How to select an industrial
heat exchanger: the 7 criteria
Selecting a heat exchanger is not a catalogue choice. It depends on seven interdependent technical criteria —and many other variables that no guide can fully capture. Field experience and deep knowledge of real equipment behaviour are just as decisive as any formula.
When someone asks "which heat exchanger do I need?", the correct answer is never a catalogue model. Nor is it a list of seven criteria. Behind every industrial process there are variables that appear on no datasheet: the actual behaviour of a fluid under variable process conditions, the experience accumulated from similar applications, the subtleties that determine whether a solution will work well in the long run. This guide describes the documentable criteria. The rest comes from deep sector knowledge.
The 7 selection criteria
Characterise the process fluid
The starting point is the precise characterisation of the two fluids that will flow through the equipment —the hot fluid and the cold fluid— under real operating conditions, not standard or laboratory conditions.
For each fluid it is necessary to determine: type (gas, liquid, saturated steam, two-phase fluid), full chemical composition, pH, suspended or fibrous solids content, dynamic viscosity and thermophysical properties —density, specific heat and thermal conductivity— at the actual working temperature. When the fluid is a mixture, the mixture properties do not always coincide with those of any of its components.
Corrosive, viscous or particle-laden fluids directly condition the admissible construction types and materials. The compatibility of a fluid with a given material depends on the exact composition, temperature and concentration: what is suitable in one environment may be completely unsuitable in a superficially similar one. A viscous fluid affects the flow regime and therefore the achievable heat transfer coefficient.
Define the temperature conditions
The inlet and outlet temperatures of each fluid (T₁ and T₂) must be established precisely. From these, the log mean temperature difference (LMTD) is derived, which is the driving force for heat transfer and the basis of the design equation Q = U · A · LMTD.
Checking the limits is as important as the central value. Maximum temperatures must be compatible with the structural material and fluid conditions; minimum temperatures, with the risk of unwanted condensation or acid dewpoint in combustion gases. The temperature at which combustion gases can condense acids in the exchanger varies depending on the fuel, air excess and other process conditions —and is one of the parameters that must be evaluated case by case.
It should be noted that working with condensing gases —including gases from the combustion of natural gas or other fuels such as diesel or fuel oil— is perfectly viable technically when the equipment is designed for this condition. In these cases, the gas outlet temperature may be below the dewpoint, and the heat exchanger must be designed to handle this.
Determine the required thermal power
The thermal power Q (kW) is the central sizing parameter. It is obtained by applying the thermodynamic formulas corresponding to the fluid type, using properties interpolated at the actual working temperature — not at ambient temperature.
- ṁ
- Mass flow rate [kg/s]. If the flow is volumetric: ṁ = ρ(T₁) · Q̇ — where ρ is evaluated at T₁, not at T_m
- cp(Tm)
- Specific heat at mean temperature Tm = (T₁+T₂)/2 [kJ/(kg·K)]
- ΔT
- |T₁ − T₂| [K]
- hfg
- Latent heat of vaporisation [kJ/kg], from IAPWS-IF97 tables. At 1 bar: 2,257 kJ/kg. At 4 bar: 2,134 kJ/kg. At 8 bar: 2,048 kJ/kg.
- ṁas
- Dry air flow rate = ṁmixture/(1+W₁), where W₁ is the inlet specific humidity
- h
- = 1.006·T + W·(2501 + 1.86·T) [kJ/kgda] — mixture enthalpy
The calculated Q value is a starting point for the technical discussion. In practice, equipment selection takes into account the progressive degradation of heat transfer over time due to fouling. How much margin is appropriate in each case depends on the fluid, operating conditions, expected maintenance frequency and knowledge of the specific application.
Establish the allowable pressure drop
The maximum tolerable pressure drop on each side of the heat exchanger (allowable ΔP) is a design parameter as important as the thermal power, but usually less well documented in initial specifications.
The ΔP directly conditions the equipment geometry: number of passes, tube length and diameter, baffle spacing and, for plate heat exchangers, the circuit configuration. A generous allowable ΔP allows higher flow velocities, better heat transfer coefficients and more compact equipment. A very tight ΔP constraint requires larger surface area equipment to achieve the same power.
The allowable pressure drop varies widely depending on process type, fluid and installation. It must be defined for each side of the heat exchanger and clearly communicated in the technical specification. Pump and fan sizing must account for the heat exchanger's contribution to the total pressure loss in the circuit.
