Question 1

What is a heat exchanger and what is it used for?

Accepted Answer

A heat exchanger is a device that transfers thermal energy between two fluids, gases or solids without mixing them, by exploiting a temperature difference between them.

Question 2

How does a heat exchanger work?

Accepted Answer

The two fluids circulate through circuits separated by a conductive wall. Heat flows from the hot fluid to the cold one by convection and conduction, until the thermal equilibrium defined by the design conditions is reached.

Question 3

What is the difference between a heat exchanger and a heat recuperator?

Accepted Answer

The term heat recuperator is a subset of the term heat exchanger: every recuperator is an exchanger, but not every exchanger is a recuperator.

Question 4

What materials are commonly used in the construction of heat exchangers?

Accepted Answer

The most common materials are carbon steel, austenitic stainless steel (304, 316L), aluminium, copper, titanium and cupro-nickel, selected according to the temperature, pressure and chemical aggressiveness of the fluids.

Question 5

What is the difference between a direct-contact and an indirect-contact heat exchanger?

Accepted Answer

In direct contact, the two fluids physically mix (cooling towers, mixing condensers). In indirect contact, a physical wall separates them and mixing is impossible.

Question 6

What is the overall heat transfer coefficient (U)?

Accepted Answer

The coefficient U expresses the amount of heat transferred per unit surface area, per unit time and per degree of temperature difference between the fluids. It is expressed in W/(m²·K).

Question 7

What is the pinch point in a heat exchanger?

Accepted Answer

The pinch point is the minimum temperature difference between the two fluids at any point in the heat exchanger. It determines the required heat transfer surface area and the maximum theoretically achievable thermal recovery.

Question 8

What is the fouling factor and how does it affect design?

Accepted Answer

The fouling factor is an additional thermal resistance included in the design to account for the reduction in efficiency caused by the build-up of deposits on the heat transfer surfaces over time.

Question 9

What are the main types of heat exchangers?

Accepted Answer

The two main families are tube-type heat exchangers (plain tubes, continuous or helical finned tubes, shell-and-tube, double-pipe coaxial) and plate-type heat exchangers (pillow plate, cross-flow, brazed/welded plates, gasketed plate-and-frame).

Question 10

What is the best heat exchanger for viscous or sediment-laden fluids?

Accepted Answer

For viscous, sticky or sediment-laden fluids, tube-type exchangers (especially shell-and-tube or double-pipe coaxial) and pillow plate exchangers offer the greatest tolerance, thanks to their wide passage surfaces and ease of cleaning.

Question 11

When is a helical finned tube heat exchanger preferable to a continuous fin exchanger?

Accepted Answer

Helical fins are the preferred choice when the gas contains particles, ash or sticky compounds, because the pitch between coil turns is adjustable and the geometry facilitates cleaning. Continuous fins offer greater compactness for clean gases.

Question 12

When is a pillow plate heat exchanger used?

Accepted Answer

The pillow plate heat exchanger is used in applications involving viscous, sticky or solid-laden fluids, and in heat transfer to granular solids, being an efficient alternative to conventional fluidised beds.

Question 13

What is the difference between a brazed-plate heat exchanger and a gasketed plate-and-frame exchanger?

Accepted Answer

Brazed plates form a rigid, joint-free assembly suitable for high pressures and temperatures, but impossible to dismantle for cleaning. Gasketed plate-and-frame exchangers can be dismantled, cleaned and extended, but tolerate lower pressure and temperature.

Question 14

When is a shell-and-tube heat exchanger used?

Accepted Answer

The shell-and-tube heat exchanger is used when high pressures or temperatures are required, when the fluid is viscous or dirty, or when a robust, long-life unit is needed in demanding industrial conditions.

Question 15

What is the difference between parallel flow, counter-flow and cross-flow?

Accepted Answer

In parallel flow, both fluids move in the same direction; in counter-flow, in opposite directions; in cross-flow, perpendicular to each other. Counter-flow maximises heat transfer and is the thermally most efficient configuration.

Question 16

When is a cross-flow heat exchanger recommended for air heat recovery?

Accepted Answer

Cross-flow is the standard solution in ventilation and air conditioning systems where heat needs to be recovered between the extract and supply air streams, without mixing risk, in a compact unit that integrates directly into the air handling unit (AHU).

Question 17

What is the difference between a gas-gas, gas-liquid and liquid-liquid heat exchanger?

Accepted Answer

The designation indicates the pair of fluids exchanging heat. Each pair has very different heat transfer characteristics and requires specific constructive types.

Question 18

What parameters are needed to size a heat exchanger?

Accepted Answer

The minimum parameters needed for thermal sizing are: the mass flow rates of both fluids, the inlet and outlet temperatures of each fluid, the working pressures, the physical properties of the fluids (density, specific heat, viscosity, conductivity) and any dimensional or maximum pressure drop constraints.

Question 19

What heat transfer surface area do I need for my application?

