Overview of Plate Heat Exchanger with Steam (PDF)

A plate heat exchanger uses metal plates to transfer heat between two fluids. Metal plates create a larger surface area for heat transfer, hence increasing the ability of heat exchange.

A plate heat exchanger is a highly efficient, compact, and robust heat exchanger in industrial applications. As the term heat exchange clarifies, the cold media become hot by heat transfer from another hot medium. So to operate there will be two streams, one stream will lose heat and the other will gain that heat. Metallic plates are used for separating these two streams. Inside the plate heat exchanger, heat transfer occurs through the plates. The plate gets heat from the hot medium and then transfers it to the cold medium.

Advantages of Plate Heat Exchanger

The main advantages of plate heat exchangers over other types of heat exchangers are:

• Simple and Compact
• Larger co-efficient of heat transfer due to more contact area
• Simple maintenance.

There are four types of Plate heat exchangers based on their construction; gasketed, brazed, welded, and semi-welded.

Basics of Heat Exchange in Plate type heat exchanger

• Two substances must have a different temperatures to exchange heat
• Heat always flows from a warmer substance to a cooler substance
• Heat flow is rapid when the heat difference is great
• During heat transfer, temperature difference decreases
• The heat transfer rate slows until the substances are at the same temperature
• Heat transfer ceases when the two substances are at the same temperature

Types of Heat Exchange

• Direct Heat exchange- The heating medium is directly mixed with the substance being heated.
• Indirect Heat Exchange- Heat energy from the heating medium is passed to the substance being heated, through a physical barrier

Plate Heat Exchanger

Fig. 1 shows the heat exchange mechanism in a plate heat exchanger.

Water Vs. Steam Heating

Features of Water as a heating medium

• Water does not change the state
• As it gives up heat energy to the secondary medium its temperature drops
• For every 1ºC drop in temperature, each kg of water will give up approximately 4.2 kJ.

Features of Steam as a heating medium

• Steam does change the state
• Heat energy passes to the secondary medium as it gives up its specific enthalpy of evaporation.
• As this specific enthalpy of evaporation is given up, the steam condenses but remains at a constant temperature.
• At a pressure of 2 bar-g, each kg of steam will give up approximately 2,100 kJ.

Terminologies of Plate Heat exchanger

• Heat Exchange Area– the total partition area available for heat energy to pass from the primary heating medium to the secondary medium, in an indirect heating application.
• ‘k’ value– the amount of heat that will pass through each m² of the partition for each ºC temperature difference between the primary heating medium and the secondary medium.
• Specific heat -the amount of heat that must be supplied to a substance to raise its temperature by 1ºC
• Dp – pressure difference
• Dt – temperature difference

Mean Temperature Difference

Heat Exchanger Over-sizing; Cause & Effect

• Calculating heat exchange area:
• ‘k’ value:

Secondary side pressure drop

• Typically, the allowable pressure loss (DP) on the secondary circuit, through a heat exchanger is 0.3 – 0.5 bar g (30 – 50 Kpa).
• Because of the high ‘k’ values attainable when using steam as the heating medium, a relatively small heat transfer area is required, which could give an unacceptably high secondary side pressure drop
• Therefore, to provide an acceptable pressure drop, the exchanger is often over-sized
• Typically, heating surface over-sizing can be in the region of 50 – 200%

Over-surfacing – something has to give!

In the above equation,

• V             – flow rate – fixed;    
• r           – density – fixed  ;    
• Cp     – specific heat-fixed;   
• Dt         – temperature rise             – fixed and    
• k   –      transfer coefficient         – fixed.

Therefore, If the heat transfer area (A) is greater than required for a given duty Dtmtd must decrease if the correct amount of heat is to be provided from the primary medium to the secondary medium.

Summary of Plate Heat Exchanger with Steam

• As a result of their extremely high thermal efficiency, and the need to keep secondary side pressure drop within acceptable limits, plate heat exchangers are often over-surfaced.
• If a heat exchanger is over-surfaced, a lower mean temperature difference will be necessary to meet the design full load condition.
• In steam applications, a lower mean temperature difference is achieved by a reduction in steam pressure, and therefore, its temperature.
• Actual steam pressure within a plate heat exchanger, at full secondary load design condition, is primarily a function of actual heat transfer surface area vs required heat transfer surface area.

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