Side-by-Side Result
Calculation Breakdown
Full Calculation Table
| Parameter | Primary Side | Secondary Side |
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How to Use the Heat Exchanger Pipe Calculator
Enter the total heat transfer rate the exchanger is designed to move. This comes from the heat source or load calculation, and must be equal on both sides since the exchanger transfers the same quantity of heat from one fluid to the other.
Choose the fluid type and enter inlet and outlet temperatures for both sides. If one side carries a glycol mixture, check the glycol concentration calculator to confirm the required concentration for freeze protection before setting temperatures.
Click Calculate Both Sides to see the required flow rate, recommended pipe size class, and a pressure drop allowance for each side. Take each side's flow rate to the pipe sizing calculator to confirm the exact pipe size based on your velocity target.
Sizing Pipe on Both Sides of a Heat Exchanger
A heat exchanger transfers thermal energy from one fluid to another without letting them mix. Each side of the exchanger has its own piping circuit, its own fluid properties, and its own flow rate, and all three can differ significantly between the two sides. Sizing both sides correctly requires running the heat transfer calculation and the pipe sizing calculation separately for each, which is what this tool does in a single step. The same fluid-to-fluid concept extends beyond hydronic loops too, showing up in refrigerant circuits on the refrigeration heat exchanger calculator.
Flow Rate Comes From the Heat Transfer Equation
The fundamental relationship between heat transfer rate, mass flow, specific heat, and temperature difference is: Q = ṁ × Cp × ΔT. Rearranging for mass flow rate gives ṁ = Q ÷ (Cp × ΔT). For water at typical hydronic temperatures, specific heat is approximately 4.18 kJ per kilogram per degree Celsius, drawn from the same kind of reference thermophysical data in NIST's Chemistry WebBook fluid properties database, so a 100 kW exchanger designed for a 10°C temperature rise needs about 2.39 kg/s, equivalent to roughly 145 L/min. The same exchanger on its primary side might run at a different temperature difference and therefore a different flow rate, which is why each side gets its own calculation here.
Why Each Side Has a Different Flow Rate
The two sides must transfer the same total heat, but the flow rates will differ whenever the inlet-to-outlet temperature difference differs between sides or the specific heat of the two fluids differs. A glycol secondary side has a slightly lower specific heat than pure water, meaning it needs more mass flow than a pure water side to absorb the same heat at the same temperature rise. This tool uses the fluid-specific specific heat for each side so both flow rates correctly reflect the actual fluid on that circuit. For systems using glycol, also check the Darcy-Weisbach calculator since glycol's higher viscosity changes the friction loss at the resulting flow rate. Air-to-air heat exchangers follow the same underlying principle on the ventilation side, which the ventilation energy recovery calculator covers for ERV and HRV core sizing.
Pipe Size and Pressure Drop Allowance
This calculator recommends a pipe size class based on targeting a velocity in the 0.5 to 1.2 m/s range for each side, which is a common design target range for hydronic branch piping. The pressure drop allowance shown is a typical figure for a plate heat exchanger at the calculated flow, and should be added to the pump head calculator total for the circuit that runs through this exchanger. Always confirm the actual pressure drop against the specific heat exchanger manufacturer's data sheet at the calculated flow rate, since it varies by model, fouling factor, and number of plates. Larger commercial plate heat exchangers are often code-stamped pressure equipment built to a standard such as the ASME Boiler and Pressure Vessel Code, Section VIII, worth confirming on the equipment nameplate for larger installations.
A Note on Accuracy
This calculator uses approximate specific heat values for water and propylene glycol mixtures at mid-range temperatures. Actual specific heat varies with temperature and glycol concentration, so for high-precision sizing or glycol systems at unusual temperatures, use the fluid manufacturer's published specific heat at the actual operating temperature. Pipe size recommendations are a starting point; confirm with the pipe sizing calculator at your preferred velocity target.
Frequently Asked Questions
The required flow rate follows directly from the heat transfer equation: Q equals mass flow rate times specific heat times temperature difference. Rearranging for mass flow, you divide the heat capacity in kilowatts by the product of the fluid specific heat and the temperature difference between inlet and outlet. For water at typical hydronic temperatures, specific heat is approximately 4.18 kJ per kilogram per degree Celsius, so a 100 kW exchanger with a 10 degree temperature difference needs about 2.39 kilograms per second, or roughly 145 litres per minute.
A heat exchanger has two independent fluid circuits that do not mix, a primary side and a secondary side, each with its own flow rate, temperature difference, fluid type, and therefore its own pipe size requirement. The two sides often have different flow rates because different fluids have different specific heat capacities. Running a separate pipe sizing calculation for each side ensures that both circuits are sized for the actual flow rate and fluid properties on that side, rather than assuming they are identical.
Pressure drop across a plate heat exchanger varies widely by model, number of plates, flow rate, and fluid viscosity. At typical residential and light commercial flow rates, many plate heat exchangers produce pressure drops in the range of 20 to 80 kPa, but the only reliable figure for any specific unit is the manufacturer's published pressure drop curve at the actual flow rate. This calculator provides a rough allowance for initial pump head estimates, which should be replaced with the actual manufacturer figure once a specific model is selected.
Yes, if the pump on a given circuit pushes fluid through the heat exchanger, the exchanger's pressure drop must be added to the total head that pump needs to develop. Enter it as the equipment pressure drop on the pump head calculator when totaling the circuit. Pumps on the opposite side of the exchanger are unaffected since each side is an independent circuit.
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