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:
- Steady-state conditions.
- Negligible heat loss to the surroundings.
- Constant fluid properties.
- Counterflow or parallel flow heat exchanger configuration.
Steps for a Basic Design:
- 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:
- CFD simulations can be used to determine and verify heat transfer and pressure drop calculations as mentioned in the post.
- 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.
- 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
- Determine Overall Heat Transfer Coefficient (U):
- Calculate the overall heat transfer coefficient () 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
- Calculate Required Area (A):
- Use the equation, Q to determine the required heat transfer area.
- The required surface area of the heat exchanger can be calculated using: Q =
- Where, is the surface area of the heat exchanger
- 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.
- 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.
- 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