[Stoves] Tilted Grate for flat-bottomed stoves

Thu Dec 19 15:59:44 CST 2013

Dear Friends

Rebecca and others requested some pictures of the grate developed to improve the heat transfer efficiency and combustion efficiency and reduce the fuel consumption of the traditional Keren Stove from Central Java, Indonesia.

First, here is the stove:

It is a simple clay shell with a too-large door. The sheet metal wired around it is to lower the top of the fuel door. It was later lowered quite a bit more.

It is sitting on a heat reflecting platform which is itself bolted to the scale. This is the system used at the YDD lab in Yogyakarta to prevent the stoves damaging the scale that sits under the whole system during tests. The scale is read by a computer every 10 seconds.

This is the same stove with the grate in it:

This is a cast iron grate and it was not as effective as the one made from round reinforcing bars. It may have to do with the spaces being too small and the top flat – we are still investigating. But you will get the idea.

Here is the grate shown upside down. You can see that the front legs are shorter than the back ones. 

The legs were made the same length and then cut so we could experiment with the tilt.

The left side is the back:

There is a re-bar version which was the proof that the concept is significantly better than letting the fire burn on a flat floor:

The left one is what NOT to make. The gap between the bars is too large. The right side has 12mm round bars and 5mm gaps. That is what works.

The grate is made in two pieces so it can be added to stoves that have a larger hole inside then the front entrance of top hole. A 20 x 40mm hole is opened at the back of the chamber right at the bottom. This allows air to get under the grate – critical to burning the char normally produced in a fire burning on a floor. The ash chokes the combustion and the char smoulders instead of burning. 

The air provided at the back to some extend prevents the inrushing air that comes through the fuel entrance to be pushed back to the centre, though the effect is not as pronounced as in a Lion Stove.

It is important to lower the pot to a clearance of about 7 or 8 mm from the stove body. This chokes the airflow to a point at which there is enough for a 6-8 kW fire, but not more. 

The combination increased the heat transfer efficiency from about 20% to 33% (consistently) and reduced the fuel consumption as well by burning virtually all the char produced on a continuous basis.

Such a grate with 12/5/12/5/12 bars and gaps can be added to any stove. It is particularly effective on stoves that have a flat floor that the fire is sitting on, meaning virtually all the clay stoves in Asia. The final version of the improved Keren stove has about ½ the fuel hole height of the original, with all of that change on the top side.

Regards

Crispin

With the WB-CSI Indonesia Stove Programme

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