ISSN 2395-1621
Improvement in the Heat Exchanger Performance by Changing the Annular Area or Shape of Heat Exchanger Tube
#1Mr.Bapusaheb D.Mahadik, #2Prof .Sapate K.D.
1bapusaheb211@gmail.com
2kdsapate09@gmail.com
#1P.G. Student, Mechanical Engineering Department, TCOE&R,
#2Mechanical Engineering Department, TCOE&R, Pune - 411 048, Maharashtra
Savitribai Phule University,Pune, India2
ABSTRACT ARTICLE INFO
Performance of the heat exchanger is possible to increase by increasing the heat transfer rate and overall heat transfer co-efficient. By changing some parameters it is possible to increase or improve the heat transfer rate and overall heat transfer co- efficient. Out of these parameters the area of heat transfer is one of the parameter and it is increase by changing the annular area or shape of the heat exchanger tube.
Keywords— Heat exchanger, Heat transfer area, Heat transfer rate, Overall heat transfer co-efficient, Shape of the tube
Article History
Received : 8th June 2015 Received in revised form : 9th June April, 2015 Accepted : 12th June, 2015 Published online : 14th June 2015 I. Introduction
Heat Exchangers are commonly used for heat transfer (HT) between two or more fluids of different temperatures. Heat exchangers serve a straightforward purpose: Controlling a system’s or substance’s temperature by adding or removing thermal energy. Although there are many different sizes, levels of sophistication, and types of heat exchangers, they all use a thermally conducting element usually in the form of a tube or plate to separate two fluids, such that one can transfer thermal energy to the other. Home heating systems use a heat exchanger to transfer combustion gas heat to water or air, which is circulated through the house. Power plants use locally available water or ambient air in quite large heat exchangers to condense steam from the turbines.
Many industrial applications use small heat exchangers to establish or maintain a required temperature. In industry, heat exchangers perform many tasks, ranging fromcooling lasers to establishing a controlled sample temperature prior to chromatography.
An exchanger’s effectiveness (ɛ) is the ratio of the actual heat transferred to the heat that could be transferred by an
.
When the hot stream exits the exchanger, it must be warmer than the inlet temperature of the cold stream. In an ideal heat exchanger, with ɛ = 1, the outgoing hot stream’s temperature equals the incoming cold stream’s temperature.
In addition, this heat exchanger’s cold stream exits at a
by the subscript “min”. Given that the temperature drop on the hot stream is greater than the temperature gain in the cold stream in this example, the product of the mass flow rate and the specific heat of the hot stream must be less than that of the cold stream, because of the required heat- transfer rate balance.
Equation for the HEAT-TRANSFER RATE OF THE HEAT EXCHANGER
The heat-transfer rate (Q) of a given exchanger depends on its design and the properties of the two fluid streams. This characteristic can be defined as:
Q=UA∆Tm
where U is the overall heat-transfer coefficient, or the ability to transfer heat between the fluid streams, A is the heat-transfer area of the heat exchanger, or in other words the total area of the wall that separates the two fluids, and ∆ Tm is the average effective temperature difference between the two fluid streams over the length of the heat exchanger.
A heat exchanger’s performance is predicted by calculating the overall heat transfer coefficient U and the area A.
From all above case it is being observed that the factors on which heat transfer rate is depends are U, ∆Tm and A. In this paper we are basically concentrating on the TOTAL HEAT TRANSFER AREA. In all above mentioned applications or all types of heat exchanger the shape of the tube is always circular, hear we use the capsule shaped tube instead of circular tube and searching effects on total heat transfer area and estimating effectiveness of the shell and tube heat exchanger.
II. TERMINOLOGY OF HEAT EXCHANGER TUBE A. CIRCULAR SHAPE TUBE (Traditional)
Do = External diameter of the circular shape tube.
Di = Internal diameter of the circular shape tube.
Ro = External diameter of the circular shape tube.
Ri = Internal diameter of the circular shape tube.
L = Length of the circular shape tube.
t = Thickness of the circular shape tube.
B. CAPSULE SHAPE TUBE (Modified)
do = External diameter of the capsule shape tube.
di = Internal diameter of the capsule shape tube.
ro = External radius of the capsule shape tube.
ri = Internal radius of the capsule shape tube.
L = Length of the capsule shape tube.
t = Thickness of the capsule shape tube.
b = Width of the flat surface of the capsule shape tube.
1. EFFECT ON THE HEAT TRANSFER AREA
Now we are deriving the ratio of the heat transfer area of CAPSULE SHAPE TUBE to the heat transfer area of CIRCULAR SHAPE TUBE. (A cap / A cir )
A. Consider a circular shaped tube, Now,
Area of the circle
Periphery of the circle
Now heat transfer area of the circular tube,
B. Consider capsule shape tube Now,
Area of the capsule,
Assume b = 4di
Since we are only changing the shape of the tube, by keeping the annular or fluid passing area as it is,
That means, AREA OF THE CIRCLE = AREA OF THE CAPSULE
i.e. A = a
Now periphery of the capsule,
Now heat transfer area of the capsule shape tube,
Now take the ratio of the heat transfer area of the capsule tube to the heat transfer area of the circular tube.
