Draft readings

Much of this work also had to be done at home, which took away precious time from our families. Their love, understanding, patience and support was indispensable for us to carry on with the seemingly endless series of chapters to be drafted, read or revised. 

Draft readings are a comparison of two pressures taken at the same elevation and are traditionally quoted in inches of water gauge. Whereas it is true that if you open an inspection port on a heater in an area of positive draft, you are liable to singe your eyebrows if you wish to really study or make sense of a set of draft readings for a furnace, you must first normalize the data. 

In Fig. 20.6, we see a simple natural-draft heater with no convective-section tubes. The laws of hydraulics tell us that fluids flow from regions of high pressure to regions of lower pressure, and yet the draft readings in Fig. 20.6 seem to contradict this principle. 

Notice that the draft readings are made at different elevations. Each measurement is in reality a comparison between the densities of the gas both inside and outside the furnace at a given elevation. The temperature difference is the main reason for difference in density inside and outside the furnace because the molecular weights of the furnace gases and of air are approximately the same. 

For each draft reading, add on the pressure exerted by the appropriate static head of air. 

Imagine Yourself to Be One of Your Audience After writing the first draft, read the report as... 

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