Patent Application
20080041087
Along the reach of the fin depth in the air flow direction 3,8, the tubes 2 may be arranged behind each other or preferably offset. In accordance with the number of tube rows, the liquid can flow through the heat exchanger one to several times.
The feed 6 of the wetting water 7 is located above the upper face 4 of the fins 5. A larger quantity of this water is output than can evaporate in the air stream 8. The excess wetting water 9 is collected in a catch bowl 10 and recirculated via a pump 11.
In accordance with the preferred embodiment, the heat exchanger 1 is a so-called “finned tube heat exchanger” in which the tubes 2 are designed as round tubes, as droplet-shaped tubes, or as oval tubes and the fins 4 produced from one piece can hold a large number of tubes 2. In the longitudinal direction of the tubes 2, several fins 5 are arranged behind each other. Their distances from each other are optimized for pressure loss and heat transfer in accordance with the respective needs.
The plane of the fins 5 is vertical and is shown on the drawing level in
Water-droplet slit and water-droplet splitting louvers 14,15 are arranged between the adjacent tubes 2 in each case in the air flow direction. The design of the water-droplet slit and water-droplet splitting louver 14, 15 in accordance with the invention is described in the following with reference to
In
As a result of a backup effect of the air stream 8 before the tube 2, the two baffles 24 flowed against first by the air, which are located next to each other in the longitudinal fin direction Y, can be positioned relatively far apart. The baffles 24 which follow in the air flow direction 8 then preferably lie in the area of the highest air flow velocity and generally in areas in which the free water droplets are transported in the cooling air. As the baffles only serve the purpose of droplet catching and droplet splitting, it is therefore not necessary for the entire air area between to adjacent tubes 2 to be covered. An air deflection is also of secondary importance, which is why the focus is on a reduction of the air-side pressure loss. Even with flows with a laminar character, new water droplets always come into immediate contact with the baffles 24. For production-related reasons, a multiple repetition of geometrically identical elementary formations assigned to the tubes 2 is advantageous.
The design of a baffle 24 is shown spatially in
The impact of a droplet 28 on the cut edge 29 presses part of the water contained in a droplet 28 onto the upper fin and part below the actual baffle. During this impact the surface tension of the water droplet 28 is affected and filmy wetting forms on the surface concerned with an enlarged working surface for the evaporation of the wetting water.
Due to a lack of mechanical resistance, the air flow between the individual baffles 33 is somewhat accelerated and directed directly at the second row of baffles 34 located in the air flow direction. Air droplets carried along which have avoided the first row of baffles 33 are caught and cut open in the course of the baffle row 34. The same process is repeated in a similar manner in the subsequent rows 35 and 36.
In accordance with the invention, the liquid coolant 45 of the primary cooling circuit P, coming from the heat source 46 and circulated by the circulation pump 47, is fed to the heat exchanger 44 with the highest cooling circuit temperature, which must always lie above the ambient air temperature to be cooled, for a purely sensible heat emission. This heat exchanger 44 has the same or a different fin geometry than the hybrid heat exchanger 43.
The high initial temperature difference (tair inlet−twater inlet) results in a very large heat flow density for the transfer to the heat from the liquid coolant via the walls of the tubes 2 to the fins 5, and ultimately to the cooling air, the flow rate of which is symbolized by the arrows 48. The liquid coolant 49 pre-cooled by means of the ambient air and without evaporation is now transferred from the heat exchanger 44 to the hybrid (water-wettable) heat exchanger. The reduced temperature of the coolant 49 at the inlet into the hybrid heat exchanger 43 now enables a primarily latent heat emission from the fins 5 to the cooling-air flow-through quantity, symbolized by the arrow 48.
The secondary cooling circuit S for wetting the fin surface of the heat exchanger 43 mainly consists of a water reservoir basin 50 with the feed pump 51 and the supply line 52 to the water feed 53. The excess wetting water 54 which has not evaporated drips over a drain plate 55 and is routed back through the return line 56 into the reservoir basin 50. A level sensor 57 regulates the water supply in the reservoir basin 50 by keeping the water level within certain limits by opening and closing the feed valve 58. By means of a drain valve 59, water with an excessively high concentration of components can be drained out of the reservoir basin 50 into a drain line 60 either conductivity-controlled or in proportion to the quantity. The overflow 61 of the reservoir basin 50 also empties into the overflow line by bypassing the drain valve 59.
