorption of heat from said fluid to be cooled; (b) second means for passing air in indirect heat transfer relationship with fluid to be cooled to remove additional heat from the fluid simultaneously with the passing of said fluid to be cooled in heat exchange relationship with said heat absorbing liquid; (c) third means for communicating with said first means to receive the heated heat absorbing liquid and dispersing the latter into the air of said second means in accordance with temperature and heat load conditions to thereby effect cooling of the heat absorbing liquid by vaporization of some of the heat absorbing liquid; and (d) means for collecting and recirculating the cooled heat absorbing liquid to the first means for cooling additional quantities of fluid to be cooled.

2. Heat exchange system as claimed in claim 1, in which the third means is constructed and arranged to disperse the heated heat absorbing liquid into the air prior to the passage of the air in indirect heat transfer with the fluid to be cooled and to avoid entrainment of droplets of heat absorbing liquid in said air.

3. A heat exchange system as claimed in claim 1 or claim 2, in which said first means includes control means for preventing flow of heat absorbing liquid in indirect heat transfer with the fluid to be cooled when the air temperature and/or heat load is sufficiently low to effect cooling of said fluid to be cooled.

4. A heat exchange system as claimed in any preceding claim, in which said first means includes a plurality of first elongate hollow members and said second means includes a plurality of second elongate hollow members and in which each of said second elongate hollow members is arranged within a first elongate hollow member to define therebetween a fluid flow path for the fluid to be cooled.

5. A heat exchange system as claimed in claim 4, in which said first elongate hollow members are polygonal in cross-section and comprise a plurality of spaced parallel passages and in which each spaced passage has arranged therein a second elongate hollow member.

6. A heat exchange system as claimed in claim 5, in which extended surface elements extend from opposite surfaces of the first elongate hollow members.

7. A heat exchange system as claimed in claim 6, in which said extended surface elements are formed from the body of the first elongate hollow members.

8. A heat exchanger comprising: (a) plurality of spaced first elongate hollow members; (b) each of said first elongate hollow members having extended surface elements projecting from at least part of its exterior surface; (c) a plurality of second elongate hollow members, one for each of said first elongate hollow members and disposed within an associated first elongate hollow member to define therebetween a first fluid flow passage; (d) each of said second elongate hollow members defining a second fluid flow passage; (e) a first inlet header means connected to receive said fluid to be cooled from a source thereof and communicating with said first fluid flow passages to conduct fluid to be cooled to the latter.

9. A heat exchanger as claimed in claim 8, in which a valve means is provided to prevent a flow of heat absorption liquid to said spray means when the temperature of the air is capable of providing sufficient heat removal from the first elongate hollow members to cool the fluid to be cooled to a pre-determined value.

10. A heat exchanger as claimed in claim 8 or claim 9, in which each of said first elongate hollow members are substantially rectangular in crosssection and have a plurality of spaced parallel passages and wherein a second elongate member of said plurality of members is disposed to define therebetween said second fluid flow passages.

11. A heat exchanger as claimed in claim 10, in which spacer means are provided to hold each said second elongate members in spaced relationship with the walls of its associated passageway of said first elongate hollow member.

1 2. A heat exchanger apparatus as claimed in any of claims 8 to 11 , in which the extended surface elements are fins which are formed integrally with the first elongate hollow members.

13. A heat exchange system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

14. A heat exchanger apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.