2. Heat exchangers are practical devices used to
transfer energy from one fluid to another
To get fluid streams to the right temperature for the
– reactions often require feeds at high temp.
To condense vapours
To evaporate liquids
To recover heat to use elsewhere
To reject low-grade heat
To drive a power cycle
What are heat exchangers for?
3. 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.
9. The aim in a heat recuperator is to transfer the heat contained in the
dryer exhaust air to preheat the drying air. In principle, there are two
types of heat recuperating systems:
Both systems are incorporated after the cyclones. However, incorporating
a bag filter prior to the heat recuperator increases the efficiency, as
deposits on the heat surface cannot be completely avoided even with
correctly selected air velocities in the dust-loaded air. It is possible to
operate the recuperator several days without cleaning, but should it
prove necessary to clean the equipment, this is done by means of a
built-in CIP system.
11. Niro offers two types of Air-Liquid-Air
• Process-Therm heat exchanger
• Hex-Tube heat exchanger
These units are much more flexible with
respect to retrofit installations. The air-
liquid-air heat recovery units consists of
two heat exchangers, one for heat transfer
between the dryer exhaust air and the
other, a finned tube heat exchanger for
preheating the inlet air to the dryer.
Typically water or water-glycol solution
is re-circulated between the two heat
exchangers as the heat transfer liquid. In
this type of heat recuperator the exhaust
air heat exchanger is placed after a bag
filter or a wet scrubber and the finned
tube heat exchanger is located after the
main inlet air filter to the dyer.
Both the Hex-Tube heat exchanger and
the Process-Therm heat recuperator units
can be cleaned during operation or shut
down and are supplied with CIP nozzles
and CIP manifold to connect to the dryer CIP system.
12. The thermal plates are assembled vertically
in compact plate banks with separation
between the plates. Exhaust air is passed
down between the plates and the heat
transfer liquid is re-circulated within the
plates. Heat transfer is a function of the
turbulence achieved in the gas stream; the
closer the plates , the higher the turbulence
and thus the heat transfer. A variable gap
allows the use of optimum conditions
avoiding the risk of plugging. The use
of individual liquid connectors allow the
plates to be isolated singly. This offers
ease of maintenance and operation but
also avoids the expense of total bank Process-Therm installation
Individual plate connections
15. Energy transfer process
Normally the heat transfer between airstreams provided
by the device is termed as 'sensible', which is the exchange
of energy, or enthalpy, resulting in a change in
temperature of the medium (air in this case), but with
no change in moisture content. However, if moisture or
relative humidity levels in the return air stream are high
enough to allow condensation to take place in the device,
then this will cause 'latent' heat to be released and the heat
transfer material will be covered with a film of water. Despite
a corresponding absorption of latent heat, as some
of the water film is evaporated in the opposite airstream,
the water will reduce the thermal resistance of the boundary
layer of the heat exchanger material and thus improve
the heat transfer coefficient of the device, and hence
efficiency. The energy exchange of such devices
now comprises both sensible and latent heat transfer; in
addition to a change in temperature, there is also a
in moisture content of the exhaust air stream.
However, the film of condensation will also slightly
pressure drop through the device, and depending
upon the spacing of the matrix material, this can increase
resistance by up to 30%. If the unit is not laid to falls, and
the condensate not allowed to drain properly, this will
fan energy consumption and reduce the seasonal
efficiency of the device.
16. · Objectives:
– Design a lightweight recuperator applicable to FTT’s Advanced
– Perform a mock build of a recuperator demonstrator
· What is heat transfer?
– The process of heat exchange between two fluids operating under
– 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
discharge air and returns it to the combustor
– Reduces heat losses and therefore increases efficiency
26. Recuperators and regenerators recover heat from the turbine exhaust and use it to
preheat the air from
the compressor before it enters the combustor, thereby saving fuel. This heat
transfer While recuperators and regenerators are quite similar thermodynamically,
they are totally
different in design.
Recuperators are conventional heat exchangers in which hot and cold gases flow
steadily on opposite sides of a solid (usually metal) wall.
Regenerators are periodic-flow devices. Fluid streams flow in opposite directions
through passages in a
wheel 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
28. In a similar manner, turbine reheat can be used to increase the power output of a large-pressure-ratio
turbine. This is the thermodynamic principle in turbojet afterburner firing. Turbine reheat increases
power, but decreases efficiency unless the turbine exhaust heat is used for additional power generation,
is 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 and
turbine discharges, thereby increasing the opportunity to use a recuperator to preheat the burner air
exhaust heat. An intercooled recuperated (ICR) machine is at present in development. The efficiency
decrease 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 extremely
small cooling passages in small blades. This results in low efficiencies, making it difficult for such turbines
to compete with high-volume production (low-cost) reciprocating (piston) engines. The low-pressureratio
recuperated cycle has greater efficiency, although at higher cost. The recuperated cycle is finding
favor 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 considered
for hybrid engine-battery vehicles. Proponents are pursuing the low-pressure-ratio recuperated gas
turbine 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 heat
exchangers. The working fluid may be any gas, and the density of the gas may be varied—to vary the
power delivered by the machine—by using a gas storage cylinder connected to the compressor discharge
36. Use in metallurgical furnaces
Recuperators have also been used to recover heat from
waste gasses to preheat combustion air and fuel for many
years by metallic recuperators to reduce energy costs and
carbon footprint of operation. Compared to alternatives
such as regenerative furnaces, initial costs are lesser,
are no valves to be switching back and forth, there are no
induced-draft fans and it does not require a web of gas
ducts spread up all over the furnace.
Historically the recovery ratios of recuperators compared
to regenerative burners were low. However, recent
to technology have allowed recuperators to
recover 70-80% of the waste heat and pre-heated air up
to 850-900 deg C is now possible.
37. Use in microturbines
Recuperators can be used to increase the efficiency of gas
turbines for power generation, provided the exhaust gas
is hotter than the compressor outlet temperature. The exhaust
heat from the turbine is used to pre-heat the air from
the compressor before further heating in the combustor,
reducing the fuel input required. The larger the temperature
difference between turbine out and compressor out,
the greater the benefit from the recuperator.  Therefore,
microturbines (<1MW), which typically have low
pressure ratios, have the most to gain from the use of a recuperator.
In practice, a doubling of efficiency is possible
through the use of a recuperator. The major practical
challenge for a recuperator in microturbine applications
is coping with the exhaust gas temperature, which can exceed
38. Use in ventilation systems
In heating, ventilation and air-conditioning systems,
HVAC, recuperators are commonly used to re-use waste
heat from exhaust air normally expelled to atmosphere.
Devices typically comprises a series of parallel plates of
aluminium, plastic, stainless steel, or synthetic fibre, alternate
pairs of which are enclosed on two sides to form
twin sets of ducts at right angles to each other, and which
contain the supply and extract air streams. In this manner
heat from the exhaust air stream is transferred through the
separating plates, and into the supply air stream. Manufacturers
claim gross efficiencies of up to 80% depending
upon the specification of the unit.
The characteristics of this device are attributable to the
relationship between the physical size of the unit, in particular
the air path distance, and the spacing of the plates.
For an equal air pressure drop through the device, a small
unit will have a narrower plate spacing and a lower air