Heat accumulator in a solid fuel boiler: calculation formula

Теплоаккумулятор в твердотопливном котле

In Ukraine, not all regions can boast of gasification. In many areas there is no gas pipeline, and electricity is supplied with frequent interruptions. In such conditions, for a comfortable home environment, the optimal solution would be installation of a solid fuel boiler. It is a metal heating device that generates thermal energy by burning solid fuels. Its operation differs from electric and gas boilers:


  1. All standard steel and cast iron boilers with volumetric combustion of fuel provide their passport efficiency in the maximum combustion mode. Since the power of the boiler is selected according to the peak heat losses, and they fall only for an insignificant period of the heating season, we have a situation when the boiler operates in incomplete power mode for most of the heating season. It is not recommended to reduce the power by less than 50-60%, which leads to excessive consumption of fuel, clogging of the furnace and chimney.
  2. The need for periodic reloading of fuel into the boiler. Most standard boilers have small combustion chambers, so the fuel loading (again, it concerns firewood) burns out in 2-3 hours. Bituminous coal burns out much longer, but it is more expensive, so wood is the leader in popularity.
  3. Today, most heating systems are designed closed, with forced circulation of the coolant. The risk of exploitation increases. In case of power outages, the circulation pump may stop or the coolant boil.


That is, safe, comfortable and economical operation is what a heat accumulator is needed for in a heating system. A buffer or heat accumulator for a solid fuel boiler is a non-enamelled metal heat-insulated container with connecting pipes for the heating system and boiler circuits.


The buffer tank for a solid fuel boiler at maximum operating power accumulates heat, and then, as required, releases it into the heating system. 1 and 2, the differences in operation are thus completely smoothed out, since no heat is lost and the fuel burns efficiently. It is extremely important to correctly calculate the buffer capacity.


Today you can see many methods for calculating storage tanks. For example, 25 or 50 liters of tank volume per 1 kW of boiler power. However, such calculations do not take into account either the temperature regime of the heating system, or the fuel used in the boilers, and, most importantly, there is no definition of the function of the tank – heat accumulation or other purposes.


So, we have a small residential building with a heated area of ​​150 sq.m. Estimated heat loss 10 kW (insulation according to modern standards). Heating radiator, temperature 70/50. We have a standard solid fuel boiler – cast iron Demrad Solidmaster 5S with a wood-fired thermal power of 23 kW. The volume of the furnace is 70 liters, the amount of loaded firewood, taking into account the density of oak firewood, is 600 kg per 1 cubic meter. and the fuel loading factor of 0.5 is: 70 x 0.6 x 0.5 = 21 kg. The calorific value of wood (dry oak), taking into account the boiler efficiency, is 3 kW per 1 kg. Thus, a one-time fuel loading will give out 21×3 = 63 kW. The fuel bookmark will burn out in 63/23 = 2.7 hours, let’s round up to 3 hours.


With a heat demand of 10 kW (peak heat loss), out of 23 kW of rated power, 10 kW is transferred to the system, and 13 kW must be stored. For 3 hours of bookmark operation, the system will consume 10×3 = 30 kW, and will store 13×3 = 39 kW, which the system will consume after the bookmark burns out for another 39/10 = 3.9, that is, almost 4 hours. When using a heat accumulator, the fuel fill will be enough for 3 + 4 = 7 hours of continuous operation.


To store 39 kW of heat, it is necessary to calculate the volume of the tank, taking into account the temperature regime of the system. The temperature in the heat accumulator cannot exceed + 95C, and the supply temperature to the system + 70C. We have a temperature reserve of 95-70 = 25C.


Heat accumulator volume = 39000 W / (1.163×25) = 1334 liters. This value is the minimum required. It can be rounded up to 1500 liters.


With an increase in the outside temperature and, accordingly, a decrease in heat demand, the operating time from charging the heat accumulator increases. Since at outdoor air temperatures of -22C and 0C, the value of heat loss differs by 2 times, then, accordingly, at a near-zero temperature, the heat supply will be enough for 7×2 = 14 hours.


At zero temperatures, the heat demand will be about 5 kW. Then the tank will store (23-5) x 3 = 54 kW of heat, which is enough to provide heat for 54/5 = 10-11 hours. The stored power increases and the temperature reserve increases.


A heating system with a buffer tank for proper functioning requires:


  1. Calculation of the amount of heat for the power reserve depending on the duration of operation.
  2. Determination of the temperature graph of the heating system.
  3. Determination of the level of heat loss in the room.


Calculate the volume of the buffer tank based on these three points. Because the power of the boiler itself is selected specifically for the volume of the buffer, and not for heat loss. With the correct scheme, the buffer should be charged up to a two-fold power reserve in one or two boiler firing.


In our case, a 23 kW boiler will provide the initial heating of the battery (at room temperature of the coolant) in (1500x (95-20) x1.163) / 23000 = 5.5 hours, that is, in almost 2 heating. In the future, during operation, the lower temperature of the coolant is unlikely to fall below 40C, and therefore the tank will heat up in one or one and a half firing, which fully meets the specified requirements.


Thus, our boiler operates with maximum efficiency throughout the entire heating season and provides quite comfortable operating conditions with a one-time boiler load operating time from 7 to 14 hours. On the face of it, the thermal accumulator performs its functions in full.


The storage tank with the “buffer tank” function means, as a rule, the protection of the solid fuel boiler from accidents associated with the boiling of the coolant. The buffer tank has various options for installation in heating systems.


The appropriate installation option is selected individually, depending on the layout of the house and other factors. It is possible to install a buffer tank in series boiler-tank-heating system as a heat accumulator. It is also possible to connect the boiler directly to the heating system, then the heat will accumulate in the buffer through the switching valves.


The buffers also serve to reduce the frequency of switching on and off (cycle) of the boiler burner.


As for calculating the volume of buffer tanks, you can find 2 options:


  • Just as with heat accumulators, a certain amount of coolant per 1 kW of boiler power.
  • The calculation formula, which is given in the documentation for many foreign boilers:


Buffer volume = 15 х Н х Qк х (1-0,3 х (Qп/Qmin), where:


Н – boiler burning time at rated power, h.

Qк – rated thermal power of the boiler, kW.

Qп – heat loss of the heated room. kW

Qmin – minimum heat output of the boiler. kW


Example. The same solid fuel boiler Demrad Solidmaster 5S with a wood-fired power of 23 kW. Burning time of the bookmark is 3 hours. The minimum boiler power, taking into account the small adjustment range, will be taken as 20 kW. The heat loss of the room is 20 kW, that is, the boiler is matched exactly to the nominal value. We have the following calculation:

Buffer volume = 15 х 3 х 23 х (1-0,3 х (20/20)) = 724 liters. Round up to 750 liters.


The formula allows you to calculate the protection against boiling, since it takes into account the power of the boiler and the duration of burning the bookmark in the maximum mode.


The examples considered give the definition of the main parameters for choosing the required volume of the tank и объясняют, по каким принципам его рассчитывают. Перед установкой данного типа теплового оборудования следует детально изучить все факторы и условия его эксплуатации.


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