Accepted Answer

The required heat transfer surface area A is calculated using Q = U · A · ΔTlm, where Q is the required thermal duty, U is the overall heat transfer coefficient and ΔTlm is the log mean temperature difference between the two fluids.

Question 20

Why are fins added to the tubes of a heat exchanger?

Accepted Answer

Fins extend the heat transfer surface on the gas side, compensating for its low convection coefficient compared to the liquid side. The objective is to balance the thermal resistances on both sides of the wall.

Question 21

How does fluid viscosity affect heat exchanger design?

Accepted Answer

The higher the viscosity, the lower the Reynolds number and the lower the convection coefficient. Very viscous fluids require exchangers with wide channels, adapted flow velocities and, often, shell heating to reduce viscosity at start-up.

Question 22

When is PED certification required for a heat exchanger?

Accepted Answer

The Pressure Equipment Directive 2014/68/EU applies to any heat exchanger with a maximum allowable pressure (PS) greater than 0.5 bar gauge.

Question 23

When is ATEX certification required for a heat exchanger?

Accepted Answer

The ATEX Directive 2014/34/EU applies when the heat exchanger is installed in a zone classified as having explosion risk due to the presence of explosive atmospheres (flammable gases, vapours, mists or dusts).

Question 24

What materials should be used for corrosive or acidic fluids?

Accepted Answer

Material selection for corrosive fluids depends on the specific acid or base, concentration, temperature and flow velocity. Common options range from stainless steel 316L through titanium, Hastelloy, Duplex to non-metallic materials (graphite, PTFE, polypropylene).

Question 25

How is the acid dew point temperature determined and why is it important in design?

Accepted Answer

The acid dew point is the temperature at which SO₃ or HCl present in combustion gases condense forming sulphuric or hydrochloric acid on metal surfaces. For combustion gases from sulphur-containing fuels, it typically lies between 120 and 150 °C.

Question 26

What heat exchanger is suitable for high-particle-content gases?

Accepted Answer

For particle-laden gases (fly ash, dust, soot, aerosols), heat exchangers with wide-pitch helical finned tubes, or multi-tube plain-tube designs, are the types offering the greatest resistance to blockage and the easiest cleaning.

Question 27

Is it possible to recover heat from the exhaust of a combustion engine or generator (Filtermist, CHP)?

Accepted Answer

Yes. The exhaust gases of a gas or diesel engine, the cooling water jacket and the lubrication oil all contain recoverable thermal energy. In combined heat and power (CHP) installations, recovery of this residual heat is the basis for calculating the overall system efficiency.

Question 28

What heat exchanger is used to cool engine and compressor oil?

Accepted Answer

Cooling of engine and compressor oil is typically carried out with compact shell-and-tube exchangers or with demountable gasketed plate-and-frame exchangers, using water or air as the secondary cooling fluid.

Question 29

What heat recovery solution is suitable for a pyrolysis plant?

Accepted Answer

Pyrolysis plants generate hot gases rich in hydrocarbons, corrosive compounds and char particles. Heat exchangers for these gases must be of the plain-tube or very wide-pitch fin type, fabricated in materials resistant to acid corrosion and coke deposit formation.

Question 30

How is a heat exchanger integrated into an SCR (Selective Catalytic Reduction) system?

Accepted Answer

In an SCR system, the heat exchanger is installed downstream of the catalytic reactor to recover thermal energy from the treated gases, or upstream to preheat the gases to the catalyst activation temperature (typically 280–420 °C for TiO₂-V₂O₅ catalysts).

Question 31

What heat exchanger is suitable for SCR (Selective Catalytic Reduction systems)?

Accepted Answer

A heat exchanger suitable for SCR systems must tolerate gases containing residual NH₃ or urea, temperatures between 200 and 550 °C, possible presence of SO₂ and catalytic particles, and must have wide-passage surfaces to prevent ammonium bisulphate deposit.

Question 32

What thermal solution is applied in a melamine plant?

Accepted Answer

In melamine plants, heat exchangers are primarily used to cool the melamine gas (CO₂ and NH₃ at 350–450 °C) at the reactor outlet, to recover the melamine condensation heat, and for the cooling circuits of the purification sections.

Question 33

How is heat recovery managed in paint booths (paint booth heat recovery)?

Accepted Answer

In paint booths, heat recovery from the extract air is technically feasible when the extracted air has been correctly filtered to remove paint particles. The recommended type is a cross-flow aluminium or stainless steel plate heat exchanger, with upstream F7–F9 class capture filters.

Question 34

What pre-filtration is required to protect a heat exchanger in a paint booth?

Accepted Answer

Protecting the heat exchanger in a paint booth requires at minimum a large-area dry filter of class F7 or higher (EN ISO 16890) installed immediately upstream of the heat exchanger.

Question 35

What heat exchanger is used for cooling electrical transformers?

Accepted Answer

Oil-cooled electrical transformers are typically cooled with oil-to-air finned radiators or with oil-to-water heat exchangers for high-power transformers (ODWF, OFWF).

