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Effectiveness of Rotary Air Preheater in a Thermal Power Plant

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 International Journal of Scientific Engineering and Technology ISSN:2277-1581
Volume No.5 Issue No.12, pp: 526-531 01 December 2016

Effectiveness of Rotary Air Preheater in a Thermal Power Plant

M. Praveen, P. S. Kishore
Heat transfer in Energy Systems, Department of Mechanical Engineering, College of Engineering (A),
Andhra University, Visakhapatnam, Andhra Pradesh, India
Corresponding Email: moyyapraveen@gmail.com

Abstract: Energy saving is one of the key issues, not only effectiveness of fixed bed regenerators for cases with equal
from the viewpoint of fuel consumption but also for the reduced periods and lengths could be compared to cases with
protection of global environment. Large quantity of hot flue unequal reduced periods and lengths. Mounika et al. [3] in
gases is released from boilers, kilns, ovens and furnaces. If their paper studied about the performance analysis of
some of this waste heat could be recovered, a considerable automobile radiator. It is similar to cross flow heat exchanger
amount of primary fuel could be saved. One of the efforts has which is designed to transfer the heat from the hot coolant
been made for heat recovery in Thermal Power Plants using coming from the engine to the air blown through it by the fan.
rotary regenerators. The present study is on the performance A small segment of the radiator is analyzed for the various
analysis of rotary air preheater in a Thermal power plant. In speed of the air striking the radiator as the vehicle moves
this thesis, the performance of air preheater is evaluated from its rest position to a certain speed. Skiepko [4] in this
based on the operating conditions such as mass flow rate and paper is to compare results obtained based on theoretical
temperature of both air and flue gases. In this analysis the modelling with directly measured experimental data on a full
variation of Reynolds number, Colburn j factor and, Friction scale operating air preheater. London et al. [5] developed
factor at the operating conditions are studied. The differential equations describing the transient behaviour of
effectiveness of the air preheater is evaluated using Kays and temperatures in rotary regenerators, obtaining analytical
London correlation and compared with the counter flow heat solutions supplemented by an experiment using an electrical
exchanger at the same operating conditions of air preheater. analog system and numerical solutions. Bahnke and Howard
The variation of pressure drop, heat transfer coefficient, and [6] obtained numerical solutions for steady state temperatures
heat transfer rate at different mass flow rate of air observed. in a rotary regenerator, including the effect of heat
conduction in the matrix material along the flow direction.
Keywords: Air preheater, Colburn j factor, Friction factor, Rajnish kumar and Kishore [7] in this paper, an experimental
Heat transfer coefficient, Pressure drop, Effectiveness. study of the condensation of water vapor from a binary
mixture of air and low-grade steam has been depicted. The
I. INTRODUCTION study is based upon diffusion heat transfer in the presence of
Rotary air preheater is the main equipment used to recover high concentration of non condensable gas. To simplify the
waste heat from the exhaust flue gases in thermal power plants. study, experimental analysis is supported by empirical
By preheating the combustion air with the hot flue gases solutions. The main objective of this work is to establish an
leaving out of the boiler, a considerable increase in efficiency approximate value for surface area and overall heat transfer
is obtained. The ducts of hot gas and cool air are arranged in coefficient of a horizontal shell and tube condenser used in
such a way that both the flue gas and the inlet air flows process space. Sepehr Sanaye and Hassan [8] in this paper the
simultaneously through the air preheater as shown in fig-1. The pressure drop and effectiveness of rotary regenerator are
air preheater absorbs the heat energy from the flue gases and important parameters in optimal design of this equipment for
stores in the matrix bed. The air from the surroundings is industrial applications. For optimal design of such a system,
allowed to flow through the matrix bed and gets heated up. The it was thermally modelled using ԑ-Ntu method to estimate its
heated air from the air prehater flows through two ducts as pressure drop and effectiveness. Sreedhar Vulloju et al. [9]
primary air and secondary air. The primary air enters into the tested air preheater elements using cold flow studies. He
mills to dry the coal and transport the pulverized coal to burner proved that performance of Ljungstrom air preheater is
and, the secondary air is used for better combustion in furnace. dependent on the heat transfer element profiles. It is
Several researchers discussed performance analysis of air necessary to develop element profiles with lesser pressure
preheater used for different purposes and developed drops for efficient heat transfer with lower power
correlations both numerically and experimentally. Air consumption to improve overall efficiency of power plant.
preheater is one of the heat exchanger, Shanker and Kishore
[1] in their paper carried out performance evaluation on charge II. AIR PREHEATER
air coolers with varying mass flow rates on hot side from and The function of air preheater is to increase the temperature of
keeping cold fluid rate is constant, and found that water-cooled the air before it enters the furnace. It is generally placed after
charge air cooler is having higher effectiveness than air-cooled the economizer, so that the flue gases pass through the
type. Saunders and Smoleniec [2] suggested that the economizer and then to the air pre heater. The Ljungstrom

