General characteristics of the
circulation pump When choosing a pump, the following characteristics should be considered:
- The circulation pump used for the heating system must withstand high temperatures( from 100 ° C).
- Head;
- Power;
- Operating flow.
And it also determines the amount of heat required for heating.
Calculation of the required power for the room
In order to determine the required amount of heat, it is necessary to know the area of the heated room. Knowing this indicator, you can refer to the standards used in the area of residence. In European countries, calculations are carried out at a rate of 100 watts per 1 m2 of the house, if there are 1-2 apartments in it, and 70 watts if it is a multi-unit house. If the room is insulated, the indicator is reduced to 30-50 W.
In the CIS countries, according to SNiP 2.04.07-86 *, it is necessary to adhere to such indicators:
- for buildings less than 3 floors - 173 W per square meter at a temperature of -25 ° C and 177 W per square meter at -30 °FROM.
- For buildings with more than 3 floors - 97 and 101 watts per square meter, respectively.
Calculating the pump's performance
To calculate the pump's output, you need to calculate its flow. There are two formulas for this. The first one is used for designing in European countries, and the second one is used in the CIS countries and is prescribed in SNiP 2.04.05-91.
P = Q /( 1.16 x ΔT)( kg / h),
where ΔT is the difference between the supply and return lines( for two-pipe wiring, if the layout is single-pipe, it is taken at 20 ° C)
1.16 -the specific heat for water, if water is not used as a coolant in the system, the value of 1.16 is replaced by the corresponding index.
P = 3.6 x Q /( cx ΔT)( kg / h),
where c is the specific heat of the substance( if water is used, we take 4.2 kJ / kg * ° C)
Calculation of pressure
ToThe coolant effectively circulates through the system, it is necessary to provide a pressure capable of fighting the hydraulic resistance.
To calculate this parameter, you need to select the most distant point from the pump in the system( far radiator).
For the network design it is possible to use the following formula:
J =( F + R x L) / p x g( m)
where:
L is the length of the section( m),
R is the water resistance in the straight pipe section( Pa / m),
p - density of the working fluid( kg / m3),
F - resistance of sewage fittings( Pa),
g - acceleration at free fall( m / s2).
These figures can be found in the catalogs and accompanying technical literature.
You can use a simplified technique, approximately estimating the resistance.
During the experiments, the following data were obtained:
- resistance in straight sections of the pipeline 105-150 Pa / m;
- in each fitting an additional 30% to losses in the straight pipe;
- thermostatic valve - 70% of the losses;
- mixer, - 20%.
The easiest way of computing was proposed by E. Busher and K. Walter.
J = R x L x k
where k is the coefficient responsible for the increased load.
It is generally accepted that in systems without complex structures and fittings, k = 1.3.If there are valves, k = 2.2.In the presence of valves and valves simultaneously - k = 2,6.
Selection of the pump model based on the calculations
In order to properly select the pump you need to make a diagram with the coordinates P and J, note the results obtained in the calculations and find the point of their intersection.
Then, in the catalogs and accompanying technical literature, choose the most satisfactory pump model.
This view has an incorrectly selected model:
And this is how it should be:
The values that have been calculated must match the characteristics of the pump as precisely as possible. If the characteristics of the pump are higher, then it will work idle, consuming excess electricity.
Checking the calculation of the characteristics of the pump for heating the house: an example
To understand how to check the calculations, let's try to figure out an example performed for all requirements of SNiP.
In the example, the calculation is carried out for a two-pipe heating system with forced circulation.
The required amount of heat was calculated - 45.6 kW.and the coolant flow rate is 2.02 m3 / h.
Also under the conditions we have a heat regulating valve and 4 sections with pipes.
All losses are supplemented by a factor of unaccounted losses( 10%).
H =( 0,141 + 0,29 + 0,63 + 0,11) * 1,1 = 1,295 m
Based on the calculation, it follows that a pump suitable for a given heating system must have the following characteristics:
Flow rate -2.02 m3 / h;
Head - 1,295 m.
Next we plot the chart and select the most suitable model.
When choosing a model, it should be borne in mind that a reserve of 1-5% is included in the calculation.
