Electric Boilers; Boiler Ratings And Efficiency - Honeywell AUTOMATIC CONTROL 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.
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.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
SIDE
OUTLET
BLOWER
ASSEMBLY
M15055
STACK
FLUE GAS:
6% OXYGEN
580°F STACK
TEMPERATURE
80% EFFICIENCY
STEAM BOILER
OIL DUAL
BURNERS
GAS
INPUT:
NATURAL GAS: 1923 CFH AT 1040 BTU/FT
2,000,000 BTUH
2,000 MBTUH
#2 OIL: 14.3 GPH
Fig. 41. Boiler Ratings and Efficiency.
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. 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.
15 PSIG
STEAM
METER
OUTPUT:
MBTUH = 1507
STEAM: POUNDS PER HOUR
1,507,000 BTUH
= 1507 LB/HR
1000 BTU/LB
HORSEPOWER:
1,507,000 BTUH
= 45 HP
33479 BTUH/HP
EDR (EQUIVALENT DIRECT RADIATION):
1,507,320 ÷ 240 BTUH/EDR = 6279 EDR
3
OUTPUT
GROSS EFFFICIENCY =
INPUT
1,507,000
= x 100 = 75.35 ≈ 75%
2,000,000
330
C2901
C2904