Calculation in Design of Heat Exchangers

• Designing a heat exchanger involves several considerations including the type of heat exchanger, its purpose, the fluids involved, heat transfer mechanisms, flow rates, and temperature differentials.
• Here’s a simplified overview of steps and equations for the design of a basic heat exchanger:

Assumptions:

• Steady-state conditions.
• Negligible heat loss to the surroundings.
• Constant fluid properties.
• Counterflow or parallel flow heat exchanger configuration.

Steps for a Basic Design:

1. Determine Heat Transfer Rate (Q):
   • Define the required heat transfer rate based on the application. This could be based on the desired temperature change or specific heating/cooling requirements.
   • You can refer the following links:
     • Heat Transfer Calculator 
     • Heat Transfer Calculator to Find Temperatures
     • Shell and Tube Heat Exchangers- Basic Calculations
     • Heat Exchanger Calculation for Thermal Design
     • Heat Exchanger Analysis
     • Shortcut for Heat Exchanger Sizing 
   • CFD simulations can be used to determine and verify heat transfer and pressure drop calculations as mentioned in the post.
2. Select Heat Exchanger Type:
   • Choose the appropriate type of heat exchanger (e.g., shell and tube, plate, or finned tube) based on factors like fluid properties, pressure drop tolerance, and application suitability.
3. Calculate Log Mean Temperature Difference (LMTD):
   • For a given flow arrangement (counterflow or parallel flow), calculate the log mean temperature difference (LMTD) using the formula ( ΔTm)
   • The LMTD Method  for the heat exchanger is given on this page
     • Calculate  the average temperature difference (LMTD) as ΔTm for a given heat exchanger:
     • Refer to the page for the LMTD calculation
     • Calculate the overall heat transfer coefficient (U)
     • Calculate heat transfer rate: Q= U A ΔTm
4. Determine Overall Heat Transfer Coefficient (U):
   • Calculate the overall heat transfer coefficient ($U$) based on the heat exchanger type and fluid properties. It’s generally a combination of individual heat transfer coefficients for the fluids and the thermal resistances across the wall.

Q= UA ΔTm

where, U =  1/ total thermal resistance from one fluid to other side fluids

1. Calculate Required Area (A):
   • Use the equation,  Q $=U\cdot A\cdot LMTD$ to determine the required heat transfer area.
   • The required surface area of the heat exchanger can be calculated using:$U\times A\times \text{LMTD}$
   • Where, $A$ is the surface area of the heat exchanger
2. Sizing and Specifications:
   • With the required heat transfer area known, specify the dimensions and other parameters of the heat exchanger to meet the application requirements.
3. Pressure Drop Calculations:
   • Calculate the pressure drop in both hot and cold fluid sides to ensure the flow rates and pressure requirements are within acceptable limits.
   • You can determine the pressure drop for each section using heat transfer coefficients given on the page.
4. Evaluate and Iterate:
   • Verify the design’s feasibility by checking pressure drop, material compatibility, and other practical aspects. Make adjustments as necessary.

• Keep in mind, that these are simplified equations and assumptions.
• In real applications, various complexities such as non-idealities, fouling, variable fluid properties, and others should be considered, often requiring more intricate calculations and iterations for accurate heat exchanger design