Part X: Heat Transfer and Refrigeration & Air Conditioning

Modes of Heat Transfer

Heat transfer occurs through three fundamental modes. Conduction is the transfer of heat through a material medium (solid, liquid, or gas) without any bulk motion of the medium itself, occurring due to molecular vibration and free electron movement, governed by Fourier's law, which states that the rate of heat conduction is proportional to the temperature gradient and the cross-sectional area, and inversely related to the material's thermal resistance. Thermal conductivity (k) is a material property expressing how readily a material conducts heat; metals generally have high thermal conductivity, while insulating materials (like glass wool, foam) have very low thermal conductivity.

Convection is the transfer of heat between a solid surface and an adjacent moving fluid (liquid or gas), driven either by an external agency (forced convection, such as a fan or pump moving the fluid) or by buoyancy differences arising from temperature-driven density differences within the fluid itself (natural/free convection). Convective heat transfer is quantified using Newton's law of cooling, which states that the rate of heat transfer from a surface to a fluid is proportional to the temperature difference between them and the surface area, with the convective heat transfer coefficient (h) capturing the combined effects of fluid properties, flow conditions, and geometry.

Radiation is the transfer of heat via electromagnetic waves, requiring no material medium at all (radiation can travel through a vacuum), and increases sharply with absolute temperature according to the Stefan-Boltzmann law, which states that the total energy radiated by a perfect emitter (a black body) per unit area is proportional to the fourth power of its absolute temperature. Real surfaces radiate less effectively than an ideal black body, characterised by their emissivity (a value between 0 and 1).

Heat Exchangers

A heat exchanger is a device used to transfer heat between two fluids without allowing them to physically mix, widely used in power plants, refrigeration systems, chemical processing, and automotive cooling systems. In a parallel-flow heat exchanger, both fluids flow in the same direction; in a counter-flow heat exchanger, the fluids flow in opposite directions, generally achieving a higher overall temperature change and better thermal effectiveness for the same heat transfer area than parallel flow, since the temperature difference between the fluids remains more uniform along the exchanger's length.

A shell-and-tube heat exchanger consists of a bundle of tubes enclosed within a cylindrical shell, with one fluid flowing through the tubes and the other flowing around them within the shell, widely used in industrial and power plant applications. A plate heat exchanger uses a stack of thin, corrugated plates to create flow channels for the two fluids, offering a high surface-area-to-volume ratio and compact size, commonly used in HVAC and food processing applications.

Refrigeration — Basic Concepts

Refrigeration is the process of removing heat from a space or substance and maintaining it at a temperature below that of its surroundings. The most common method used in practical refrigeration and air conditioning is the vapour compression refrigeration cycle, which consists of four main components operating in a closed cycle: the compressor (compresses low-pressure refrigerant vapour into high-pressure vapour, raising its temperature), the condenser (rejects heat from the high-pressure vapour to the surroundings, condensing it into high-pressure liquid), the expansion device (such as a thermostatic expansion valve or capillary tube, which throttles the high-pressure liquid to a low pressure and correspondingly low temperature), and the evaporator (absorbs heat from the space or substance being cooled, evaporating the low-pressure liquid refrigerant back into vapour, which then returns to the compressor to repeat the cycle).

The coefficient of performance (COP) of a refrigeration system is defined as the ratio of the refrigerating effect (heat absorbed at the evaporator) to the work input required (typically the compressor's work), and is the key measure of a refrigeration system's efficiency; a higher COP indicates a more efficient system for a given cooling requirement.

Refrigerants are the working fluids used in refrigeration cycles. Older refrigerants such as CFCs (chlorofluorocarbons, e.g., R-12) have been phased out internationally due to their ozone-depleting potential; HCFCs (e.g., R-22) are being phased down due to both ozone depletion and global warming concerns; and modern alternatives include HFCs (e.g., R-134a), which have zero ozone-depleting potential but significant global warming potential, and increasingly, natural refrigerants (such as ammonia, CO2, and hydrocarbons) and low-GWP synthetic refrigerants, driven by international environmental agreements such as the Montreal Protocol and the Kigali Amendment.

The vapour absorption refrigeration system provides an alternative to vapour compression, using a heat-driven absorption/desorption process (commonly with an ammonia-water or lithium bromide-water pair) instead of a mechanical compressor, making it well suited to applications where waste heat or low-cost thermal energy is available, though generally with a lower COP than vapour compression systems.

