What is Fouling In Heat Exchanger | Forms Of Fouling

INTRODUCTION
Fouling is a general term that includes any kind of deposits of extraneous material that appears on the heat transfer surface during the lifetime of the heat exchanger.
Fouling reduce heat transfer across the exchanger surface hence reduces efficiency of the heat exchanger. The foulingdeposits also reduce flow cross-section area causing a pressure deferential across the heat exchanger which in turn increasing on the fan power required. It might also eventually block the heat exchanger.
Different kinds of fuel produce different degrees of fouling .most fuel produce just soft black soot that get deposited on the exchanger surface.
This can easily be removed by brushing and sand washing. However lower grade fuel oil(principally no.6.oil or resid)contain large quantities of alkaline sulfates and vanadium pentoxide that causes scaling due to their lower fusion temperatures.

The forms of fouling may therefore include

Particulate fouling.
Scaling/precipitation.
Chemical/corrosion fouling.
Solidification.

FORMS OF FOULING

Scaling/precipitation: –

Scaling/precipitation occur as a result crystallization of dissolved substance on to the heat transfer surface. These deposits can be removed by scratching or by cleaning via chemical treatment. This is the most common type of fouling.
Scaling/precipitation can be reduced by treating the fluid flowing past the heat exchanger before it reaches the heat exchanger surface.

Particulate fouling
This result from the accumulation of solid particles suspended in the process fluid onto the heat transfer surface. Such solid particles can be removed by use of filters to treat the process fluid before it reaches the heat exchanger surface.

Chemical /corrosion fouling
In this case, the surfaces are fouled by accumulation of the products of chemical reactions on the surfaces. This form of fouling can be avoided by coating the heat exchanger surfaces by glass. Heat exchanger surfaces can also be fouled by growth of algae in warm fluids (chemical fouling) which can be prevented by chemical treatment.

Solidification fouling
The crystallization of a pure liquid or one component of the liquid phase on a sub cooled heat transfer surface.
The mechanism of fouling is complicated and no reliable techniques are available but there are means of reducing fouling. The methods mostly used to reduce fouling include use of filters and increasing the fluid flow to ensure turbulent flow.

Fouling tendencies depends on the type of heat exchanger and the fluids. During the design stage certain considerations may help minimize fouling experienced in the field:

If possible, allocate the more fouling fluid to the tube side
Design for a fouling fluid velocity of 5 ft/sec on the tube side and 3 ft/sec on the shell side
Try to keep the fluid velocity constant
Allow for easy access for cleaning
In water service, ensure the tube wall temperature is not too high to create salt deposits or render treatment chemicals ineffective
Do not throttle water flows in winter time