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Combustion & Clean Air Breakthroughs.

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How to Balance a Forced Draft / Induced Draft (FD / ID) Heater.

The successful operation of an FD/ID system begins with the heater and burner design specifications. One of the main objectives of this type of unit should be uniform air flow to all of the burners in the system. This requires that the burner and heater manufacturers work together to understand the system and how it is supposed to operate.

The main problem with common air ducts is the large variation in air flow along its length. At the beginning of the duct, the air flow and velocity are at their peak, but as burners “consume” air along the duct length the main duct flow decreases until it reaches zero at the very end. With the variation in airflow, there are variations in air velocities, which cause changes in dynamic pressures and ultimately in static pressures. At the point where the velocity becomes zero, typically at the end of the duct, the static pressure is the highest. This pressure is called the stagnation pressure.

This variation in static pressures along a duct is the reason for poor flow distribution through the burners. One way to reduce the variations is tapering the air duct so that the air velocity remains constant. Another key item to ensure uniform airflow is the air pressure drop across the burners. If the burners use enough pressure loss the variations in dynamic and static pressures inside the air duct become much less important and have less impact on the burner air flow. This helps to distribute the air uniformly. For process burners, a good design drop is I .5 to 2.0 inches of the water column. The goal should be to keep at least 1.0 inch of positive pressure in the ductwork upstream of the burners.

Since the air ducts cannot always be tapered well enough the design should include “balancing” dampers in all the main duct runners supplying air to the burners. All the main runners should be equipped with pressure taps for the installation of draft gages. The burners should also have pressure taps for draft measurement.

The use of CFD modeling or a Cold Flow Model is essential to design the system properly. The model can show the errors in the design and how to correct these deficiencies.

Effect on Operations

A balanced draft heater that has non-uniform air flow to the burners may create problems such as:

  • Danger of explosion due to incomplete combustion
  • Poor flame patterns
  • Flame impingement on the tubes
  • Instability in the flame
  • High convection section temperatures
  • Carbon Monoxide and Unburned Hydrocarbons in the stack

Corrective / Preventive Actions

Once the burners have been installed, and the system is ready for start-up the initial balancing should be done on cold air without the burners in service. The forced draft fan should be running at a rate comparable to the normal duty of the heater if possible. The induced draft fan should be bypassed with the air exiting out of the stack like a natural draft heater. All the burner air dampers should be fully open at this time.

Using the balancing dampers on the main duct runners the first step is to achieve uniform pressure in all the runners. The goal is to achieve air pressure readings, using a Magnehelic pressure gage, that is within + / – 5%.

The probe used to obtain the data should be a pitot tube with the static pressure connected to the high-pressure side of the gage. The gage should have a pressure range corresponding to the burner pressure drop or loss. Normally for a process burner, a gage that has a range of 0-5 inches of the water column is sufficient.

After uniform air pressures have been achieved in the main ducts, then individual burners can be adjusted. The goal again is to achieve static pressure readings that are + / – 5%. This requires starting by measuring the air pressure in the burner plenums, with the burner dampers full open, and recording your data. Generally, you will have to pinch down on the burners near the end of the runner duct. This forces more air upstream to the burners closer to the main duct.

Once this initial balancing is done and the damper positions are documented the heater is ready for start-up. After the heater is at normal rates and combustion air temperatures, further adjustments may be necessary to improve the flames.

Caution should be used when adjusting burners using preheated combustion air, with the heater in service. The air pressure taps provided on the burner plenums will be under positive pressure. This means that hot air will be forced out when the cap is removed.

Finally, remember always to make adjustments in a slow and steady progression. If you make too many moves at the same time, the heater could go into a state of unstable or non-steady operation. After adjusting always wait 10-15 minutes to give the system time to “line out” to the new conditions.

Effective and Safe Troubleshooting.

Frequently, the operator of the heater must be trained to use knowledge of the equipment and process unit to make adjustments that bring operations back to the required capacity desired by plant management.

It is essential that troubleshooting be done in a systematic, well-organized fashion. Effective and safe troubleshooting involves four basic steps:

  1. Recognizing the problem
  2. Observing indications of the problem
  3. Identifying solutions for the problem
  4. Taking corrective action

When a problem is noted, it is necessary to evaluate its likely effect on the process or product being produced.  Some solutions may require the heater to be shut down for the problem to be resolved.

Once a cause has been determined, standard procedures should be followed to solve the problem. All personnel involved should be aware of the problem, the planned corrective actions, the ways that safety is addressed, the expected results, and the proper action to take should the problem worsen or not be solved.

Did you know that API 556 recommends an oxygen analyzer every 30 feet for cabin type haters? Properly trained personnel could reduce operational costs. Check out the John Zink Institute for the latest training in process burners and heaters.