Heat Exchanger Calculator
Calculate heat exchanger thermal duty, required flow rates, and outlet temperatures using the LMTD method. Supports counter-flow, parallel-flow, and cross-flow arrangements.
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Heat exchanger resultHow It Works
The Heat Exchanger Calculator uses fundamental heat transfer equations to determine thermal duty, outlet temperatures, or required flow rates. The Logarithmic Mean Temperature Difference (LMTD) method is used to account for the varying temperature difference along the length of the exchanger.
Thermal Duty Equation
The heat transfer rate (thermal duty) is calculated using:
Q = ṁ × Cp × ΔT
Where:
- • Q = Thermal duty (heat transfer rate) in watts
- • ṁ = Mass flow rate in kg/s
- • Cp = Specific heat capacity in J/(kg·K)
- • ΔT = Temperature difference between inlet and outlet
LMTD Calculation
The Logarithmic Mean Temperature Difference accounts for the non-linear temperature profile:
LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂)
For counter-flow: ΔT₁ = Th,in − Tc,out and ΔT₂ = Th,out − Tc,in. For parallel-flow: ΔT₁ = Th,in − Tc,in and ΔT₂ = Th,out − Tc,out.
Required Surface Area
The heat transfer area needed is derived from:
A = Q / (U × LMTD)
Where U is the overall heat transfer coefficient in W/(m²·K) and A is the required heat transfer surface area in m².
Typical U Values
Reference overall heat transfer coefficients for common configurations:
| Configuration | U (W/(m²·K)) |
|---|---|
| Water to Water | 800 – 1500 |
| Water to Oil | 100 – 350 |
| Steam to Water | 1000 – 6000 |
| Steam to Oil | 50 – 300 |
| Gas to Gas | 10 – 40 |
| Gas to Water | 10 – 250 |
Fluid Properties
Specific heat capacity (Cp) values used for preset fluids:
| Fluid | Cp (J/(kg·K)) | Density (kg/m³) |
|---|---|---|
| Water | 4186 | 998 |
| Glycol 30% | 3850 | 1038 |
| Glycol 50% | 3480 | 1070 |
| Steam | 2010 | 0.6 |
| Oil | 2000 | 870 |
FAQ
Here you will find the answers to the frequently asked questions about heat exchanger calculations.
Frequently Asked Questions
What is LMTD and why is it used in heat exchanger design?
LMTD (Logarithmic Mean Temperature Difference) is a logarithmic average of the temperature difference between the hot and cold fluids along the length of the heat exchanger. It is used because the temperature difference varies along the exchanger, and a simple arithmetic average would overestimate the effective driving force. LMTD provides an accurate representation for sizing heat exchangers when inlet and outlet temperatures are known.
What is the difference between counter-flow and parallel-flow heat exchangers?
In counter-flow arrangement, the hot and cold fluids flow in opposite directions, which produces a higher LMTD and more efficient heat transfer. The cold fluid outlet can approach the hot fluid inlet temperature. In parallel-flow arrangement, both fluids flow in the same direction, resulting in a lower LMTD. The cold fluid outlet temperature is limited to the hot fluid outlet temperature. Counter-flow is generally preferred for maximum heat recovery.
What is the overall heat transfer coefficient (U value)?
The overall heat transfer coefficient (U) represents the total thermal resistance between the two fluids, including convection on both sides and conduction through the tube wall. It is measured in W/(m²·K) and depends on fluid properties, flow velocities, tube material, and geometry. Higher U values indicate more efficient heat transfer. Typical values range from 10 W/(m²·K) for gas-to-gas exchangers to over 5000 W/(m²·K) for condensing steam to water.
How does fouling affect heat exchanger performance?
Fouling is the accumulation of deposits on heat transfer surfaces, which adds thermal resistance and reduces the effective U value. Common types include scaling from hard water, biological growth, corrosion products, and particulate deposition. Engineers typically apply a fouling factor (Rf) to account for this degradation when sizing heat exchangers. Regular cleaning schedules and proper water treatment help maintain performance and extend equipment life.
How do I determine the thermal duty of a heat exchanger?
Thermal duty (Q) is the rate of heat transfer in the exchanger, calculated as Q = mass flow rate × specific heat capacity × temperature change for either the hot or cold side. In a balanced system, the heat lost by the hot fluid equals the heat gained by the cold fluid. Thermal duty is typically expressed in kilowatts (kW) or BTU/h and is the primary parameter for selecting and sizing heat exchangers for a given application.
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