Correct heat exchanger sizing is one of the most important steps in any thermal system design. An undersized unit will fail to meet the required duty; an oversized unit wastes capital cost, increases pressure drop, and can cause operational problems such as flow instability or refrigerant maldistribution.
This guide walks through the key parameters and steps involved in sizing a brazed plate heat exchanger for a typical HVAC or industrial application.
Step 1: Define the Thermal Duty
The starting point for any heat exchanger sizing is the thermal duty — the amount of heat to be transferred. This is expressed in kilowatts (kW) or megawatts (MW) and is calculated from the flow rates and temperature changes on one side of the exchanger.
Q = ṁ × Cp × ΔT where Q = heat duty (kW), ṁ = mass flow rate (kg/s), Cp = specific heat capacity (kJ/kg·K), ΔT = temperature difference (K)
Example: A district heating substation needs to cool primary water from 80°C to 55°C at a flow rate of 5 m³/h. The thermal duty is approximately: Q = (5000/3600) × 4.18 × (80-55) = 145 kW
Step 2: Define the Secondary-Side Parameters
Once the primary-side duty is established, the secondary-side inlet and outlet temperatures and flow rate must be defined. This allows calculation of the heat balance and verification that the thermal duty is consistent on both sides.
For the example above, if the secondary circuit (building heating) enters at 45°C and must leave at 70°C, the required secondary flow rate can be calculated from the same formula.
Step 3: Calculate the Log Mean Temperature Difference (LMTD)
The LMTD is the effective average temperature driving force for heat transfer across the exchanger. For a counterflow heat exchanger (the normal configuration for plate heat exchangers):
LMTD = (ΔT1 – ΔT2) / ln(ΔT1/ΔT2) where ΔT1 = hot inlet – cold outlet, ΔT2 = hot outlet – cold inlet
For the district heating example: ΔT1 = 80 – 70 = 10°C, ΔT2 = 55 – 45 = 10°C. In this case LMTD = 10°C (the symmetric case).
A lower LMTD means a larger heat exchanger is required. A tighter approach temperature between the two outlet streams increases the LMTD and generally improves system efficiency, but drives up heat exchanger size and cost.
Step 4: Specify Fluid Properties and Working Conditions
The heat exchanger selection requires accurate fluid property data for both circuits:
- Fluid type (water, glycol solution, refrigerant, oil, etc.)
- Concentration (for glycol solutions)
- Maximum and minimum operating temperatures
- Maximum allowable pressure drop on each circuit
- Maximum working pressure
- Fluid cleanliness / fouling tendency
Step 5: Use Selection Software
Manual heat exchanger sizing from first principles is complex and iterative. In practice, heat exchanger manufacturers provide selection software that performs these calculations automatically and identifies the correct model, plate count, and configuration.
HEXONIC’s CAIRO online selection platform allows engineers to input the thermal duty, fluid parameters, and pressure constraints and receive an optimised heat exchanger selection. CAIRO covers the full range of HEXONIC products including brazed plate, plate and frame, and shell and coil exchangers.
- Access CAIRO at: cairo.hexonic.com
- Input required: thermal duty, fluid types, inlet/outlet temperatures, max pressure drop, working pressure
- Output: recommended model, plate count, thermal performance data, pressure drops, dimensional drawing
Step 6: Verify the Selection
Before finalising the selection, verify:
- The thermal performance meets the required duty at the worst-case operating conditions (not just design point)
- Pressure drops are within the allowable limits for the pump selection
- The working pressure rating exceeds the maximum system pressure including hydraulic shock allowance
- The connections and dimensions are compatible with the installation space
- The model selected is approved for the fluids and temperatures specified (especially for food or pharmaceutical duty)
Common Sizing Mistakes to Avoid
- Undersizing for peak load conditions — always size for the maximum expected duty, not just the average
- Ignoring fouling allowance — for non-clean water circuits, add a fouling factor to the selection
- Specifying too tight an approach temperature — this drives up size and cost disproportionately; 3–5°C approach is typical for HVAC
- Not accounting for glycol concentration — glycol/water mixtures have lower Cp and higher viscosity than pure water, significantly affecting sizing
- Neglecting pressure drop — an exchanger selected purely for thermal performance may generate excessive pressure drop, requiring a larger pump or a longer, fewer-plate model
Use HEXONIC’s CAIRO online selection tool for instant heat exchanger sizing — or contact our engineering team directly for complex applications and custom requirements.
Frequently Asked Questions
How many plates does a brazed plate heat exchanger typically have?
This depends entirely on the duty. Small DHW applications may require as few as 10–20 plates. Large district heating substations or industrial applications may use 60–100+ plates. The HEXONIC CAIRO selection software calculates the optimum plate count for any given duty.
Can I use the same heat exchanger for different duties?
A heat exchanger is sized for a specific set of operating conditions. Operating significantly outside its design envelope — particularly at much lower flow rates or very different temperature levels — can cause operational issues such as maldistribution or insufficient turbulence. Always re-verify the selection if system operating parameters change.
What information do I need to get a heat exchanger quote from HEXONIC?
To provide an accurate quotation, HEXONIC requires a process data sheet specifying: fluid types and properties, inlet and outlet temperatures for both circuits, flow rates, maximum allowable pressure drop, maximum working pressure, and any material or certification requirements. Our engineers will review your data and propose the optimum solution.
