Flame structure, Voltage | Middleby Marshall PS224 PS310, PS570

SECTION 3 - SERVICING COMPONENTS

3.Pilot and Proof of Pilot Flame Rectification

With standing pilots, heat is a necessary ingredient for proper thermocouple opera- tion. But this is not the case with IIDS (Intermittent Ignition Device Systems) when flame conduction or rectification is used. To better understand the principles of flame conduction and rectification, we must first understand the structure of a gas flame. See Figure 66.

Figure 66

Flame structure

Figure 67 - Flame rectification

Figure 68 - Pilot and probe

With the proper air-gas ratio to give a blue pilot flame, three zones exist.

Zone 1: An inner, fuel-rich cone that will not burn be- cause excess fuel is present.

Zone 2: Around the inner, fuel-rich cone is a blue enve- lope. In this area is a mixture of vapor from the fuel-rich inner cone and the secondary, or surrounding, air. This is where combustion occurs.

Zone 3: Outside the blue envelope is a third zone that contains an excessive quantity of air and will not burn.

Of concern is the second, or combustion area. This is where the burning occurs and is the area that is of prime importance for good flame sensor location.

Flame Rectification

With flame rectification, two probes with different sur- face areas are exposed to a flame - in this case, the pilot flame. The probe with the larger surface area at- tracts more free electrons and, as a result, becomes the negative probe. Therefore, current is conducted through the flame from the positive probe to the nega- tive probe. See Figure 67.

Note also that the AC voltage sine wave has not changed, but the negative portion of the current sine wave has been chopped off. This positive portion now represents a DC current. This is the phenomenon of flame rectification.

To apply this principle to an IID (Intermittent Ignition Device - in this case, the pilot/ignitor assembly), a pilot and flame sensor have been substituted for the two probes (See Figure 68). After the pilot is ignited, a DC current flow of 2.0mA (microamps) or more is conducted through the flame, from the flame sensor (the positive probe) to the pilot tip (the negative probe). The pilot tip, acting as the negative probe, completes the circuit to ground. The IID sensing circuit uses this DC current flow to energize a relay and open the main burner gas valve.

THE FOLLOWING CONDITIONS WILL HAVE A DIRECT BEARING ON EVERY IID APPLICATION:

Voltage

The supply voltage to the ignition controls should be within the following ranges:

•120VAC controls – 102 to 132VAC

•24VAC controls – 21 to 26.5VAC

24VAC systems should use transformers that will pro- vide adequate power under maximum load conditions.

Gas Pressure

Inlet gas pressures

•Natural gas, Wayne burner - 6-12” W.C. (14.9-29.9 mbar)

•Propane, Wayne burner - 11½-12” W.C. (28.7-29.9 mbar)

•Midco burner (all gases) - 6-14” W.C. (14.9-34.9 mbar)

Regulated gas pressures

•Natural gas, Wayne burner - 3.5” W.C. (8.7 mbar)

•Propane, Wayne burner - 10” W.C. (24.9 mbar)

•Midco burner (all gases) - 3-5” W.C. (7.5-12.5 mbar)

Pilot gas pressures

• Natural gas, Wayne burner - 3½-4” W.C. (8.71-9.95 mbar)

• Propane, Wayne burner - 8-10” W.C. (19.9-24.9 mbar)

• Midco burner (all gases) - 5-6” W.C. (12.5-14.9 mbar)

PS224 PS310, PS220, PS555, PS200, PS360 specifications

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Middleby Marshall PS224 PS310, PS570 Flame structure, Flame Rectification, Voltage, Gas Pressure

Models: PS224 PS310 PS220 PS555 PS200 PS360 PS570