Over the past decade, thermal analysis of machineries of industrial plants has started to receive more attention.
In fact, with the increasing requirements for compactness, energy efficiency, cost reduction, lightweight design, extreme temperatures and the need to fully exploit new topologies and materials, it is now necessary to analyse the thermal circuits to the same extent as the other aspects.
Considerable portion of the electrical and mechanical power in a typical industrial unit is eventually converted to the heat.
In many applications, the maximum temperatures reached during operation can impact reliability, sometimes forcing users to de-rate or modify thermal arrangement.
Design engineers of high-performance industrial machinery or equipment for use in any applications (particularly thermally-demanding applications) require accurate thermal models of heat transfer processes and temperature destructions.
With a thorough understanding of thermal arrangement, design engineers can take steps to optimise thermal structure such as to minimise the temperature differences between the machinery casing and the environment (or a heat sink) for more efficient and more reliable operation.
For many machineries and equipment, the accurate temperature distribution is critical for operational and safety reasons to control the peak temperature for hazardous locations, to predict the fatigue under the heating-up and cooling-down processes, and to calculate the number of consecutive starts.
Dimensional changes can occur due to temperature deformations at the machinery elements. Considering tight working fitness of various components in modern machineries and complex equipment, it is necessary to take into account the actual variation of the installation and the operation gaps considering the thermal arrangement.
Electric motors have extensively used in industrial plants. A typical electric motor having an assumed efficiency of around 94-98% that dissipates around 2-6% of totally energy as the heat. Main portion of this heat is absorbed by the electric machine cooling systems; others are rejected from the casing to the surrounding environment. For large electric motors, closed-loop cooling-water systems are quite common.
By means of proper thermal management (for example, using proper insulating materials or cooling systems), the working temperature of all machineries and equipment parts should be stabilized within the defined limits. As a very rough indication, for a properly-designed machinery (or equipment), around 1 - 4% of the total power is rejected as heat to the surrounding.
Thermal lumped-parameter method is a popular method used for the equipment thermal analysis.
Machinery or equipment can be described as a thermal model including many lumped elements and with the help of linear networks of heat resistances of each element, neighbouring elements generate and store heat.
Lumped parameters are calculated from the size-related information, thermal characteristics of materials and various heat transfer coefficients.
Detailed features are lumped into a model with averaged properties. The temperature at every node in the thermal model can be calculated by solving energy balances.
It is necessary to account properly for the radial heat transfer, heat generation distribution, heat radiation, contact conductance and surface convection modelling.
Convection heat transfer usually play important roles in an equipment thermal analysis. For machineries, the forced convection heat transfer due to the rotor rotation (and also the convection around the machine ends) should be properly modelled.
Nowadays modern thermal analysis softwares are developed for the machineries and various mechanical equipment. The users input geometric data for the design under consideration using the radial and cross-sectional graphical editors.
ENTROPY BALANCE STUDIES
Entropy balances (based on the “Second Law of Thermodynamic”) allow for a calculation of the entropy generation or, equivalently, the energy destruction in a piece of equipment. Evaluation of the entropy generation, the energy destruction and their distribution throughout machinery (or mechanical equipment) thermal system allows for an identification of mechanisms that contribute most to the overall irreversibility and destruction of the useful work.
This analysis can provide a diagnostic tool for identification of areas where potential thermal performance improvements can be done.
Generally high temperature differences between components and fluids (main stream fluid, cooling water system, lubrication oil stream, surrounding environment, etc) can result in the energy destruction.
For example, machinery arrangements that result in the high temperatures at bearing systems can cause high temperature difference between the lubrication oil and the surfaces in contact with the lubrication oil. Considerable amount of heat can be transferred to the lubrication oil that can cause the lubrication oil problems, heat waste, requirements for very expensive lubrication oil system, large lubrication oil pumps, expensive lubrication oil coolers, or special lubrication oil requirement. The same is true for the closed-loop cooling-water systems. Considerable amount of energy is also destroyed near the outer housing (or casing).
