Air Cooled Surface Condenser

Abstract

An air-cooled surface condenser has a supply of cooling air which is varied in accordance with the temperature of the condensate to thereby diminish the possibilities of freezeup at below normal ambient temperatures. The condenser has at least two downflow tubes and one reflux tube with the tubes so arranged that the cooling air travels first over the reflux tube and then sequentially over the first and second downflow tubes, and with the tubes being of such dimensions that condensate is formed in the reflux tube. With this arrangement the air supply can be varied in response to drop in condensate temperature at the first downflow tube and thus assure that the air, before reaching those portions of the first downflow tube where condensate has formed, has a temperature which has been elevated by the latent heat of condensation within the reflux tube.

Description

BACKGROUND OF INVENTION

The field of invention may be defined as the art of air-cooled surface condensers in which the air supply is varied in accordance with condensate temperature for the purpose of avoiding freezeup.

A great number of condensers are intended for outdoor installation and depend upon the outdoor air for a coolant. Freezeup of the condenser is a generally recognized problem and a variety of design features have been suggested as possible solutions to this problem of freezeup.

A number of prior art designs have suggested the variance in either the fin dimensions or tube diameter between those tubes which are first impinged by the coolant air and those which are subsequently impinged by the air after its temperature has been elevated because of the initial pass. These suggestions have been successful to some degree; however, the variance in tube design from row to row is considered a disadvantage from a manufacturing and cost standpoint.

SUMMARY OF INVENTION

The concept underlying the present invention is considered to be control of the cooling airflow in response to condensate temperature so that latent heat of condensation in one or more of the reflux tubes (which, because of their steam-condensate counterflow, are less susceptible of freezeup than a downflow tube) elevates the temperature of the cooling air before it passes over those sections of subsequent downflow tubes in which complete condensation occurs at low temperatures.

In order to so utilize the latent heat, the condenser embodying the present invention includes at least two downflow tubes which are located on the downstream side of one or more reflux tubes with respect to airflow over the tubes. Further, the airflow is controlled in response to condensate temperature changes in the first downflow tube (i.e. the first with respect to the airflow) to assure that the air which impinges those sections of the downflow tubes in which full or complete condensation has taken place has first passed over sections of the reflux tube in which full or complete condensation has taken place.

Other objects and advantages will be pointed out in, or be apparent from, the specification and claims, as will obvious modifications of the embodiment shown in the drawing.

DESCRIPTION OF THE DRAWING

The drawing is a cross-sectional view of a condenser section taken in the plane of airflow, and the levels at which condensation has been completed in the tubes at various airflow temperatures are shown by the dotted lines.

DESCRIPTION OF PREFERRED EMBODIMENT

The illustrated condenser has top and bottom headers 10 and 12, respectively, which are connected by rows (one row only being shown) of four finned condenser tubes of the usual design. The top header is divided into an inlet chamber 14 which is connected to a source of steam or vapor and an exit chamber 16 which is provided with appropriate exit ports for the noncondensable gases. The bottom header has only one chamber 18 which provides communication between all of the condenser tubes and which serves to collect and drain the condensate.

The condenser may be classified as a mixed flow condenser in that it incorporates two downflow tubes 20 and 22 and two reflux tubes 24 and 26. A variable speed or variable volume blower 30 is positioned to provide cooling airflow in a path generally transverse to the tubes 20, 22, 24 and 26. The blower is connected to a temperature sensing device 32 which senses the condensate temperature which may accumulate in a small cup 34 below tube 20 or, if may be located inside the bottom end of tube 20. Temperature sensing device 32 can be of conventional design, e.g. a thermocouple arrangement and for that reason has not been shown in detail. Since in operation of the condenser the downflow tubes do at times differ in their functions, it is best to classify them in accordance with their position in the airflow path and to thus refer to tube 20 as the first downflow tube and to tube 22 as the second downflow tube. On the other hand, one or more reflux tubes can be used in proper operation of the condenser and, thus, such a classification will not be made in respect to the reflux tubes.

