The management of thermal and mechanical issues for control panels, local panels, and machinery panels is becoming more important due to increasing heat fluxes, continued miniaturization, and higher speeds and variable-speed capabilities of machineries:
- Thermal considerations are concerned with critical temperatures of sensitive electrical/electronic components.
- Mechanical considerations are focused on the housing and enclosures of the electrical, electronics, and control systems, and the ability of this housing/enclosure to maintain the integrity and reliability of all items and systems under various loading conditions such as dynamic loads, wind, shock, vibration, possible random loads and others.
- Thermomechanical management is concerned with the impact of thermal loads on the mechanical behavior of the system. For instance, as the result of generated heat and associated high temperatures, there could be thermal expansions and overstressing whereas these high stresses or their cyclic effects can damage parts and components.
Thermal, mechanical, and electrical issues can no longer be treated separately, as thermal and mechanical configurations are very much application driven. Higher power ratings and higher heat dissipation levels require that thermal behaviors are assessed with electrical and control behaviors. This article discusses the thermal and mechanical considerations for local panels, control panels, and machinery panels.
Thermal considerations
The electrical, electronics, and control items have been vulnerable to high temperatures and thermal failures. There are many possibilities of resonances and damages for the electrical, electronics, and control items inside the local control panel of modern variable-speed machineries and other packages/equipment.
At the same time, they produce some heat, which if not managed properly, can cause high temperatures and operational problems. To the extent possible, the thermal path resistance should be minimized from the heat-generating elements to the heat dissipating surfaces in order to increase the heat transfer. The heat transfer of an item is closely related to its configuration and orientation. For example, radiation heat transfer is directly related to its surface orientation. A good recommendation is to orient heat radiating surfaces such that they face an unencumbered view of cooler surfaces.
Contact interface resistance. In many panels, components and parts should be attached together by bolting, fasteners, or similar means to develop the required configuration. Therefore, there are contact interfaces between parts and pieces. For example, power supply units should be bolted to the baseplate. This requires two separate surfaces be joined at an interface that is interrupting to some extent the heat flow across this interface. Because of surface irregularities, the actual contact area is much smaller than the apparent contact area. Due to a smaller actual contact area and also gaps between surfaces, there is a thermal resistance in the contact interface. In other words, the interface leads to less heat transfer and higher temperature rises.
As more pressure is applied to the interface, surface asperities deform and provide a larger contact area, and the gaps shrink. All these factors lead to a reduction of interface thermal resistance. Interface materials such as special polymers might be used to increase heat transfer in contact interfaces. This provides a better heat conduction path (through gaps) that help lower the thermal resistance.
Interface thermal resistances between different parts and components can significantly affect heat transfer paths and overall thermal management of the panel. A lot of engineering and research effort is currently being applied to the development of interface materials (such as interface polymers) that can be used to minimize interface thermal resistance.
Convection heat transfer. Convection heat transfer is extremely important for the thermal performance of control panels and local panels. In convection heat transfer, the configuration and orientation of parts and subsystems are critical. Properly oriented boards, parts, and pieces allow for effective internal convection heat transfer to circulate the medium/air and lower the overall temperatures.
Free convection is an effective cooling method for systems with relatively low component densities, which is the case in many panels. As a rule of thumb, the heat flux estimate for free convection is 1500 W/m2 for each electronic/control board; for instance, about 45 W for a 300mm × 100mm board. In general, control panels and local panels should be sufficiently large, and boards and parts should be located with sufficient distances and spacing from each other.
For panels, when possible, the vertical dimension should be longer (say at least twice as long) as the horizontal dimensions to offer better overall heat transfer and thermal management. An efficient heat transfer path should be ensured for each major heat source. The highest power-dissipating pieces and boards should be placed near the walls of the enclosure to shorten the heat transfer paths.
Fins. Fins greatly improve the efficiency of convection cooling. However, there are a variety of fin shapes and configurations and a proper one should be selected for each application. The proper selection of fins is particularly important for the internal hot spots and external surfaces of the enclosures. The orientation of the fins, usually parallel to gravity, is crucial to developing proper flow patterns.
Forced-convection cooling. A variety of forced-convection cooling techniques are available, and two commonly used methods are direct flow and cold plates. In direct flow, air (or another coolant such as an inert gas) is forced directly over parts, boards, and components to cool them. Great care should be taken for pressure drop/losses; flow paths; component spacing and distribution. The fan and the overall panel (system) would interact with each other. Fan sizing and selection would be critical as many aspects of operation and forced-convection cooling should be considered. Matching the system impedance to the fan performance curve is only one aspect of the fan selection.
