I. Determination of Basic Process Data (Prerequisites for Model Selection)
1. Clearly define the properties of the hot and cold media (determining equipment materials and structural boundaries).
First, it is essential to identify the specific media on both the hot and cold sides-such as process water, circulating water, steam, thermal oil, acidic or alkaline solutions, or brine.
Different media affect the equipment in distinct ways; for instance, chloride-containing media can easily cause pitting corrosion, necessitating the use of materials like 316L stainless steel or titanium.
If the media contain suspended particles or substances prone to crystallization, consider increasing the channel gap or reducing the flow velocity to prevent clogging.
If food-grade or pharmaceutical-grade requirements apply, the design must meet sanitary standards (e.g., allowing for disassembly and cleaning, and ensuring a structure free of dead zones).
2. Specify inlet and outlet temperature conditions (which determine the driving force for heat transfer).
Inlet and target outlet temperatures for both the hot and cold sides must be provided (e.g., cooling from 80°C to 50°C, heating from 20°C to 60°C).
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· The temperature difference directly determines the driving force for heat transfer; a smaller temperature difference requires a larger heat transfer area.
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· It is also necessary to determine whether there are any "temperature approach limitations"-for instance, if process requirements dictate that the cold-side temperature cannot exceed a certain limit.
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· If phase changes (such as condensation or evaporation) are involved, the temperature profile must be specified separately.
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3. Determine flow rate and operating load (determines heat exchange scale)
Mass or volumetric flow rates for both the hot and cold sides must be provided, as these are the core inputs for calculating the thermal load.
It is necessary to distinguish between three operating conditions: maximum load, normal load, and minimum load.
If the system involves fluctuating operation (such as intermittent production), adaptability to dynamic operating conditions must be considered.
The accuracy of the flow rate data directly determines whether the heat exchanger is oversized or undersized.
4. Define system boundaries and constraints (engineering constraints).
Clearly define the maximum allowable pressure drop range-for example, ≤50 kPa or a limit determined by pump capacity.
→ This parameter directly dictates the design of the plate channels and the selection of flow velocities, serving as a critical boundary condition that influences system energy consumption.
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Clearly define installation space constraints, including equipment height, inlet/outlet orientation, and maintenance clearance.
→ Spatial conditions will directly dictate the dimensions of the heat exchanger frame and the feasibility of future maintenance.
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Determine whether future capacity expansion is permitted (which affects whether additional plate capacity needs to be reserved).
→ This factor determines whether a modular design should be adopted to allow for increased heat transfer capacity at a later stage.
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Determine whether the system operates continuously (which affects maintenance and redundancy design).
→ The operating mode influences whether redundant equipment configurations are required and how cleaning and maintenance cycles are established.
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Thermal Calculations and Determination of Heat Transfer Requirements (Core Design Phase)
1. Thermal load calculation Q (determines the equipment's "capacity class")
Basic calculation formula:
Q = m × Cp × ΔT
Calculations should be performed separately for the hot and cold sides; theoretically, the principle of energy conservation must be satisfied.
Adjustments should be made to account for heat loss or process-related heat release/absorption.
The Q value is the primary parameter determining the heat exchanger surface area.
2. Calculation of Logarithmic Mean Temperature Difference (LMTD) (Driving force analysis)
Uses the counter-flow calculation formula:
LMTD = (ΔT1 - ΔT2) / ln(ΔT1 / ΔT2)
ΔT1 and ΔT2 represent the temperature differences at the two ends.
A smaller LMTD indicates a weaker temperature-difference driving force, requiring larger equipment.
Plate heat exchangers typically employ a counter-flow arrangement to maximize the LMTD.
3. Flow correction factor F (structural influence factor)
• When the fluid has a multi-pass or non-ideal flow pattern, a correction factor needs to be introduced.
• Typical range: 0.75–1.0.
• Multi-pass structures typically have a lower F-value, but can improve velocity distribution.
• The manufacturer's software must be used in conjunction with the design.
Plate Specifications and Structural Conversion (From Theory to Equipment)
1. Plate type selection (determines efficiency and voltage drop)
• Herringbone plates: High heat exchange efficiency, but relatively large pressure drop.
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• Shallow corrugated plates: Low pressure drop, suitable for high flow rate, low resistance systems.
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• Deep corrugated plates: Strong anti-clogging capability, suitable for fouling conditions.
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Plate angle (e.g., 30°/60°) affects turbulence intensity and drag characteristics.
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2. Plate size and heat exchange area are matched.
• Different manufacturers use different plate areas (0.1–3 m²/plate).
• Larger plates are suitable for high-flow-rate systems, while smaller plates are suitable for precision control systems.
• Equipment size and maintenance costs must be considered when selecting a plate.
• For the same heat exchange area, different plate sizes can lead to significant structural differences.
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