/ Heating and cooling processes

Heat transfer

Heat dissipation

Coolers

Heat exchangers

A basic grouping of thermal systems may be established according to whether heat exergy, Q, work energy, W, or mass, mi, is transferred through the frontier of the system of interest ('' if going out, '+ if going in):

  • (Q) Heat dissipation systems: coolers. Heat flows out of the system naturally or accelerated.
  • (Q+) Heat generation systems: heaters, furnaces, boilers, heat pumps. Heat flows into the system.
  • (W) Power generation systems: heat engines, chemical engines, propulsion. Work flows out of the system.
  • (W+) Cold generation systems: refrigerators and air conditioners. Work flows into the system to produce cold (considered above if for heating).
  • (mi) Mixture separation and chemical synthesis by thermal processes: dryers, distillers and reactors. An specific substance must go out of the system.
  • (mi+) Mixing and dilution: preparation (compounding) or dispersion (waste disposal). An specific substance must go into of the system.

Thermal processes are those giving way to thermal effects. Natural thermal effects include, besides the tendency for temperature gradients to die out in an isolated system (strict thermal effects), the tendency for velocity gradients to die out in a suitable reference frame (for an isolated system), and the tendency for concentration gradients (really chemical potential gradients, in absence of external force fields) to die out for an isolated system. Artificial thermal effects aim at forcing the gradients not to die out (at expense of some external exergy consumption).

Heat transfer

Heat-transfer problems may arise in typical thermal applications, like heating and cooling; e.g., the defrosting problem in refrigeration and air conditioning, due to thermal insulation of the ice layer (frosty ice conductivity can be as low as that of wood), admits several solutions, all of them controlled by heat transfer. Any temperature-measure involves some heat transfer problem. Besides, many other heat-transfer problems come from non-thermal pursuits; for instance:

  • Cooling electronic equipment. Microprocessor computing power is limited by the difficulty to evacuate the energy dissipation (a Pentium 4 CPU at 2 GHz in 0.18 mm technology must dissipate 76 W in an environment at 40 ºC without surpassing 70 ºC). Most electronics failures are due to overheating by improper ventilation or fan malfunction. Bipolar junctions in silicon wafers fail to keep the energy gap between valence and conduction electrons above some 400 K, but at any working temperature there is some dopant diffusion and bond-material creep, causing some random failures, with an event-rate doubling every 10 ºC increase. Electrical powers up to a few watts can usually be dissipated by natural convection to ambient air, and up to few hundred watts by forcing air with fans (cheap, but noisy and wasteful), and liquid cooling is usually needed beyond 1 kW systems.
  • Cooling rubbing parts. In a mechanical transmission, the oil loses its lubricating capacity if overheated; in a hydraulic coupling or converter, the fluid leaks under the pressure created. In an electric motor, overheating causes deterioration of the insulation. In an overheated internal-combustion engine, the pistons may seize in the cylinders.
  • Lamp design. The size of an incandescent lamp is governed by heat transfer (the filament needs a bulb to keep away from oxygen, but the bulb-size is large in glass bulbs to avoid glass softening, and small in halogen lamps to be hot enough to maintain the halogen cycle inside, in spite of the little size for the electrical resistance R that must be fed at low voltage for the same power P=VI=V2/R).
  • Materials processing like casting, welding, hot shaping, crystal growth, etc. Materials machining is limited by the difficulty to evacuate the energy dissipation. And not only engineering materials: food processing and cooking, dish washing, cloth washing, drying and ironing, and many other house-hold tasks, are dominated by heat transfer.
  • Energy conversion devices, like solar collectors, combustors, nuclear reactors, etc.
  • Environmental sciences like meteorology, oceanography, pollutant dispersion, forest fires, urban planning, building, etc.

Heat dissipation

The case study here is a system with a given energy release (internal or at the frontier), in the presence of an infinite environment, and the goal is to get rid of that energy without allowing the temperature of the system to rise too much. Energy release in an isolated system, e.g. by electrical dissipation or chemical or nuclear reaction, will force temperature increments, phase changes and material dissociations, increasing the aggression (rising pressure and chemical attack) to the assumed isolation boundary. However, much sooner than that material decomposition, most systems will end to work as assumed (electronic run-outs, mechanical creeping, dilatation stresses, etc.). Some systems must operate at very high temperatures, so they must be at the same time cooled and heated, as for incandescent lamps (in other cool-lamps the problem is only heating up the electrodes).

