COMMON PROBLEMS IN HEAT EXCHANGER

Fouling

EFFECT OF FOULING ON THE THERMOHYDRAULIC PERFORMANCE OF HEAT EXCHANGERS

1. The fouling layers on the inside and the outside surfaces are known generally to increase with time as the heat exchanger is operated.

2.  There is an increase of the surface roughness, thus increasing frictional resistance to flow,

and fouling blocks flow passages; due to these effects, the pressure drop across the heat

exchanger increases.

3.  Fouling may create a localized environment where corrosion is promoted.

4.  Fouling will reduce the thermal efficiency of heat exchangers, which in turn affect the

subsequent processes or will increase the thermal load on the system.

MECHANISMS OF FOULING

1. Particulate fouling

Particulate fouling may be defined as the accumulation of particles suspended in the process streams onto the heat transfer surfaces. 

2. Reaction fouling

Deposits formed by chemical reactions at the heat transfer surface in which the surface material itself is not a reactant are known as chemical reaction fouling. Polymerization, cracking, and coking of hydrocarbons are prime examples of reaction fouling.

3. Corrosion fouling

Corrosion fouling is due to the deposition of corrosion products on heat transfer surface. In this category of fouling process, the heat transfer surface material itself reacts to produce corrosion products, which foul the heat transfer surface.

4. Precipitation fouling

This type of fouling mostly takes place in cooling-water systems, when water-soluble salts, predominantly calcium carbonates, become supersaturated and crystallize on the tube wall to form scaling.

5. Biological fouling

The attachment of microorganisms (bacteria, algae, and fungi) and macro organisms (barnacles, sponges, fishes, seaweed, etc.) on heat transfer surfaces where the cooling water is used in as drawn condition from river, lake, sea and coastal water, etc.

6. Solidification fouling

The freezing of a liquid or of higher-melting constituents of a multicomponent solution on a subcooled heat transfer surface include frosting of moisture in the air, freezing of cooling water in low-temperature processes.

CLEANING OF HEAT EXCHANGERS

·        Off -Line Mechanical Cleaning

1.       Manual cleaning

2.       Automatic jet cleaning

-Advantages

1.      Simplicity

2.      ease of operation

3.      capability to clean even completely blocked tubes.

-Disadvantages

1.      this method may damage the equipment

2.      it does not produce a chemically clean surface

3.     the use of high-pressure water jet or air jetmay cause injury and/or accidents to personnel engaged in the cleaningoperation.

·        Off -line Chemical Cleaning

1. Circulation: This method involves the filling of the equipment with cleaning solution and circulating it by a pump. While cleaning is in progress, the concentration and temperature of the solution are monitored.

2. Fill and soak cleaning: This may be repeated several times until satisfactory results are achieved. However, this method is limited to small units only.

-Advantages

1. Uniform cleaning and sometimes complete cleaning.

2. Sometimes chemical cleaning is the only possible method.

3. Capable of cleaning inaccessible areas.

4. Moderate cleaning cost and longer intervals between cleaning.

-Disadvantages

1. Chemicals used for cleaning are often hazardous

2. Non-environmental friendly

·        Online Cleaning Methods

1. Upstream filtration

Cooling-water fouling can be controlled, and in some cases eliminated, by adequately filtering the intake water.

2. Flow excursion

In this method, the instantaneous flow is increased to remove the fouling deposits. This method is particularly applicable to a heat exchanger fouled badly due to the effects of low velocity.

3. Air bumping of heat exchangers

This technique involves the creation of slugs of air, thereby creating localized turbulence as slugs pass through the equipment.

4. Reversing flow in heat exchangers

The cleaning effect is achieved by changing the flow direction in the heat exchanger so that the dirt accumulated in the inlet region and the heat exchanging channels is flushed out.

5.  Automatic tube cleaning systems such as

Sponge rubber balls cleaning system: The cooling-water flow forces the balls through the tubes and the deposits on the tube walls are wiped out.

Advantages

1. Extended run time between cleaning intervals

2. Convenient to install

3. Does not require any plant shutdown

4. Can save time and labour

Disadvantage

1. the initial cost may be very high in certain cases.

 COST IMPOSED DUE TO FOULING

1. Capital expenditure

2. Energy costs

3. Maintenance costs

4. Cost of production loss

5. Extra environmental management cost

Leakage

There are two types of leaks in heat exchangers: internal and external. Most time it is from gasket problem. It can cause low pressure drop because not all the flow is entering the tubes that should. And leakage problem is worst in high pressure condition.

Improper venting

Proper venting is a startup necessity. Improper venting usually occurs on startup and is recognized by poor heat transfer and a high pressure drop. 

Case study

Refferences

[1] Maurice Stewart, Oran T. Lewis, Heat Exchanger Equipment Field Manual Common Operating Problems and Practical Solutions, 2013.

[2] Kuppan Thulukkanam, Heat Exchanger Design Handbook Second Edition, 2013.