What is the function of economizer in steam boiler?

Answer 1

Basically . economiser is located in between exit of boiler and entry of air preheater. when the flue gases are coming out from boiler they take away a lot of heat. economiser utilize this heat from flue gases. and these heat is used to heat the feed water which is going to feed the bolier. hence by using economiser the coal consumption required to heat the feed water is reduced hence cost reduced.

Answer 2

In an air-handling system that provides cooled air to occupants of a building, most of the air is returned from the space back to the air handler to be reused. This is called Return Air, or RA. However, some fresh air (called Outside Air, or OA) must be mixed in. The OA prevents buildup of carbon dioxide in the building, which can be harmful to the occupants. The ratio of RA to OA that is fed into the air handler is usually controlled by dampers.

In places that are very hot outside, the control system closes the OA damper down until the bare minimum OA is flowing, and opens the RA damper to supply the rest. This is because it takes more energy (and thus more money) to cool the hot OA versus re-cooling the RA coming from the space. Local laws require a certain amount of fresh air per occupant, this is the minimum allowed if there are people inside.

However, during the cooler months, the OA may actually be cooler, and thus more economical to use than RA. The economizer is part of the control system that senses this condition, opens the OA damper and closes the RA damper, using the cooler OA instead of mechanical cooling to condition the space.

Note that the economizer usually does not make the decision based on temperature (dry-bulb) alone. This is because the amount of energy in the air is partly from the amount of water vapor. Thus if we have two air masses at the same temperature, but one contains more moisture (higher relative humidity or RH), the air with more moisture will require more energy to cool. The excess moisture is condensed out, which takes energy.

So, when making the decision whether to use the economizer mode or not, the control system needs to know not only temperature, but also RH to calculate the total heat (sensible plus latent) or enthalpy of both airstreams. It then selects the air (either OA or RA) with the lowest enthalpy. If it selects RA, then the economizer is said to be 'off', if it selects OA, then the economizer is said to be 'on'.

Of course, when the economizer is on, mostly (or all) OA is fed into the space. The air pressure would build up too high if the RA had no place to go, so there is a third damper, called Exhaust Air, or EA. When the RA damper closes, the EA damper opens, allowing the RA to exhaust to atmosphere.