heat exchanger

Question 1

How is fluid flow controlled in a plate and frame heat exchanger? What limitation does this place on the operating conditions?

Answer 1

Controlled by the positioning of the gaskets on each plate Limitation is temperature as a result of the available gasket materials

Question 2

Why does the LMTD in a shell/tube heat exchanger require a correction factor for a multi-pass configuration, but not for a single pass configuration?

Answer 2

Correction is for the co-current flow section in a multipass HeX A single pass HeXD can be arranged to be close to counter-current flow only

Question 3

The viscosity of heavy crude oil decreases rapidly as its temperature increases. How does this complicate the calculation of the tubeside heat transfer coefficient in a shell/tube heat exchanger?

Answer 3

hi is a function of viscosity - usually viscosity is taken to be a constant over the length of the tubes, but this cannot be the case for heavy crude oil

Question 4

A BEU heat exchanger is found to have developed leaks in some of the tubes. What can the maintenance staff do to address this problem? How could this affect heat exchanger performance?

Answer 4

This is a U-tube heat exchanger so removing tubes and replacing them is mostly not possible (except for the very outer layer of tubes. The only remedy is to plug the leaking tubes. This reduces the heat transfer area available by a little, so the number that can be plugged is limited.

Question 5

A correlation for nucleate boiling of octane on a horizontal steel tube has predicted a heat transfer coefficient that is an order of magnitude lower than that which was experimentally measured. What is the most likely reason for this?

Answer 5

That the correlation doesn’t allow for the surface finish of the steel tube i.e. the roughness of the surface.

Question 6

pressure drop over tube size from 2 to 4

Answer 6

original pressure*(pass1/passoriginal)^3

Question 7

some issues when choosing a type of heat exchanger

Answer 7

how hot/cold are the streams ? (insulation, safety etc)
 how viscous are the streams ?
 how corrosive are the streams ? (construction material)
 can it be fabricated from the required material ? how dirty are the streams ? (fine solids, bacteria etc)
 how often is cleaning needed ?

Question 8

plate fin heat exchanger

Answer 8

advantages
 aluminium gives high heat transfer rates large heat transfer surface area per unit volume little plant space required and low weight  can be used at cryogenic temperatures
for liquid gases production (e.g. liquid air)
disadvantages
 small channels are easily blocked
 the channels are difficult to clean

Question 9

air heat exchangers

Answer 9

disadvantages
 seasonal air temperature
variations require control
systems (for the fans) to maintain outlet temperature they cannot be close to large buildings because the
buildings can deflect hot exit air back to the inlet side18/71
 capital costs are higher (they are bigger
because air has low heat capacity)

Question 10

Why are 2 passes better than single pass

Answer 10

fluid velocity in a tube is doubled. increases the rate of heat transfer & reduces the amount of
dirt settling in the tubes
disadvantage - pumps need to work harder due to increased pressure. higher energy cost

Question 11

co-current flow

Answer 11

driving force for heat transfer (T) decreases along the tube

Question 12

counter current flow

Answer 12

driving force (T) changes much less than for co-current flow

Question 13

baffles function

Answer 13

 force the flow to “zig-zag” and so raise fluid velocity increased velocity increases heat transfer rate
 also support the tubes and stop them sagging or vibrating