Page 131 - Boiler_Operators_Handbook,_Second_Edition

116 Boiler Operator’s Handbook

that combustion air as an input. However, simple input the steam in the steam tables in the appendix, subtract
output efficiency calculations only include the heating the enthalpy of the water supplied to the plant and mul-
value of the fuel. They’re used to avoid measuring the tiply by the number of pounds of steam produced to get
energy added by fan motors and pump motors along an output in Btu. Divide by 100,000 to convert to therms
with that hotter combustion air. Combustion efficiency and one million for decatherms or million Btu.
calculations will show a negative loss because the tem- If you’re getting condensate back, you’ll have to
perature of the hotter air is subtracted from the tempera- meter it or subtract makeup and blowdown from steam
ture of the flue gas. output to determine the quantity of it. Use the enthalpy
For reasons I don’t understand everyone concen- in the steam tables for water at the condensate tempera-
trates on boiler efficiency when it doesn’t change very ture. Multiply by pounds of condensate returned to get
much and has little to do with the overall “plant effi- Btu. Adjust that result to match your output units and
ciency” which the boiler operator should be attending subtract from the steam output to get plant output.
to. This is a bigger problem when there is so much con- Maybe you’re generating electricity too, use the
fusion over what boiler efficiency really is. Two identical conversion and add that to your output.
boilers in different plants can have the same boiler ef- For hot water plants determine the water flow rate.
ficiency and combustion efficiency but one will produce Hopefully it is constant. Convert gallons per minute to
less usable energy than the other because it has a higher pounds per hour then multiply by the number of hours
blowdown rate. The energy absorbed by the water and in the day, week, or month you’re evaluating. One gpm
steam in the boiler (ASME definition) includes the heat is approximately 500 pounds per hour so multiplying
added to the blowdown water. Two plants with identi- gpm by 500 is close enough. The time period is deter-
cal boilers and loads can have different plant efficiencies mined by how you measure your fuel usage. If you’re
simply because one plant doesn’t have water softeners relying on the gas billing it’s usually the month and
so it must blow down more. Maybe they both have you’ll use 720 or 744 hours depending on the month (ex-
softeners but one has very little condensate return; it cept February which will be 672 or 696). Once you have
must heat the makeup water to replace that condensate the number of pounds you were pumping around you
and blow down more. Those and other variations can multiply it by the temperature difference of the water.
produce plants with boilers having an 80% efficiency After all, the definition of a Btu is the amount of heat
operating with a plant efficiency as low as 40%. Take a required to raise the temperature of water one degree.
plant with a mismatch between equipment and load and You’ll have to use an average temperature for re-
that plant efficiency can be as low as 20%. turn water (or supply water if you control on the return
So what’s “Plant Efficiency?” It’s the amount of temperature) to calculate the output. Since the loads
heat you deliver to the facility, the usable heat you gen- swing, a Btu meter, which constantly performs that cal-
erate, divided by the energy used in the plant. What you culation, should be an integral part of your plant so you
deliver to the facility is your output. I like to use energy can measure your output.
in steam or hot water going down the pipe to the plant That’s it, plant efficiency is your output divided
less the energy in the condensate or return water. That by input. You can calculate it regularly or use some of
way my output is what the facility is using. The energy the rate measurements we’re about to cover. So, what
used in the plant includes electric power in addition to do you do with it? You compare it! By measuring your
fuel. plant efficiency you’re developing a measure that will
A kilowatt-hour is 3,413 Btu. Multiply the kWh allow you to determine, first and foremost, if the plant
on your electric bill by that number to know how many performance is consistent, increasing, or decreasing. You
Btus were added by electricity. If you’re firing gas and want to produce the highest efficiency or highest rate of
want to deal in therms then multiply the kWh in your output per unit of input that you can. It’s called burning
electric bill by 34.13 to convert the electricity use to less fuel and using less electricity while still satisfying
therms. If you’re larger and use decatherms or millions the load.
of Btu multiply it by 3.413. With identical units you can So, you measure it to determine where you are.
add your electrical and fuel energy inputs to the plant to You’ll discover that running one boiler instead of two
get the total energy used. makes a big difference. You’ll find out when you shut
If you deliver steam to the facility and get nothing down the continuous blowdown heat recovery system
back you’re a 100% makeup plant and the energy you’re that it costs a lot more to operate without it. However,
delivering is all in the steam. Look for the enthalpy of continuous blowdown saves more money in water than