The following table gives the draught area and heating surface of the various-sized boiler tubes and flues:

External Diameter.

Draught Area in Square Inches.

Draught Area in Square Feet.

Outside Heating

Surface in Feet per Foot of Tube in Length.

Number of

Tubes in One

Square Foot of

Draught Area.

5/8..............

.1636

..............

.1963

1..............

.575

.0040

.2618

250.0

1..............

.968

.0067

.3272

149.3

1..............

1.389

.00964

.3927

103.7

1..............

1.911

.0133

.4581

75.2

2..............

2.573

.0179

.5236

55.9

2..............

3.333

.0231

.5891

43.3

2..............

4.083

.0284

.6545

35.2

2..............

5.027

.0349

.7200

28.7

3..............

6.070

.0422

.7854

23.7

3..............

7.116

.0494

.8508

20.2

3..............

8.347

.0580

.9163

17.2

3................

9.676

.0672

.9818

14.9

4..............

10.93

.0759

1.0472

13.2

4..............

14.05

.0996

1.1781

10.2

5..............

17.35

.1205

1.3090

8.3

6..............

25.25

.1753

1.5708

5.7

7..............

34.94

.2426

1.8326

4.1

8..............

46.20

.3208

2.0944

3.1

9..............

58.63

.4072

2.3562

2.5

10..............

72.23

.5016

2.6180

2.0