liminfoHeat Exchanger Calculator
Free web tool: Heat Exchanger Calculator
LMTD Results
About Heat Exchanger Calculator
The Heat Exchanger Calculator is a free, browser-based engineering tool that performs thermal analysis of shell-and-tube or plate heat exchangers using two industry-standard methods: the Log Mean Temperature Difference (LMTD) method and the Number of Transfer Units (NTU) - Effectiveness method. For the LMTD method, you input the inlet and outlet temperatures of both the hot and cold fluid streams along with their mass flow rates and specific heat capacities to compute the LMTD, heat duty on each side, and the required overall heat transfer coefficient-area product (UA). For the NTU-effectiveness method, you provide the UA value along with fluid properties and inlet temperatures to determine the number of transfer units, heat capacity ratio (Cr = Cmin/Cmax), exchanger effectiveness, and actual heat transfer rate.
This tool is used by chemical engineers, mechanical engineers, HVAC designers, and process engineers who need rapid preliminary sizing calculations or verification of existing heat exchanger performance. Both counter-flow (where hot and cold fluids flow in opposite directions, giving the highest theoretical effectiveness) and parallel-flow (where fluids enter from the same end) configurations are supported with the correct formulas applied for each case. Counter-flow exchangers achieve higher effectiveness for a given UA and are the preferred configuration in most industrial applications.
All calculations use SI units internally — temperatures in degrees Celsius, flow rates in kg/s, heat capacities in kJ/(kg·K), heat duty in kW, and UA in W/K. The LMTD is computed using the correct logarithmic formula with a special case handler for when the two terminal temperature differences are nearly equal (to avoid division by zero). The NTU-effectiveness formula for counter-flow uses the standard exponential model, with a separate linear approximation when Cr equals 1 (balanced flow). All processing runs entirely in your browser with no server communication.
Key Features
- LMTD method: calculates log mean temperature difference, heat duty (hot and cold sides), and required UA
- NTU-effectiveness method: calculates NTU, heat capacity ratio Cr, effectiveness, and actual heat transfer rate
- Supports both counter-flow and parallel-flow configurations with correct formulas for each
- Inputs for both hot and cold fluid: inlet/outlet temperatures (°C), mass flow rate (kg/s), specific heat Cp (kJ/kg·K)
- UA input for NTU method — independently adjustable from the temperature inputs
- Heat capacity rates Ch and Cc computed and displayed alongside Cmin/Cmax ratio
- Special case handling for balanced flow (Cr ≈ 1) to prevent numerical instability
- Compact, tab-based interface allowing quick switching between LMTD and NTU modes
Frequently Asked Questions
What is the LMTD method for heat exchangers?
The LMTD (Log Mean Temperature Difference) method calculates heat duty as Q = UA × LMTD. The LMTD is the logarithmic average of the temperature differences between the hot and cold streams at each end of the exchanger. It is used when both inlet and outlet temperatures are known, to find the required UA for a given duty.
What is the NTU-effectiveness method?
The NTU-Effectiveness (Number of Transfer Units) method calculates exchanger performance when only inlet temperatures are known. NTU = UA / Cmin, and effectiveness (ε) is the ratio of actual heat transfer to maximum possible heat transfer. It is used for exchanger rating (predicting outlet conditions given a known UA) rather than sizing.
What is the difference between counter-flow and parallel-flow?
In counter-flow, hot and cold fluids enter from opposite ends, so the largest temperature driving force is maintained across the exchanger. This gives the highest LMTD and effectiveness for a given UA. In parallel-flow, both fluids enter from the same end, producing a declining temperature difference that limits maximum effectiveness to 50% when Cr = 1.
What does UA mean in heat exchanger calculations?
UA is the product of the overall heat transfer coefficient U (W/m²·K) and the heat transfer area A (m²). It represents the thermal conductance of the exchanger. A higher UA means more heat can be transferred for the same temperature difference. In the LMTD method, the required UA is computed from the duty and LMTD; in the NTU method, UA is an input used to find effectiveness.
What is heat capacity rate and why does Cmin matter?
Heat capacity rate (C) is the product of mass flow rate and specific heat capacity: C = m × Cp (W/K). Cmin is the smaller of the two fluid heat capacity rates. The maximum possible heat transfer (Qmax) is Cmin × (Thi - Tci) because the stream with the smaller heat capacity rate undergoes the largest temperature change. NTU is always based on Cmin.
Why does the LMTD calculator require outlet temperatures for both fluids?
The LMTD method requires all four terminal temperatures (hot in/out, cold in/out) because LMTD is derived from the temperature differences at both ends of the exchanger. If outlet temperatures are unknown, you would need to use the NTU method with an assumed UA value, or iterate until the energy balance and LMTD equations are simultaneously satisfied.
What is the Cr = 1 special case in the NTU formula?
When Cr (heat capacity ratio Cmin/Cmax) equals 1, the standard counter-flow NTU formula contains a 0/0 indeterminate form. In this balanced-flow case, the effectiveness simplifies to ε = NTU / (1 + NTU). This calculator detects when |Cr - 1| < 0.001 and applies this special case formula automatically.
Can this calculator be used for condensers or evaporators?
Phase-change exchangers like condensers and evaporators have one fluid at near-constant temperature (isothermal phase change), meaning Cmax approaches infinity and Cr approaches 0. In that case, NTU = -ln(1 - ε) and the NTU formula simplifies for both counter and parallel flow. While you can approximate this by setting the phase-change fluid Cp very high (simulating large Cmax), a dedicated condenser/evaporator tool would be more appropriate for rigorous design.