The significance of the present invention can best be appreciated by comparing the operating characteristics of the condenser as the cooling air temperature is lowered from a maximum design temperature to those extremely low temperatures at which condensate solidification might normally pose a problem. In operation, steam passes through downflow tubes 20 and 22, changes direction in chamber 18, and proceeds upwardly through tubes 24 and 26. The four tubes of the condenser are of such design that at maximum air temperature, maximum vapor temperature, and maximum load (measured by the pounds of material which can be removed by condensation) condensation will take place in all tubes and will have been completed at levels a and b in tubes 24 and 26. The difference in the levels a and b is, of course, caused by the fact that both tubes receive steam at the same temperature and that tube 26 is cooled by air at a higher temperature than tube 24.

The condensate in tubes 24 and 26 will flow downwardly and counter to the steam flow out of chamber 18 and the noncondensable gases will be conveyed out through the outlet of exit chamber 16. At all temperatures some condensation will take place in all tubes. As the cooling air temperature decreases from the maximum design temperature to a lower temperature, the location of complete condensation falls to levels c and d in tubes 24 and 26. With further decrease in cooling air temperature a point is reached where complete condensation will start to take place in the first downflow tube 20 while some condensation continues to take place in both of the reflux tubes as complete condensation will not be occurring in the second downflow tube 22. The reason for this phenomenon is that at each tube the airstream temperature gradient is different. Steam entering reflux tubes 24 and 26 is at the same temperature, which, when disregarding heat losses in chamber 18 and pressure differences between tubes 20 and 22, may be assumed for purposes of this illustration to be at the temperature of the steam leaving tubes 20 and 22. However, despite the identical initial steam temperature, the increase in air temperature caused by the condensation in tube 24 will result in a significantly smaller steam-air gradient at the bottom portion of tube 26 and, thus, at low air temperatures will cause full condensation at level f which is above level e of tube 24.

The air flow reaching the first downflow tube may be divided into a "cold" section, which has passed over those sections of the reflux tubes which are above levels f and e and a "warm" section, which is that air which has passed over the reflux tube sections between their inlets and levels f and e. As the air reaches the first downflow tube 20, the upper or "cold" section will have been only slightly heated by tubes 24 and 26, but the lower or "warm" section will have benefited from the latent heat of condensation which has been transferred from the portions of tubes 24 and 26 which are below levels f and e. Thus, the steam-air temperature gradient at that portion of the first downflow tube which is impinged by the "cold" airflow will be sufficient to cause complete condensation at level g in tube 20. However, the latent heat of condensation of the steam in the first downflow tube which is transferred to the "warm" portion of the air flow at that tube, plus the heat which has already accumulated because of passage over tubes 24 and 26 will sufficiently decrease the steam-air gradient at the second downflow tube to prevent full condensation in that tube.

Subcooling takes place below level g in tube 20 and the temperature sensing device 32 must reduce the airflow to prevent the condensate being subcooled to the freezing point at the bottom of tube 20. If not, ice will form in the lower portion of the tube 20, i.e. below level g.

This invention proposes to solve this problem of freezeup due to subcooling by controlling the flow of air, and thus the amount of heat transfer, such that the complete condensation level in the reflux tubes overlaps the complete condensation level in the downflow tubes; as illustrated so that level f (or f and e ) is higher than level g. With that arrangement, the "warm" portion of the airflow which impinges on the lower portion of the first downflow tube prevents subcooling to a degree which would otherwise cause freezing of the condensate in the tube section below level g. The prevention of full or complete condensation in the second downflow tube is, of course, essential since that tube must supply steam to the reflux tubes to permit condensation therein and, thus, the transfer of latent heat to the flowing air at the lower sections of the reflux tubes.

With a decrease in the cooling air temperature the full condensation levels e and f in tubes 24 and 26 will drop whereas the level g of the first downflow tube 20 will rise. If the point is reached where the levels e and f in tubes 24 and 26 a below the level g in tube 20, a portion of the condensate within tube 20 will be subjected to the "cold" portion of the airflow which will increase the subcooling of the condensate. This can result in solidification of the condensate at the bottom of tube 20.

