Lab Manual 2

KC21001 LABORATORY 4 LAB MANUAL 2 Group 1

1. 2. 3. 4.

 Adam bin Abdul Abdul Rahman Corezza Adele Chin Nigel Pearce Paramanathan Rasrina bte Razin Wong

BK13110006 BK13110006 BK13160546 BK13160546 BK13110277 BK13110277 BK13110367 BK13110367

Experiment HE3: Double Pipe Heat Exchanger 

Date: 01/04/2015 Objectives 1. To investigate the effects of cold water flow rate on the heat-transfer efficiency (n), real heat-transfer coefficient (k re re) and theoretical heat-transfer coefficient (k  th th). 2. To investigate the best conditions of heat exchanger in the double tube heat exchanger: compare between co-current and counter-current configurations, compare between laminar and turbulent flows.

Theory and Explanation Heat is normally transfer from high temperature region to a lower temperature region until it reaches thermal equilibrium as stated in the second law of thermodynamics. Heat exchangers are a device that exchange the heat between two fluids of different temperatures that are separated by a solid wall. The temperature gradient, or the differences in temperature facilitate this transfer of heat. Transfer of heat happens by three principle means: radiation, conduction and convection. In the use of heat exchangers radiation does take place. However, in comparison to conduction and convection, radiation does not play a major role. Conduction occurs as the heat from the higher temperature fluid passes through the solid wall. To maximize the heat transfer, the wall should be thin and made of a very conductive material. The biggest contribution to heat transfer in a heat exchanger is made through convection. The double-pipe heat exchanger is one of the simplest types of heat exchangers. It is called a double-pipe exchanger because because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first. This is a concentric tube construction. Flow in a double-pipe heat exchanger can be co-current or counter-current. There are two flow configurations: co-current is when the flow of the two streams is in the same direction, counter current is when the flow of the streams is in opposite directions. Co-current Operation

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KC21001 LABORATORY 4 LAB MANUAL 2

In co-current operation, the hot and cold fluid streams flow in the same direction across the heat transfer surface (the two fluid streams enter the heat exchanger at the same end).

Figure 1:  Mechanism of co-current flow arrangement in double tube heat exchanger

Counter-current Operation In counter-current operation, the hot and cold fluid streams flow in opposites directions across the heat transfer surface. The fluids travel roughly perpendicular to one another through the exchanger. The counter current design is most efficient, in that it can transfer the most heat from the heat (transfer) medium.

Figure 2: Mechanism of counter-current flow arrangement in double tube heat exchanger

The determination of the overall heat-transfer coefficient is necessary in order to determine the heat transferred from the inner pipe to the outer pipe. This coefficient takes into account all of the conductive and convective resistances (k and h, respectively) between fluids separated by the inner pipe, and also takes into account thermal resistances caused by fouling (rust, scaling, i.e.) on both sides of the inner pipe. The type of fluid flow is an important factor in the determination of the heat transfer coefficient. Two major classifications of fluid flow occur which greatly affect coefficient. If the fluid moves in parallel layers with molecular momentum transfer between adjacent layers, one speaks of laminar flow; if mixing of the fluid layers occurs because of bulk fluid motion we call it turbulent flow. The only part of the overall heat-transfer coefficient that needs to be determined is the convective heat-transfer coefficients. Correlations are used to relate the Reynolds number 2

KC21001 LABORATORY 4 LAB MANUAL 2

to the heat-transfer coefficient. The Reynolds number is a dimensionless ratio of the inertial and viscous forces in flow . Re =

 .  

For laminar flow, the rate of heat transfer is predominantly governed by thermal conduction between the layers of the fluid. Heat transfer for turbulent flows, however, is largely a result of gross fluid motion, that is, convective movement of the fluid; thermal conductivity, in this case, plays a minor role in the transfer of heat. For a plate exchanger: Nu = 0,383 Re

0.65

x Pr0.4

For a tube exchanger: Nu = 4 Pr1/3

(laminar flow)

Nu = 0.0225 Re Where, Nu =

0.8

    ; Pr = ; v =  ;     

x Pr0.4

D is

(turbulent flow)

the diameter of the tube.

The observed viscosity in some dimensionless numbers is very temperature-sensitive. Laminar flow : Re < 2100 Turbulent flow : Re > 2100 Hydraulic diameter: δhydro

=4

σ P

Where  = section perpendicular to the flow, and P = wet perimeter 3.464 P 2

De =

 – d0

  d 0

Passage surface, aCT =

 Dc  P 

( P   d 0 ) B

The power transmission of fluid: Ф f  = ρf Qf Cpf (ts-te)

cold water receive power

Ф c  = ρcQcCpc(Te-Ts)

hot water transfer power Ф  =

Heat-transfer efficiency, n  =

 c



2

   f    c

Heat-transfer coefficient:

3

 f  

KC21001 LABORATORY 4 LAB MANUAL 2

 

(a) Real heat-transfer coefficient, k re  =

Where S = 2LR m, R m =

 R1



R2

2

S 

ln



T 1

T 2 T 1  T 2

, ΔT1 = Te – ts and ΔT2 = Ts – te

(b) Theoretical heat-transfer coefficient, 1

k th =  Rm hc  R1 1

 Rm ln 

 R 2  R1

 



 Rm  R 2h f  

Where λ = 43 kcal/hm∙K

Experimental Procedure 1. Connect the cold water tube to prepare a co-current configuration system. 2. Supply pressure and make sure the four indicators to indicate the ambient temperature. 3. Supply pressure to the thermos regulator group to open the valve of the hot water flow meter for a flow ranging between 20 to 40 L/h. 4. Regulate the inlet hot water temperature to Te=80oC. 5. Wait for the temperature of the hot water to stable by observe the indicators relating to the hot water. 6.  Adjust the hot water flow rate to 40 L/h. 7. Open the cold water valve and adjust to 10 L/h. 8. Wait for the four indicators for about 5 minutes to stabilize before record the values. 9. Increase the cold water flow rate to 20, 30, and 40 L/h. 10. Repeat step 1-9 for counter-current configuration. 11. Cut off the supply of thermos regulator, close the two hot and cold water valves, and switch off the general supply of the unit.

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KC21001 LABORATORY 4 LAB MANUAL 2

Result (a) Co-current configuration q (L/h)

40

40

40

40

d (L/h)

10

20

30

40

Hot water inlet temperature, Te (oC) Hot water exit temperature, Ts (oC) Cold water inlet temperature, te (oC) Cold water exit temperature, ts (oC) (b) Counter-current configuration q (L/h)

40

40

40

40

d (L/h)

10

20

30

40

Hot water inlet temperature, Te (oC) Hot water exit temperature, Ts (oC) Cold water inlet temperature, te (oC) Cold water exit temperature, ts (oC)

References 1. Holman, J. P. (2010). Heat Transfer 10th Edition. Singapore: McGraw-Hill. 2. Perry H. Robert & Green W. Don. (2008). Perry’s Chemical Engineers’ Handbook 8 th  Edition . Singapore: McGraw-Hill Publications. 3. Williams, J. (2002). Double-Pipe Heat Exchanger. Project No. 1H Laboratory Manual .

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