Transcript oil heat

Oil Heat
North Seattle Community College HVAC Program
Instructor – Mark T. Weber, M.Ed., CMHE
Oil - 1
Objectives
• After studying this chapter, you should
be able to:
– Describe various types of fuel oil
– List and describe the characteristics
associated with fuel oil
– Describe different methods for storing fuel
oil
– Explain how oil tanks are sized
Objectives (cont’d.)
– Explain the importance of periodic oil tank
inspection
– Describe how fuel oil and air are prepared
and mixed in the oil burner unit for
combustion
– List products of combustion of the fuel oil
– List the components of gun-type oil burners
– Describe basic service procedures for oil
burner components
Objectives (cont’d.)
– Sketch wiring diagrams of the oil burner
primary control system and the fan circuit
– State tests used to determine oil burner
efficiency
– Explain corrective actions that may be taken
to improve burner efficiency, as indicated
from the results of each test
– Describe preventive maintenance procedurs
Safety Checklist
• Do not reset any primary control too
many times because unburned oil may
accumulate after each reset
• Do not start a burner if heat exchanger
is cracked or otherwise defective
Safety Checklist (cont'd.)
• Do not start burner with the fuel pump
bypass plug in place unless the oil
delivery system is configured as a twopipe system with both a supply and
return line
• When conducting flue-gas efficiency
tests, avoid burns by not touching the
hot flue-gas pipe
Safety Checklist (cont'd.)
•
•
•
•
Keep your distance from the ignition arc
Observe all electrical safety precautions
Inspect oil burners and lines for leaks
When installing oil tanks, make certain
that all codes are followed
• Oil tanks need to be inspected
periodically to minimize tank leakage
Introduction to Oil-Fired
Forced-Warm-Air Furnaces
• Oil-fired forced-warm-air furnaces have
two main systems:
– A heat-producing system, consisting of the
oil burner, fuel supply components,
combustion chamber, and heat exchanger
– A heat-distributing system, consisting of the
blower, and other related components
• When thermostat calls for heat, ignition
system is powered and oil burner motor
starts
Physical Characteristics
• Lowboy
– Often used if there is not much headroom
– May have a cooling coil on top to provide air
conditioning
• Upflow
– Vertical furnace in which the air is taken in
at the bottom, forced across the heat
exchanger and out the top
Physical Characteristics
(cont'd.)
• Downflow
– Looks similar to an upflow furnace except
that air is drawn in from the top and forced
out the bottom
• Horizontal
– Usually installed in a crawlspace under a
house or in an attic
– Available with right-to-left or left-to-right
airflow
Fuel Oil
• Delivered to the customer in liquid form
and stored in tanks either aboveground
or underground
• Oil used in residential and most
commercial systems in the United
States: basically diesel fuel that has
been dyed red for tax purposes
• Derived from crude oil by a process of
distillation called cracking
Fuel Oil (cont'd.)
• There are six grades of fuel oil and a
numbering system, 1 through 6, is used
to identify each grade
– The lower-numbered oils are called light oils
because they weigh less per gallon than the
higher-numbered oils
– The lightest of these, No. 1, is most
commonly known as kerosene
– Fuel oil No. 2 is known as pentane and is
the most commonly used
Fuel Oil (cont'd.)
• The ASTM sets standards for acceptable
ranges for fuel oil characteristics:
– Flash point
• Has to do with the maximum safe storage and
handling temperature for the fuel
• Lowest temperature at which vapors in air above
the fuel ignite briefly when exposed to a flame
– Ignition point
• A few degrees higher than flash point
• Flame keeps burning because vapors keep
rising from liquid
Fuel Oil (cont'd.)
– Viscosity
• Thickness of the oil under normal temperatures
• Heavier oils are thicker at the same
temperatures
• Expressed in Saybolt Seconds Universal (SSU)
– Carbon residue
• Amount of carbon left in a sample of oil after
boiling in an oxygen-free atmosphere
• Properly burned oil has no appreciable residue
Fuel Oil (cont'd.)
– Water and sediment content
• Attempt to ensure that water, sediment, and
other contaminant levels are as low as possible
• Sediment can form when rust forms on internal
pipe; tank surfaces and sludge can form when
water condensation reacts with the fuel oil
– Pour point
• Lowest temperature at which the fuel can be
stored and handled
• No. 2 oil is one of the lower pour point fuels; can
be used down to 20°F
Fuel Oil (cont'd.)
