Thermal Control System

To ensure product quality and health safety, temperature control throughout the cold chain is of utmost importance. While many factors affect quality, and introduce losses in fresh products, temperature is primary to the rate of microbial activity, which cause spoilage of most fresh food products. It is vital to monitor full-time temperature history, to properly control the process of short- and medium-distance distribution situations. Metabolic activities accelerate as storing temperatures rise, while short hiccups in the control of the cold chain, invites rapid deterioration in product quality.   The flow of thermal energy (heat) is a process that is either transient or steady. Thermal control provides for maintenance of a system, within the perimeters of certain extremes, while the system is active. Thermal control retains the specified temperature stability for delicate equipment, so that they function with the greatest efficiency. The technology employed in thermal management, is based on thermodynamics and heat transfer.   Thermal control systems are employed in a wide range of industries, to keep component systems within predefined temperature ranges. Spacecraft are susceptible to temperature fluctuations, just like temperature-controlled vehicles. There is however, an external environment to contend with. This environment varies with solar or planetary flux, and the internal heat generated by the craft itself.   Thermal control is central to optimum performance and mission success. Subjecting any component to temperature extremes, can have an impact on component integrity and performance. In fact, certain components have stringent temperature ranges, while in operation.

Key considerations in thermal control systems design

To design effective thermal control systems, the following must be taken into account:

Environmental interaction

This considers all environmental influences, on the external areas of equipment. The options are to protect from the equipment or improve interactions, to improve performance. The main objectives of environment interaction are to increase or reduce absorbed environmental fluxes and increasing heat losses, to the environment.

Heat collection

Here, heat dissipated from the equipment it is created in, is removed to avoid unwelcome increases in temperature.

Heat transport

Heat is taken from point of generation, to a radiating device.

Heat rejection

Heat collected and transported is rejected, an apt temperature to a heat sink that usually supports the space environment. The rejection temperature is relative, to the amount of heat involved, the temperature to be controlled and the temperature of the radiation-fed environment.

Heat provision

This maintains a desirable temperature level, where heat is required. Adequate heat storage must be taken into consideration here too.

Active vs. Passive thermal management in electronic systems

Thermal technologies give us a great variety of options, to keep electronics cool. The thermal control apparatus can be works in two basic ways:

• Protecting equipment from overheating, either through thermal insulation from external heat fluxes, or proper removal of heat from internal hotspots.
• Protecting equipment from temperature extremes – cold (by thermal insulation from outer sinks or heat absorption from external sources), and hot (from internal sources).

Electronic components emit heat by default, and this must be adequately managed. This matters more as power densities increase, and components such as microprocessors draw more power, to meet functional and computational requirements of high-powered applications. Combining high-powered parts and more complex circuit boards, results in systems that produce even more heat.  Heat is a main cause of electronic failure, and the need arises for effective thermal design. Components operating at excessive temperatures or high localised temperatures, risk functional defectiveness and diminished operational life, due to component failure and degradation. For expected performance and reliability, heat must be diverted from critical parts and system hot spots, to the ambient environment.  This cooling process employs conduction, convection, and radiation. Radiation is the least-applied portion in the overall thermal load. To effectively manage heat, how optimally have heat transfer mechanisms been integrated into each level of electronic packaging? These systems are component, board, and system levels.

Passive thermal management

This comprises of cooling technologies, depending on the thermodynamics of heat transfer mechanisms, being least expensive, and easiest to implement. The components of such system are:

• Multi-layer insulation – protects a temperature-controlled unit, from external heating or cooling sources.
• Coatings – alters thermo-optical properties of external surfaces.
• Thermal fillers – enhances thermal coupling at key interfaces, like the thermal path between an electronic unit and its radiator.
• Thermal washers – reduces thermal coupling at predefined interfaces.
• Thermal doublers – these cover the radiator surface, with the heat the equipment disseminates.
• Mirrors – either optical solar reflectors (OSR) or secondary surface mirrors (SSM) that enhance heat rejection capacity of external radiators, while reducing absorption of external solar fluxes.
• Radioisotope heater units (RHU) – produces heat for thermal control systems (TCS) purposes. Deployed in certain planetary and exploratory missions.

Important examples include heat spreaders, heat pipes, and thermal interface materials (TIMs). 

Active thermal management

This on the other hand, must introduce energy (often from an external device), to complement the heat transfer process. This increases the rate of fluid flow during convection, dramatically increasing the rate of heat removal. They however must operate with electricity, and can introduce audible noise to the system. The component of active thermal management systems include:

• Resistive electric heaters controlled by a thermostat, to sustain equipment temperature, above its lower limit.
• Fluid loops that transfer heat to radiators. The heat is generated within the equipment. Fluid loops can be single-phase controlled by pumps), and two-phase (comprised of heat pipes (HP), capillary pumped loops (CPL) and loop heat pipes (LHP)).
• Louvers – change heat direction capacity to space, due to temperature.
• Thermoelectric coolers

Active thermal control systems are more complex and expensive. They include forced air, forced liquid, and solid-state heat pumps.

Temperature requirements

Instruments and equipment are the major drivers, for thermal control systems design. The aim of the system is to keep all instruments performing, within their permitted temperature ranges. Electronic instruments have a rather stringent temperature range. This makes it crucial to ensure these temperature conditions are met, as and when due.  Acceptable temperature ranges include:

• Batteries, with a really narrow operating range, typically between -5 and 20 °C
• Propulsion components, with a normal range of 5-40 °C for safety reasons, with a wider acceptable range
• Cameras, -30 – 40 °C
• Solar arrays, with more expansive range of –150 -100 °C
• Infrared spectrometers, -40 – 60 °C

Thermal control systems are very delicate systems, with far-reaching complexity levels. They can be too complex and expensive for adaptation in regular life. Otherwise, they might have a potential in the refrigeration processes of the cold chain that makes perishables available around the world, using refrigerated containers (reefers), refrigerated trucks, vans, and other temperature-controlled equipment.  If you want to buy a used refrigerated van, a used freezer van, a new refrigerated van or a new freezer van call, Glacier Vehicles on 0208 668 7579.

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