ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
328
Fig. 41. Boiler Ratings and Efficiency.
Some modular boilers have very small storage capacity and
very rapid heat transfer so water flow must be proven before
the burner is started.
BIOLER
SECTIONS
BURNER
M15055
FLUE GAS
COLLECTOR
HEAT
EXCHANGER
FLUE GAS
CLOSE-OFF
SIDE
OUTLET
BLOWER
ASSEMBLY
GENERATING
CHAMBER
REGULATING
CHAMBER
WATER
LEVEL
ELECTRODES
C2901
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.
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.
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.
FLUE GAS:
6% OXYGEN
300°C STACK
TEMPERATURE
80% EFFICIENCY
STEAM BOILER
STACK
DUAL
BURNERS
OIL
GAS
200 kPa
STEAM
METER
INPUT:
NATURAL GAS: 15.8 L/s AT 37 kJ/L
586 kW
#2 OIL: 0.9 L/s
OUTPUT:
kW= 441.5
GJ/h= 1.589
STEAM: KILOGRAMS PER HOUR OR
GRAMS PER SECOND
C4089
441 389 J/s
2 325 800 J/kg
POWER: 441 kW
=
=
0.1898 kg/s
190 g/s (683 kg/h)
GROSS EFFICIENCY = OUTPUT = 441 kW
X 100 = 75.26 ≈ 75%
INPUT 586 kW
To Next Page
Loading...
Need help?
General
