Method for controlling cascade boiler system

1. A method for controlling a cascade boiler system comprising a primary side including a plurality of boilers, a secondary side including a load, and a hydro-separator disposed between the primary side and the secondary side to correct a flow rate, the method comprising:

a) operating the number of boilers set in an initial operation state;

b) detecting a supply water temperature and a returned water temperature of the primary side of the hydro-separator and a supply water temperature and a returned water temperature of the secondary side, and calculating a flow rate corrected by the hydro-separator using the detected temperatures;

c) calculating a set temperature serving as the supply water temperature of the primary side that is able to maintain the supply water temperature of the secondary side when the supply water temperature of the secondary side is within a set range of a target temperature while maintaining the initial operation state; and

d) calculating the number of boilers that are able to maintain the calculated set temperature, and controlling an operation of the boilers according to the number.

2. The method for controlling the cascade boiler system according to claim 1 , wherein, after d), whether an event of varying an operation condition occurs is determined, and when the event occurs, the number of boilers that are operating is adjusted according to a type of the event.

3. The method for controlling the cascade boiler system according to claim 1 , wherein, in a), all of the plurality of boilers are operated to reduce a time that the supply water temperature of the secondary side reaches the target temperature.

4. The method for controlling the cascade boiler system according to claim 1 , wherein a supplementary flow rate of the hydro-separator calculated in b) is calculated by the following Equation 2,

F 3=( F 1×( T 1− T 3))÷( T 3− T 4)  Equation 2

wherein F 3 is a supplementary flow rate of the hydro-separator, T 1 is a supply water temperature of the primary side, T 3 is a supply water temperature of the secondary side, T 4 is a returned water temperature of the secondary side, and F 1 is a flow rate of the primary side that is a sum of pump capacities of the boilers.

5. The method for controlling the cascade boiler system according to claim 1 , wherein the set temperature calculated in c) is calculated by the following Equation 3,

T 1 n=T 3+(( F 3÷ F 1)×( T 3− T 4))  Equation 3

wherein T 1 n is a set temperature, T 3 is a supply water temperature of the secondary side, F 1 is a flow rate of the primary side, F 3 is a supplementary flow rate of the hydro-separator, and T 4 is a returned water temperature of the secondary side that is equal to a supplementary water temperature of the hydro-separator.

6. The method for controlling the cascade boiler system according to claim 5 , wherein, in d), the number of boilers operated to maintain the set temperature serving as the supply water temperature of the primary side is calculated by substituting a value obtained by multiplying a flow rate of one boiler that is constant and the number of boilers that are operating with the flow rate F 1 of Equation 3.

7. The method for controlling the cascade boiler system according to claim 2 , wherein, in the event, a sum of maximum capacities of the boilers that are currently operating is smaller than the set temperature, or

a sum of minimum capacities of the boilers that are currently operating is larger than the set temperature.