Heat Recuperators Air-to-Air, Air-Liquid-Air

A Presentation recupretor Submitted to: Dr. Erfan Uckan Associate Professor Heat exchangers are practical devices used to transfer energy from one fluid to anotherTo get fluid streams to the right temperature for the next processreactions often require feeds at high temp. To condense vapoursTo evaporate liquidsTo recover heat to use elsewhereTo reject low-grade heatTo drive a power cycleWhat are heat exchangers for? what is Recupretor

A recupretor is a one in which the two fluids are separated at all time by a solid barrier.A recuperator is a special purpose counter flow energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat

Recuperators are often used in association with the burner portion of a heat engine, to increase the overall efficiency. For example, in a gas turbine engine, air is compressed, mixed with fuel, which is then burned and used to drive a turbine. The recuperator transfers some of the waste heat in the exhaust to the compressed air, thus preheating it before entering the fuel burner stage. Since the gases have been pre-heated, less fuel is needed to heat the gases up to the turbine inlet temperature. By recovering some of the energy usually lost as waste heat, the recuperator can make a heat engine or gas turbine significantly more efficient.Heat exchangersRecuperatorsRegeneratorsWall separating streamsDirect contactMost heat exchangers have two streams, hot and cold, but some have more than twoMain Categories Of ExchangerRecuperators/Regenerators

Other types of gas-to-gas heat exchangers

- Run around coil

-Thermal wheel, or rotary heat exchanger (including - enthalpy wheel and desiccant wheel) - Heat pipe - Radiation Recuperator - Convection RecuperatorHeat RecuperatorsAir-to-Air, Air-Liquid-Air

The aim in a heat recuperator is to transfer the heat contained in thedryer exhaust air to preheat the drying air. In principle, there are twotypes of heat recuperating systems: Air-to-Air Air-Liquid-Air- Process-Therm- Hex-TubeBoth systems are incorporated after the cyclones. However, incorporatinga bag filter prior to the heat recuperator increases the efficiency, asdeposits on the heat surface cannot be completely avoided even withcorrectly selected air velocities in the dust-loaded air. It is possible tooperate the recuperator several days without cleaning, but should itprove necessary to clean the equipment, this is done by means of abuilt-in CIP system.

Heat Recuperators

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Niro offers two types of Air-Liquid-Airheat recuperators:

Process-Therm heat exchanger Hex-Tube heat exchangerThese units are much more flexible withrespect to retrofit installations. The air-liquid-air heat recovery units consists oftwo heat exchangers, one for heat transferbetween the dryer exhaust air and theother, a finned tube heat exchanger forpreheating the inlet air to the dryer.Typically water or water-glycol solutionis re-circulated between the two heatexchangers as the heat transfer liquid. In this type of heat recuperator the exhaustair heat exchanger is placed after a bagfilter or a wet scrubber and the finnedtube heat exchanger is located after themain inlet air filter to the dyer.Both the Hex-Tube heat exchanger andthe Process-Therm heat recuperator unitscan be cleaned during operation or shutdown and are supplied with CIP nozzlesand CIP manifold to connect to the dryer CIP system.The thermal plates are assembled vertically in compact plate banks with separationbetween the plates. Exhaust air is passeddown between the plates and the heattransfer liquid is re-circulated within theplates. Heat transfer is a function of theturbulence achieved in the gas stream; thecloser the plates , the higher the turbulenceand thus the heat transfer. A variable gapallows the use of optimum conditionsavoiding the risk of plugging. The useof individual liquid connectors allow theplates to be isolated singly. This offersease of maintenance and operation butalso avoids the expense of total bank

