Importance of Streams in Superheater, Economizer

The number of streams in a superheater or economizer is a very important piece of information, and plant engineers should know how many streams are used in their superheater and economizer. Often, when there is a tube failure or a problem with superheater tube wall or temperature, the first parameter one estimates is the tube-side heat transfer coefficient and velocity. In order to do that, information on streams is essential. Streams are the number of tubes through which the entire tube-side fluid flows. From this, one may estimate the flow per tube (w = flow per tube = total tube-side flow/number of streams).

Flow per tube determines the fluid velocity, heat transfer coefficient, as well as pressure drop, and hence, one should understand the importance of this term. Plant engineers should also review the drawings provided by boiler supplier and find out how many streams are used in their superheater and economizer. Sometimes, this information is not clearly indicated or provided by the boiler suppliers, and plant engineers also don’t understand the significance of this. Sometimes baffles may be used in the superheater headers, which may decrease the streams by half or even a third; having these data handy will help plant engineers evaluate their superheater or economizer performance when occasion arises such as when there is a superheater tube failure or overheating of tubes. They may also use this information to estimate the tube-side pressure drop and cross-check with field data to ensure that the steam-side pressure drop inside the superheater is reasonable.

The author has seen many superheaters fail in operation due to wrong selection of streams by the boiler supplier or lack of understanding of streams while designing the superheater leading to overheating of tubes, stagnation of flow, or even reverse flow. Selection of streams is done based on many considerations in superheaters such as velocity, pressure drop, and height of superheater (if tubes are vertical and superheater steam flow is in downward direction). Chapter 3 discusses a superheater tube failure problem due to low steam-side pressure drop. Since gas flow to steam flow ratio may vary depending upon the application, the number of tubes across the boiler width need not be the

Appendix B: Tube-Side Heat Transfer Coefficients and Pressure Drop 385

same as the number of streams. It can be a fraction or a multiple of the number of tubes wide. Some examples of superheater (and economizer) geometry are presented as follows to illustrate the concept of streams.

1. Superheater with horizontal tubes : In this case (Figure B.4a), we have a superheater in staggered arrangement. If there are, say, 30 tubes across the header in each row, then there are a total of 30 × 2 = 60 streams. Some engineers call the number of tubes in the same plane as starts. So there are 30 tubes across and 2 starts. The bottom line is that if 200,000 kg/h is the total flow of steam in this superheater, then 200,000/60 = 3,333 kg/h is the average flow per tube. This is w to be used in the

calculation of h c or ΔP. Figure B.4a shows only four streams (2 × 2).

2. Vertical tube superheater: There are 18 tubes across, but by reviewing the header draw-

ing (Figure B.4b), the plant engineer notices a baffle plate in the inlet header, and hence, there are nine streams only. The steam inlet and exit nozzles are on the same header. The other header is just a turnaround header, and so there is no baffle. Steam flows through nine tubes or nine streams in counter-flow direction and then makes a 180° turn and returns to the same header in parallel-flow fashion. This is done some-

times to lower the superheater tube wall temperatures (by increasing h c ). Hence, if

27,000 kg/h is the total steam flow, the average flow per tube will be 3,000 kg/h.

3. Economizer (or superheater) with vertical tubes: In this drawing of a gas turbine HRSG

economizer with vertical tubes (Figure B.4c), there are 12 tubes carrying water (across the width), but 24 tubes/row across the boiler width. All tubes are in counter-flow arrangement. Twenty-four tubes across are required from gas velocity considerations, but steam velocity consideration requires only half that number. This arrangement differs from that in Figure B.4b in that we have complete counter-flow in this case, while it is party counter- and partly parallel-flow in ear- lier case. The LMTD values will accordingly be different and have to be evaluated. One may note that a collection header is required at the bottom for each row of tubes as there are incoming and outgoing tubes in each header. It is a more compli- cated arrangement to have streams as a third of the number of tubes wide. Hence, this arrangement is suitable for the number of streams half the number of tubes wide. If streams are the same as the number of tubes wide, then we can eliminate the collection header at the top of each row of tubes.

4. Economizer with low number of streams: In the case of Figure B.4d, there are eight tubes

per row in the economizer with vertical downward gas flow, but the water flow requires only four streams. Hence, each tube is bent in the horizontal as well as in the vertical plane. Similarly, there can be 30 tubes across while streams can be 15 or

10 or 5 or even 6 or even 30 streams depending on water velocity required. Typically, 1–2 m/s is a good starting value for the velocity of water at 100% load. This arrangement is seen in gas turbines where the ratio of gas flow to steam flow is so large that the gas velocity demands a large number of tubes in the cross section while the water velocity requirement dictates a few tubes only. Figure shows eight tubes across with four streams. Note that in this type of arrangement, the transverse and longitudinal spacing are determined by the bend radius available for a given tube size.