Since we are changing only shape of the tube, by keeping the length of the both tube is same.
Therefore LENGTH OF CIRCULAR TUBE = LENGTH OF CAPSULE TUBE
i.e. L = L
From above eqn. it is concluded that the heat transfer area of the given tube is increased by nearly about 43% by only changing the shape of the tube, without changing the annular or fluid passing cross sectional area of the tube, length of the tube & wall thickness of the tube & by keeping all other parameters of the exchanger as it.
2. EFFECTE ON THE HEAT TRANSFER RATE
Now we are computing the ratio of the heat transfer rate through CIRCULAR SHAPE TUBE to the heat transfer rate through CAPSULE SHAPE TUBE.
We have the eqn. for heat transfer rate as given below, For the circular cross sectional tube,
Where,
Qcir = Heat transfer rate for the circular c/s tube.
L = Length of tube.
K = Thermal conductivity of mtl.
∆T = Temp. diff. betn inlet & outlet temp. of the tube.
r1 = Internal radius of the tube.
r2 = External radius of the tube.
Qslab = Heat transfer rate for the flat c/s area.
t = Thickness of the slab or tube.
A. Now consider a tube of circular c/s.
Assume,
Now the heat transfer rate for the circular tube.
OR
Where,
LMA = Log mean area for the circular c/s tube.
Now computing the (LMA)cir,
Put the value of the Ro, Ri & LMA in Eqn. 2 we get,
B. Now consider a capsule shaped tube,
We have,
We have, &
Now consider the circular portion from the capsule.
OR
Now computing the (LMA)cir,
Put the value of the ro, ri & (LMA)cir in Eqn. 4 we get,
Now consider the flat portion from the capsule.
We have, b = 4di & di = 0.3645Do
Put the value of A & t in above eqn. we get,
Total heat transfer rate for the capsule shaped tube is,
Now take the ratio of Qcap to Qcir.
From above eqn. it is seen that the heat transfer rate through capsule shaped tube is nearly 41% more than the circular shaped tube. And it is achieved by only changing the shape of the tube without changing the annular or fluid flowing area & thickness of the wall of the tube as well as by keeping all other parameters of the exchanger as it 3. EFFECT ON THE OVERALL HEAT TRANSFER COEFFICIET
Now we are computing the ratio of the overall heat transfer
OR
Eqn. for flat portion or slab,
A. Now consider the tube of circular shape, In previous eqn. we are determined some values and they are as follow,
Di = 0.9Do, Ri = 0.45Do, Ro = 0.5Do, t = 0.05Do.
We know the overall heat transfer coefficient for circular shape is,
OR
Now we are converting the circular tube in the flat slab that is we are calculating the LMA for circular tube.
Now LMA for circular tube,
B. Now consider the tube of capsule shape.
In previous eqn. we determine the some values and they are as follow,
di = 0.3645Do, do = 0.4645Do, ri = 0.1822Do, ro = 0.2322Do, t = 0.05Do.
For capsule shaped tube,
After calculating the LMA for circular tube.
Now consider the circular portion of the capsule tube.
Now LMA for circular tube,
Now area of the flat portion of the tube,
We have, b = 4di & di = 0.3645Do
Therefore total area of the capsule tube is,
Now put this value of Acap & t = 0.05Do in eqn. 7 We get,
Now compare the eqn. of Ucap and Ucir to each other we get,
From above eqn. it is seen that the heat transfer rate through capsule shaped tube is nearly 41% more than the circular shaped tube. And it is achieved by only changing the shape of the tube without changing the annular or fluid flowing area & thickness of the wall of the tube as well as by keeping all other parameters of the exchanger as it.
III. CONCLUSION
From the eqn. A it is concluded that if we just change the shape of the circular tubeto capsule tube by keeping its annular or fluid passing area as it is, the total heat transfer area of the tube is increased nearly by 44%.
Heat transfer rate (Q) and overall heat transfer coefficient (U) of the heat exchanger are directly proportional to the total heat transfer area (A) of tube, as area is increased,
heat transfer rate and overall heat transfer coefficientalso increases, which are proved by deriving the eqn. B and eqn. C. which shows that heat transfer rate and overall heat transfer coefficient are increased nearly by 41%.
From all these discussion finally it is seen that, if we just change the shape of the tube of the shell and tube heat exchanger by maintaining all other parameter constant the overall performance of the heat exchanger (Which is totally depends on Area, heat transfer rate and overall heat transfer coefficient) is improved.
IV. REFERANCES
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