1. HEAT EXCHANGER

2. Heat exchangers
Heat Exchanger(HE) is process equipment designed for the effective transfer of heat energy between two fluids ; a hot fluid and a coolant.
The purpose may be either to remove heat from a fluid or to add heat to a fluid.
Notable examples are :
Boilers (evaporators) super heaters and condensers of a power plant.
Automobile radiators and oil coolers of heat engines.
Evaporator of an ice plant and milk-chiller of a pasteurizing plant.
Condensers and Evaporators in refrigeration units
Water and Air heaters or coolers.

3. CLASSIFICATION OF HEAT EXCHANGERS
Many types of HE have been developed to meet the widely varying applications based upon their,
Operating principle.
Arrangement of flow path.
Design and certain constructional features.
The HE can be classified into the following categories:

4. 1. Nature of Heat Exchange process
Based upon nature of Heat Exchange process, the heat exchangers are classified into direct contact type, regenerators and recuperators.
fig -Direct contact type or open HE

5. 1. Nature of Heat Exchange process
In Direct contact or open heat exchangers, the energy transfer between the hot and cold fluids is brought about by their complete physical mixing; there is simultaneous transfer of heat and mass.
In a regenerator, the hot fluid is passed through a certain medium called matrix. The heat is transferred to the solid matrix and accumulates there; the operations called heating period.

6. In a recuperator, the fluids flow simultaneously on either side of a separating wall; the heat transfer occurs between the fluid streams without mixing or physical contact with each other.
The wall provides an element of thermal resistance between the fluids and the heat transfer consist of :
Convection between hot fluid and the wall.
Conduction through the wall.
Convection between wall and cold fluid.
Such exchanger are used when the two fluid cannot be allowed to mix.

7. 2. Relative direction of motion of fluids
Based on direction of motion of fluids HE are classified in three categories: parallel flow, counter flow , and cross flow.
In the co-current or parallel flow arrangement, the fluids enter the unit from the same side, flow in the same direction and leave from the same side.
Fig: parallel flow arrangement

8. 2. Relative direction of motion of fluids
In the counter-current or counter-flow arrangement, the fluids enter the unit from the opposite ends, flow in the opposite direction and leave from the opposite ends.
Fig : counter-flow arrangement

9. 2. Relative direction of motion of fluids
In the cross flow arrangement, the two fluids are directed at right angles to each other.
Fig : cross flow arrangement

10. 3. Mechanical design of heat exchange surface
Concentric tubes: two concentric pipes are used, each carrying one of the fluids. The direction may be unidirectional or counter flow arrangement.
Shell and tube: one of fluids is carried through a bundle of tubes enclosed by a shell. The other fluid is forced through the shell and flows over the outside surface of tubes.

11. Multiple shell and tube passes: The two fluids may flow through the exchanger only once(single pass), one or both fluids may traverse the exchanger more than once(multi-pass).
Fig : Multiple shell and tube passes

12. 4. Physical states of heat exchanging fluids (condensation & evaporation)
Condenser: The hot fluid remains at constant temperature all along its passes through the heat exchanger, while the temperature of other fluid (coolant water) gradually increases from inlet to outlet.
Evaporator: During heat exchange in the evaporation of water into steam, the cold-fluid (boiling water) evaporates at constant temperature while the temperature of hot gases continuously decreases from inlet to outlet.

13. The HE can be further classified on the basis of following design parameters:
Temperature and pressure levels of the fluid.
Corrosiveness, toxicity and scale forming tendency of the fluids.
Economic considerations such as cost, ease of manufacture, space, life etc.

14. Performance Analysis
Fig. represent the block diagram of HE. The indicated parameter are:
m= mass flow rate (kg/s)
c = specific heat (j/kg-deg)
t = fluid temp.(◦c)
∆t = Temp. drop or rise of a fluid across the HE
Fig: Block dia. of a HE

15. 1. The hot fluid gives up heat 𝑄 ℎ = 𝑚 ℎ ∗ 𝑐 ℎ 𝑡 ℎ1 − 𝑡 ℎ2 2
1. The hot fluid gives up heat 𝑄 ℎ = 𝑚 ℎ ∗ 𝑐 ℎ 𝑡 ℎ1 − 𝑡 ℎ2 2. The coolant picks up heat 𝑄 𝑐 = 𝑚 𝑐 ∗ 𝑐 𝑐 𝑡 𝑐2 − 𝑡 𝑐1 3. The structure of the HE transfer the heat from the hot fluid to the coolant 𝑄 𝑒𝑥 =𝑈 𝐴 𝜃 𝑚 where 𝑈=overall heat transfer coeff. Between the 2 fluid A = Effective heat transfer area 𝜃 𝑚 = Appropriate mean temp. difference across the HE structure From energy balance the heat given up by the heat fluid is picked up by the coolant on being transferred through the HE. 𝑄 ℎ = 𝑄 𝑐 = 𝑄 𝑒𝑥

16. Condensation and Evaporation
Fig. (a) Fig(b)

17. Counter flow and Parallel flow

18. Logarithmic Mean Temperature Difference (LMTD) Method
Exchange between two fluids, the temp. of the fluids change in the direction of flow and consequently there occurs a change in the thermal load causing the flow of heat.
fig.(a ) represents the temp. conditions existing in surface condenser or fed water heater .
The hot fluid is steam and the cold fluid is water. Here the temp. of steam remains constant but the temp. of water is progressively rising.
During heat exchange in evaporation of water into steam fib(b) , the water evaporate at constant temp. and the temp. of hot gases continually decreases in flowing from inlet to outlet

