Enter Heat Exchanger Data
Use kJ, kg, seconds, kelvin differences, cubic metres, and kilopascals. Temperatures are entered in degrees Celsius.
Example Data
| Input | Hot Stream | Cold Stream | Unit |
|---|---|---|---|
| Mass flow rate | 1.20 | 1.50 | kg/s |
| Heat capacity | 4.18 | 4.18 | kJ/kg·K |
| Inlet temperature | 150 | 25 | °C |
| Outlet temperature | 90 | 73 | °C |
| Inlet pressure | 300 | 200 | kPa |
| Outlet pressure | 280 | 185 | kPa |
| Specific volume | 0.001 | 0.001 | m³/kg |
Understanding Exergy in Heat Exchangers
Exergy measures the useful work potential carried by energy. A heat exchanger transfers thermal energy between streams. Its first law performance may look excellent. Yet useful work potential can still disappear. That disappearance is exergy destruction. It occurs because real heat transfer needs temperature differences. Pressure drops, mixing, friction, and external heat leaks also create losses. Studying exergy reveals where improvements matter most. It helps engineers compare designs beyond simple heat duty. A smaller temperature driving force usually reduces irreversibility. Better flow distribution also protects available energy.
Reading the Important Results
The calculator evaluates each stream at inlet and outlet conditions. It uses local ambient temperature and pressure as reference conditions. Stream exergy includes thermal and pressure contributions. The thermal part depends on temperature relative to the environment. The pressure part uses specific volume and pressure difference. Hot-stream exergy normally falls as it cools. Cold-stream exergy normally rises as it warms. Their difference is not automatically destruction. The external heat term must also be included. Entropy generation gives the most direct measure of irreversibility. Multiply generated entropy by ambient absolute temperature to obtain destroyed exergy. Results support retrofit priorities, operating targets, and equipment choices across the whole plant.
Formula Used
Temperatures use kelvin. Heat capacity uses kJ per kilogram kelvin. Specific volume uses cubic metres per kilogram. Pressures use kilopascals. The heat-transfer exergy term is EQ = Q[1 − T0/Tb]. Positive Q enters the exchanger. Entropy generation is Sgen = Σm cp ln(Tout/Tin) − Q/Tb. Exergy destruction is ED = T0Sgen. A valid passive exchanger should not produce negative destruction.
Checking Energy and Exergy Closure
The hot-side duty equals mh cp,h(Th,in − Th,out). The cold-side duty equals mc cp,c(Tc,out − Tc,in). For an insulated exchanger, these values should be close. A heat leak changes the expected balance. The calculator reports the energy residual. Large residuals suggest inconsistent measurements, incorrect units, or unaccounted losses. It also reports an exergy-balance closure value. Small closure supports the selected assumptions. Larger closure can arise from unmatched energy data, variable heat capacities, phase change, or ignored kinetic energy. Use laboratory data carefully when pressure changes are substantial.
How to Use This Calculator
Enter the ambient reference temperature and pressure first. Then enter temperatures, pressures, flow rates, heat capacities, and specific volumes for both streams. Use positive flow rates. Enter external heat transfer as positive when heat enters the exchanger. Choose the boundary temperature associated with that transfer. Select the flow arrangement for terminal approach reporting. Submit the form to view results above the inputs. Compare destroyed exergy with heat duty and thermal effectiveness. Check energy residual before trusting efficiency. Export the displayed values for reports. This tool uses constant properties and neglects kinetic and potential energy changes. Use detailed property software for steam, refrigerants, reacting fluids, or phase change.
Frequently Asked Questions
1. What is exergy destruction?
It is useful-work potential lost through irreversibility. In a heat exchanger, finite temperature differences, friction, pressure drops, mixing, and unwanted heat transfer generate this loss. It is commonly reported in kilowatts.
2. Why is the ambient reference state important?
Exergy is measured relative to the environment. Changing ambient temperature or pressure changes the calculated work potential. Use reference conditions that represent the actual surroundings or the accepted design standard.
3. Can this calculator handle heat loss to surroundings?
Yes. Enter a negative external heat-transfer value for heat leaving the exchanger. Enter a positive value when heat enters it. The boundary temperature determines the associated exergy transfer.
4. What does a negative exergy-destruction result mean?
A passive heat exchanger should not have negative exergy destruction. Check all temperatures, signs, flow rates, the boundary temperature, and the reference state. A large energy imbalance can also create this result.
5. Why are temperatures converted to kelvin?
Exergy and entropy relations use absolute temperature. Kelvin prevents invalid logarithms and preserves thermodynamic meaning. You may enter Celsius values because the calculator converts them internally.
6. When should I use specific volume?
Use a representative specific volume for liquid-like streams. It estimates pressure-related exergy. For ideal gases, real gases, steam, refrigerants, or phase-changing fluids, use property data instead of a constant-volume approximation.
7. Does the calculator support phase change?
Not accurately. Condensing or evaporating streams need enthalpy and entropy values from reliable property tables or software. Constant heat capacity and specific volume assumptions can be misleading during phase change.
8. What is exergy efficiency here?
It is the cold-stream exergy gain divided by the exergy supplied by the hot stream and any incoming heat transfer. It is a practical indicator, but interpretation requires a physically consistent energy balance.
9. What does thermal effectiveness show?
Thermal effectiveness compares the calculated cold-side duty with the maximum duty allowed by inlet temperatures and the smaller heat-capacity rate. It complements exergy analysis but does not measure irreversibility directly.
10. Why is the energy residual shown?
The energy residual checks whether hot-side duty plus external heat equals cold-side duty. A small value supports the measurements. A large value usually indicates wrong units, missing heat loss, or inconsistent readings.
11. Can I use this for design decisions?
Use it for screening, comparisons, and early troubleshooting. For final equipment sizing, use temperature-dependent properties, pressure-drop models, fouling factors, detailed geometry, and validated process data.