Page 44 - Boiler plant and distribution system optimization manual

Energy Management Basics for Boilers and Systems
tion temperature. This is usually done by apply-
ing heat to steam after it has been removed from
contact with the water.
This phenomena also occurs when steam
pressure is reduced by a pressure reducing valve
in the steam distribution system.
ACCOUNTING FOR HEAT IN
STEAM AND WATER
The heat content of water and steam is ex-
pressed in British thermal units (Btu) per pound
and is known as Enthalpy expresses by the sym-
bol “h” in tables and formulas.
The “Steam Tables” contain the basic informa- carbon dioxide which can cause severe pitting and
tion on the energy properties of steam. The infor- corrosion to the boiler and piping systems.
mation in this book was obtained from works of
Keenan and Keys “Thermodynamic Properties of
Steam” published by John Wiley and Sons, Inc.
Enthalpy of Saturated Liquid (h ) is the
fg
heat required to rise the temperature of one pound
of water from 32°F to the saturation temperature.
This property is sometimes known as the Heat of
the Liquid.
Enthalpy of Evaporation (h ) is the amount
fg
of heat of heat required to change one pound of
water at the saturation temperature to dry satu-
rated steam at the same temperature. This is also
known as the Latent Heat of Evaporation.
Enthalpy of Saturated vapor (h,) is the heat
required to change one pound of water at 32 F
into dry saturated steam. It is the sum of the en- It is a sure sign that steam traps are leaking steam.
thalpies of saturated water and evaporation. It is
also known as the Total Heat of Steam.
Table 3.1 lists the HHV for fuels and Table
3.2 shows the Btu values for steam and Table 3.3
the values for water. This information combined
serves as a common basis for Input-Output ener-
gy calculations.
The steam tables also show the saturation
temperature changes with pressure. At atmo-
spheric pressure water boils at 212 Deg F [100C]
and at 250 psig the boiling temperature goes up to
29
400 Deg F [204C]. The working temperature of the
steam is raised by increasing pressure {Table 3.4)
ENERGY RECOVERY IN THE
CONDENSATE SYSTEM
On the condensate recovery side of the steam
system, there is an important rule of thumb to
know about. Heating water for use in the boiler
uses a significant percentage of the total energy
in the steam, so it is important to get condensate
back to the plant as hot as possible.
The water being fed to the boiler must be
heated to the boiling point to drive off oxygen and
The hotter the returning condensate is, the
higher the efficiency of the system.
The Rule of Thumb for feedwater heating:
——————————————————————
Every 11 Deg F [6 Deg C) that has to be added
to the boiler feed water reduces efficiency by 1 %.
——————————————————————
Starting with the cold water side of the sys-
tem 50 Deg F [10C], water is raised to approxi-
mately 220 Deg F [104C] in the feed water heater,
depending on the type of system, for injection
into the boiler which takes one Btu per pound per
degree F. In this case 170 Btus per pound of water.
This amounts to 15% of the energy in the steam.
Unfortunately, there can be another reason
for boiling hot water returning to the boiler room.
CALCULATING FUEL SAVINGS AND
LOSS BASED ON EFFICIENCY CHANGE
There is an important difference between ef-
ficiency improvement and fuel savings. An effi-
ciency increase from 80% to 81% is a 1% efficiency
improvement. It is a proportional increase of 1%
out of 80% (1180%) or 1.25. This represents a 1.25%
fuel savings from the 1% efficiency improvement.