NOZZLE MIX SYSTEMS
FOR MOST ACCURATE CONTROL WITH HIGH PRESSURE GAS, USE INDIVIDUAL CROSS-CONNECTED LINE REGULATORS
16 osi -- high 0.5 osi -- low
air
3D
5D
above 2 psi
spring setting 19 osi
-- high -- low
4 pipe dia.
16 osi -- high 0.5 osi -- low
10 pipe dia.
The usual method, previously discussed, gives the good control required for most applications, but the pressure drops may be too high for the degree of turndown control required on some precision jobs. Without the dotted impulse line, assume the 7218 is sized for a 2 osi drop plus a 1 osi safety factor, the underlined 19 osi supply pressure gives a drop across the 7218 at the 16 osi high fire, of 3 osi. At the 0.5 osi low fire, drop across the air/gas ratio regulator is 18.5 osi. If application requires best possible control at low firing rates, a suitable line regulator may be used and cross-connected to the controlled air supply. The dotted lines show the added cross-connection, and the circled numbers show the new pressure. This arrangement gives a 3 osi drop across the air/ gas ratio regulator at all times, making control of the low fire gas flow much easier. This is about a 6 to 1 turndown for the regulator, as opposed to about 15 to 1 previously. This system requires a line regulator for each zone of control.
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