The usual environment is an infinite gaseous atmosphere, although special problem where the system is embedded in the hydrosphere or the lithosphere are treated similarly. Even for outer-space applications, the philosophy is the same except that only heat dissipation by radiation to the background universe at 3 K is ultimately available.

The basic mechanism for heat dissipation from a system to the atmosphere is by natural convection, i.e. by heat transfer from the surface (wet area A) of a hot system (at T=T0+T) to a dilatable atmosphere (at T=T0, with thermal expansion ) in the presence of the gravity force (buoyancy is proportional to these three effects: T,  and g). The overall heat transfer is , where K is the global thermal transmittance, that depends on the material properties of system and atmosphere, flow properties, and temperatures, usually modelled in three separate terms (conduction, convection, and radiation), although real heat transfer processes are usually a combination of them

Cooling by natural convection is a key role of ventilation, usually combined to heating and refrigeration under the term HVAC-engineering (HVAC, or HVAC&R, stands for Heating, Ventilation, Air-Conditioning and Refrigeration). When fresh air is not available (as in space and underwater vehicles), there is a need of more complex environmental control for life support systems (ECLSS). Besides air quality, environmental control demands proper illumination, proper acoustics, and other ergonomic and aesthetic conditioning.

Ventilation, i.e. the removal of ambient air, has several objectives:

  • Sanitary renovation of breathable air in habitable spaces; i.e., removing foul air (contaminated with CO2, odours and particles) and supplying fresh air, at a rate of at least 2 litres of fresh-air per second per person (10 L/s for smokers; ASHRAE-1989 standard was 7 L/s). Airliner standard cabin-air supply is 9.4 L/s per person (20 cfm), half fresh and half recycled (filtered).
  • Convective cooling of living and powered systems; i.e., removing heat from any operating item (persons, animals, plants, electrical devices, chemical processes, and so on). The rate of ventilation required depends on dissipated power and maximum allowed temperature. Most electrical appliances come with ventilation grids, which must be left unobstructed for operation.
  • Convective entrainment of matter in air-consuming, vapour-generating and dust-generating processes, to keep operating the process; e.g. to keep a fire burning, to get rid of odours, etc.

When the basic natural-convection cooling must be enhanced, the approach may be:

  • To decrease unwanted energy inputs: shade from sun rays, avoid nearby hot objects.
  • To increase the thermal transmittance of the sink (the atmosphere):
  • by avoiding restrictions to air motion: leave ample room between the system and nearby walls, floor and ceilings.
  • by choosing a location where some natural air draught already exists.
  • by forcing an artificial air flow (this is an active system that requires a continuous expense of energy).
  • by forcing an artificial intermediate fluid flow to be later cooled (an active system).
  • by enhancing the radiative coupling with the surroundings, increasing the emissivity for infrared radiation and decreasing the absorptivity for visible radiation.
  • To increase the system area wetted by the atmosphere (A), either by adding fins to the system, or by placing the system on a larger heat conducting plate that acts as an intermediate sink.
  • To increase the thermal transmittance of the source (the system), i.e. to efficiently move the dissipated energy from the inside to the surface:
  • by increasing the thermal conductivity of the system, enhancing joint conductance or adding internal conductors.
  • by increasing the convection inside a system (e.g. with internal fans that, although contributing to the heat source, redistribute more evenly thermal energy).
  • by enhancing the radiative coupling in the insides (e.g. painting internal surfaces black).
  • by a special thermal bridge, as a heat-pipe, inside which a working fluid boils in one side and condenses on the other end.
  • To procure endothermic physicochemical reactions, as the evaporative cooling in animal sweating and industrial cooling towers.

Coolers

A cooler is an artificial device to enhance natural cooling, preventing excessive temperatures, and may be active (i.e. powered; e.g. a fan, a pumped heat exchanger) or passive (un-powered, as the fins added to extend a surface, cold plates and radiators, a heat-pipe, a thermo-siphon heat exchanger). Natural cooling is termed ventilation.

Sometimes the name cooler is improperly used as synonymous of refrigerator (or even freezer). A cooler just accelerates the natural process of thermal relaxation of a hot system, whereas a refrigerator has to overcome the natural process of thermal relaxation of a cold system, necessarily expending some external exergy.

A typical cooling system is the water ‘refrigeration’ system for the car engine, a pressurised water loop with a pump forcing water through the outside walls of the combustion cylinders and then through a forced-air ‘radiator’, to provide thermal conditioning to working materials.

