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Oil Mechanical

Oil Mechanical. Chimneys & Draft. Introduction. A path must exist, to remove combustion gases. Chimney design can not be changed. You must be able to get the best efficiency under all conditions. Draft. Is suction of air up the chimney. Created by Heat or Natural air currents. Draft.

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Oil Mechanical

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  1. Oil Mechanical Chimneys & Draft

  2. Introduction • A path must exist, to remove combustion gases. • Chimney design can not be changed. • You must be able to get the best efficiency under all conditions.

  3. Draft • Is suction of air up the chimney. • Created by Heat or Natural air currents.

  4. Draft • If created by heat it is known as a thermal draft. • If created by air currents it is known as a natural draft. • Wind blowing across a chimney is current draft

  5. Draft • If adding a fan or Blower (draft inducer) • The induced draft serves as a booster. • The forced draft handles all combustion gases. And forcing it out of the building.

  6. Why is draft needed • Slight vacuum must be present in combustion area. • Without a vacuum, the flame will pulsate & rumble • Also overheating of the nozzle & chamber.

  7. Draft measurement • Measured in inches of water column. • One inch of water column is the pressure difference required to lift a column of water one inch.

  8. Draft measurement • One psig = 27.7 inches of water column. • Typical oil stack draft should -0.04 inches of water column (-0.03,-0.05)

  9. Measurement • Acquired by use of a draft gauge. • Reading: • Suction=negative pressure=pulling force at over the fire & the stack

  10. Measurement • If positive pressure exists: • No venting can take place. • Air from outside is coming the stack (down draft)

  11. Combustion gases • Are hotter than out side air. Hotter means lighter. Lighter means automatic rise up the chimney. Resulting in suction at the over fire.

  12. Determining factors • One CFM of air at 60 degrees F • One CFM of air at 600 degrees F • Chimney Height With correct flue: Higher chimney= Greater draft

  13. Determining factors • Hot gases=hotter chimney=increased draft. • As weather changes, On a cold day you will have greater draft. Greater temperature difference between chimney & outside air. • Warm day you will have a lower draft. Less temperature difference chimney & outside air

  14. Determining factors • Since a chimney is a permanent structure we consider it to be constant. • Variables are the temperature of flue gas & the outdoor temperature. • These variable are constantly changing.

  15. Determining factors • Outdoor temperature is beyond our control. Temperature of flue gas we can control. • Adjusted by addition or deletion of combustion. Every time you change burner adjustment you effect the draft

  16. Determining factors • Chimneys must be warm & dry. Must extend 2ft above surrounding objects. Also vertical & straight.

  17. Draft regulation • Since a chimney is cool at start up. Draft will increase as temperature rises. • Over year’s of operation draft varies from: Low of .02 to a High of .14

  18. Draft regulation • Change is not wanted. They give a end result of: Smoke change, reduced efficiency & a high draft at shut off which equals heat loss up the chimney.

  19. Draft effect on burner air • Vary the draft to compensate for any changes. A drop in draft effects air flow into the chamber. • After proper draft is obtained the air supply is set. • An over fire draft should be .01 to .02

  20. Draft regulators • Known as a by-pass or air bleed. Uses a swinging door with a counter weight. • As the door opens room air mixes with flue exhaust. • Cooling stack gas temperature lowering draft & reducing pressure by allowing air to enter.

  21. Draft regulators • Door opens & closes due to changes in room & chimney pressures regulating the draft. • Proper name is the barometric draft regulator. • A draft regulator can only reduce over fire draft. It can not increase stack draft.

  22. Measuring & Adjusting • Use of a draft gauge. Check for over fire draft. Shaft must be horizontal. • Door must be free. Also operate burner for a minimum of five minutes.

  23. Over fire draft • Lowest possible level .01 to .02 • If over adjust regulator counterweight to open. Keep in mind a second regulator may be needed. • If below adjust counterweight to slightly close door.

  24. Draft in stack • Greater than over fire draft (.02 to .04) • Normally (.03 to .06 (try for .05) • If stack is too high there is a indication of a dirty heat exchanger. Soot will build up.

  25. Draft regulator • This will increase restrictions, air delivery falls off, smoke is the end result. • If lower this is a indication of a restriction.

  26. Chimney & stack sizing • One square inch for every 16,500 btu input. Never override local codes. • Stack sizing same size as unit collar. Never undersize. • Rule of thumb 6’’ under 1.0 gph, 7’’ up to 1.5 gph 8’’ under 1.5 to 2.0 gph

  27. Stack installation • As short as possible, uniform in size, use 45’s instead of 90’s. • Firmly join all sections with 3 screws, & cement all holes even at the chimney.

  28. Draft regulator installation • Mounts between chimney and stack control. Must be a minimum of 18’’ from control. It must be level. • Must be 18’’ minimum from combustible surfaces. Should 10’’ larger than the stack.

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