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## PowerPoint Slideshow about ' Heat Transfer ' - gigi

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### Heat Transfer

Flow Inside Heat Exchangers

Flow inside Heat Exchangers

- In Heat Exchangers, one fluid is cooled while the other fluid is heated without direct contact between fluids.

- There are two types of flow inside Heat Exchangers.

(Co current Flow)

2- Countercurrent Flow

W,Cp,T2

W,Cp,T1

W,Cp,T1

W,Cp,T2

w,cp,t1

w,cp,t1

w,cp,t2

w,cp,t2

T1

T1

T2

T2

temperature

temperature

t2

t2

t1

t1

x

x

- Typically in heat exchangers, the heat lost by the hot fluid is gained by the cold fluid through indirect contact (neglecting any heat losses), an overall Energy Balance Equation can be written as:

Q=w cp(t2-t1)=W Cp (T1-T2)=UoAotm

- Uo:Overall heat transfer coefficient

- Ao:Heat transfer area

- tm:Average temperature difference

- Rwallcan be neglected compared with convection resistances.

hi

ho

- Where is the heat transfer coefficient of the inner fluid based on the outside area

- For a differential element inside the exchanger, with a thickness dx, T & t are the mean values of the hot and cold fluids respectively, t=T-t

T

t

dQ

dx

- The differential rate of heat transfer dQ is
- dQ=Uo Do dx t

dQ=

T1

T1

T2

T2

t2

t2

t1

t1

temperature

temperature

Q=0 Q Qtotal

Parallel Flow

Countercurrent Flow

Q=0 Q Qtotal

Parallel Flow thickness dx, T & t are the mean values of the hot and cold fluids respectively,

Countercurrent Flow

=Constant

T1

T1

T2

T2

t2

t2

temperature

t1

t1

temperature

Q=0 Q Qtotal

Q=0 Q Qtotal

Where thickness dx, T & t are the mean values of the hot and cold fluids respectively, is the LMTD: Logarithmic Mean Temperature Difference

Example (1) thickness dx, T & t are the mean values of the hot and cold fluids respectively,

A hot fluid enters the exchanger at T1=300oF and leaves at T2oF, A cooler fluid enters the exchanger at t1=100oF and leaves at t2oF

a) If T2=200oF and t2=170oF

b) If T2=t2=182oF

c) If T2=110oF and t2=233oF

In case of Isothermal Fluids thickness dx, T & t are the mean values of the hot and cold fluids respectively,

If a cold fluid is heated from 100oF to 275oF by condensing steam at 300oF, Calculate LMTD for both cases

300

300

300

300

275

275

100

100

temperature

temperature

Countercurrent Flow

Parallel Flow

200 25

200 25

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