Heat exchangers

Heat exchangers are off-the-shelf equipment targeted to the efficient transfer of heat from a hot fluid flow to a cold fluid flow, in most cases through an intermediate metallic wall and without moving parts. We here focus on the thermal analysis of heat exchangers, but proper design and use requires additional fluid-dynamic analysis (for each fluid flow), mechanical analysis (for closure and resistance), materials compatibility, and so on.

Heat losses or gains in a whole heat exchanger (except in open-flow types), can be neglected in comparison with the heat flow between both fluids; i.e. a heat exchanger can be assumed globally adiabatic.

Although heat flows from hot fluid to cold fluid by thermal conduction through the separating walls (except in direct-contact types), heat exchangers are basically heat convection equipment, since it is the convective transfer what governs its performance. Convection within a heat exchanger is always forced, and may be with or without phase change of one or both fluids. When one just relies in natural convection to the environment, like in the space-heating home radiator, or the domestic fridge back-radiator, they are termed 'radiators' (in spite of convection being dominant), and not heat exchangers.

The basic designs for heat exchangers are the shell-and-tube heat exchanger and the plate heat exchanger, although many other configurations have been developed.

Main types can be grouped according to flow layout in:

  • Shell-and-tube heat exchanger (STHE), where one flow goes along a bunch of tubes and the other within an outer shell, parallel to the tubes, or in cross-flow (Fig. 1a shows a typical example of STHE; details presented below).
  • Plate heat exchanger (PHE), where corrugated plates are held in contact and the two fluids flow separately along adjacent channels in the corrugation (Fig. 1b shows details of the interior of a PHE; more details are presented below).
  • Open-flow heat exchanger, where one of the flows is not confined within the equipment (or at least, like in Fig. 1c, not specifically piped). They originate from air-cooled tube-banks, and are mainly used for final heat release from a liquid to ambient air, as in the car radiator, but also used in vaporisers and condensers in air-conditioning and refrigeration applications, and in directly-fired home water heaters. When gases flow along both sides, the overall heat-transfer coefficient is very poor, and the best solution is to make use of heat-pipes as intermediate heat-transfer devices between the gas streams; otherwise, finned separating surfaces, or, better, direct contact through a solid recuperator, are used.
  • Contact heat exchanger, where the two fluids enter into direct contact (simultaneous heat and mass transfer takes place). Furthermore, the contact can be continuous, i.e. when the two fluids mix together and then separate by gravity forces, as in a cooling tower, or the contact can be alternatively with a third medium, usually solid, as in regenerative heat exchangers (RHE), like the rotating wheel shown in Fig. 1d (the hot gas heats the wheel whereas the cold gas retrieves that energy). When the heat-exchange process between the hot and the cold fluids is delayed significantly, the term 'thermal energy storage' is used instead of RGE. There is always some contamination by entrainment of one fluid by the other, although many times it is irrelevant (as in air-conditioning heat-recuperators), or even intended (as in cooling towers). Notice also that, if the mixed-up fluids do not separate, as in open feed-water heaters or in evaporative coolers, the device is not named heat exchanger but just heater or cooler.

Fig. 1. Types of heat exchanges: a) shell-and-tube, b) plates, c) open-flow, d) rotating-wheel.

Additionally, heat exchangers may be classified according to the type of fluid used (liquid-to-liquid, liquid-to-gas, gas-to-liquid, gas-to-gas), according to phase changes (vaporisers, condensers), according to relative flow direction (counter-flow, co-flow, cross-flow), according to area density (transfer area per unit volume) or channel size, etc. In terms of the smallest hydraulic diameter of the two flows, Dh, or the area density,  (typical correlation is =3/Dh), heat exchangers may be grouped as:

  • Conventional or non-compact heat exchangers, if Dh5 mm, or <400 m2/m3.
  • Compact heat exchangers, if 1<Dh/mm<5, or 400 m2/m3<3000 m2/m3. Many times, the terms compact-heat exchanger (CHE) and plate-heat-exchanger (PHE) are used indistinguishably.
  • Meso heat exchangers if 0.2<Dh/mm<1, or 3000 m2/m3<10 000 m2/m3.
  • Micro heat exchangers if Dh0.2 mm (or >10 000 m2/m3). Human lung alveoli are typically 0.2 mm in size and have some 15 000 m2/m3.

Back to index

Heating and cooling processes1