– Ash content
• Amount of noncombustible materials contained
in the fuel oil
• Can be abrasive and wear down burner
components
– Distillation quality
• Describes ability of the oil to be vaporized
• Lighter oil turns into vapor easier than heavier oil
Oil Storage
• When possible, install tanks above
ground for environmental reasons
• Follow all local codes when installing
• Oil tank sizing
– Tanks are available in sizes from about 100
gallons to over 1000 gallons
• Two main issues in selecting tank size are
frequency of oil deliveries and quality of oil with
respect to storage time
• Tank should hold about 1/3 of the annual oil
consumption
Oil Storage (cont'd.)
• Underground tanks
– Tanks installed underground must be
protected against corrosion
– The most popular types are the STI-P3
tanks and fiberglass tanks
– See Figure 32-7 in text
Oil Storage (cont'd.)
• Aboveground tanks
– Typically fabricated from 12 gauge steel and
hold between 275 and 330 gallons of oil
– Polyethylene/steel tanks: tank within tank
• Galvanized steel outer tank protects the
polyethylene inner tank and provides secondary
containment in the event of a leaking inner tank
• Indoor oil storage
– Reduces temperature fluctuation as well as
moisture and condensation inside the tank
Oil Storage (cont'd.)
• Oil tank inspection and maintenance
– Aboveground tanks should be painted
periodically and inspected on a regular
basis
– STI-P3 tanks should be checked to ensure
protective anodes are in good working order
– Tank piping should be inspected regularly
for imperfections and leaks; always check
for loose or missing vent and fill caps
Oil Storage (cont'd.)
– Tanks should be checked for water at least
once a year
• If water is present, the source of water should be
identified and the situation corrected
• Tanks are tested for water content by placing a
water-sensing paste on the oil tank stick
– Visually inspect aboveground tanks for
damage to pipes, caps, oil lines, tank legs,
and gauges and check for traces of oil
under the tank
Fuel Oil Supply Systems
• Two common piping configurations:
– One-pipe system: only one pipe runs
between the oil storage tank and the oil
burner
• As oil is needed, it flows from tank to burner
Fuel Oil Supply Systems
(cont’d.)
– Two-pipe system: one pipe is the supply and
the other acts as the return
• Oil flows at maximum capacity to the burner
where the system uses what it needs and the
rest flows back to the tank via the return line
– One-pipe system is used when tank is
located above the oil burner; two-pipe when
tank is below
Fuel Oil Supply Systems
(cont'd.)
• Key points about fuel oil lines and piping:
– Oil lines should be sized at a minimum of
3/8” OD copper lines
– Oil lines can be constructed of wrought iron,
steel, or brass pipe connected with
malleable fittings; never use cast-iron fittings
– Never use PVC pipe for oil lines
– Use only flare connections to join oil lines
Fuel Oil Supply Systems
(cont'd.)
– When needed, use only a non-hardening,
oil-resistant pipe joint compound
– When installing oil lines, make the runs as
short as possible, use as few fittings as
possible, and avoid kinking the lines
– Be sure that all oil lines are secured and
protected from damage
– Ensure lines run outside are well insulated
Fuel Oil Supply Systems
(cont'd.)
– Underground oil lines should be installed
with secondary containment
– There should be a shutoff valve on the
suction line at the oil burner
– Fill and vent pipes should pitch toward tank
opening to prevent formation of oil traps
– A vent alarm should be installed in all tanks
at the vent opening
Fuel Oil Supply Systems
(cont'd.)
• The oil dearator allows a one-pipe
system to function as a two-pipe system
• Auxiliary fuel systems get the fuel supply
to the burner if it is too far above the tank
• Fuel line filters remove fine solid
impurities
• Oil safety valve (OSV) stops oil flow if
there is a leak in the suction line
Preparation of Fuel Oil for
Combustion
• Liquid oil must be atomized (broken up
into droplets)
• Oil droplets mix with air, which contains
oxygen
• High-pressure gun-type oil burners feed
oil through a nozzle under pressure and
force air through a tube surrounding it
By-Products of Combustion
• The correct ratio of fuel to air is required
for proper combustion
– 1 lb of fuel oil must be mixed with about 3 lb
of oxygen for complete combustion to occur
• Air is approximately 79% nitrogen and 21%
oxygen; 192 ft³ are needed to burn a single
pound of fuel oil
– Most oil burners provide 50% excess air
• Needs 21.6 lb or 288 ft³ of air for 1lb fuel oil
– By-products are heat, flue gases, CO2, and
water vapor
Gun-Type Oil Burner
• Oil and air are forced into the burner for
mixing and ignition
• Flame retention burner design improves
on conventional design
• Main parts:
– Burner motor provides power for fan/pump
– Burner blower forces air into the chamber
– Fuel oil pumps deliver oil to the chamber
– Nozzle atomizes air prior to ignition
Gun-Type Oil Burner (cont'd.)
