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We are often asked why we believe our Eddy Draft
Regulator Technology can increase thermal efficiency in situations where stack
dampers, oxygen trim systems and forced or induced burners are in use.
The answer is found in the over-all effect the stack
may be having on the pressure and velocity of combustion and secondary air.
In most cases this is a “hidden”
effect.
Whether the Eddy
Draft Regulator (EDR) can have an adequate positive effect on heater and
boiler operations and efficiency will depend on the existing stack pressure and
surrounding conditions, such as temperature and wind conditions. In well over
90% of all cases the Eddy
Draft Regulator can help.
The pressure of the stack by itself works on the heat
and air in the heater or boiler 100% of the time, except when an off-cycle stack
damper completely cuts off the stack-flow on standby mode. The losses on the
off-cycle are a minor part of total stack losses. Even if there is no off-cycle
damper in place, the EDR
will reduce this part of heat loss, without
any moving parts.
Manual damping of stack-flow is inadequate, and often
contributes more to inefficiency than to efficiency. The damper is set at any
given time to a certain pressure and flow position during a certain firing rate.
If anything at all changes during this set position, such as a gust of wind,
temperature drop, firing rate modulation, etc., the fuel-air mixture will change
and inefficiency (and even sooting) may occur without the operator even being
aware of it. In many heaters and boilers, the dampers cause severe and excessive
levels of carbon monoxide, without it showing up as visible sooting. This is
both health-threatening and very inefficient.
Automatic damper systems sometimes work reasonably
well, but have two major problems when it comes to efficiency.
One is the fact that anything needing adjustment and having moving parts
will often get out of adjustment or will often malfunction without being
detected. This is regularly
observed in the real world by our technicians. The second is the fact that the
response time of such mechanisms leaves a gap of inefficiency which is
considerable. By the time a change is called for by the system, the fuel-air and
velocity situation has already been in a state of change for several seconds or
more, causing an imbalance for a time. Over time this slow-response time makes a
major difference in the operation and energy usage of a heating or boiler
system.
Oxygen trim systems are sophisticated mechanisms with
the capability of analyzing stack gases and adjusting the fuel-air mixture
through actuators. These have the
same problems as the automatic dampers. Often building engineers are forced to
disconnect them entirely, because they go out of calibration easily, shut down
during electrical storms, or simply do not live up to expectations. Even when
working properly the response time from analysis of the gases to actual
adjustment in the fuel-air mixtures is wasteful. Furthermore, the oxygen trim
system only deals with the oxygen percentage content in the stack gas, not
the proper volume of fuel and air for the heater or boiler.
The problem of fuel-air ratio vs. volume/velocity is
seldom considered by operators, technicians, engineers and boiler/HVAC service
personnel. Often the main concern
is the oxygen and CO2 percentages (without CO) in the flue gas. This
is a major factor, but may be misleading. For instance, let’s imagine a
boiler with a forced-draft burner attached to a tall stack. The burner is set at
3% oxygen because that is the lowest setting at any firing rate where the
fluctuation margin is safe. But, 3% of what
source of oxygen? In most applications, we find it is 3% of whatever
volume/velocity that is developed by the combination of the pull of the stack
(negative pressure) and the push of the forced-draft burner (positive). The net
stack pressure will probably show a heavy negative reading, with an excessive
volume/velocity of air passing through the system. This
causes a dramatic decrease in efficiency.
In most cases, the fuel mixture has been set to “catch
up” with the air volume/velocity through
the system. This is backwards.
The volume/velocity may be up to double the amounts needed for proper
efficiency. Furthermore, with weather, atmospheric conditions and wind
changes, the balance is upset even more. What if the EDR were set into the flue stack to “tune” it to the proper pressure conditions for the heater or
boiler? Instead of a significant
negative pressure, it would be tuned to a slight
negative pressure (for atmospheric boilers) under all conditions. With power
boilers like Superior, Cleaver Brooks, etc., the EDR would be set to a
small positive pressure. The burner could then be re-set to the 3% oxygen
reading, providing a dramatic reduction in fuel consumption. The resulting heat
would actually be greater, due to the stabilized air flow. In most cases, the
oxygen percentage can now be even further reduced
because of the stability of the air-fuel mixture, once again increasing
efficiency.
