Total Expenditure by main purpose of the trip

The firing range of an Forced draught burner is a representation in the Cartesian plan of an area, showing the pressure of the combustion chamber on the Y axis and the thermal output on the X axis; this area indicates working conditions under which the burner guarantees combustion corresponding to the thermo-technical requirements. The firing range is obtained referring to data gained from experimental trials, which are correct in a prudent sense.

Diagram 14 shows the representation of the firing range of a series of diesel oil-fired burners.

Quite often, the firing range of just one burner is not illustrated, but rather a whole series, as in the diagram above.

The output can be expressed in kW or in kg/h of fuel burnt, while the pressure is expressed in either mbars or in Pa.

The firing range is obtained in special test boilers according to methods established by European legislation, in particular:

• EN 267 standard for liquid fuel burners;

• EN 676 standard for gaseous fuel burners;

These standards establish the dimensions that the test combustion chamber must have.

Diagram 15 shows the graph indicating the dimensions of the test furnace for forced draught burners powered by liquid or fuel gas.

The graph represents the average dimensions of commercial boilers; if a burner is to operate in a combustion chamber with distinctly different dimensions, preliminary tests are advisable.

The firing range is determined experimentally under particular atmospheric pressure and combustion supporter air temperature test conditions. All the graphs showing the firing range for a forced draught burner must be accompanied by pressure and temperature indications, generally corresponding to a pressure of 1000 ⁽³⁾ mbar (100 m above sea level) and combustion supporter air temperature of 20°C.

If running conditions are considerably different from the test conditions, certain corrections must be made, as shown in chapter 3 of this manual.

Burners operating chances: a) one-stage, b) two-one-stage, c) progressive two-one-stage, d) modulating

Diagram 12

(3) Normal pressure at 100 m above sea level.

Start up

Start up 1st stage

Start up 1st stage

Start up Stop

Start up

Layout of two monobloc (RL and RS series) burners and dual bloc (TI) burner Diagram 13

RS series

TI series

RL series

FR P.E. cell GF Gas filter BP Pilot burner

V1,V2 Delivery oil valves PA Air pressure switch C2 Oil modulating cam PV Nozzzle holder PC Leak detection control device C3 Gas modulating cam

AD Air damper C Anti-vibrant joint D Gas distributor

M Air fan and pump motor PCV Gas pressure governor LPG Low pressure gas governor P Pump with oil filter and PG Minimum gas pressure switch MM Oil delivery gauge

pressure regulator

MT Two-stage hydraulic ram PGM Maximum gas pressure switch MR Oil return gauge

V Supply air fan RG Gas flow regulator (butterfly valve) PO Maximum oil pressure switch VS Gas safety valve C1 Air moudulating cam SI Ionisation probe

VTR Combustion head SM Cam’s servomotor VP Pilot vaves

regulation screw

U,U1,U2 Nozzles VR Gas regulation valve VU Nozzle’s safety valve

The burner should be chosen so that maximum required load falls within the burner's firing range. The firing point is found by tracing a vertical line in correspondence with the required output value and a horizontal line in correspondence with the pressure in the combustion chamber; the intersection point between the two lines is the system firing point, including the burner and the heat generator.

As far as the choice of single-stage burners is concerned, the firing point can be in any point of the burner's firing range.

For two-stage burners the firing range is ideally divided into two areas, left (zone A) and right (zone B) of the vertical line traced for the point corresponding to the maximum head available, as indicated in Diagram 16.

The firing point corresponding to the maximum output and, consequently, to operate in the 2nd stage, must be chosen within zone B.

Zone B provides the maximum output of the burner in relation to the combustion chamber pressure.

The 1st stage output should be chosen within the minimum/maximum declared formula and normally falls within zone A. The absolute lower limit corresponds to the minimum value of zone A. However, in certain cases, for example where the use of two-stage burners is required in domestic hot water boilers, it is advisable not to go below 60-65% of maximum output in the first stage, and, due to condensation problems, to maintain flue temperature around 170-180°C at maximum load and at 140°C at 65% of load.

As far as progressive or modulating two-stage burners are concerned, the burner should be chosen in a similar manner to two-stage burners. In modulating burners, the nearer the firing point is to the maximum output values of the firing range, the higher the modulating formula of the burner. The modulating formula is defined as the turn down ratio between the maximum output and the minimum output expressed in proportion (e.g. 3:1 or 5:1).

0 1 2 3 4 5 6 7

50 100 150 200 250 300 350 400 450 500 550 600

kW

Combustion chamber pressure (mbar)

RLS 50 RLS 38

RLS 28

Firing ranges of Riello RLS series dual fuel burners

Diagram 14

Test combustion chamber for burners Diagram 15

d = diameter of the flame tube Heat output (kW) X Lenght of the flame tube (m)Flame tube firing intensity (kW/m3)

The firing range in cartesian format can only be determined for monobloc forced draught burners, where the coupling of the combustion head with the fan is defined by the burner manufacturer. The situation changes for dual bloc burners, as the combination of the combustion head and the fan is delegated to the design engineer. In this case, the firing range is characteristic only for the combustion head and determined in relation to the maximum and minimum fuel output allowed to the head itself.

For example Diagram 17 shows the firing ranges for combustion heads in the Riello TI Series Burners, where the darker area represents the range of optimum choice

recommended by the manufacturer.

The choice regarding the size of the combustion heads should be made solely in relation to the output and the temperature of the combustion supporter air.

2.3 TYPICAL SYSTEM LAYOUT