The cooling air entering the heat exchanger, symbolized by the arrows 63, has the same state for both heat exchangers 44, 43. However, for the sensible heat transfer in the heat exchanger 44, primarily the air temperature, and for the primarily latent heat transfer in the hybrid heat exchanger 43, the air temperature with the related moisture content, must be included in the calculation of the thermal output.
The content of the mixed-air water vapor, formed from the exhaust-air streams (arrows 65 and 66), at the transition from the cooler to the ambient air is decisive for the prevention of a visible, disturbing plume of steam as the result of condensation of the water vapor of the exiting cooling air, symbolized by the arrow 64, in the cooler ambient air. The air exiting from the heat exchanger 44, symbolized by the arrow 65, which is heated without adding water, is greatly reduced in its relative moisture content at the outlet. On the other hand, the relative moisture content of the air exiting the heat exchanger 43, symbolized by the arrow 66, is increased greatly, or up to almost 100%, by evaporation of water.
These two air streams (arrow 65, 66) are sucked in by the fan 62 and thoroughly mixed by its rotation, symbolized by arrow 67. This results in a relative water vapor content that lies between the sensible and latent heat transfer of the heat exchangers 44,43 and far from the mist area with 100% saturation. A re-condensation of the cooling air in the area around the cooler can therefore certainly be excluded.
Another advantageous property of the liquid heat exchangers 44 and 43 connected in series on the coolant side is the division of a high total cooling range between the coolant entry and exit temperature to a sensible heat sink in the high temperature range and a latent heat sink in the lower temperature range with a smaller cooling range in each case. This may exclude damaging influences on the tube-fin connection or even on the entire heat exchanger geometry as a result of excessively high thermal stress.
The high specific air throughput for sensible and latent heat transfer in the design point also causes an increase in the air throughput for purely sensible heat transfer in the lower air temperature range. This in turn causes a shifting of the switchover point from purely sensible to sensible/latent heat transfer in the upper air temperature range. The extended drying operation with purely sensible heat transfer saves a great deal of expensive, processed additional water over the year, and therefore leads to economical cooling operation of the system.
In two sensible and latent heat exchangers connected in series, the coolant of a liquid coolant mass flow from a cooling circuit of a process of 17.05 kg/s with an ambient temperature state of 91° F. (33° C.)/38% relative humidity and by means of the fan with a sucking action for the two heat exchangers is to be cooled down from 149° F. (65° C.) to 86° F. (30° C). This is equivalent to a thermal output of 2,250 kW.
In the sensible heat exchanger first charged with the liquid coolant, into which the coolant enters at 149° F. (65° C.), it is cooled down to 123.94° F. (51.08° C.) with the air mass flow of 45.36 kg/s sucked in by the fan with an air-side pressure loss of 118 Pa.
The water pre-cooled to 123.94° F. (51.08° C.) then flows to the latent heat exchanger of the same geometry and the same dimensions. There it is cooled down to 86° F. (30° C.) with an air mass flow of 29.53 kg/s sucked in by the same fan with an air-side pressure loss of 118 Pa by means of evaporation of water from a secondary circuit. The smaller air mass flow with latent heat transfer is due to the somewhat increased pressure loss of the water-droplet slit and water-droplet splitting louver required for effective wetting.
The initial air state at the inlet into the two heat exchangers 44, 43 of 91° F. (33° C.)/38% relative humidity changes in the process at the outlet of the sensible heat exchanger 44 to 133.52° F. (56.4° C.)/11% relative humidity, and at the outlet of the latent heat exchanger 43 to 90.5° F. (32.5° C.)/94% relative humidity. The two air streams with the different air quantities as a result of different pressure losses in the fin gaps of the two heat exchangers, and with different air states as a result of sensible cooling in the first and latent cooling in the second heat exchanger, are mixed in the cooling air fan and reach a mixed air state of 114.62° F. (45.9° C.)/31% relative humidity at the outlet of the cooler and at the transition to the ambient air respectively. With the very large difference of 69% relative humidity to the saturation limit of 100% relative humidity, the formation of visible plumes is completely excluded.
At a constant cooling capacity of 2,250 kW but dropping temperatures of the ambient air, the switchover point to purely sensible heat transfer in both heat exchangers and at the same fan performance as in the design point is reached at an ambient air temperature of 55.94° F. (13.3° C.). In moderate climatic regions, this lies far above the annual average temperature.
It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.
