AP08 This mixer, with 1/4" BSP threads, utilises a special tee unit with machined air nipple and venturi sections. Unless a specific capacity and air pressure are noted on the customer's order, a pilot sized unit will be supplied. It is important if a pilot sized mixer is ordered for sizing on site, the customer should be aware that a definite ratio exists between the air nipple and the venturi and this must be maintained through the range. Please ask for more information.

As there is no integral gas adjuster fitted to the AP08, we recommend the customer fits our fine control needle valves, part number NV8, for complete gas and air control.

SINGLE VALVE ADJUSTMENT

Single valve control is possible by fitting a "zero governor" to the gas inlet line. This negates the need to adjust both the air and the gas for high and low fire. The gas pressure is reduced to zero at the mixer inlet; the negative pressure produced by the mixer venturi action induces the correct amount of gas.

ADJUSTING THE AIR / GAS RATIO

It is possible to adjust the air/gas ratio even if the flame cannot be seen as is common on sealed-in tunnel burners. A JM1B or BB1 tip is used and is connected into the mixture line as shown. Light the main burner and the test tip and adjust for the desired flame characteristic.

PREMIX BURNERS

Premix burners are possibly the most common industrial burner type in use and have several important advantages:

• Higher mixture pressures enable higher combustion chamber and process heat inputs
• A wide variety of flame shapes are possible to suit all applications
• Easy adjustment of the air/gas ratio and flame configuration
• Simplified burner trains

A basic premix burner train will consist of the burner tip (flame holder), air/gas mixing chamber, air and gas control valves and frequently a gas proportionator valve (also known as a zero governor).

THE MIXING UNIT

This is the heart of a successful system. Premix burners require the air and gas to be mixed in precise proportions to enable accurate adjustment of the flame. A well engineered mixer will also ensure there is an auto proportioning effect over the entire range. The air jet and venturi sections are machined according to the total burner port area, mixture pressures, air and gas delivery pressures and likely manifold back pressure. A mixer will only cover a certain range of burner capacities i.e. a small burner requires a small mixer. An incorrectly sized mixer or a simple tee arrangement can not only restrict adjustment but more importantly can cause unsafe conditions to occur. It is important to check for a high negative pressure (suction) on the gas inlet to the mixer with air on maximum.

BURNERS

Premix burner tips can vary from simpler open type cast iron tips to multiport line burners. The common principle with all these burner ends is the requirement for flame retention. This will ensure the flame stays on the tip/s and can be pilot flame strips or holes or even a particular tunnel shape.

SAFETY SYSTEMS

A safety light flap check valve is required by the safety authorities, as a minimum, to be fitted into the gas line (unless a gas proportionator is fitted) to stop the ingress of air in the event of a blockage up stream. Full automatic ignition and flame safety is frequently fitted to automate the process and provide quick lockout on  flame out or process stop. The safety module controls solenoid valves fitted to the gas train and high temperature ignition and sensing probes mounted to the tip supply the high tension spark and complete the flame sensing rectification circuit. We can also provide flame out alarms if preferred. More technical information is available on any of the above concepts.

MULTIPLE BURNER SYSTEMS

It is common on particular processes to have separate burner tips fitted to a common manifold to convey the mixed air and gas to the burner tip/s. It is imperative these are correctly sized relative to the burner port area and mixture pressure. A badly engineered manifold system will cause flash back (detonation in the pipe), back flow and inadequate mixing. Hoses must be used that have minimal pressure drop and there must not be valving in the mixture line from the mixer to the burner tip.

TROUBLE SHOOTING PREMIX BURNERS

In addition to operational faults detailed in our "Fault Finding" data sheet, there are particular conditions that may be associated with these burners. Inadequate turn down and lightback are the most common.

INADEQUATE TURNDOWN

This will be apparent as an inability to control from a high to a low flame on ratio and is usually caused by lack of adequate negative pressure on the gas inlet to the mixer. Other factors are:

• Deterioration in the air supply possibly caused by a buildup of dirt or diminution of the air supply or blower motor, bearings or belts if fitted. A pressure check should be performed on the air line just prior to the air orifice with the burner or air rate on maximum. The mixture pressure should also be measured and compared with the air pressure. Although other factors can have an effect on this ratio an air pressure of 3.5 kPa should provide an approx. mixture pressure of 1.3 kPa.
  
• Deterioration of the burner port or the mixer air orifice. This could involve wearing from the constant flow of dirt in the air or a buildup on these ports and will cause the fine ratio mentioned above to not be maintained. The burner must be disassembled and the mixer and burner port accurately measured.
  
• Deterioration of the gas proportionator if fitted. Again this should be carefully disassembled by an approved person with the valve and valve seat checked and the diaphragms and the spring tension checked. Check for sticking on the seat and loss of spring tension.
  
• Similar conditions to "Burner lightback" detailed below.

Reducing the air orifice size by a small amount will improve the negative pressure providing the other factors have been checked first.

BURNER LIGHTBACK

This occurs when the burning velocity of the mixture is less than the flame speed of the gas and can be caused by the following:

• A misalignment of the air jet to venturi in the mixer. It is important these are concentric, do not have sharp edges and have a smooth surface.
  
• An unsuitable air jet/venturi relationship. This can be caused by wear or incorrect initial or post machining.
  
• The gap between the air jet and the venturi is too large. This should only be approx. 2 mm or less.
  
• The burner port area is too large and/or the burner manifold area is out of ratio with the total burner port area.
  
• The downstream components provide excessive restriction i.e. too many bends or the mixture pipe is too small.
  
• Inadequate burner tip design for the mixture pressure developed particularly the flame retention.

The converse where the flame "lifts off" the burner tip can also occur and will be due to a high air velocity or blocked retention holes (inadequate flame retaining ability).

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