An article on Plate Heat Exchanger with Steam: Part 1 of 2

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. The purpose of this article is to: –

  • Provide basic heat exchange knowledge
  • Show why plate heat exchangers are often over-surfaced on steam applications and explain the effect on steam pressure
  • Show how to predict actual steam pressure within an exchanger
  • Show how to predict when ‘stall’ may occur in variable secondary load applications
  • Provide guidance on selecting the correct control solution

Basics of Heat Exchange

  • Two substances must have a different temperature 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
  • 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)

A typical Plate Heat Exchanger
Fig. 1: A typical Plate Heat Exchanger

Water Vs. Steam Heating (Fig. 2)

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.
Water vs Steam as Heating Medium
Fig. 2: Water vs Steam as Heating Medium

Terminology

  • 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 which must be supplied to a substance to raise its temperature by 1ºC
  • Dp – pressure difference
  • Dt – temperature difference

Mean Temperature Difference (Fig. 3)

Mean temperature difference
Fig. 3: Mean temperature difference

Heat Exchanger Over-sizing; Cause & Effect

  • Calculating heat exchange area:
  • ‘k’ value:
Heat Exchange Area Calculation

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

  • 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.
  • On 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.

Few more Resources for you..

Various Articles related to Heat Exchangers
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Piping Design and Layout
Piping Stress Analysis
Piping Materials

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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