HT UNIT-5 TEST

Air is best heated with steam in a heat exchanger of

• plate type.

• double pipe type with fin on steam side.

• double pipe type with fin on air side.

• shell and tube type.

Shell side pressure drop in a shell and tube heat exchanger does not depend upon the

• baffle spacing & shell diameter.

• tube diameter & pitch.

• viscosity, density & mass velocity of shell side fluid.

• none of these.

For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area

• increases

• decreases

• remains constant

• passes through a maxima

Hot water (0.01 m3 /min) enters the tube side of a counter current shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m3/min) of density 800 kg/m3 and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately

• 23

• 32

• 53

• 35

In a heat exchanger, floating head is provided to

• facilitate cleaning of the exchanger.

• increase the heat transfer area.

• relieve stresses caused by thermal expansion.

• increase log mean temperature gradient.

Baffles in the shell side of a shell and tube heat exchanger

• increase the cross-section of the shell side liquid.

• force the liquid to flow parallel to the bank.

• increase the shell side heat transfer co-efficient.

• decrease the shell side heat transfer co-efficient.

Air is to be heated by condensing steam. Two heat exchangers are available (i) a shell and tube heat exchanger and (ii) a finned tube heat exchanger. Tube side heat transfer area are equal in both the cases. The recommended arrangement is

• finned tube heat exchanger with air inside and steam outside.

• finned tube heat exchanger with air outside and steam inside.

• shell and tube heat exchanger with air inside tubes and steam on shell side.

• shell and tube heat exchanger with air on shell side and steam inside tubes.

Pick out the wrong statement.

• The controlling resistance in case of heating of air by condensing steam is in the air film.

• The log mean temperature difference (LMTD) for counter flow and parallel flow can be theoretically same when any one of the fluids (hot or cold fluid) passes through the heat exchanger at constant temperature.

• In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.

• Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature.

Finned tube heat exchangers

• give larger area per tube.

• use metal fins of low thermal conductivity.

• facilitate very large temperature drop through tube wall.

• are used for smaller heat load.

In a shell and tube heat exchanger,

• the temperature drops in the two fluids and the wall are proportional to individual resistances.

• the temperature drop is inversely proportional to the resistance across which the drop occurs.

• there is no relationship between temperature drop and resistance.

• the relationship is not generalised.