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Efficiency
Always makes sense |
| The EDR technology provides a thermodynamic
solution to the losses associated with 3 Categories of
Efficiency with the existence draft conditions in the flue
stack. The EDR reduces or eliminates excessive draft conditions
and by doing so, optimizes all aspects of boiler efficiency. |
Combustion
Efficiency
The efficiency of the combustion process is directly
proportional to draft conditions in the flue. Insufficient draft
prevents adequate air supply for the combustion process and
results in smoky, incomplete combustion. Excessive draft,
however, is common in 85% of commercial and industrial boiler
systems. Too much cold combustion intake air replaces the hot,
high velocity flue gases, making the flame cooler and resulting
in a poor combination fuel -to-air ratio. The flame pattern is
usually altered, inhibiting the fuel from burning properly. The
EDR establishes draft conditions, which allow the boiler to
operate at 2% to 4% oxygen, which maintains maximum combustion
efficiency. |
Thermal
Efficiency
The dwell time of combustion gases in the boiler's heat transfer
regions directly affects the amount of energy transferred to
useful output rather than sent up the stack unused. An
uncontrolled flue stack literally pumps the heat from the
combustion equipment at high velocity out into the atmosphere.
When hot flue gases travel too fast through the flue stack, the
heat exchanger cannot capture the heat for use, as it should.
Also the colder air being pulled through the heating equipment
drags along the surface as it travels, insulating the metal heat
exchanger surface from the hotter gases, prohibiting the heat in
the hot flue gases from passing into the heat exchanger. The EDR
increases the dwell time of combustion gases in the boiler's
heat transfer region and reduces the amount of energy sent up
the stack unused. Additionally, it reduces excess air which acts
as an insulating blanket between the burner flame and the heat
transfer area, while still allowing the manufacturer designed
draft to pass through the boiler. |
Off
Cycle Efficiency
Between firing cycles, the transfer of heat up the open flue
stack loses substantial energy. All this means that more fuel is
required to produce the same problem that vent damper systems
attempt to prevent. The EDR reduces off-cycle heat -loss
resulting in shorter recovery time and shorter boiler run times. |
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"Fundamentals
of Boiler Efficiency"
By Exxon Company Technical Services (Excerpts with comments
added under each) |
| Draft Measurements Draft
Determines the rate at which combustion gases passes through the
furnace or boiler .Excessive draft can produce an increase in
the stack temperature and will also reduce the percentage of CO2
in the flue gas.
Inadequate draft Can result in insufficient
combustion air and smoky operations. At perfect draft, the flame
bushes out to nearly fill the firebox, giving maximum time for
clean burn-out. |
| Comment: Both "excessive "
draft and "inadequate" draft result in loss of
efficiency. "Perfect draft, then would be the least draft
required to cause a correct combustion of fuel and safe venting
of gases. |
| Stack In addition to
providing draft, the stack also provides a passageway for the
escape of exhaust gas and any residual heat to the atmosphere.
It is here that the major loss of heat occurs. Some heat loss is
unavoidable, and even the most efficient boilers will lose
between 6 and 13%, depending on the hydrogen content of the fuel
burned. Higher heat losses occur if the stack gas is hotter than
need be, or if there is more volume than there should be, Both
of these situations can be remedied (within the limits of
equipment design) by careful attention to the amount of excess
air used in combustion. |
| Comment: Since the major loss of heat
occurs through the stack, it is very important to address this
loss. By reducing the total heat/volume loss, increased
efficiency will be achieved. |
| Flame Temperature The
flame temperature is the highest temperature produced in
combustion. Theoretically the maximum flame temperature occurs
when the air and fuel are mixed in exact stoichiometric
proportions. Addition of excess air or fuel only provides more
material to absorb heat from the combustion reaction. (When
draft is properly controlled, both combustion efficiency AND
thermal efficiency are increased. The fuel burns closer to the
exact stoichiometric balance, and the additional heat produced
is retained and put to use by the heat exchanger instead of
being "sponged" by excess air or fuel.)
Stack Losses The above tests aid in
determining the source and cause of heat losses. It will be
found that the main source of heat loss is through the stack.
All other losses are small in comparison to the stack loss.
Additional heat is lost by radiation from the furnace walls and
by blowdown from the steam drum.
Thermal stack loss is equal to the heat
required to raise the flue gas from room temperature to the
stack temperature measured after the last boiler heat transfer
surface. Stack losses can range from 30% in boilers that are
poorly designed, maintained, or operated to 7% in large
efficient boilers equipped with economizers and air heaters to
extract all possible heat. The latent heat of the water in the
stack is also considered a loss. |
| Comment: A key challenge and objective
in vented heating equipment is to prevent all of the stack loss
possible, thus saving fuel and money, as well as the
environment. |
