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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. The term economizer is used for other purposes as well. Boiler, power plant, heating, refrigeration, ventilating, and air conditioning (HVAC) may all use economizers. In simple terms, an economizer is a heat exchanger.
Steam boilers use large amounts of energy raising feed water to the boiling temperature, converting the water to steam and sometimes superheating that steam above saturation temperature. 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.
An indirect contact or direct contact condensing economizer will recover the residual heat from the combustion products. A series of dampers, an efficient control system, as well as a ventilator, allow all or part of the combustion products to pass through the economizer, depending on the demand for make-up water and/or process water. The temperature of the gases can be lowered from the boiling temperature of the fluid to little more than the incoming feed water temperature while preheating that feed water to the boiling temperature. High pressure boilers typically have larger economizer surfaces than low pressure boilers. Economizer tubes often have projections like fins to increase the heat transfer surface on the combustion gas side. On average over the years, boiler combustion efficiency has risen from 80% to more than 95%. The efficiency of heat produced is directly linked to boiler efficiency. The percentage of excess air and the temperature of the combustion products are two key variables in evaluating this efficiency.
The combustion of natural gas needs a certain quantity of air in order to be complete, so the burners need a flow of excess air in order to operate. Combustion produces water steam, and the quantity depends on the amount of natural gas burned. Also, the evaluation of the dew point depends on the excess air. Natural gas has different combustion efficiency curves linked to the temperature of the gases and the excess air. For example, if the gases are chilled to 38 °C and there is 15% excess air, then the efficiency will be 94%. The condensing economizer can thus recover the sensible and latent heat in the steam condensate contained in the flue gases for the process. The economizer is made of an aluminium and stainless steel alloy. The gases pass through the cylinder, and the water passes through the finned tubes. It condenses about 11% of the water contained in the gases.
Economizers were eventually fitted to virtually all stationary steam engines in the decades following Green's invention. Some preserved stationary steam engine sites still have their Green's economisers although usually they are not used. One such preserved site is the Claymills Pumping Engines Trust in Staffordshire, England, which is in the process of restoring one set of economisers and the associated steam engine which drove them. Another such example is the British Engineerium in Brighton & Hove, where the economiser associated with the boilers for Number 2 Engine is in use, complete with its associated small stationary engine. A third site is Coldharbour Mill Working Wool Museum, where the Green's economiser is in working order, complete with the drive shafts from the Pollit and Wigzell steam engine.
Economizers are commonly used as part of a heat recovery steam generator (HRSG) in a combined cycle power plant. In an HRSG, water passes through an economizer, then a boiler and then a superheater. The economizer also prevents flooding of the boiler with liquid water that is too cold to be boiled given the flow rates and design of the boiler.
A common application of economizers in steam power plants is to capture the waste heat from boiler stack gases (flue gas) and transfer it to the boiler feedwater. This raises the temperature of the boiler feedwater, lowering the needed energy input, in turn reducing the firing rates needed for the rated boiler output. 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.
Air-side economizers can reduce HVAC energy costs in cold and temperate climates while also potentially improving indoor air quality, but are most often not appropriate in hot and humid climates. With the appropriate controls, economizers can be used in climates which experience various weather systems.
When the outside air's dry- and wet-bulb temperatures are low enough, a water-side economizer can use water cooled by a wet cooling tower or a dry cooler (also called a fluid cooler) to cool buildings without operating a chiller. They are historically known as the strainer cycle, but the water-side economizer is not a true thermodynamic cycle. Also, instead of passing the cooling tower water through a strainer and then to the cooling coils, which causes fouling, more often a plate-and-frame heat exchanger is inserted between the cooling tower and chilled water loops.
Good controls, and valves or dampers, as well as maintenance, are needed to ensure proper operation of the air- and water-side economizers.
An economizer display takes advantage of the fact that refrigeration systems have increasing efficiencies at increasing pressures and temperatures. The electricity the gas compressor needs is strongly correlated to both the ratio and the difference, between the discharge and the suction pressures (as well as to other features like the Heat capacity and the type of compressor). Low temperature systems such as freezers move less fluid in same volumes. That means the compressor's pumping is less efficient on low temperature systems. This phenomenon is notorious when taking in account that the evaporation temperature for a walk-in freezer at may be around . Systems with economizers aim to produce part of the refrigeration work on high pressures, condition in which gas compressors are normally more efficient. Depending on the application, this technology either allows smaller compression capacities to be able to supply enough pressure and flow for a system that normally would require bigger compressors, increases the capacity of a system that without economizer would produce less refrigeration, or allows the system to produce the same amount of refrigeration using less power.
The economizer concept is linked to subcooling as the Subcooled liquid line temperature is usually higher than that on the evaporator, making it a good place to apply the notion of increasing efficiencies.Danfos, The Influence of Subcooling on Refrigeration Control Quality. Page 2. [1] Recalling the walk-in freezer example, the normal temperature of the Subcooled liquid in that system is around or even higher (it varies depending on the condensing temperature). That condition is by far less hostile to produce refrigeration, than the evaporator at .
booster has a Heat exchanger that provides subcooling for the compressed fluid.]] A standard two staged cycle of this kind has an expansion valve that expands and modulates the amount of refrigerant incoming at the interstage. As the fluid arriving at the interstage expands, it will tend to evaporation, producing a temperature drop and cooling the second compressor's suction when mixed with the fluid discharged by the first compressor. This kind of set-up may have a heat exchanger between the expansion and the interstage, which may be a second evaporator to produce an additional refrigeration effect, though not as cool as the main evaporator (for example to produce air conditioning or for keeping fresh products). A two staged system is said to be set-up as a booster with subcooling, if the refrigerant arriving at the interstage passes through a subcooling heat exchanger that subcools the compressed fluid arriving at the main evaporator of the same system.Danfos, The Influence of Subcooling on Refrigeration Control Quality. Page 6. [2]
economizer reduces the amount of in the system.]]Economizer screw compressors are built by several manufacturers like Refcomp, Mycom, Bitzer and York. These machines merge both compressors of a two staged system into one screw compressor with two inputs: the main suction and an interstage side entrance for higher pressure gas. This means there is no need to install two compressors and still benefit from the booster concept.
There are two types of economizer setups for these compressors, flash-gas and subcooling. The latter works like a two staged booster with subcooling. The flash economizer is different because it doesn't use a heat exchanger to produce the subcooling. Instead, it has a flash chamber or tank, in which flash gas is produced to lower the temperature of the compressed fluid before the expansion. The flash-gas that is produced in this tank leaves the liquid line and goes to the economizer entrance of the screw compressor.
Recently, machines exclusively designed for this purpose have been developed. In Chile, the manufacturer EcoPac Systems developed a cycle optimizer able to stabilize the temperature of the liquid line and allow either an increase in the refrigeration capacity of the system, or a reduction of the electricity.Pesca y Medioambiente Nº45, New Challenges on Refrigeration, Cycle Optimizers.[3] Such systems have the advantage of not interfering with the original design of the refrigeration system and are a way to expand a single staged system that does not possess an economizer compressor.
An internal heat exchanger is simply a heat exchanger that uses the cold gas leaving the evaporator coil to cool the high-pressure liquid that is headed into the beginning of the evaporator coil via an expansion device. The gas is used to chill a chamber that normally has a series of pipes for the liquid running through it. The superheating then proceeds on to the compressor. The subcooling term refers to cooling the liquid below its boiling point. of subcooling means it is 10 °F colder than boiling at a given pressure. As it represents a difference of temperatures, the subcooling value is not measured on an absolute temperature scale, only on a relative scale as a temperature difference.
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