IJSET@2016 doi : 10.17950/ijset/v5s12/1201 Page 526

 International Journal of Scientific Engineering and Technology ISSN:2277-1581
Volume No.5 Issue No.12, pp: 526-531 01 December 2016

Air preheater is a try sector type as shown in fig-2 with rotor temperature without extracting the heat. To reduce the air
diameter and hub diameter are 8.89m, 1.42m respectively. The leakage seals are provided. It is an implied requirement that
height of the heating elements of three sections are respectively the rotating parts should have some working clearance
450mm, 1050mm, and 300mm from top to bottom of the rotor. between the static parts to avoid any interference between
The cold end heating elements are made of Corten steel while them. Here, in air preheaters, rotors are constructed to have
the hot and intermediate end heating elements are made of high thermal expansion and these gaps are close with the
Carbon steel. The heating elements present in all three sections flexible seal leaves. Major types of seals used in thermal
are double undulated element profiles having hydraulic power plant are radial seals, axial seals, bypass seals, and
diameter (Dh) is 0.7475 mm. The total mass of the air preheater circumferential seals.
is approximately 144 tons. Total surface area of the air
preheater or total matrix surface area is 17000 m2, and the III. DESIGN THEORY OF AIR PREHEATER
matrix bed porosity is 0.78. The differential equations are subject to the normal
idealizations made in the design theory. The differential
equations can be made dimensionless to arrive at the
dimensionless groups for the effectiveness - number of
transfer units (ԑ-Ntuo) or the reduced length - reduced period
(A-П) methods of solution. The ԑ-Ntuo method is usually used
for rotary regenerator. It can be shown that the effectiveness
of a counter flow regenerator is a function of six
dimensionless variables.
ԑ = f ( Ntuo, (hA)* , Cr*, C*, λ, Ak* )
The most important assumption in the above relation is that
there is no pressure or carryover leakage. In a direct contact
type recuperator the effectiveness is only a function of Ntu o
Fig-1: Flow passages in the rotary regenerator and C*. The parameters (hA)* and Cr* arise because of the
heat storing nature of a regenerator. The longitudinal heat
conduction effect is accounted for by λ and Ak*. An
advantage of this particular choice of dimensionless
parameters is that the influence of (hA)* and Ak* on the
effectiveness are small and that the solution method parallels
that of a recuperator for the case of infinite Cr* (infinite
rotational speed), so the effectiveness as a function of the six
above-mentioned parameters. In the case of zero longitudinal
heat conduction the functional relationship for the
effectiveness becomes
ԑ = f ( Ntuo, (hA)* , Cr*, C* )
As mentioned earlier, the wall temperature profile in a
regenerator (in the absence of longitudinal wall heat
conduction) is going to be dependent on the thermal
conductance’s (hA)h and (hA)c between the matrix wall and
the hot or cold fluids. For a high temperature regenerator, the
thermal conductance will not only include convection
conductance but also radiation conductance. The
dimensionless group that takes into account the effect of the
Fig-2: Air preheater
convection conductance ratio is (hA)*. Lambertson (1958)
The rotary air preheater is a counter flow regenerative rotary
and others have shown through a detailed analysis that (hA) *
type heat exchanger. Specially corrugated heating elements are
has a negligible influence on the regenerator effectiveness for
tightly placed in the sector compartment of the rotor. The rotor
the range 0.25 ≤ (hA)* < 4. Since most regenerators operate
turns at a speed of 1.42 rpm and is divided into gas channels
in this range of (hA)*, fortunately, the effect of (hA)* on the
and air channels. The air side is made of primary air channels
regenerator effectiveness can usually be ignored.
and secondary air channels. When gas flows through the rotor,
it releases heat and delivers it to the heating elements and then Convection conductance ratio (hA)*=
the gas temperature drops, when the heated elements turn to Now the effectiveness of a counter flow regenerator is a
the air side, the air passing through them is heated and its function of three dimensionless variables.
temperature is increased. By continuing maintaining such a ԑ = f ( Ntuo, Cr*, C* )
circulation, the heat exchange is achieved between gas and air. For specified Ntuo, C*, and Cr*, the effectiveness generally
Usually air leaks in to the gas in the air preheater due to decreases with decreasing values of (hA)*, and the reverse
pressure differences. This leakage air decreases the flue gas occurs for large values of Ntuo and C* ≈ 1. However, the