The configuration of parts and pieces surrounding the fan are important. For instance, the boards and items near the fan should not hinder the air flow at the suction or discharge of the fan. The layout of parts and components should allow proper flow to the fan and from it. The generated flow by the fan should be properly distributed in the panel and cool down the entire panel. To choose an optimum fan, the operating point (head-flowrate of the overall panel system) should be within the fan’s optimum operating range, close to the fan’s best efficiency point.
The flow prior to reaching the fan and immediately leaving it would impact the fan performance curve and the overall cooling operation. Care should be taken to check the flow, spacing, and potential restrictions around the fan to assess how the flow develops and behaves at the upstream and downstream of the fan.
Cold plates cool components indirectly by either air or other fluids. In a cold plate system, heat is transferred to coolant through a heat sink. Such a system requires careful flow assessments to prevent either choking or lack of pressure as well as measures to prevent coolant leakage.
Local effects. Local effects are extremely important as localized high temperatures in a part, piece, or board could be detrimental. Localized temperature rises should not exceed the specified maximum limit. In fact, many reported failures and problems were related to hot spots and localized high temperatures. A proper solution, whether fins, an internal low-velocity fan or others should be incorporated so that localized hot spots do not develop. Ideally, high-powered heat sources should be mounted with proper provisions (such as copper straps) and proper thermal pads. In many panels, all high-powered heat sources are attached directly to the internal surface of the external heat sink.
Mechanical considerations
Local panels have usually been fabricated from thin-plates or shells (typically folded thin plates). The plate is usually thin, as a rule of thumb somewhere around 1.6mm to 8mm. They should resist different operational and environmental loadings such as weight of content, dynamic and cyclic loads, vibration loads, wind loads, shock, impact, accidental loads, and random forces. Operational loads and forces produce different effects such as bending moment and others.
The choice of the layout and arrangement between possible alternatives and the choice of details, dimensions, stiffeners, etc., depend mainly on operation and functionality. The overall configuration is also dependent on finding the most economical layout of the panel considering different factors such as previous successful experiences, manufacturing requirements, and client/operator preferences.
The mechanical behavior of the panel as a body fabricated from folded thin plates should be considered in three levels: the plate element level, the subassembly level, and the entire panel level. The mechanical behavior of plates and plated bodies normally depends on a variety of influential factors, including geometric and material properties, loading characteristics, boundary conditions, initial imperfections, and local irregularities such as openings, holes, and perforations. These holes and perforations have been important for control panels and local panels, as many have large openings for instruments, doors, indicators, and MHIs (machine human interfaces). Sharp edges should be avoided.
Dynamic considerations are extremely important, as sensitive controls, electronics, and boards can easily be damaged by dynamic forces. Understanding accurate natural frequencies and conducting modal analysis have been critical, especially to avoid resonance issues. Rigidity and generally high stiffness are important as low stiffness might reduce the natural frequencies and increase the risk of resonances. Although proper analysis particularly modal studies are needed to avoid any resonance and dynamic problems. Strong and heavy-duty panels are always recommended as far as permitted by weight limitations and cost considerations. Simultaneously resonance prevention and dynamic considerations are important.
Conclusion
One of major problems associated with practical reliability and overall mechanical/thermal effects is the lack of accurate and complete material data including thermal, mechanical, and dynamic data. Improvements in material characterization techniques and acceptance criteria, especially in areas such as interfacial adhesion, are critical. For instance, properties of many polymers have been shown to be strongly temperature dependent.
Modern technologies pose significant challenges to engineers responsible for management of thermal and mechanical issues in the local panels. A good way forward is to adopt a totally integrated assessment approach where thermal, mechanical, and dynamic issues are considered all together. Correct evaluations and assessments of the thermo-mechanical reliability of micro-components require special knowledge on characteristics of electronic/control items (such as boards) as well as appropriate failure criteria to achieve reliable failure predictions and then failure prevention.
This story originally appeared in the September 2022 issue of Plant Services. Subscribe to Plant Services here.
About the Author: Amin Almasi
Amin Almasi is a machinery/mechanical consultant in Australia. He is chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE) in addition to a M.Sc. and B.Sc. in mechanical engineering and RPEQ (Registered Professional Engineer in Queensland). He specializes in mechanical equipment and machineries including centrifugal, screw and reciprocating compressors, gas turbines, steam turbines, engines, pumps, condition monitoring, reliability, as well as fire protection, power generation, water treatment, material handling and others. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 200 papers and articles dealing with rotating equipment, condition monitoring, maintenance, condition monitoring, fire protection, power generation, water treatment, material handling and reliability. Contact him at [email protected].