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Cross-connected air/gas regulator system
The
cross connected regulator system is the most common system for
keeping air and gas in the proper ratio at all firing rates.
The regulator is the heart of the system. An air impulse line carries the air pressure down to one
side of a diaphragm in the cross connected regulator.
Gas pressure in the line downstream of the regulator is
applied to the other side of the diaphragm.
If the gas pressure is not equal to the air pressure, the
diaphragm will move to open or close the regulator and bring the
pressures into balance.
To check the air/gas ratio regulator,
hook up
a manometer to the air line and the downstream gas line.
If the pressures are equal, the manometer reads zero.
When the control valve moves, expect the changing
gas pressure to lag behind the changing air pressure by about a
half inch W.C. But there should not be any violent swings of an inch or two.
Erratic movements mean the regulator is sticking or you
have holes in the big diaphragm or in the small stem sealing
diaphragm.
Dirt and trash in the piping
Dirt and trash in the piping is a common problem in
combustion systems. Air
going into the blower should come from outside the building or
else a filter should be used.
Outside air is usually pretty clean, but it can be dirty if
there is a dirt road nearby.
The inlet should at least be covered with a screen to stop
birds and papers. The
inlet should also be at least 15 ft. or so above the ground
because most dirt and paper is close to the ground. If there is a filter, it must be kept clean. Checking pressure at the blower outlet will
indicate when
the filter needs cleaning , but check pressure when the
burners are on high fire. There
is very little pressure loss even in a dirty filter when the
burners are off or on low fire.
Usually the gas supply is clean and dry, but it can be
dirty in some locations. Also,
at start up or any time the piping system has been worked on, you
can get trash in the gas or air piping.
This can be anything from pipe dope or Teflon tape, to paper, rags and gloves.
Pilots
Pilots and pilot components have many small passages and therefore
are easily upset by dirt and trash. Pilots are probably the major cause of trouble in
combustion systems. The
best solution is to eliminate them and use spark ignited burners.
When pilots are causing shutdowns, it is often necessary to
take everything apart including the piping and clean it. If you
are having a lot of problems with dirt in a pilot
system, you should install separate filters for the pilot air.
You can also install solenoid valves in the air lines to
shut off flow when the pilots are off so that the piping stays clean
longer.
Another problem with pilots is that many blowers pulsate
when the burners are on low fire. This causes the pilot flame to pulsate and to change ratio
when it pulses. The
pulsing of the blower can usually be stopped by venting some of
the blower output when the burners go to low fire. Another solution is to put a pressure regulator on the
pilot air line.
Deterioration of components
After a period of service, combustion problems can be
caused by deterioration of burner casting, burner tiles, or burner
flame shielding tubes. The
only way to detect this type of thing is to carefully inspect the
component for obvious deterioration. Depending on the
component, it may be necessary to carefully disassemble the
component to see the problem - such as with failing regulator
diaphragms.
Flame Supervision
The flame supervising system is another major source of
problems. Although this is a big subject, the most common maintenance items are:
- Clean the lens on the flame sensor
- Adjust the flame so it can be read by the sensor, making sure
that it
is the correct color (ratio) and check milliamps from the sensor
- Replace the flame sensor
- Replace the flame relay
- Too much heat on the sensor
Leaks
Leaks in the furnace may make difficult or impossible purging an
atmosphere furnace to low oxygen level. For indirect
fired radiant tube atmosphere furnaces and direct fired
non-atmosphere furnaces, leaks can also cause problems in
achieving load temperature uniformity. Leaks can be caused
by one or more of the following:
- Door seals
- Shaft seals on fans, dampers, etc.
- Gaskets on any component plugs or access covers
- Cracked welds in the shell or areas that were never welded
- Cracked radiant tubes
- Packing on radiant tubes - for expansion
The latest design, materials
and equipment specifications should be obtained from the company
before any reliance is placed on the enclosed since changes may
occur due to product improvement. | 
