HEAT EXCHANGER

A heat exchanger is an equipment in which two fluids are separately circulated in adjacent spaces so that some of the heat in the fluid at the higher temperature is transferred into the fluid at the lower temperature. The fluid having the higher temperature is therefore cooled and the fluid having the lower temperature is heated.

  The heat from a fluid can therefore be transferred to another colder fluid by means of:

a.     Separately circulating them in adjacent spaces - heat flowing from a “hot” fluid through a tube wall or other membrane to another cooler fluid.

b.      Mixing them together - heat flowing from a hot fluid to a cooler fluid due to mixing of one with the other.

SHIPBOARD HEAT EXCHANGERS ARE CLASSIFIED UNDER THE FOLLOWING CATEGORIES

•         COOLER:  THE TEMPERATURE OF A HOT LIQUID IS LOWERED BY TRANSFERRING PART OF ITS HEAT TO SEAWATER.

•         CONDENSER: HEAT FROM A HOT GAS OR VAPOUR IS TRANSFERRED TO SEAWATER TO THE EXTENT THAT THE GAS OR VAPOUR LIQUIFIES.

•         HEATER: THE TEMPERATURE OF A LIQUID OR GAS IS RAISED BY TRANSFERRING ENERGY FROM A HOT MEDIUM, USUALLY STEAM.

•         EVAPORATOR: HEAT ENERGY IS TRANSFERRED TO A LIQUID UNTILL IT EVAPORATES FROM LIQUID STATE TO GASEOUS STATE.

Theory of heat exchanger

    In all heat exchangers the hot fluid flows from the hot fluid to a cooler one through an intermediate heat-conductive wall which takes up some intermediate temperature. The temperature profile across an element of wall surface is shown.

  FLOW PATTERNS

Counter-flow or contra-flow, heat exchangers are those where the two fluids in opposite direction.The fluids flow strictly in parallel with one another, the flow system is termed co-current or parallel flow.

PERFORMANCE

THE FOLLOWING FACTORS AFFECT THE PERFORMANCE OF HEAT EXCHANGERS.

1)    AIR LOCKING OF FLUID SPACES.

2)    FOULING OF HEAT TRANSFER SURFACES.

3)    IN TWO-OR MULTI PASS HEAT EXCHANGERS, FLUID BY-PASSING TUBES BECAUSE OF  DIVISION PLATE JOINT FAILURE OR HOLED DIVISION PLATES.

FOULING

     FOULING MAY BE DEFINED AS ANY UNDSIRABLE DEPOSITS ON A  HEAT TRANSFER SURFACE WHICH INCREASE RESISTANCE TO BOTH  HEAT TRANSFER AND FLUID FLOW.

1)    PRECIPITATION FOULING:  THIS RELATES TO THE PRECIPITATION OF DISSOLVED SUBSTANCES ON THE HEAT TRANSFER SURFACE.

2)    PARTICULATE FOULING: THIS OCCURS WHEN FINELY DIVIDED SOLIDS (RUST,DUST SAND ETC.) SUSPENDED IN THE PROCESS FLUID ACCUMULATE ON THE HEAT TRANSFER SURFACE.

3)    BIOLOGICAL FOULING: THIS RELATES TO BIOLOGICAL ORGANISMS WHICH ADHERE TO THE HEAT TRANSFER SURFACE. EG. BARNACLES & OTHER MARINE ORGANISMS.

      Fouling occurs when a fluid goes through the heat exchanger, and the impurities in the fluid precipitate onto the surface of the tubes.

 Precipitation of these (impurities) can be caused by:

1.     Frequent use of the Heat Exchanger

     2. Not cleaning the Heat Exchanger regularly

     3. Reducing the velocity of the fluids moving through the heat

    exchanger.

    4. Over-sizing of the heat exchanger

Effects of fouling are more abundant in the cold tubes of the heat exchanger, than in the hot Tubes.

This is because impurities are less likely to be dissolved in a cold fluid.

This is because solubility increases as temperature increases.

Fouling reduces the cross sectional area for heat to be transferred and causes an increase in the resistance to heat transfer across the heat exchanger.

 This is because the thermal conductivity of the fouling layer is low. This reduces the overall (heat transfer coeifficient) and efficiency of the heat exchanger.

 This in turn, can lead to an increase in pumping and maintenance costs.

Maintenance

Plate heat exchangers need to be dissembled and cleaned periodically.

 Tubular heat exchangers can be cleaned by such methods as acid cleaning,( sandbalasting, high pressure water jet) sandblasting, high-pressure water jet, bullet cleaning, or drill rods.

In large-scale cooling water systems for heat exchangers,(water treatment) water treatment such as purification, addition of (chemical)chemicals, and testing, is used to minimize fouling of the heat exchange equipment.

 Other water treatment is also used in steam systems for power plants, etc. to minimize fouling and corrosion of the heat exchange and other equipment.

A variety of companies have started using waterborne oscillations technology to prevent (biofouling)biofouling. Without the use of chemicals, this type of technology has helped in providing a low-pressure drop in heat exchangers.