Mechanical Engineering

A Cupola or Cupola furnace is a melting device used in foundries that can be used to melt cast iron, ni-resist iron and some bronzes. The cupola can be made almost any practical size. The size of a cupola is expressed in diameters and can range from 6 inches to 13 feet. The overall shape is cylindrical and the equipment is arranged vertically, usually supported by four legs. The overall look is similar to a large smokestack.

Cupola and Cupolet operation. CUPOLA VRS CUPOLET Both are the same in most respects from the tuyeres down. The cupolet is short and has a lid to maintain pressure and heat inside the furnace. The cupola has a tall stack and no lid. The operation varies only in how the furnace is charged. The cupolet is a batch melter. The iron charge is bashed very small and the full amount of metal to tap can be charged at one time. When charging be sure not to let the coke bed get too low. If the tap is 100lbs you can often charge the full amount all at one time or as much as will fit in the furnace. Watch the tuyeres as the iron rains and when it stops its time to tap. The cupola is charged in smaller amounts in sequence. If the tap is 100lbs then charge 25lbs iron then 5lbs coke then iron then coke etc. To know when to tap keep time with the charges. Usually 7-10minuts to melt a charge. Mark the time of the first iron charge and watch the tuyeres for the first drops iron. This will give you an idea of how fast you are melting. The slag hole is often used to determine when to tap. As the well fills with iron the slag will run. When iron runs from the slag hole the well is full and the furnace is taped. Conversely cupolets don't always have a slag hole. After a tap the tap hole is left open to run off the slag. Be sure not to over fill a furnace with no slag hole or iron and slag will pour into the wind box. AIR As a general rule of thumb, more is better. If there is any doubt you probably need more. 2 or 3 electric or gas leaf blowers (gas is not recommended due to the fact that you have GAS at an iron pour) would be ok to use on a 16inch furnace. Squirrel cage blowers are no good. Pressure is critical not just volume. In general, for a 16inch bore furnace, 600-900 CFM and 2+ inches of water. (as read on a manometer). Default to more air. BOTOM SAND Ram the sand so that it slopes to the tap. In general, the steeper the slope the better. In some cupolas the sand slopes from the height of the slag hole to the tap hole. However! A steep slope greatly increases the hydraulic pressure at the tap hole and bots have been known to break out. This can happen at any moment. Use anything you want just be sure it cannot run out the bottom like an hourglass. COKE When loading the furnace before burn in use softball size coke (bed coke) and fill to the tuyere level. Then use coke about half that size to fill the rest. Be sure to make room at the tap for burn in and taping. Build a cave at the bottom and also at the tuyeres. Always avoid blocking holes. BURN IN The burn in is the most critical time of the iron pour other than the tap. Almost all the problems that might arise are often linked with a poor burn in. burning in with a gas burner (propane or other) is the easiest way. It allows you to make the coke bed just right and if dun well will not burn away a lot of coke as the furnace reaches temperature. Start every burn in with the burner in the tap hole. Be patient as it can take a long time. The burn in is not just to light the coke, its also to soak the furnace with heat. Watch the tuyeres and note the color of the coke. When the coke is bright orange or yellow you may turn on the blast and remove the burner. NEVER EVER put iron in the furnace until the coke is too bright to look at with the naked eye. Even then the furnace may need time to soak with heat. If iron is charged to soon it will melt and freeze in the well. The furnace needs to be hot enough to melt iron and hotter to keep it liquid until the tap. During the initial burn in the center of the coke may be yellow and the coke near one or more tuyeres may be cold. The blast can be turned on with the tuyeres open and the burner can be held in the cool tuyere. Do this until all the tuyeres are evenly hot and then close the tuyeres. Start with lower blast volume as to not blow out the fire. In a pinch a furnace can be started with wood or charcoal. Briekets make a lot of ash and it can cause clogging. Hard wood charcoal is preferred, its hotter and isn't so ashy. Burring in with wood or charcoal prevents you from packing the bed so watch out for obstructions in the well and tuyeres. TUYERES Keep them clear and free of slag and coke. If there is glass in them be sure to open them the moment there is no blast air this is less critical with non glass. Also the tuyeres stay open whenever the blast air is off, except for short times during charging and taping etc. Opening the tuyeres prevents the build up of explosive gases. It also prevents the furnace heat from breaking the glass. FLUX About a 1/2 lb to every 100lbs of iron. Bash it small and charge it before the iron. It cleans the iron and helps to remove silica. Flux makes the slag flow and helps later when cleaning out the furnace by making the slag easer to brake out. CHARGE In general the furnace should be charged as soon as there is room to do so, iron then coke. If time is needed between taps just replenish the coke that burns. Often the blast is turned off for charging. Bash the iron small. The largest chunk of iron should be no bigger than the size of a serving spoon. Never try to melt sprew cups or thick things. Cupolas can melt bigger chunks than cupolets. Steel is ok to melt but it needs more heat to melt and is usually only supplementary. TAP The furnace should be taped as fast as is safe. A well-run furnace should be able to be taped with a handspike and a hammer. Also a pointy length of rod is good. When taping, hold the ladle on its side to avoid getting crumbly bot in it as the tap is cleared. Always tap in an upward or level direction to avoid breaching the bottom sand. Never ever tap down ward. Quite often the tap needs to be poked a bit after the iron starts flowing to clear slag and obstructions. BOTOM DROP Lookout it's hot!

If the water content of the steam is 5% by mass, then the steam is said to be 95% dry and has a dryness fraction of 0.95.

Dryness fraction can be expressed as:

ζ = ws / (ww + ws) (1)

where

ζ = dryness fraction

ww = mass of water (kg, lb)

ws = mass of steam (kg, lb)

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