| Boiling Point No. 29 - December 1992 |
by Liu Hongpeng, Bureau of Energy and EP, Ministry of Agriculture, No. I 1, Nongzhanguan Nanli, Beijing 100026, People's Republic of CHINA
What Kang Lianzao is and its general structure
A bed connected with a cooking stove is called Kang-Lianzao and can help keep people warm when they are sleeping or resting on the bed by using the surplus heat' from the stove. It is used in northern areas of China where the temperature is lower than that in other areas in winter. It is not only the main kind of house facility for sleeping in northern, rural areas but also the cooking appliance of daily use.
The overall heat efficiency of the traditional Kang-Lianzo is only about 40%, so people always use more fuel to maintain the room at a comfortable temperature and so waste fuel.
Some problems of the traditional Kang-Lianzao:
1. Temperature unwell distributed - the high temperature smoke from the stove is detained in the front part of the Kang because of the dividing walls and the soot/ ash deposits. The surface temperature of Kang at the stove end is higher than at the other end. The temperature can reach 90°C in the hottest areas and even destroy the bedding.
2. Waste heat - because the smoke moves in narrow, straight channels, the rate of smoke flow is too quick and it does not have enough time to transfer heat to the Kang. The smoke in chimney still remains high temperature.
3. Low heat capacity of Kang body - this results in waste heat and too high temperatures.
Since 1985 the local governments have responded to the national conservation and rural energy construction policy to implement a new technique line for both heating and cooking. To promote heat efficiency, industrialized production and commercial distribution were supported by new Kang-Lianzao social popularization.
The traditional Kang Lianzao stove used fuel with a heat efficiency of 10-15% and without a grate fuel could not be burnt completely. On the other hand, it requires more hot smoke flow into kang to heat the bed. The improved stoves have grates with reduced combustion chamber and controlled air inlet and passage to kang. An area ratio of 1:1 for bars and gaps in grate is strongly recommended to ensure adequate primary air.
The traditional kang was installed directly on the ground and so radiated only from the top surface. The suspended kang is installed above the ground and so radiates from top and bottom surfaces.
It can increase the room temperature by 4-50C. The heat comes from smoke flow of the stove and depends on the exchange time and the heat transfer area. The heat transfer time depends on the velocity of smoke flow. When normal burning starts, the smoke flow velocity reaches 3m/sec in the channel. The high temperature smoke flows into the kang, spreads out and its velocity drops quickly to about 0.1 m/sec after 1-1.5 metres in the bed. Due to turbulence the smoke fills the spaces and has time to transfer its heat.
Construction Material to Hold Heat
The temperature of the top surface of the kang should be 25-300C. The temperature increase from one fuel burning is 8- 10°C. Thus the construction materials must be good heat absorbers and conductors, have mechanical strength and long life and be available at affordable prices. Concrete is the first choice for the kang top followed by clay adobe.