Heat Exchangers

Question 1

2. Describe, with the aid of sketches, a plate type heat exchanger.

Answer 1

A plate heat exchanger consists of a series of thin corrugated metal plates between which a number of channels are formed, with the primary and secondary fluids flowing through alternate channels. Heat transfer takes place from the primary fluid steam to the secondary process fluid in adjacent channels across the plate. A corrugated pattern of ridges increases the rigidity of the plates and provides greater support against differential pressures. This pattern also creates turbulent flow in the channels, improving heat transfer efficiency, which tends to make the plate heat exchanger more compact than a traditional shell and tube heat exchanger. The promotion of turbulent flow also eliminates the presence of stagnant areas and thus reduces fouling. The plates will usually be coated on the primary side, in order to promote the drop wise condensation of steam.

Question 2

3. With reference to a low-pressure fresh water evaporator:
   a) Explain why the water at the evaporator outlet is unsuitable for drinking; (8) b) State the treatment that may be used to make the water potable. (8)

Answer 2

a) At the evaporator outlet the water has been boiled, but it still isn’t safe to drink. This is due to it being boiled at a much lower temperature than 100℃, usually 55℃, which doesn’t sterilise it. This means bacteria dangerous to human health can still be present in this water, as it isn’t tested before it enters the fresh water generator.
b) Ultra-Violet sterilisation.
Addition of mineral salts.
Chlorination.
Katadyn Silver Ion.

Question 3

5. State FOUR advantages and FOUR disadvantages of plate type heat exchangers.

Answer 3

Advantages:
Larger surface area for heat exchange.
Smaller and more compact than a shell or tube type.
Allows turbulent flow.
Usually made of titanium which is strong, has hard wearing and has good thermal conductivity.
Disadvantages:
Expensive to manufacture and repair.
Large surface area means lower pressure.
Not enough head pressure on low pressure systems to push liquid through all plates. Use special gaskets which can only be used at low temperatures and cannot be used with steam.