When the
volume and velocity of air through a system is reduced and balanced, “dwell
time” is increased. This
means the hot gases from combustion are retained in the heat-exchanger
convection area longer, transferring more heat to the intended target, rather
than escaping through the stack. In
naturally drafted (atmospheric, as opposed to power drafted) equipment
this will usually raise the stack temperature by 50 to 150 degrees.
In power drafted boilers, the stack temperature will often stay the same
or decrease by 30 to 100 degrees, depending on the efficiency of the
heat-transfer surfaces and the number of passes. This increase in “dwell
time” not only guarantees more complete combustion of the fuel, but also
provides much greater thermal efficiency. By going
from a negative to a positive stack pressure the flame changes from a this,
quickly moving "rope" type flame to a slower moving, larger diameter
"bush" type flame.
The bush type flame makes far more contact with the fire tube walls,
allowing more heat transfer. Many
times, a turbulator is installed in an attempt to address this problem.
We essentially, and much more effectively turbulate the entire heat
transfer area of the boiler without causing negative side effects (hot spots,
warp tubes) that are commonly associated with turbulators. A properly tuned stack is the answer.
To achieve optimum efficiency and longevity of
fossil-fueled heating/process equipment, first tune the entire exhaust system.
If the pressure is correct, and remains constant,
the rest of the equipment has a chance to work close to its design
specifications. A simple flue
pressure gauge or manometer can display the stack pressure on both the off
(standby) mode and the run cycle. On
the off mode, the stack pressure should normally be slightly negative. On the
run cycle it should be slightly negative for atmospherically-drafted equipment
(with draft diverter at 90% full). On power-drafted equipment, a neutral or
slightly positive pressure is normal, depending on the system and burner design. A strong negative stack pressure (usually -.05 IWC and
greater) becomes an energy thief, and will often cause inefficiencies in all
aspects of the combustion/thermal system.
When the EDR is used to tune or “normalize” the stack, all other controls and dampers may become
totally unnecessary, but may generally be left in place. In some cases, the use
of dampers and induced-draft fans may need to be adjusted or even discontinued
altogether. This can be
determined by the installing technician.
Some may argue that it does not make sense to cut the
stack draw (pressure) when there is a forced or induced-draft system in place.
Most of these draft-assistance fans are for the purpose of overcoming
interior flow-resistance inside a boiler, and to mix fuel and air properly.
They, too, are affected by stack pressure and changes in weather.
It is much like a defroster fan in an automobile.
It is designed to deliver forced air at a given volume/velocity when the
windows are up, and the “pull” pressure (negative) in the car is zero. But
what happens when the back window is rolled down? Does the volume/velocity of
the fan delivery change? Obviously,
it increases. It may, in fact, instantly double. The same is true of the stack
affect on a heater or boiler, either with or without a power assisted draft.
Now, consider the effect of the wind blowing at 30 mph across the top of
a stack. This could be compared to driving a car at 30 mph
with the back windows open.
The theory and practice of tuning a stack is to “normalize”
the flow and pressure of air and flue gases. In this way the stack removes less heat with lower
volumes, at the slowest airflow speed possible. This is how the equipment was designed to operate.
In other words, our goal is “lab-type
efficiency” in real-world conditions. The
EDR comes closer than any other known technology to help achieve optimum
efficiency through flue stack tuning/balancing.
The stainless steel construction makes the
EDR a life-time, non-corrosive product in virtually all stack conditions.
Because it has no moving parts it is fail-safe, never needs attention or
maintenance, uses no energy, and
works 100% of the time to increase efficiency.
In most cases it is out of sight and out of mind.
This can be helpful when different technicians are servicing the heaters
and boilers. They like to tinker and investigate strange,
new equipment. This can be
detrimental to any equipment’s use and proper adjustment. The EDR is set for the life of the stack in almost every case.
Occasionally it may be re-set if the size or rating of the equipment is
radically changed, but for the most part it is a permanent part of the stack. It
is a true “install and forget it” technology.
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