Process-Therm installationIndividual plate connections

Energy transfer process

Normally the heat transfer between airstreams providedby the device is termed as 'sensible', which is the exchangeof energy, or enthalpy, resulting in a change intemperature of the medium (air in this case), but withno change in moisture content. However, if moisture orrelative humidity levels in the return air stream are highenough to allow condensation to take place in the device,then this will cause 'latent' heat to be released and the heattransfer material will be covered with a film of water. Despitea corresponding absorption of latent heat, as someof the water film is evaporated in the opposite airstream,the water will reduce the thermal resistance of the boundarylayer of the heat exchanger material and thus improvethe heat transfer coefficient of the device, and hence increaseefficiency. The energy exchange of such devicesnow comprises both sensible and latent heat transfer; inaddition to a change in temperature, there is also a changein moisture content of the exhaust air stream.However, the film of condensation will also slightly increasepressure drop through the device, and dependingupon the spacing of the matrix material, this can increaseresistance by up to 30%. If the unit is not laid to falls, andthe condensate not allowed to drain properly, this will increasefan energy consumption and reduce the seasonalefficiency of the device. Objectives: Design a lightweight recuperator applicable to FTTs Advanced EngineConcept Perform a mock build of a recuperator demonstrator What is heat transfer?

The process of heat exchange between two fluids operating underdifferent temperatures A device that implements this process is a heat exchanger What is a recuperator?

Waste heat recovery heat exchanger Utilizes the hot turbine exit gases to heat a portion of cooler compressordischarge air and returns it to the combustor Reduces heat losses and therefore increases efficiency

Recuperators and regenerators recover heat from the turbine exhaust and use it to preheat the air fromthe compressor before it enters the combustor, thereby saving fuel. This heat transfer While recuperators and regenerators are quite similar thermodynamically, they are totallydifferent in design.

Recuperators are conventional heat exchangers in which hot and cold gases flowsteadily on opposite sides of a solid (usually metal) wall.Regenerators are periodic-flow devices. Fluid streams flow in opposite directions through passages in awheel with heat storage walls. The wheel rotates, transferring heat from one stream to the other.Regenerators usually use a nest of very small parallel passages oriented axially on a wheel which rotates

In a similar manner, turbine reheat can be used to increase the power output of a large-pressure-ratioturbine. This is the thermodynamic principle in turbojet afterburner firing. Turbine reheat increasespower, but decreases efficiency unless the turbine exhaust heat is used for additional power generation, asis the case with a combined cycle, or is used with a recuperator to preheat combustor inlet air.Intercoolers and reheat burners increase the temperature difference between the compressor andturbine discharges, thereby increasing the opportunity to use a recuperator to preheat the burner air withexhaust heat. An intercooled recuperated (ICR) machine is at present in development. The efficiencydecrease at part load of an ICR gas turbine is much less than of conventional simple cycle machines.Small gas turbines have uncooled turbine blades as a result of the difficulty in manufacturing extremelysmall cooling passages in small blades. This results in low efficiencies, making it difficult for such turbinesto compete with high-volume production (low-cost) reciprocating (piston) engines. The low-pressureratiorecuperated cycle has greater efficiency, although at higher cost. The recuperated cycle is findingfavor in programs for small (under 300-kW) gas turbines used for stationary power.Because of their compact size, low emissions, and light weight, gas turbines are also being consideredfor hybrid engine-battery vehicles. Proponents are pursuing the low-pressure-ratio recuperated gasturbine as the way to obtain high efficiency and low emissions in a compact power plant.An ingenious gas turbine cycle is the closed cycle in which the working fluid is sealed in the system.Heat is added to the fluid with an externally fired heater and extracted from the fluid through heatexchangers. The working fluid may be any gas, and the density of the gas may be variedto vary thepower delivered by the machineby using a gas storage cylinder connected to the compressor discharge

Use in metallurgical furnacesRecuperators have also been used to recover heat fromwaste gasses to preheat combustion air and fuel for manyyears by metallic recuperators to reduce energy costs andcarbon footprint of operation. Compared to alternativessuch as regenerative furnaces, initial costs are lesser, thereare no valves to be switching back and forth, there are noinduced-draft fans and it does not require a web of gasducts spread up all over the furnace.Historically the recovery ratios of recuperators comparedto regenerative burners were low. However, recent improvementsto technology have allowed recuperators torecover 70-80% of the waste heat and pre-heated air upto 850-900 deg C is now possible.

Use in microturbines

Recuperators can be used to increase the efficiency of gasturbines for power generation, provided the exhaust gasis hotter than the compressor outlet temperature. The exhaustheat from the turbine is used to pre-heat the air fromthe compressor before further heating in the combustor,reducing the fuel input required. The larger the temperaturedifference between turbine out and compressor out,the greater the benefit from the recuperator. [1] Therefore,microturbines (