Heat Exchanger - 2

LMTD & Correction Factor (F)

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---------------------------------------------------- (1)

Q = Heat transferred in H-E-X

U = Overall Heat transfer coefficient

F = Correction Factor.

 = Logarithmic Mean Temperature Difference (LMTD)

* LMTD is dependent only on inlet and outlet temperatures and is independent of type of HE used.

where, 

1-stands for Inlet                                                  

2-Stands for Outlet

• Normal Practice if to calculate LMTD for counter flow and apply correction factor F to it.

Fig.1. LMTD for single pass STHE

Fig.2. LMTD for 1/2 STHE

F (Correction Factor)

• Shows departure form true counter current flow
• F is correction factor for multi-pass and crossflow heat exchanger and given for two-pass shell-and-tube heat exchangers in equation (4) below.
• 0 < F < 1
• fn { fluid Temperature, no. of passes }
• F > 0.8, if not then redesign the HEX with more no. of passes or larger LMTD

Find F value:

1. Find R using:

------------------------- (2)


 
     2. Find P using:

Heat Exchanger - 1

Heat Exchanger – Classification

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Applications :

• Air Conditioners
• Refrigeration
• Natural gas processing
• Petrochemical Plants
• Power Plants
• Radiators (cars, bikes, IC Engines and so on.)
• 99% of Chemical plants

Types :

• Basic Classification:

Recuperative:

• Fluid flows in separate conduits
• Heat transfer from hot fluid to cold fluid across the walls

Plate H-E-X

• Plate & Frame

Fig. 1 .Plate & Frame H-E-X

1. Series of rectangular parallel plates
2. Held firmly with each other.
3. Sealed by gaskets.
4. Hot fluid passes through alternate pairs of plates transferring heat to cold fluid in adjacent spaces.
5. Cleaning easy