ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
330
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. The terms Btuh (Btu per
hour) and MBtuh or MBH (thousand Btuh) indicate boiler input
rate. Input ratings are usually shown on the boiler (or burner)
nameplate. The terms hp (boiler horse power), EDR (equivalent
direct radiation), and pounds per hour (steam) indicate boiler
output rate.
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.
A testing procedure issued by the US Department of Energy in
1978 measures both on cycle and off cycle losses based on a
laboratory procedure involving cyclic conditions. This result is
called the AFUE (Annual Fuel Utilization Efficiency) rating or
seasonal efficiency, which is lower than gross efficiency.
FLUE GAS:
6% OXYGEN
580°F STACK
TEMPERATURE
80% EFFICIENCY
STEAM BOILER
STACK
DUAL
BURNERS
OIL
GAS
15 PSIG
STEAM
METER
INPUT:
NATURAL GAS: 1923 CFH AT 1040 BTU/FT
3
2,000,000 BTUH
2,000 MBTUH
#2 OIL: 14.3 GPH
OUTPUT:
MBTUH = 1507
STEAM: POUNDS PER HOUR
HORSEPOWER:
1,507,000 BTUH
33479 BTUH/HP
1,507,000 BTUH
1000 BTU/LB
= 45 HP
= 1507 LB/HR
EDR (EQUIVALENT DIRECT RADIATION):
1,507,320 ÷ 240 BTUH/EDR = 6279 EDR
GROSS EFFFICIENCY =
x 100 = 75.35 ≈ 75%
1,507,000
2,000,000
C2904
OUTPUT
INPUT
=
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