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)
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.
- 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)
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.
- 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.
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