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close this book Boiling Point No. 31 - August 1993
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Research & Development

Chimney Design

Update of an article by M Crowther in BP 28 page 3 "Chimneys and Hoods for Smoke Removal". Since publication of Crowther's article about the design of hoods and chimneys and his theoretical analysis, Peter Young of ITDG has used the mathematical model to produce a chart for the design of chimneys.

For a hood with a chimney to work without spilling smoke into the room it must not be too short or too small in diameter and there must be sufficient heat released from the fire to produce an effective draught. The design calculations are complex and to assist in this process a computer

model has been developed by M E Crowther,

Coal Research Establishment.

With the aid of this model a chart has been constructed to show the hood face area required to remove the smoke from an open fireplace given varying heat outputs and with different chimney heights.

Ed Note: For effective hood and chimney design M E Crowther recommends that the minimum draught should not be less than 0.5mm of Water Gauge. Chimney draughts less than 05mmWG are likely to be affected by a gentle breeze and smoke will spill into the room.

Conclusions on Design

The bold shaded parts on table 1 represent a reasonably safe working area, ie the chimney is likely to work in a gentle breeze. The italic area represents a very marginal performance and is not advisable. The general principle to bear in mind is: the taller the chimney and the more heat that is released the better the chimney will function. For the conditions specified, chimneys below 4m and with less than 3kW of heat released will not be reliable.

Table 1 Specific Conditions

Chimney Dia: 200mm, Velocity of air across the face: 0.2m/sec, Ambient Air Tamp: 28C

Table 1- Hood Face Areas in Square Metres

Sensible Heat

Chimney Heights

 

2.5m

3.0m

3.5m

4.0m

4.5m

5.0m

 

2.0kW

Face area(sqm)

0.269

0.278

0.288

0.299

0.310

0.310

 

Draught mm/wg

0.263

0.306

0.345

0.382

0.415

0.460

2.5kW

Face area (sqm)

0.288

0.296

0.306

0.315

0.325

0.336

 

Draught mm/wg

0.302

0.353

0.405

0.445

0.487

0,525

3.0kW

Face area (sqm)

0.302

0.318

0.327

0.336

0.346

0.356

Draught mm/wg

0.340

0.390

0.444

0.495

0.543

0.588

3.0kW

Face area (sqm)

0.313

0.328

0.344

0.353

0.362

0.371

 

Draught mm/kg

0.377

0.435

0.486

0.544

0.589

0.649

4.0kW

Face area (sqm)

0.329

0.343

0.348

0.366

0.375

0.384

Draught mm/wg

0.406

0.470

0.591

0.591

0.652

0.709