Air Conditioning — Basic Psychrometrics

Psychrometry is the study of the properties of moist air (a mixture of dry air and water vapour) and is fundamental to air conditioning system design. Key psychrometric properties include dry-bulb temperature (the temperature measured by an ordinary thermometer), wet-bulb temperature (the temperature measured by a thermometer with a wetted wick, reflecting the cooling effect of evaporation), relative humidity (the ratio of the actual water vapour present in the air to the maximum the air could hold at that temperature, expressed as a percentage), and dew point temperature (the temperature at which moist air becomes saturated and begins to condense moisture upon further cooling).

A psychrometric chart graphically represents these interrelated properties, allowing designers to determine the state of moist air and to analyse air conditioning processes such as sensible heating/cooling (temperature change with no change in moisture content), dehumidification (moisture removal, typically accompanying cooling below the dew point), and humidification (moisture addition).

Air conditioning systems are broadly classified as comfort air conditioning (maintaining conditions for human comfort in residential, commercial, and office spaces) and industrial air conditioning (maintaining specific temperature/humidity conditions required for a manufacturing process, such as in textile mills, pharmaceutical plants, or precision manufacturing, regardless of human comfort considerations).

Practice Questions — Heat Transfer and RAC

1. Conduction heat transfer occurs due to molecular vibration and electron movement, governed by:
   (a) Fourier's law (b) Newton's law of cooling (c) Stefan-Boltzmann law (d) Boyle's law
2. Convection heat transfer driven by an external fan or pump is termed:
   (a) Forced convection (b) Natural (free) convection (c) Radiation (d) Conduction
3. Radiation heat transfer, unlike conduction and convection, can occur:
   (a) Through a vacuum, with no material medium required (b) Only through solid materials (c) Only through liquids, never gases or solids (d) Only at absolute zero temperature
4. The Stefan-Boltzmann law states that black body radiation is proportional to:
   (a) The fourth power of absolute temperature (b) The temperature directly, with no power relation (c) The square root of temperature (d) The inverse of temperature
5. In a counter-flow heat exchanger, the two fluids flow:
   (a) In opposite directions (b) In the same direction (c) Perpendicular to each other exclusively, with no other arrangement possible (d) Only intermittently, never continuously
6. In the vapour compression refrigeration cycle, the evaporator's function is to:
   (a) Absorb heat from the space being cooled, evaporating the refrigerant (b) Compress refrigerant vapour to high pressure (c) Reject heat to the surroundings by condensing refrigerant (d) Throttle high-pressure liquid to low pressure
7. The coefficient of performance (COP) of a refrigeration system is the ratio of:
   (a) Refrigerating effect to work input (b) Work input to refrigerating effect (c) Condenser heat rejection to evaporator heat absorption (d) Compressor speed to refrigerant flow rate
8. R-134a, a common modern refrigerant, has:
   (a) Zero ozone-depleting potential (b) High ozone-depleting potential, identical to CFCs (c) No global warming potential of any kind (d) No practical use in refrigeration systems
9. Relative humidity is defined as the ratio of:
   (a) Actual water vapour present to the maximum the air could hold at that temperature (b) Dry-bulb temperature to wet-bulb temperature (c) Dew point temperature to dry-bulb temperature (d) Air pressure to water vapour pressure only
10. The vapour absorption refrigeration system is particularly suited to applications where:
    (a) Waste heat or low-cost thermal energy is available (b) No heat source of any kind is available (c) Only mechanical compression is permitted, with no thermal input (d) Extremely high COP is the only consideration, regardless of energy source

Answer Key: 1.(a) Fourier's law governs conduction heat transfer. 2.(a) Forced convection is driven by an external fan/pump. 3.(a) Radiation can occur through a vacuum. 4.(a) Stefan-Boltzmann law: radiation proportional to T⁴. 5.(a) In counter-flow, fluids flow in opposite directions. 6.(a) The evaporator absorbs heat from the space being cooled. 7.(a) COP = refrigerating effect / work input. 8.(a) R-134a has zero ozone-depleting potential. 9.(a) Relative humidity = actual vapour / maximum vapour capacity at that temperature. 10.(a) Vapour absorption systems suit applications with available waste/low-cost heat.