Specifically, in order to avoid the exposure of tube 20 below level g to the "cold" portion of the airflow, probe 32 is designed to sense the temperature of the condensate as it leaves tube 20 and to cause the blower to decrease the airflow at times when the condensate temperature drops sufficiently low to indicate that level g is approaching level f. A decrease in airflow, either by decrease in blower speed or volume of air intake, will in turn cause an increase in the air temperature at tubes 24 and 26 and thereby raise the full condensation levels in the reflux tubes and correspondingly lower the full condensation level in tube 20. Thus, the sequential arrangement of the tubes and the control of the fan in response to the downflow tube condensate temperature permits the system to be maintained in a balance which maintains a sufficiently wide "warm" air path to cover at least that portion of the first downflow tube in which condensation has been completed. Temperature sensing device 32 by responding to change in temperature of the condensate at the bottom of tube 20 automatically keeps the system in balance upon changes in air temperature or upon changes in vapor temperature or load.

In analyzing the above, it should be noted that the freezeup problems which are encountered in the downflow tubes result from the fact that condensate flow is in the same direction as the steam or vapor flow. The condensate flow in the reflux tubes is in the opposite direction of the steam or vapor flow. For this reason the freezeup problems, in absence of a maintenance of the aforementioned balance, will normally be first encountered at the bottom of the first downflow tube and not in the reflux tubes, despite the fact that the air which impinges upon the reflux tubes is at a lower temperature than that which impinges upon the first downflow tube.

Claims

1. An air cooled surface condenser of the type in which the cooling airflow is varied in response to temperature changes in the condenser tubes in order to render the condenser operable at below maximum cooling air temperatures, the condenser having: first and second downflow tube means communicating at their top ends with a source of vapor and communicating at their bottom ends with a condensate collecting chamber; reflux tube means communicating at the bottom end thereof with said collecting chamber and said first and second downflow tube means and communicating at its top end with an exit passage from the condenser; airflow generating means operable to provide a cooling airflow across said two downflow tube means and said reflux tube means; said tube means being so positioned relative to each other and with respect to the direction of airflow that the cooling air initially impinges on said reflux tube means, thereafter impinges on said first downflow tube means and subsequently impinges on said second downflow tube means; and temperature change sensing means positioned to sense temperature changes within the condenser and operably connected to said flow generating means to cause said generating means to vary said airflow in response to such

2. A condenser according to claim 1 wherein: said first and second downflow tube means and said reflux tube means are so dimensioned as to cause condensation to be completed within said reflux tube means as well as within said first downflow tube means at relatively low cooling air temperatures; said temperature change sensing means positioned to sense temperature changes in the condensate of said first downflow tube means; and wherein said flow generating means diminishes said cooling airflow in response to a decrease in said first downflow tube means condensate temperature below a

3. A condenser according to claim 2 wherein said first and second downflow tube means and said reflux tube means are generally of equal length and extend generally parallel to each other between a top header and a bottom header of the condenser and being arranged at an angle to the horizontal, with said top header having an inlet chamber and an exit chamber and with said inlet chamber including means for communication with said source of vapor and with said two downflow tube means, and with said exit chamber being provided with exit port means to permit escape of the noncondensables from the condenser; and with said bottom header being provided with said condensate collecting chamber which is in communication

4. A condenser according to claim 3 wherein said reflux tube means has a plurality of reflux tubes which are aligned in succession with respect to said cooling airflow, and wherein the number of reflux tubes equals the sum of the tubes contained in said first and second downflow tube means.

5. A condenser according to claim 4 wherein a separate condensate collecting means is provided at the exit of said first downflow tube means, said separate collecting means intercepting the condensate of said first downflow tube means; wherein, said temperature change sensing means is positioned to sense the condensate temperature in said separate

6. A condenser according to claim 1 wherein said temperature change sensing means is operative to control said airflow generating means such that the level of complete condensation in said reflux tube means overlaps the level of complete condensation in said first downflow tube means so that the temperature of the cooling air impinging on said first downflow tube means below said complete condensation level will have been raised by passage over the portion of said reflux tube means below said complete condensation level of said reflux tube.