Heat Exchanger/FWG/Sewage Treatment

Question 1

1. Sketch a cross-section of a horizontally mounted two pass, tubular heat exchanger suitable for lubricating oil,
   showing the flow path of the fluids (8)

Answer 1

Question 2

2. Describe with the aid of sketches a plate type heat exchanger (8)

Answer 2

A plate heat exchanger is a type of heat exchanger that uses metal plates usually titanium to
transfer heat between two fluids.

This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates.
The pressed pattern on plates surface induce turbulence and minimise stagnant areas and fouling.

Advantages are they can be easily cleaned, and damaged plates can be easily replaced.

The disadvantages are they use very expensive plate metal, the reassembly is time consuming and space is required to open up cooler for cleaning.

Question 4

3. With reference to a low pressure, fresh water evaporator
   a. Explain why the water at the evaporator outlet is unsuitable for drinking (4)
   b. State the treatments that may be used to make the water potable (4)

Answer 4

3. With reference to a low pressure, fresh water evaporator
   a. Explain why the water at the evaporator outlet is unsuitable for drinking (4)
   • Water within the evaporator is not boiled at 100degrees so it is not sterile
   • The water is untested and untreated for bacterial growth
   • Natural salts have been removed

b. State the treatments that may be used to make the water potable (4)
• Ultra-Violet sterilisation
• Addition of mineral salts
• Chloride dosing 2ppm minimum
• Silver ion dosing 0.1ppm minimum

Question 5

4. Sketch an aerobic sewage treatment plant (8)

Answer 5

The Trident sewage treatment unit shown above consists of three chambers.
Sewage enters the aeration chamber via a coarse mesh filter where large solids are broken down. The aeration chamber
is where the main biological action takes place. Here air blowers mounted on the outside of the unit oxygenate and stir
the effluent and bacteria mix via a series of pipes and nozzles. The sewage remains in this aeration tank for some time.

Incoming sewage displaces some effluent of the settling tank (or hopper) where under inactive conditions biological floc,
activated sludge and bacteria settle out and are returned to the aeration chamber via air lift pumps also driven by the
blowers.
A second transfer pipe scum’s the surface of the settling tank and returns it back to the aeration chamber.

This returned sludge contains the bacteria to digest the incoming sewage. Thus the importance of this floc return can be seen