US2535047A - Air preheater for steam generating plants - Google Patents

Description

Dec. 26, 1950 D. DALIN ET AL AIR PREHEATER FOR STEAM GENERATING PLANTS Filed Nov. 22, 1944 Patented Dec. 26, 1950 AIR PREHEATER FOR STEAM GENERATING PLANTS David Dalin', Ronninge, and Gustav Vilhelm. Ha-gby, Ostertalje, Sweden Application November 22, 1944, Serial No. 564,584 In Sweden November 6, 1943 1 Gl'aim. 1

The present invention relates to a heat exchange apparatus and refers more particularly to a heat exchanger especially adapted for air preheating by means of hot flue gases.

The object of this invention is to provide an efficient. simple and compact air preheater by which desired overall plant efficiency can be materially improved without producing undesirable gas pressure drop or draft losses within the flue gas passage.

While past preheater constructions have employed tubes as a means for exchanging heat be tween the hot gases and the air to be heated, it has been consistently the practice to have both ends of the tubes anchored to tube sheets. As a result, the stresses resulting from expansion and contraction of the tubes constantly loosen the tube sheets.

The present invention overcomes this object on by using U-shaped tubes anchored only at their open ends with the turn or bight oi the U entirely free;

A further object of the present invention is to provide an air preheater unit which may be very easily installed or removed from its position within a flue gas passage.

With the above and other objects in view, which will appear as the description proceeds, this invention resides in the novel construction and arrangement of parts substantially as hereinafter described, and more particularly defined by the appended claims, it being understood that such changes in. the. precise embodiment of the here nafter disclosed invention may be made as come within the scope of the claims.

The accompanying drawing illustrates one complete example of the physical embodiment of the invention constructed in accordance with the best mode so far devised for the. practical application of the principles thereof, and: in which:

F gure 1 is a vertical sectional view of an air preheater embodying the present invention;

Figure 2 is an end view of the preheater apparatus taken from the outlet end thereof; and

Figure 3 is a top plan view of the preheater apl paratus showing the arrangement of the biiurcated inlet and outlet air passages.

Referring now more particularly to the accompanying drawing in which like numerals indicate like parts the numeral l3 designates a section of a duct hav ng an inlet 8 and an outlet 9 at opposite ends thereof. The duct section serves as a passage through which hot flue gases pass and may be located at any point along the flue gas passage, but preferably at a point where the flue gas is the hottest, since the hotter the flue gas passing through the air preheater the less chance there is for the formation of condensation within the preheater.

Located within this section of the duct are three groups of U-shaped tubes 12, b and c, all of the tubes of which hang from a flat tube sheet 4 which forms the topwall for the preheater portion of the flue gas duct. These groups of tubes are spaced from one another along the path of the gases with group 11 located farthest downstream near the outlet 9.

Above the top wall of the tube sheet a is a housing i I having a plurality of spaced apart straight vertical partition walls [2 which extend transversely of the duct l3. These partition walls divide the housing into six adjacent transverse headers, two of which are designated by the numeral l, two by the numeral 6 and the two remaining ones i and 6. The headers land 6 are arranged alternately, the headers i being connected with a common air inlet chamber 21, located at one side of the duct l3, and the headers 6 being connected with a common outlet chamber 22, located at the other side of the duct l3.

Each of the tube groups is composed of a plurality of U-shaped tubes l, with the extremities of their legs 2 and 3 fixed to the tubes sheet 4 so that the anchored upper ends of the tubes open to the headers. The tubes hang from the tube sheet A and have their closed ends or bights' 5 located adjacent to the bottom Wall of the duct in the manner shown in Figure 1.

One leg of each of the U-shaped tubes of groups 1a, and b communicates with an i-nletheader I and the other leg thereof communicates with the adjacent outlet header 6, upstream therefrom.

Thus, with each of the tubes of the groups a and I) having one end communicated with an inlet header 5 and its other end communicating with the adjacent outlet header ii (upstream therefrom) it will be readily understood that cold air entering the inlet 2! flows into both headers l, which may be considered branches of the inlet, through the U-shaped tubes of groups a and b to be heated by the hot flue gases flowing through the flue gas duct and over the tubes, then flows into the warm air headers 5 to be discharged through outlet 22. Attention is directed to the fact that the tubes are so disposed that their bights are parallel with the longitudinal axis of the flue gas duct, and that all of the tubes are wholly contained within the flue gas passage.