19. LMTD = 𝜃 𝑚 = 𝜃 1 − 𝜃 2 log 𝜃 1 𝜃 2 = 𝑡 ℎ1 − 𝑡 𝐶2 − 𝑡 ℎ2 − 𝑡 𝑐1 log 𝑡 𝑐2 − 𝑡 𝑐1 𝑡 ℎ2 − 𝑡 𝑐1
If the variation in the temp. of the fluids is relatively small, then temp. variation curves are approximately straight line and sufficiently accurate results are obtained by taking the Arithmetic Mean Temp. Difference (AMTD).
AMTD= 𝜃 1 + 𝜃 2 2

20. Heat exchanger effectiveness (€): the effectiveness of a heat exchanger is defined as the ratio of the energy actually transferred to the maximum theoretical energy transfer.
€= 𝑄 𝑎𝑐𝑡 𝑄 𝑚𝑎𝑥 = 𝑎𝑐𝑡𝑢𝑎𝑙 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟
The actual energy transfer is given by the product of the capacity rate and the temperature difference for either fluid,
Qact= mhch(th1 – th2)
= mc cc (tc2 – tc1)
The maximum available temperature difference= (th1 – tc1)
Maximum possible energy transfer becomes
Qmax = Cmin (th1 – tc1)

21. Effectiveness of Heat exchanger is then:
€= mh ch (th1 – th2) Cmin (th1 – tc1) = ch (th1 – th2) Cmin (th1 – tc1)
€= mc Cc (tc2 – tc1) Cmin (th1 – tc1) = ch (tc2 – tc1) Cmin (th1 – tc1)
Since either the hot or cold fluid may have the minimum value of capacity heat rate, there are two possible values of effectiveness
 Cc<Ch: € ℎ = (th1 – th2) (th1 – tc1)
Cc<Ch : € 𝑐 = (tc2 – tc1) (th1 – tc1)

22. Effectiveness and number of transfer units[NTU]
The concept of LMTD will not work when outlet temp of both the fluids are not known.
Normally in many mechanical application we do not know exit temp of both the fluids . At these situation we use NTU approach which is based on the concept of capacity ratio, effectiveness and number of transfer units.
From energy balance the heat given up by the hot fluid is picked up by the coolant on being through the heat exchanger. Thus,
𝑄 ℎ = 𝑚 ℎ ∗ 𝑐 ℎ 𝑡 ℎ1 − 𝑡 ℎ2 = 𝑄 𝑐 = 𝑚 𝑐 ∗ 𝑐 𝑐 𝑡 𝑐2 − 𝑡 𝑐1 = 𝑄 𝑒𝑥 =𝑈 𝐴 𝜃 𝑚

23. Capacity Ratio(C): The product of m. c (mass
Capacity Ratio(C): The product of m*c (mass * specific heat) of a fluid flowing in a heat exchanger is repeatedly encountered and is termed as the capacity rate. It indicates the capacity of the fluid to store energy at a given rate
Capacity rate of the hot fluid , 𝐶 ℎ = 𝑚 ℎ 𝑐 ℎ
Capacity rate of the cold fluid , 𝐶 𝑐 = 𝑚 𝑐 𝑐 𝑐
The capacity ratio C is defined as the ratio of the minimum to maximum capacity rate.
If 𝑚 ℎ 𝑐 ℎ > 𝑚 𝑐 𝑐 𝑐 : C = 𝑚 𝑐 𝑐 𝑐 𝑚 ℎ 𝑐 ℎ
If 𝑚 ℎ 𝑐 ℎ < 𝑚 𝑐 𝑐 𝑐 : C = 𝑚 ℎ 𝑐 ℎ 𝑚 𝑐 𝑐 𝑐

24. Number of Transfer Units (NTU): the number of transfer units (NTU) is a measure of the size of heat exchanger; it provides some indication of the size of the heat exchanger. It is defined as:
NTU = 𝑈𝐴 𝑚 𝐶 𝑐 𝑐 = when 𝑚 ℎ 𝑐 ℎ > 𝑚 𝐶 𝑐 𝑐
NTU = 𝑈𝐴 𝑚 ℎ 𝑐 ℎ = when 𝑚 ℎ 𝑐 ℎ < 𝑚 𝐶 𝑐 𝑐
The denominator is always the smaller thermal capacity rate and therefore
NTU = 𝑈𝐴 ( 𝑚𝑐) 𝑚𝑖𝑛 = 𝑈𝐴 𝐶 𝑚𝑖𝑛

25. Effectiveness for the parallel flow heat exchanger
Effectiveness for the Counter flow heat exchanger
Effectiveness(€) = 1− 𝑒 (−𝑁𝑇𝑈 1−𝐶 ) 1−𝐶 𝑒 (−𝑁𝑇𝑈 1−𝐶 )
Where ,
C = 𝐶 𝑚𝑖𝑛 𝐶 𝑚𝑎𝑥 , NTU = 𝑈𝐴 𝐶 𝑚𝑖𝑛

26. Limiting Values of capacity ratio
Case 1 : When C = 0
for Boiling & Condensation for one fluid change of Temp is zero that means capacity rate Cmax
C = 𝐶 𝑚𝑖𝑛 𝐶 𝑚𝑎𝑥 = 0
For Both Parallel and Counter Flow
€ = 1- 𝑒 (−𝑁𝑇𝑈)

27. Case 2 : When C = 1
In gas turbine , exhaust gases after expansion in the turbine are used to heat the compressed air so both the fluids have same capacity rate,
Therefor C = 𝐶 𝑚𝑖𝑛 𝐶 𝑚𝑎𝑥 = 1
For parallel flow Heat Exchanger
€ = 1− 𝑒 (−2𝑁𝑇𝑈) 2
For Counter Flow Heat Exchanger
€ = 𝑁𝑇𝑈 1+𝑁𝑇𝑈