Process Equipment and Plant Design
In the case of a shell and tube heat exchanger, the logarithmic mean temperature difference

May be either more or less than geometric mean and arithmetic mean value depending upon whether the flow of stream is co-current or counter-current
Is always more than arithmetic mean value and the geometric mean value
Is always less than arithmetic average value
Is always less than arithmetic mean value, but more than geometric mean value

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Process Equipment and Plant Design
In the design of a shell and tube heat exchanger, the corrosion allowance

For carbon steel and cast iron pressure parts is 1.5 mm (except for tubes) and for severe conditions it is 3 mm
For internal cover and tube sheet is provided on both the sides
All of these
Need not be provided for non-pressure parts like tie rods, spacers, baffles, supports etc

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Process Equipment and Plant Design
Generally, elliptical dished heads

Resist half the pressure rating compared to hemi spherical head provided on the cylindrical shell of the same thickness and diameter
All of these
Are manufactured on 2:1 ratio of major to minor axis and is recommended to be used for pressure vessels operating above a pressure of 1.5 MN/m²
Are approximately as strong as seamless cylindrical shell having the corresponding I.D and O.D

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Process Equipment and Plant Design
Number of evaporators which can be used in service in a multiple effect evaporation system is limited by the

Practical limit reached, due to low heat transfer rate because of inadequate temperature difference
Both A & B
Economic limit reached, when the plant cost exceeds the saving of steam
Neither A nor B

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