FORCED AIR/GAS MIXERS

TECHNICAL HINTS

ASPIRATORS

The AP range of air/gas mixers is designed to mix low pressure air, up to approx. 28 kPa, with any clean commercially available gas. The resulting mixture, under pressure, is delivered to a pre mix type burner or series of blast type burner tips. A complete range from 8mm to 80mm BSP is available. The smaller sizes are frequently used with higher pressure compressed air and greater outputs than indicated are possible. Please specify air pressure and capacity when ordering.

OPERATION

The energy of the air stream passing through a correctly sized jet firing into the matching venturi creates a negative pressure on the gas line. The gas is induced in the correct proportion and the air and gas are mixed in the venturi throat. A gas adjusting valve is provided to control the gas flow and to compensate for gasses with varying heating values or specific gravity.

SIZING

aspirate.jpg (10079 bytes)

We offer a sizing service where we can accurately size the air and the venturi orifice to suit your requirements.

We require the gas type, capacity and the air pressure. Comprehensive sizing charts are available on request.

 

MODEL
NUMBER

MIXTURE OUTLET
(BSPM)

AIR INLET
(BSPM)

GAS INLET
(BSPM)

CAPACITY MJ/HR
@3.5 kPa AP

OVERALL
LENGTH mm

WEIGHT
kg

AP08

8mm

8mm

8mm/F

2 - 8

80

140g

AP15

15mm

15mm

10mm

8 - 40

125

750g

AP20

20mm

20mm

15mm

35 - 80

150

950g

AP25

25mm

25mm

20mm

75 - 150

185

1.15

AP40

40mm

40mm

25mm

140 - 340

270

3.75

AP50

50mm

50mm/F

32mm

320 - 700

350

5.75

AP80

80mm

80mm/F

50mm/F

700 - 1800

500

13.0

HIGH AIR PRESSURE AP MIXERS

Frequently the smaller AP mixers are used in high pressure air situations where blast tips or premix type burner bars are employed. Economical, fully adjustable burners result that can be used in a variety of situations such as metal melting, preheating, spot heating, high gas input baking etc. The most common are the AP08 and the AP15 although theoretically any of the mixer units can be sized for compressed air. Due to the resultant high air volumes it is usual to revert to low pressure air blowers for larger air requirements.

ap08.jpg (8259 bytes)

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. zero.jpg (14549 bytes)


Temperature control is simple to install using this system with an automatic control valve in the air line only. Modulating the air only would then produce the same flame characteristics over the entire range.

ADJUSTING THE AIR / GAS RATIO

testip.jpg (14328 bytes)

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:

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:

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:

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).

Click here to go to the top or click here to return to Air and Gas Mixing Devices.

Copyright Australian Combustion Services Pty Ltd 1997-2004