UEi Test Instruments SMART BELL - Manual (Page 13)

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• The three T’s of combustion
– Time
• Amount of time that the fuel and oxygen are together in the combustion chamber
– Temperature
• How high the temperature is determines the rate of oxidation, or speed of the combustion
– Turbulence
• How well the fuel and air are mixed
• These three factors are all interrelated, and will move your results along the combustion curves.
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Other parameters measured include net temperature, draft and efficiency.
Net Temperature
Net temperature is the difference between the combustion air entering the combustion chamber and the flue gas tem-
perature past the heat exchange. This is used to determine how efficient the system is extracting heat from the combus-
tion process in addition to the performance of the combustion process. On sealed systems that have ducted inlet air for
combustion air, the net temperature must compare this air stream temperature with the flue gases. If the appliance simply
uses room air for the combustion air, our analyzers have an internal temperature sensor in the handset, so it will use this
temperature when calculating net temperature.
The most accurate results for efficiency are obtained when measuring flue gases at the point where flue temperature (not
flame temperature) is the highest.
Draft
Draft is the difference between the ambient pressure level and the pressure level in the flue. This is created either by the
natural buoyancy of the hot gases created in combustion lifting, or by an inducer fan that assists the flow of flue gases up
the stack. Most combustion equipment will specify the amount of draft that is required for proper operation.
Draft helps draw combustion air into the combustion chamber, and also helps in mixing the fuel and oxygen. Without
proper draft, the combustion process can spill poisonous byproducts into the space where the appliance is located. This
can be a risk to those in the area, or create a danger to residents or employees working near the combustion equipment.
Efficiency
Efficiency is a measure of how well the fuel is burned to create heat, and how well the generated heat is captured for the
intended use.
The information used to create this value are based on the fuels heating value, the heat lost up the flue and the gas
components in the flue gas. The original method to determine efficiency included many manual methods and lookup
charts. As an example you would measure the CO2 level and the stack temperature and then reference a slide scale that
would give you the relative efficiency number. UEi’s electronic combustion analyzers perform the measurements on a
continuous basis, and can calculate the efficiency as adjustments are being made. With this information you are able to
provide a before and after comparison of the combustion equipment in relatively little time as part of normal servicing.
NOTE: Combustion analyzers measure the combustion efficiency. This is NOT the same as the AFUE indicated by the
manufacturer. The AFUE is derived through an extensive process, and includes many additional parameters such as start-up and
shut-down losses and shell losses as an example.
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