– Air tube delivers air to the chamber
– Electrodes vaporize atomized oil droplets
and provide spark for ignition
– Ignition transformer provides high voltage to
electrodes
– Solid state igniters are found in newer oil
burners, replacing ignition transformers
– Primary control unit provides means for
operating the burner and a safety function to
shut down the burner if icombustion does
not occur
Gun-Type Oil Burner (cont'd.)
Figure 32-30 Diagram that compares the flames created
by conventional and flame-retention oil burners
Oil Furnace Wiring Diagrams
• Includes wiring for the fan, the oil burner
primary, and the 24-volt control circuits
• The limit switch is a safety that passes
power to the primary control
• On a call for heat, power is passed to the
ignition transformer, burner motor, and
fuel valve
• The safety device will shut the burner
down in case of a problem
Safety Controls
• Stack switch safety control
– Positioned in the flue pipe
– Bimetal heated by combustion flue gases
• Hot contacts allow the burner motor to continue
operation
• Cold contacts allow current to flow through
safety switch heater
• Cad cell safety control
– When the burner fires, the cad cell senses
light and its resistance drops, causing the
triac to open
Combustion Chamber
• Atomized oil is burned in suspension in
the combustion chamber
• Oil that is not completely ignited will hit
the chamber walls and condense
– Oil vapor hitting chamber walls and
condensing will lower combustion efficiency
• The burner must be matched to the
chamber
• The chamber is a box to contain the fire
Heat Exchanger
• Transfers heat from combustion to the air
that is circulated to heat the structure
– Separates flue gases from the air circulated
to heat the structure
– Should be inspected for cracks during
normal service
• Most state codes prohibit repairing cracks by
welding
• Correct airflow across the heat
exchanger is important
Condensing Oil Furnace
• More efficient than conventional furnaces
• Combustion system includes:
– Burner and related components, combustion
chamber, as many as three heat
exchangers, and vent fan and pipe
• In third heat exchanger, temperatures are
reduced below dew point
• Heated air circulation system includes:
– Blower fan, housing, motor, plenum, and
duct system
Service Procedures
• Pumps: connect vacuum and pressure
gauges to pump
– Pressure should be 0 psig if tank is above
burner
– If tank is below burner, pressure should not
be lower than 17 in. Hg
• 1 in. Hg for each foot of vertical lift
• 1 in. Hg for every 10 feet of horizontal run
– Nozzle pressure should be steady at 100
psig or 140 psig depending on the burner
assembly
Service Procedures (cont'd.)
• Burner motors: press reset button to see
if motor will start
– Check for voltage between orange and
white wires of the primary control
• If there is no voltage, the primary control needs
to be checked
– Check for voltage between black and white
wires on the burner motor
• If there is voltage and the motor does not turn,
the motor is defective
Service Procedures (cont'd.)
• Nozzles: do not try to repair a nozzle
– Nozzles are often replaced annually
– Always make certain to use the correct
nozzle size
– Use proper tools to remove nozzles
– Overheating nozzles can result in oil
breakdown
– After-drip can result in the nozzle getting
clogged
Service Procedures (cont'd.)
• Ignition system
– Transformer
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•
Turn power off and swing the transformer back
Shut off fuel supply to the burner
Restore power to the burner
Check for output voltage of 10,000 volts
– Electrodes
• Ensure the spark gap settings for proper position
• Check insulation for cracks
• Keep electrodes clean
Combustion Efficiency
• Areas to test for proper combustion:
– Draft: determines rate at which flue gases
pass through the furnace
• Too much increases stack temperature, too little
can result in pressure and smoke
– Smoke: indicates incomplete combustion
and fuel waste
– Net stack temperature
– CO2: good indicator of combustion efficiency
• Low reading indicates fuel oil has not burned
completely
Summary
• Oil-fired furnaces are made up of heatproducing and heat-distributing systems
• No. 2 fuel oil produces about 140,000
Btu/gal
• Fuel oil must be atomized before
combustion
Summary (cont’d.)
• Typical oil burner contains a motor,
blower, fuel pump, nozzle, electrodes
and primary control
• Nozzles rated by flow rate, angle, and
spray pattern
• Ignition transformers and electronic
igniters provide the high voltage needed
for spark ignition
Summary (cont’d.)
• The primary control controls burner
operation and safety functions of the
system
• Primary controls can use cad cells or
stack relays
• The resistance of a cad cell decreases
as the amount of light sensed increases
Summary (cont’d.)
• The stack relay responds to the stack
temperature
• Condensing furnaces are more efficient
than conventional furnaces
For more information please contact
Mark T. Weber, M.Ed, CMHE
At
North Seattle Community College
WWW.NorthSeattle.edu
[email protected]