5. Economizer or superheater with multiple streams: In this example of an economizer

(Figure B.4e) or superheater, there are more streams than the tubes across. If the number of tubes wide is say 10, the streams are a multiple of it, 20. There can be

30 or even 60 streams depending on the ratio of gas to water flow. In a few waste

386 Appendix B: Tube-Side Heat Transfer Coefficients and Pressure Drop

Steam in Steam out S 2 Steam in

Inlet header

Water inlet Water outlet

Gas flow

View B 2

View A

Drains

(c)

FIGURE B.4

(a) Horizontal tube superheater. (b) Vertical tube superheater with baffle in header. (c) Vertical tube superheater in a waste heat boiler.

Appendix B: Tube-Side Heat Transfer Coefficients and Pressure Drop 387

Gas in Water out

Water in

Out Water in

out

Water in Gas out

Superheater inlet

Superheater exit

(f )

Inlet

Outlet (g)

Gas ﬂow

FIGURE B.4 (Continued)

(d) Streams are a fraction of number of tubes wide. (e) Horizontal gas flow, horizontal tube superheater. (f) Horizontal gas flow, horizontal tube superheater. (g) Counter-flow staggered finned tube economizer.

(Continued)

388 Appendix B: Tube-Side Heat Transfer Coefficients and Pressure Drop

From desuperheater Steam drum Top header

Sat steam

Flue gas ﬂow Upper + SH header

+ Lower

Bottom header

SH header +

Mud drum (h)

To desuperheater

Main steam outlet Pass plate

w Gas ﬂo

Coil layout (i)

side elevation

FIGURE B.4 (Continued)

(h) Vertical tube horizontal gas flow superheater. (i) Horizontal gas flow, horizontal tube economizer.

Appendix B: Tube-Side Heat Transfer Coefficients and Pressure Drop 389

heat boilers, the flue gas flow will be quite small while the steam generation or water flowing in the economizer or condensate heater will be very large requiring multiple streams. Figure shows four streams (2 × 2).

6. Horizontal gas flow superheater with multiple tubes on each header : In this horizontal gas flow, horizontal tube superheater header (Figure B.4f), there are 2 tubes in each horizontal plane, and if there are 30 tubes along the header length, the number of streams is then 60. (Two starts on the header and 30 tubes across.) This could be arranged in parallel- or counter-flow fashion depending on steam and tube wall temperatures. If there are four starts on the header, then the streams will be 30 × 4 = 120.

7. Vertical gas flow counter-flow economizer : In this counter-flow economizer (or superheater) (Figure B.4g), there are 20 tubes/row and 20 streams in staggered arrangement of tubes.

8. Inverted loop superheater : This is a common arrangement in package boilers (Figure B.4h). We have an inlet and exit header with multiple starts. The number of streams is decided based on steam velocity, pressure drop, tube wall temperature considerations, gravity versus friction loss at low loads, and height of the tubes. At the steam inlet, we have 6 starts with 5 tubes along the gas flow for a total of

30 streams. There is a baffle in the header after the 10th tube, so the 30 streams can reverse and flow up and down again. This is a commonly used arrangement in package boilers. The steam from the desuperheater has only 24 streams for illus- tration purposes. Here we have a baffle after the eighth row in the header.

Note that due to tube wall temperature limitations, the first stage is counter- flow while the second stage is in parallel-flow. Thus, plant engineers should spend some time understanding their superheater flow configuration, which will help them evaluate its performance and those like this. As discussed in Chapter 3, at low loads, if the gravity loss in downward flow direction is larger than the steam pressure drop, then stagnation or reverse flow may occur. Hence, this is also a consideration for selecting the steam flow per tube or the number of streams.

9. Horizontal gas flow, horizontal tube economizer : In this economizer, the arrangement is uncommon (Figure B.4i). It has horizontal tubes and horizontal gas flow with

a two-pass design! Sometimes due to layout considerations, this arrangement is warranted. There are 18 streams (9 × 2) in staggered arrangement. The water flows down from top-to-bottom header and then from bottom-to-top header. There is a baffle in the top header. Note that the total length along gas flow direction is small. We may have any number of such passes depending on the duty.

Importance of Streams in Superheater, Economizer