Electric Boilers; Boiler Ratings And Efficiency - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
Some modular boilers have very small storage capacity and
very rapid heat transfer so water flow must be proven before
the burner is started.
FLUE GAS
CLOSE-OFF
HEAT
EXCHANGER
FLUE GAS
COLLECTOR
BIOLER
SECTIONS
BURNER
Fig. 39. High Efficiency Modular Boiler.

ELECTRIC BOILERS

Electric boilers heat water or produce steam by converting
electrical energy to heat using either resistance elements or
electrodes. Electric boilers are considered to be 100 percent
efficient since all power consumed, directly produces hot water
or steam. Heat losses through the jacket and insulation are
negligible and there is no flue.
Electrode boilers (Fig. 40) have electrodes immersed in the
water. Current passes through the water between electrodes
and the resistance of the water generates heat. Electrode boilers
are available in sizes up to 11,000 kW. Resistance boilers have
the resistance (heating) elements immersed in but electrically
insulated from the water and are manufactured in sizes up to
3000 kW.
SIDE
OUTLET
BLOWER
ASSEMBLY
M15055
FLUE GAS:
STACK
6% OXYGEN
300 C STACK
TEMPERATURE
80% EFFICIENCY
STEAM BOILER
OIL DUAL
BURNERS
GAS
INPUT:
NATURAL GAS: 15.8 L/s AT 37 kJ/L
586 kW
#2 OIL: 0.9 L/s
GROSS EFFICIENCY = OUTPUT = 441 kW
INPUT
Fig. 41. Boiler Ratings and Efficiency.
Electric elements and electrodes are grouped to provide four
or more stages of heating. A step controller responding to steam
pressure or hot water temperature activates each stage of heating
as required to heat the building.
GENERATING
CHAMBER
REGULATING
CHAMBER
WATER
LEVEL
ELECTRODES
Fig. 40. Electrode Steam boiler.

BOILER RATINGS AND EFFICIENCY

Boilers can be rated in several ways. Figure 41 shows
commonly used ratings and terms. Boiler input rate is in
kilowatts (kW). Input ratings are usually shown on the boiler
(or burner) nameplate. The boiler output rate is given in grams
per second (g/s).
Gross efficiency is output (steam or water heat content and
volume) divided by fuel input (measured by a fuel meter at
steady-state firing conditions). The efficiency as indicated by
flue gas conditions does not take into account jacket and piping
losses so is usually higher than the gross efficiency.
200 kPa
STEAM
METER
OUTPUT:
kW= 441.5
GJ/h= 1.589
STEAM: KILOGRAMS PER HOUR OR
GRAMS PER SECOND
441 389 J/s
=
0.1898 kg/s
2 325 800 J/kg
=
190 g/s (683 kg/h)
POWER: 441 kW
X 100 = 75.26 75%
586 kW
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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C2901
C4089