Tag Archive for: turn down ratio

Turndown Ratio (TDR) in Cooking and Heating Stoves

/0 Comments/in

https://www.inchcalculator.com/wp-content/uploads/2016/06/heating-btu-climate-zones-650x488.jpg

A cook stove can require a ~three to one turndown ratio to boil quickly and then not burn rice, tomato sauce, etc. High power boils food, ~one third of the high firepower simmers it to completion.  With a lid, even lower amounts of energy can maintain simmering temperatures. When the stove cannot turn down sufficiently the pot with lid boils constantly.

An adequate turndown ratio is necessary in heating stoves as well. According to the map above, a stove needs a 2 to 1 TDR (30BTU per sq. ft. in Zone 1 up to 60BTU in Zone 5) to respond to changes in climate. Personal temperature preferences, house sizes, heat loss per hour, etc. also vary. A nationally sold stove needs a wide TDR to keep everybody at desired temperatures during cold seasons.

Traditional gas furnaces only operate at high power but for short amounts of time. A thermostat turns the heater on and off fairly frequently. Since pellet and log burning biomass-heating stoves do not easily turn off and on, they need to deliver adjustable high and low power. Then the house is not too cold or too warm. ARC tries to provide a three to one turndown ratio in cooking stoves. Heating stoves may require a wider range.

A chef cooks at a stove with a flaming wok

Turn Down Ratio: Cooking and Heating Stoves

/0 Comments/in ,

The successful stove delivers the needed amount of heat to perform a task. In Haiti, our little charcoal stove could not bring the big pots of rice and beans to boil although the thermal efficiency at low power (simmering) was above 40%.

In the same way, a tight, well-insulated house requires a low amount of heat to stay warm. A “leaky” house needs higher firepowers to replace the constant flow of hot air lost through cracks under doors, etc.

Generally, a good stove has a minimum three to one turn down ratio. Heating stove experts have suggested a high power of 5 pounds burned per hour and a low power of 1.5 pounds. If a heating stove cannot turn down sufficiently, the tight, well-insulated house gets too warm. On the other hand, a “leaky’ house needs a big fire.  To save fuel, a tight house is more important than a new stove.

To boil 5 liters of water in less than 25 minutes in an uncovered pot, low mass cook stoves with tight pot skirts typically need a high power between 3kW and 2.5kW. However, firepower is often a lot higher when cooks are trying to get food on the table. In our experience, many cooks prefer ~ 5kW. 

Experiments at ARC have shown that with a low mass stove, lid, tight skirt on a 5 liter pot, it takes only ~0.4 kW to maintain a 97°C simmering temperature. But, cooking requirements vary a lot from country to country. Chinese cookstoves tend to use 10-15kW and may not need low power. 

Village cooks in Southern India cooked with many small pots and often did not bring the water to a full boil. For India, Dr. K. K. Prasad proposed, “…an ideal burner design with the power output ranging from 2.64 kW to 0.44 kW… (Prasad and Sangen, 1983, pp. 108-109). Dr. Baldwin adds, “One of the most important factors determining field performance of a stove is the firepower it is run at during the simmering phase. Because simmering times tend to be long, quite modest increases in firepower above the minimum needed can greatly increase fuel consumption.” (Baldwin, 1987)

With careful operation, the heat exchanger efficiency of houses and pots combined with delivering the appropriate firepower largely determines the fuel used per task.