Economizer

Economizers (US and Oxford spelling), or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid.

Boiler, power plant, heating, refrigeration, ventilating, and air conditioning (HVAC) may all use economizers.

Heat transfer efficiency is improved when the highest temperatures near the combustion sources are used for boiling and superheating, while using the residual heat of the cooled combustion gases exhausting from the boiler through an economizer to raise the temperature of feed water entering the steam drum.

Economizer tubes often have projections like fins to increase the heat transfer surface on the combustion gas side.

The percentage of excess air and the temperature of the combustion products are two key variables in evaluating this efficiency.

Combustion produces water steam, and the quantity depends on the amount of natural gas burned.

Natural gas has different combustion efficiency curves linked to the temperature of the gases and the excess air.

[citation needed] The first successful economizer design was used to increase the steam-raising efficiency of the boilers of stationary steam engines.

It consisted of an array of vertical cast iron tubes connected to a tank of water above and below, between which the boiler's exhaust gases passed.

The most successful feature of Green's design of economizer was its mechanical scraping apparatus, which was needed to keep the tubes free of deposits of soot.

Economizers were eventually fitted to virtually all stationary steam engines in the decades following Green's invention.

Modern-day boilers, such as those in coal-fired power stations, are still fitted with economizers which are descendants of Green's original design.

Economizers are commonly used as part of a heat recovery steam generator (HRSG) in a combined cycle power plant.

Economizers lower stack temperatures which may cause condensation of acidic combustion gases and serious equipment corrosion damage if care is not taken in their design and material selection.

Good controls, and valves or dampers, as well as maintenance, are needed to ensure proper operation of the air- and water-side economizers.

For example, for a walk-in freezer that is kept at −20 °F (−29 °C), the main refrigeration components would include: an evaporator coil (a dense arrangement of pipes containing refrigerant and thin metal fins used to remove heat from inside the freezer), fans to blow air over the coil and around the box, an air-cooled condensing unit sited outdoors, and valves and piping.

Systems with economizers aim to produce part of the refrigeration work on high pressures, condition in which gas compressors are normally more efficient.

The economizer concept is linked to subcooling as the condensed liquid line temperature is usually higher than that on the evaporator, making it a good place to apply the notion of increasing efficiencies.

The fluid that arrives at the interstage of both compressors comes from the liquid line and is normally controlled by expansion, pressure and solenoid valves.

A standard two staged cycle of this kind has an expansion valve that expands and modulates the amount of refrigerant incoming at the interstage.

The flash gas that is produced in this tank leaves the liquid line and goes to the economizer entrance of the screw compressor.

[5] The above systems produce an economizer effect by using compressors, meters, valves and heat exchangers within the refrigeration cycle.