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Kiln Connection: Burner Ratio Control

May 1, 2002
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Fuel fired kilns depend on a variety of means to control the burner air-to-fuel ratio. These systems vary considerably, but all of them require a good understanding of the concepts of flow and pressure. Over the next several columns, we’ll discuss the many systems that are commonly found on kilns for ceramic firing. This column will consider a basic ratio control system and examine the components that make it work.

The Fundamental of Flow

The flow of air and fuel gases follows standard flow laws, and the laws governing pressure and flow are straightforward. This makes the setup and control of burners simple and accurate, as long as certain rules and precautions are followed. For starters, the flow of gas and air are reflected in the formula:

Flow is proportional to (Pressure)1/2

Just knowing this simple relationship can be very helpful. For example, if you know the flow value at a given pressure, this formula allows you to readily calculate the flow at another pressure, or vice versa. As an example, if a system flow is 500 units at a pressure of 1 psig, and we increase the pressure to 4 psig, the new flow, as long as nothing else has changed, will be 1000 units, as seen below:

1000 = 500 (4/1)1/2

Normally, the units of flow are standard cubic feet per hour, which is a cubic foot of the gas volume at standard temperature (60?F) and pressure (14.7 psia). Keep this formula in mind whenever you are considering fluid flow, since it will be useful as long as the temperature of the flowing gases and the system pressures are relatively constant.

Figure 1. A proportional control system is designed to provide constant ratio control.

Proportional Control

The combustion schematic in Figure 1 is a simple arrangement that shows a proportional control system. It is designed, in its most basic form, to provide constant ratio control, i.e., a set ratio of air-to-gas regardless of firing rate. And it only works due to the flow law that we defined above. This combustion system consists of the following bits of hardware:
  • Combustion air fan. This fan delivers combustion air to the burner system at a reasonably constant pressure.
  • Motor operated air control valve. The valve modulates the flow of air to the burner.
  • Gas proportioning regulator/ratio regulator. This device is the “heart” of the system. Developed decades ago, it is still an accurate and economical means of controlling the proportion of gas to air.
  • Limiting orifice. This device introduces a pressure drop in a flow line to allow for balancing or adjusting flow rate. In laymen’s terms, it is a needle valve.
Of these components, the gas proportioning regulator bears further discussion. The regulator has an impulse line that is connected from the combustion air line to the regulator port. As the combustion air flow varies in accordance with burner input, the pressure applied to the regulator port also varies. This pushes down on the diaphragm, which opens the valve, causing more gas to flow. The increased gas flow creates a pressure rise at the outlet of the regulator, and through internal port, this pressure pushes up on the underside of the diaphragm until the gas pressure at the regulator output is just equal to the impulse pressure from the combustion air. A spring is in place to balance the weight of the diaphragm and the valve, so that the pressure of the air impulse and gas pressure are the only factors affecting the operation of the regulator.

At this point, the regulator has achieved its purpose: to create the same level of gas pressure at the regulator outlet as the impulse pressure that is applied to the regulator impulse. This remarkable device continues to be the most widely used device in fuel-to-air ratio control.

Troubleshooting this regulator is simple. Since the gas outlet pressure at the regulator should be equal to the impulse pressure, one need only connect a suitable pressure monitor at both locations and compare pressures; they should be equal.

This is just a start—there are many different regulator types, including bias, multiplication, fixed output, differential and combination sets. Next time we’ll look at the exact way that the ratio regulator proportions air-to-fuel ratio in a variety of systems.

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