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NORA Oilheat Technicians Manual

Chapter 7 Combustion Figure 7-24: An analyzer Valve Scale Flexible Diaphragm Chapter 7—Combustion 7-23 the percentage of oxygen (O2) in the combustion gases. For each 1% decrease in oxygen levels introduced into the combustion process, efficiency increases by up to 1%. While some excess air is needed for complete combustion, we must set the air to the manufacturer’s specifications to ensure maximum efficiency. Another problem with excess air is that it cools the flame. This creates incomplete burning in the combustion area and carbon monoxide levels can rise. Therefore we must exactly control the amount of excess air we allow into the burner. The only way we can achieve this delicate compromise between smoke and efficiency is by using combustion test equipment. You cannot see flue gases; the only way you can be sure you are right is testing. CO2 measurement The Orsat CO2 analyzer, named for its inventor, uses chemicals that absorb CO2 from a mixture of gases without absorbing any other gas. The chemical usually used is potassium hydroxide (KOH) because it has the capacity to absorb large amounts of CO2. When it does so, it expands. By measuring how much it expands we can determine the amount of CO2 in the gases. There are two main parts to this analyzer: 1. The sampling pump: A. The sample tube that is inserted into the stack gases, or replaced by a longer tube for over-fire sampling. B. The yarn filter and water trap, which stops soot and water from entering the analyzer. C. The sample pump, a rubber bulb with rubber flapper suction and discharge check valves that allow flow in only one direction into the analyzer. D. The rubber connector, which seals the sampling pump system to the analyzer. 2. The analyzer: A. A body, which has two cavities or “cups” at the top and the bottom, connected by a narrow tube with an adjustable scale alongside. (Figure 7-24). B. A valve system that either seals the gases and liquid inside the analyzer, or else lets a sample be pumped into the top cavity while the narrow tube and the lower cavity are sealed off. C. A diaphragm, or flexible disc in the bottom cavity, which prevents a vacuum from forming inside the analyzer and lets the liquid in the bottom cavity be drawn up into the narrow tube after CO2 is absorbed. Using a CO2 analyzer A. Prime and “wet” the instrument by tipping it over and then back once, and allow the fluid to drain from the upper cavity while holding the instrument at a 45- degree angle. Next, hold upright and depress valve on top several times and release the valve. Loosen the lock nut on the sliding scale on the right side. Slide the scale until “0” lines up with top of fluid in the center tube. Tighten the locknut. B. Insert the sampling tube into the stack to draw the gases to be sampled through the sampling hole in the stack. This gives you a stack CO2 reading. However, under certain conditions, it is desirable to determine the over-fire CO2. To do this you must replace the short metal sampling tube with a ¼" metal tube about 30" long. This must extend through the sampling hole in the fire door to a point above


NORA Oilheat Technicians Manual
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