Evaluate the construction material
The choice of material for tubes (or plates), headers and shell is one of the decisions with the greatest long-term impact. Operating temperature, pressure and the chemical nature of the fluid —including pH, presence of halides, sulphur compounds or other aggressive species— are factors that must be considered together, not independently.
The following table gives indicative information on some of the most common materials used in industrial heat exchangers. The ranges shown are general references and do not replace specific verification for each application, fluid and operating conditions. The actual compatibility of a material with a given fluid depends on multiple factors beyond temperature limits:
| Material | Indicative T range | Behaviour with chlorides | Common use |
|---|---|---|---|
| AISI 316L | up to ~500°C | Limited; sensitive to high concentrations or temperatures | Chemical, food, general service |
| AISI 304 | up to ~500°C | Lower resistance than 316L | General service in less demanding environments |
| Titani Gr. 2 | up to ~300°C | Excellent in most conditions | Seawater, corrosive environments |
| Cu-Ni 90/10 | up to ~300°C | Good tolerance | Marine cooling |
| Hastelloy C-276 | up to ~370°C | Excellent in highly aggressive environments | Strong acids, highly corrosive environments |
| Acer C P265GH | up to ~300°C | Not recommended in corrosive environments | Standard shell, non-corrosive fluids |
The combination of materials between the fluid-contact parts —tubes, shell, tube sheets— requires attention when materials of different nature are used in the presence of an electrolyte, as this can activate galvanic corrosion mechanisms.
Evaluate cleaning and maintenance requirements
The tendency of the fluid to deposit fouling is a selection criterion, not a subsequent operational consideration. Its magnitude is highly variable: there are processes with extremely clean fluids that generate virtually no fouling, and processes where fouling is rapid and intense. This variability means that general values applicable to all cases cannot be established.
Fouling tendency conditions the admissible construction type. Processes with high risk of fouling or solid precipitation require equipment that allows physical access to the exchange surface for cleaning. In some continuous production processes, it may make sense to provide operational redundancy to allow cleaning without stopping the process.
Check the applicable regulations (PED)
The European Pressure Equipment Directive 2014/68/EU (PED) establishes the essential safety requirements for heat exchangers that exceed the thresholds defined in Annex II. The information in this article is indicative and is based on the regulations in force at the time of writing; regulations may be amended and it is the reader's responsibility to verify the updated version applicable to their case.
Equipment classification in Categories I to IV determines the required conformity assessment module, the necessary technical documentation and the possible involvement of a Notified Body (NoBo). The main classification criteria are: fluid type (Group 1 — flammable, toxic or oxidising; Group 2 — others), maximum allowable pressure PS [bar] and internal volume V [litres] or nominal diameter DN. The equipment is classified for the higher-risk side (tubes or shell).
Category III or IV equipment —typically steam or Group 1 fluids at significant pressures or volumes— requires a Notified Body (NoBo) to be involved in the certification process and in the final inspection before CE marking. PED classification and compliance with its requirements is not optional: it is a legal requirement for putting the equipment into service in the European Union.
This guide covers seven criteria that can be partially documented and quantified. But the appropriate selection of an industrial heat exchanger also depends on variables that appear on no datasheet: the actual behaviour of a fluid under variable process conditions, the experience accumulated from applications with similar characteristics, the nuances that determine whether a solution will work well in the long run. No document can substitute for deep sector knowledge and its applications.
Thermal power estimation (support for Criterion 3)
The calculator applies the Criterion 3 formulas with fluid properties interpolated at the actual process temperature. The result is a starting point to guide the first technical discussion. For a full sizing, the Technical Office works directly with your process data.
The calculator obtains Q from fluid, flow rate and temperatures, with properties interpolated at actual temperature. It does not calculate U, LMTD, surface area, pressure drop or incorporate fouling or geometry: these steps require the actual process data and application knowledge. If you have a Q and want to go further, the Technical Office handles the full sizing.
ρ at Tinlet · cp at Tm · Temperature-interpolated properties · Result with no normative validity
| Parameter | Calculated value |
|---|
Result obtained with properties interpolated from reference tables (VDI Heat Atlas 2010 / Eastman / CRC Handbook). Does not incorporate U, LMTD, fouling or geometric parameters. To move from Q to a real equipment specification, contact the Technical Office.
The BOIXAC Technical Office works with the real data of your process to identify the appropriate heat transfer solution.