IJSET@2016 doi : 10.17950/ijset/v5s12/1201 Page 527

 International Journal of Scientific Engineering and Technology ISSN:2277-1581
Volume No.5 Issue No.12, pp: 526-531 01 December 2016

influence of (hA)* on ԑ is negligibly small for 0.25 ≤ (hA) * < Now let us present approximate formulas to compute
4, as shown by Lambertson, among others. When Cr* tends to effectiveness (ԑ) for a wide range of Cr* and C*. The
∞, the effectiveness ԑ of a regenerator approaches that of a influence of Cr* on effectiveness (ԑ) can be presented by an
recuperator. The difference in ԑ for Cr* ≥ 5 and that for Cr*= ∞ empirical correlation for ԑ ≤ 90% by Kays and London.
is negligibly small and may be ignored for the design purpose. Now finally the effectiveness of air preheater is given by
ԑ = ԑcf [1- ] (11)
IV. ANALYSIS OF AIR PREHEATER
The performance of air preheater can be analyzed by putting Where
the necessary equations in order as below: ԑcf = (12)
Frontal or face area of air preheater is given by
Cr = (M Cm N)
Afr = Rotor cross sectional area (fraction of rotor face area Heat transfer rate (Q) from the flue gas stream to the air
not covered by radial seals) stream is given by
(1)
Rotor cross sectional area = disk area – hub area ԑ=
The flue gas and air side frontal areas are proportional to their ԑ=
respective flow split ratio. The flow split ratio of air preheater
is 1:1 Q = ԑ Qmax (13)
Now hot side frontal area, and cold side frontal is given by The expression for evaluating pressure drop by kays &
London correlation
Afrh = ( ) Afrc = ( ) Pressure drop in the flue gas flow is given by
(2)
∆pf= ] (14)
Mass velocity of flue gases is given by
Gf = Pressure drop in the air flow is given by
∆pa = ] (15)
(3)
Reynolds number of flue gases is determined by Effectiveness (εo) of the Counter flow heat exchanger is
Ref = given by
εo = (or) (16)
(4)
Colburn j factor and Friction factor data for the matrix surfaces
in rotary regenerators are presented as experimental V. RESULTS AND DISCUSSION
correlations in terms of Reynolds number. These correlations
for some matrix surfaces are given by Kays and London. The
rotor of this air heater is enclosed in a causing, the cross
section of it can be square, triangle, or orthogonal. For large
diameter rotor the orthogonal casing is provided.
Colburn j factor of flue gases (jf) is given by
jf = exp[ E + F (ln Ref) + G (ln Ref)2 + H (ln Ref)3 ] (5)
Friction factor of flue gases (ff) calculated from the following
correlation
ff = exp[ A + B (ln Ref) + C (ln Ref)2 + D (ln Ref)3 ]
(6)
Stanton number of flue gases (Stf) is given by
Stf = ( jf Prf-2/3) (7) Fig-3: Variation of Colburn j factor of air Vs Reynolds
The heat transfer coefficient of flue gases (hf) can be calculated number of air
from the following correlation
hf = (Stf Gf Cpf ) (8)
Similarly the above calculations can be done to air side also.
Modified number of transfer units is given by
Ntuo = (9)

Surface area of the both streams are proportional to the

respective flow split ratio, so that
Af = , and Aa =
Now overall heat transfer coefficient (Uo) is given by
= + (10) Fig-4: Variation of Colburn j factor of flue gases Vs
Reynolds number of flue gases

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 International Journal of Scientific Engineering and Technology ISSN:2277-1581
Volume No.5 Issue No.12, pp: 526-531 01 December 2016

The variation of Colburn j factor with Reynolds number of Reynolds number of flue gases increases due to decrease in
both streams is shown in the Fig-3 and Fig-4. The result shows viscosity, so there is a decrease in Friction factor. The range
that the Colburn j factor decreases with increase in Reynolds of Reynolds number in this analysis is less, hence the curve
number. Colburn j factor is an exponential function of looks like approximate linear.
Reynolds number. With increase in the mass flow rate of air
entering into the air preheater the Reynolds number of air
increases there by the Colburn j factor decreases. In this
analysis mass flow rate of flue gases entering into the air
preheater is same. The flue gases temperature decreases while
passing through the air preheater, so viscosity of the of the flue
gases decreases. Reynolds number of flue gases increases due
to decrease in viscosity, so there is a decrease in Colburn j
factor. The range of Reynolds number in this analysis is less,
so the curve looks like approximate linear.

Fig-7: Variation of Air stream pressure drop (Pa) Vs Mass

flow rate of air (kg/s)

The variation of air stream pressure drop (∆p) with mass flow
rate of air (ma) is shown in the Fig -7. The result shows that
the air stream pressure drop increases with increase in mass
flow rate of air. Pressure drop is a polynomial function of
mass velocity (G) and Friction factor. As mass flow rate of
Fig-5: Variation of Friction factor of air Vs Reynolds number air increases mass velocity increases, but there is decrease in
of air Friction factor. The decrease in Friction factor is negligible
when compared to increase in mass velocity. The range of
mass flow rate in this analysis is less, so the curve looks like
approximate linear.

