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Maritime Forum


HEAT EXCHANGERS 28 Mar 2017 04:47 #4767

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With reference to oil coolers:

(a) How and why turbulent flow is provided in heat ex changers.

(b) If we eliminate the sacrificial anode how we compensate them.

(c) What is guided flow in heaters.

(d) Allowance given for tube expansion.
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HEAT EXCHANGERS 06 Apr 2017 08:15 #4775

(a) Why turbulent flow:
A common situation encountered by an engineer is heat transfer to fluid flowing through a tube.

This can occur in heat exchangers, boilers, condensers, evaporators, and a host of other process equipment.

The study of heat transfer can be done either in laminar flow or in turbulent flow. Both conditions show different performance characteristics of the heat transfer.

Under turbulent flow conditions, the increase in heat transfer rate is more significant than that under laminar flow conditions.

This is due to the increase in the Reynolds number of the flowing fluid in turbulent flow.

Laminar flow develops an insulating blanket around the channel wall and restricts heat transfer. Conversely, turbulent flow, due to the agitation factor, develops no insulating blanket and heat is transferred very rapidly.

How the turbulent flow is achieved

1.Higher the velocity more the turbulent the flow will be.
2.The jumbled, tumbled flow pattern can disrupt much of the stationary fluid film. Built-in obstructions to flow, called turbulators, disrupt laminar flow, thereby improving heat transfer. Although these obstructions to flow increase pressure drop through the heat exchanger, the improvement in heat transfer more than compensates for the higher pressure drop.

(b) Impressed current corrosion protection system (ICCP) is used if there is no sacrificial anode fitted.

Impressed current cathodic protection works by delivering controlled amounts of DC current to the surfaces submerged in water with the aid of ultra-reliable zinc electrodes as well as combined anodes of metal oxide. The electrical current that is continuously regulated and monitored by the ICCP system helps prevent the electrochemical mechanism of galvanic corrosion prior to its attack.

A heat exchanger is a device for transferring thermal energy from one medium to another. It generally consists of two channels or system of channels, one for each medium, and separated from each other by partitions through which heat is transferred from the hot medium to the cold medium.

Most coolers used on board ship transfer heat from a hot fluid to sea water. For the main propulsion engine of the motor ship, the engine jacket water, lubricating oil and charge air must be cooled and generally also water or oil used in cooling the pistons.

The flow path of the fluids are fixed by the division plates in the heat exchanger heads for the cooling fluid and the internal baffles or tube support shets within the body of the heat exchanger for warm fluid:
The division plates in the heads or boxes fix the position of the inlet and outlet branches for the fluid passing through the tubes.

The internal baffles fix the position of the inlet and outlet branches for the fluid passing through the body of the heat exchanger on the outside of the tubes.

If the heat exchanger heads do not contain any division plates, he fluid passing through the tubes enters at one end of the heat exchanger and leaves at the other. This arrangement is referred to as a single pass heat exchanger. If the heat exchanger is a double pass type, a division plate is fitted in one head. The inlet and outlet connections for the fluid passing through the tubes are fitted on this head. The division plate prevents the fluid bypassing and causes it to pass through half the tubes in the heat exchanger which is referred to as the inlet bank. After the fluid passes through the inlet bank it enters the other head which is just a bobbin piece and a cover. The direction of fluid flow is reversed in this head or box and it passes back through the outlet bank of tubes and leaves at the outlet branch. The fluid has passed through the tubes in two different paths from which it gets the name two pass, or double pass.

In recent designs, the guided flow concept has been introduced, i.e. a secondary cooling surface in the form of radial fins integral with the tubes between which flow is guided radially, alternately out and in from section to section. This gives better heat transfer surface and better heat transfer, lower metal surface temperature and increase performance.

In a multipass shell and tube heat exchanger, the problem of differential expansion between the shell and tube passes is taken care of by using a U-bend or floating head tube sheet.
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