Question 36

What heat exchanger is suitable for hygienically demanding applications (pharmaceutical, food industry)?

Accepted Answer

In pharmaceutical and food applications, heat exchangers must meet hygienic design criteria: smooth surfaces without dead corners, finishes Sa 2.5 or Ra ≤ 0.8 µm, materials certified for food contact (AISI 316L, 304), FDA-grade silicone or PTFE gaskets, and compatibility with CIP and SIP.

Question 37

How much fuel can be saved by installing an economiser on a boiler?

Accepted Answer

As a practical rule of thumb, for every 6 °C rise in feedwater temperature, boiler fuel consumption decreases by approximately 1%. An economiser that raises the temperature by 60 °C can represent savings of 8–10% of annual fuel cost.

Question 38

What is the typical return on investment for an industrial heat recovery heat exchanger?

Accepted Answer

In continuously operated installations (>4,000 h/year) with recovered thermal duties of hundreds of kW to MW, the return on investment typically falls between 1 and 3 years, depending on energy cost, recovered duty and equipment cost.

Question 39

How do heat exchangers contribute to CO₂ emission reduction?

Accepted Answer

Lower fuel consumption translates directly into fewer CO₂ emissions per unit of useful energy produced. In installations subject to emissions trading (EU ETS), every emission reduction has an additional direct economic value.

Question 40

What is the difference between thermal efficiency and heat exchanger effectiveness (NTU-ε method)?

Accepted Answer

Thermal efficiency compares the heat actually transferred to an external reference value. The NTU-ε effectiveness compares the heat actually transferred to the maximum theoretically transferable between the two fluids in that specific exchanger.

Question 41

Under what conditions is low-temperature heat recovery economically viable?

Accepted Answer

Low-temperature heat recovery (40–100 °C) is viable when the available temperature differential is sufficient (>10–15 °C), the flow rate is high, the number of operating hours is large, and there is a productive use for the recovered heat within the same plant or in a nearby circuit.

Question 42

How is excessive fouling detected in an operating heat exchanger?

Accepted Answer

Operating indicators of excessive fouling include: reduction in cold-fluid outlet temperature, rise in hot-fluid outlet temperature, increase in pressure drop on the affected side, and measurable reduction in transferred thermal duty compared to design values.

Question 43

What cleaning methods are available for industrial heat exchangers?

Accepted Answer

The main methods include chemical circulation cleaning (CIP), mechanical cleaning using brushes or scrapers (for tubes), high-pressure water washing, compressed air soot blowing (for the gas side), and manual cleaning for dismountable units.

Question 44

What symptoms indicate an internal leak in a heat exchanger (cross-contamination)?

Accepted Answer

An internal leak manifests as the presence of tracers from fluid A in the circuit of fluid B: colour change, odour, chemical composition change, pH shift or presence of specific tracer compounds. Detection can be confirmed by hydrostatic pressure test or helium leak test.

Question 45

When should gaskets be replaced in a gasketed plate-and-frame heat exchanger?

Accepted Answer

Gaskets should be replaced when they show visible signs of deterioration (cracks, excessive hardness, permanent set), when a hydrostatic test reveals leaks, or preventively when the maximum service life recommended by the manufacturer is reached.

Question 46

How is a hydrostatic pressure test carried out on a heat exchanger?

Accepted Answer

The hydrostatic test involves filling the circuit under test with water, fully purging all air, gradually applying the test pressure, holding it for a specified period and verifying the absence of leaks and permanent deformation. The applicable test pressure depends on the applicable standard and equipment type; for PED equipment it typically falls between 1.25 and 1.43 times the maximum allowable pressure (PS).

Question 47

What vibrations can a gas flow induce in a heat exchanger, and how are they prevented?

Accepted Answer

Gas flow through a tube bundle can induce mechanical vibrations through Kármán vortex shedding, especially when the vortex shedding frequency coincides with the natural vibration frequency of the tubes. Consequences include mechanical fatigue, fretting wear and, in severe cases, tube failure.

Question 48

What is the typical service life of an industrial heat exchanger?

Accepted Answer

The service life of an industrial heat exchanger depends critically on the materials selected, the actual operating conditions and the maintenance programme applied. There is no universal range: each unit ages according to its specific process environment.

Question 49

How do start-stop cycles affect heat exchanger integrity?

Accepted Answer

Start-stop thermal cycles generate fatigue stresses from differential thermal expansion and contraction between heat exchanger components. In applications with many daily cycles or large temperature gradients during start-up, thermal fatigue sizing is as important as thermal duty sizing.

Question 50

How can I obtain a custom heat exchanger for my application?

Accepted Answer

To obtain a custom heat exchanger, you need to provide the supplier technical office with the process data for each fluid (flow rate, inlet and outlet temperatures, pressure, composition and physical properties), dimensional and pressure drop constraints, and applicable regulatory requirements.