Fig-6: Variation of Friction factor of flue gases Vs Reynolds

number of flue gases

The above Fig-5and Fig-6 shows the variation of Friction Fig-8: Variation of Heat transfer coefficient of air Vs Mass
factor with Reynolds number of both streams. The result shows flow rate of air
that the Friction factor decreases with increase in Reynolds
number. Friction factor is an exponential function of Reynolds The above Fig-8 shows the variation of heat transfer
number. With increase in the mass flow rate of air entering into coefficient of air (ha) with mass flow rate of air (ma). The
the air preheater the Reynolds number of air increases there by result shows that the heat transfer coefficient of air (h a)
the Friction factor decreases. In this analysis mass flow rate of increases with increase in mass flow rate of air. Mass velocity
flue gases entering into the air preheater is same. The flue of air increases with increase in mass flow rate of air, hence
gases temperature decreases while passing through the air the heat transfer coefficient of air increases.
preheater, so viscosity of the of the flue gases decreases.

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 International Journal of Scientific Engineering and Technology ISSN:2277-1581
Volume No.5 Issue No.12, pp: 526-531 01 December 2016

side was decreased by 2%, heat transfer coefficient of air

increased by 1%, and pressure drop of air increased by
34.4%.
2. Mass flow rate of air increases from 165 kg/s to
184.21 kg/s. Then the Heat transfer rate is increased by
13.7%.
3. Number of transfer units increases from 4.34 to
4.79. Then the effectiveness by Kays and London
correlation is increased by 3.3%.
4. Heat capacity rate ratio increases from 0.9 to 0.99,
then the effectiveness of the air preheater by Kays and
London correlation decreased by 5%.

NOMENCLATURE
Fig-9 Variation of Heat transfer rate (kW) Vs Mass flow rate
A overall heat transfer surface area
of air (kg/s)
Afr rotor cross sectional area, (m2)
C flow stream heat capacity rate, (W/K)
C* heat capacity rate ratio (Cmin/Cmax)
Cmin minimum of Ch and Cc, (W/K)
Cmax maximum of Ch and Cc, (W/K)
Cr total heat capacity rate of a matrix, (W/K)
Cr* total matrix heat capacity rate ratio (Cr/ Cmin)
Cp specific heat at constant pressure, (J/kg-K)
Cm specific heat of matrix, (J/kg-K)
D rotor diameter or disk diameter, (m)
d hub diameter, (m)
Dh hydraulic diameter, (m)
F Friction factor
G mass velocity, (kg/m2-s)
h convective heat transfer coefficient, (W/m2-K)
*
Fig-10: Variation of Effectiveness Vs Ntuo (hA) symmetry factor related to thermal resistance (hA)
on the Cmin side/ (hA) on the Cmax side)
The above Fig-9 shows the variation of heat transfer rate (Q) J Colburn j factor for heat transfer (St Pr 2/3)
with mass flow rate of air (ma). The result shows that the heat L disk height, (m)
transfer rate increases with increase in mass flow rate of air m mass flow rate, (kg/s)
due to the rate of heat transfer is directly proportional to the M mass of matrix bed, (kg)
mass flow rate. N rotational speed, (rpm)
The above Fig-8 shows the variation of effectiveness (ε) with Ntuo number of transfer units
Ntuo. The result shows that the effectiveness by Kays and P pressure, (Pa)
London correlation increases with increase in number of Pr Prandtl number (μCp/k)
transfer unit (Ntuo), similarly the effectiveness of counter flow Q rate of heat transfer (kW)
heat exchanger also increases with Ntuo. From the figure we Re Reynolds number (GDh/μ)
noticed that the effectiveness by kays and London is always T temperature, (oC)
more than the effectiveness by counter flow heat exchanger. Uo overall heat transfer coefficient, (W/m2-K)
Therefore the performance of air preheater is better than the Greek symbols
counter flow heat exchanger. Μ dynamic viscosity, (N-s/m2)
ԑ Effectiveness
VI. Conclusions ρ fluid density, (kg/m3)
The main points from the present study can be concluded as ρm matrix density, (kg/m3)
follows: σ porosity
1. Mass flow rate of air varies from 165 to 191.67kg/s ∆p pressure drop, (Pa)
and mass flow rate flue gases kept constant at 166.67 kg/s, Subscripts
the thermal performance of the air preheater was analyzed a air
using ԑ-NTU method and results obtained are Colburn-j f flue gases
factor on air side was decreased by 13.74%, Colburn-j factor o outlet
on flue gas side was decreased by 2.04%, Friction factor on I inlet
air side was decreased by 13.45%, Friction factor on flue gas max maximum
min minimum

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 International Journal of Scientific Engineering and Technology ISSN:2277-1581
Volume No.5 Issue No.12, pp: 526-531 01 December 2016

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