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The Grate Diameter

The property that determines the grate diameter is the combustion intensity in units of W/cm2. A tentative suggestion is due to Verhaart (1981). On the basis of a few experiments, he concluded that combustion intensity or power flux, p, for stoves is

with chimney         p = 50 W/cm2

without chimney         p = 10 - 15 W/cm2

Since that time, Sangen (1983) undertook a systematic compilation of all the experimental work done by the Woodburning Stove Group at Eindhoven and Apeldoorn. His work has been updated and presented in Table 1. The table lists two quantities one based upon the grate area and the other based upon the combustion volume. We will consider the latter in the following section. There is no obvious figure that one could recommend from the list above. Verhaart's recommendations seem too optimistic for the chimney stoves and too pessimistic for the non-chimney stove. A second point about the stoves compared is that several of them have no grates. While we pointed out that the combustion intensity can be higher for an open fire with grate , the data in the above table do not necessarily support that finding. For example Tungku Lowon and Shielded fire have the same combustion intensity. The last point in this connection is the combustion quality. The combustion intensity needs to be correlated with it and the above table does not include that information. The whole area requires much more careful research that should include more carefully the fuel specification.

Table 1: Experimental Data on Combustion Intensities
Design Type Combustion Intensity
  W/cm2 l/kW
With Chimney
Experimental Stove 19.8 0.40
Delepeleire -VanDaele stove 481 0.04
Family Cooker 49.2 0.22
Nouna Stove* 9.6 1.03
Heavy Experimental Stove 19.2 0.63
Without Chimney
Tungku Lowon* 27.5 0.65
Cylindrical Combustion chamber 52.7 0.65
Shielded Fire 27.5 0.44
Open fire 31.4 0.39
* No grate

In the absence of more reliable data we can do no better than using the following figures as the starting point for the design process. The designer has to verify through experiment and modify the design if necessary.

With Chimney                 p = 40 W/cm2

Without Chimney          p = 20 W/cm2

Thus the area of the grate is given by


Where the values of p from Eqs.(1) have to be used according to whether the stove has a chimney or not.