This construction permits the heating of large 3 quantities of air within a relatively short length of gas passage, and also enables the tubes to freely expand or contract without interfering with adjacent tubes and without affecting the tube sheet 4 to which the tubes are secured.

In addition the tubes of a particular row of any one of the groups may be replaced without necessitating removal of all of the tubes of a single group.

Since the air to be heated enters at the inlet 2| and flows into the tubes from the headers 1 a desirable counterflow is achieved, i. e. the air as it is heated moves upstream with respect to the gas flow, especially as the air passes through the horizontal bights 5 of the tubes.

The third tube group c can be used in conjunction with the tube groups a and b or independently thereof. It is preferably located upstream of the tube groups a and b. Tube group 0 is similar in construction to groups a and b and has a cold air header 1 and a warm air header 6 with which the legs of the U-shaped tubes of the group communicate. Circulation of air to be heated through tube group 0 is the same as through groups a and b, with cold air entering header 1 passing through the tubes and into the header 6.

Normally the groups a and b are employed for heating primary air and the third group 0 is employed for heating secondary air, since the secondary air must necessarily be hotter than the primary air. The air preheater comprising the present invention is primarily intended to be used in plants having stoker firing, for example in plants in which the primary air need only be heated to a temperature between 102 C. and 105 C., but in which the temperature of the secondary air should be as high as possible, and in which the quantity of primary air is considerably greater than the quantity of secondary air.

In this type of plant and in the type of plants designed to use different kinds of fuels, thus necessitating different quantities of secondary air, and in which the same grate is used in the combustion of the fuels, there may be provided a connecting duct with regulating valves between the secondary air outlet duct and the primary air outlet duct. Such a connecting air duct is indicated by 23 in Figure 3 of the drawing. The use of connecting passage is usually required when the quantity of secondary air needed for the boiler varies, and serves in the type of apparatus illustrated to transfer a portion of the secondary air to the primary air passage. In this way that portion of the air preheater intended for heating of secondary air may always be completely utilized even though the quantity of secondary air required by the boiler is comparatively small.

From the foregoing description taken in connection with the accompanying drawing, it will be apparent to those skilled in the art that this invention provides an air preheater having many advantages over devices of this type heretofore available, since the air preheater unit is so compact as to require very limited space and to permit highly efiicient heat exchange between the flowing media.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is:

An air preheater for steam generating plants comprising: a duct through which hot gases are adapted to flow, said duct having a flat top wall at one portion thereof; two groups of U-shaped tubes wholly inside the duct and one downstream from the other, said tubes having the extremities of their legs fixed to said flat top wall portion and opening therethrough, and each group of tubes extending across the duct from side to side thereof with the legs of the tubes of each group lying at opposite sides of a plane transverse to the duct so that the tubes occupy planes extending substantially in the direction of gas flow in the duct; a housing connected to said top wall portion at the exterior of the duct; spaced apart straight walled partitions in said housing extending transversely of the duct and located between the groups of tubes and substantially in said transverse planes between the legs of the tubes of each group, said partitions defining two pairs of separate but adjacent headers, each header extending transversely across said fiat top wall portion of the duct over and in communication with the open ends of the tubes at one side of said transverse plane; an air inlet chamber communicating with one set of alternate headers at one side of the duct; and an air outlet chamber communicating with the other set of alternate headers at the opposite side of the duct.

DAVID DALIN GUSTAV VIIJ-LELM HAGBY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,421,247 Keenan, Jr June 27, 1922 1,673,122 Mills June 12, 1928 1,744,452 Dowd, Jr Jan. 21, 1930 1,765,675 Jacobus June 24, 1930 1,833,314 Bruce Nov. 24, 1931 1,919,029 Lucke July 18, 1933 2,008,255 Larkin July 16, 1935 2,029,284 Armacost Feb. 4, 1936 2,063,441 Kerr Dec. 8, 1936 2,279,518 Olson Apr. 14, 1942 2,423,997 Ruegg July 15, 1947

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