Inputs
Formula Used
Sensible heating method
Base Duty (kW) = ṁ × Cp × (Tout − Tin)
where ṁ is mass flow (kg/s), Cp is specific heat (kJ/kg·K), and temperature rise is in K (same increment as °C).
Enthalpy rise method
Base Duty (kW) = ṁ × (hout − hin)
where h is specific enthalpy (kJ/kg).
Adjustments
With losses = Base × (1 + Loss%/100)
With margin = With losses × (1 + Margin%/100)
Required duty = With margin ÷ Efficiency
How to Use This Calculator
- Pick a method: use sensible when you know temperatures and Cp, or enthalpy when you have h-values from a steam table or process data sheet.
- Enter mass flow and select the correct unit. The calculator converts everything to kg/s internally.
- Provide temperatures or enthalpies depending on your chosen method.
- Add efficiency, expected distribution losses, and a safety margin for sizing.
- Press Calculate to see results above the form. Use the download buttons for CSV and PDF reports.
Example Data Table
| Case | Method | ṁ (kg/s) | Tin (°C) | Tout (°C) | Cp (kJ/kg·K) | Efficiency | Loss % | Margin % | Required Duty (kW) |
|---|---|---|---|---|---|---|---|---|---|
| A | Sensible | 1.50 | 60 | 90 | 4.186 | 0.90 | 3 | 10 | 237.14 |
| B | Sensible | 0.80 | 45 | 75 | 4 | 0.88 | 5 | 15 | 131.73 |
| C | Enthalpy | 1.20 | — | — | — | 0.92 | 2 | 8 | 179.61 |
Example values are illustrative for estimating and verifying output behavior.
Professional Guide: Reheater Duty in Construction Systems
Reheaters are commonly applied in building services and industrial facilities to lift the temperature of a working fluid after a pressure drop, heat loss, or a control-stage reduction. In hydronic HVAC, a reheater may restore supply water temperature after mixing, long pipe runs, or heat recovery. In process utilities, it can stabilize downstream conditions and protect equipment from condensation or thermal shock.
This calculator estimates reheater duty using two robust approaches. The sensible heating method is best when you know inlet and outlet temperatures and can assume a representative specific heat. The enthalpy rise method is preferred when you have thermodynamic property data (for example, from a specification sheet or steam tables), because it inherently captures phase effects and property variation.
For field sizing, always account for real-world penalties. Distribution losses can come from uninsulated fittings, valve bodies, and long headers, while performance efficiency captures heat exchanger effectiveness and control limitations. A practical safety margin helps cover fouling, seasonal extremes, and future load growth. The final “Required duty” output applies these adjustments to produce a conservative design duty suitable for selection and scheduling.
Example calculation: Using the sensible method with ṁ = 1.50 kg/s, Tin = 60 °C, Tout = 90 °C, and Cp = 4.186 kJ/kg·K, the base duty is: 1.50 × 4.186 × (90−60) = 188.37 kW. With 3% losses and 10% margin, demand becomes 213.46 kW. Dividing by 0.90 efficiency gives a required duty of 237.18 kW (≈ 809.27 kBtu/h). This aligns with the example table above and illustrates how small percentages materially change final equipment sizing.
In commissioning, compare calculated duty against measured flow and temperature rise to validate sensors, balancing, and valve authority. In design, document assumptions (units, Cp source, loss allowance, margin rationale) so reviewers can trace results. For procurement packages, exporting the PDF summary supports submittals, scope coordination, and change control.
FAQs
1) Which method should I choose?
Use sensible heating when you know temperatures and Cp. Use enthalpy rise when properties come from data sheets or steam tables, especially near phase change or wide temperature ranges.
2) What Cp value should I use?
For water near room temperature, Cp ≈ 4.186 kJ/kg·K is common. For glycol mixes or process fluids, use manufacturer data or a verified property source for the operating range.
3) Why does efficiency increase required duty?
Efficiency represents how much of the supplied heat becomes useful heating. If efficiency is 0.90, you must supply more than the useful demand, so required duty equals demand divided by efficiency.
4) How do I estimate distribution losses?
Start with 1–5% for well-insulated systems and increase for long runs, poor insulation, or many fittings. If you have heat-loss calculations, enter the equivalent percentage to match them.
5) What safety margin is reasonable?
Many projects use 5–15% depending on uncertainty, fouling risk, and future expansion. Coordinate with design criteria, client standards, and equipment turndown so oversizing does not harm control.
6) Can I use volume flow instead of mass flow?
This calculator uses mass flow. Convert volume flow using density at operating conditions. For water, mass flow ≈ density × volume flow; density is near 1000 kg/m³ but varies with temperature.
7) How should I validate results onsite?
Measure flow and temperatures (or properties) across the reheater and compute ṁ×Cp×ΔT or ṁ×Δh. Compare to equipment ratings and check for sensor bias, air binding, or control instability.