OUTDOOR MACHINERY INSTALLATION
An important decision for every piece of equipment (or machinery) can be “Outdoor” or “Indoor” installation. Indoor installation usually seems safer, more reliable and better, but not always. The first option should usually be “Outdoor” installation in an industrial plant.
This can make the access, installation, operation and maintenance easier. Enclosure design can be very expensive and complex for industrial units. Sometimes conflicting requirements for the enclosure design, noise control, ventilation system, civil design, support design, various maintenance access and different crane access can result in an extremely expensive enclosure for machinery or equipment.
The enclosure volume should be minimised to keep cost down, however this may compromise access and other requirements. Access requirements for installation and major overhaul could be problematic if not addressed properly. Various new risk issues may be introduced by an indoor concept.
The “Outdoor” installation is the best solution for many machineries or equipment. The noise protection based on an initial noise study (with proper margin) can be included in the machinery supplier’s scope (with machine localised noise protection).
All maintenance (routine maintenance, overhaul, etc) can be done by proper mobile cranes. In future, if a temporary shelter will be installed, it will improve package reliability.
Of course there are some exceptions to the above-mentioned “Outdoor” concept. In some cases, environmental conditions, operation concepts, local rules, applicable codes, or project specifications mandate an “Indoor” installation. For these indoor machines or equipment, proper enclosure, proper thermal management and suitable ventilation system should be provided.
ENCLOSURE VENTILATION SYSTEM
Improper ventilation in an industrial equipment enclosure can result in elevated operating temperatures that reduce equipment life, generally increase maintenance and repair costs and yield an unreliable system. The initial cost of a good ventilation system is generally far less than the cost of a hot operating environment.
Usually optimised solution should be found between the accuracy of the heat load calculations and the ventilation system design margins. For machineries including electrical machine, the ventilation fan should be designed considering hazardous area classification requirements. For example, the air flow should exceed 12-times the volume air change rates per hour to maintain a zone-2 hazardous area classification specified for some industrial plants that contain flammable or combustible materials.
The ventilation system should be kept as simple as possible. The ventilation design criteria are as follows:
1- Positively pressurised forced ventilation system through the enclosure should be designed to provide sufficient air volume for maintaining internal enclosure temperature rise below certain level.
2- It is recommended to utilise two (2) or more ventilation units (usually two, each supplying 70% of the total airflow through the enclosure). This option may result in around 30-60% greater purchase cost compared to one (1) system, but this design increases reliability and improves the air distribution.
3- The inlet and exhaust systems should be properly positioned at opposite ends of the enclosure with the intake usually at low levels and outlet at high level to provide good airflow characteristics through the enclosure. Cooling air should be taken at a relatively low point, but not so low to pick up dirt or dust.
4- Regarding large machinery trains (that contain electric machines) in zone 2 hazardous area enclosure, a very rough ventilation sizing method (only suitable for basic design stage or proposal stage) is to select two identical fan systems each sized around 14-20 times enclosure volume air change per hour (say 2×15 times the volume per hour).
The first case study is presented for a 3 MW electric motor driven natural gas compressor for fuel gas of an industrial plan.
Heat generated from the compressor skid, electric motor, and ancillary facility/piping systems are 22 kW, 55 kW and 13 kW, respectively. The total heat rejection is calculated around 90 kW (around 3% of the train rated power). In this case study, generated heat by the compressor casing, gear unit, electric motor and ancillary facility/piping constitutes roughly 13%, 12%, 61% and 14% of the total generated heat, respectively.
The second case study is presented for a totally-closed enclosure for a 3.4 MW electric motor driven pump. The heat rejections from the pump, electric motor and other ancillary facility/piping systems are calculated 15 kW, 45 kW and 13 kW, respectively.
The total generated heat is around 73 kW which is approximately 2.1% of the total pump train rated power.
CONCLUSION AND FINAL NOTE
In this article, a comprehensive set of guidelines, methods and practical notes on the thermal analysis of machineries and industrial equipment have been presented. In the lumped-parameter thermal model, the fluid and solid components of machinery (or mechanical equipment) are lumped into fair number of nodes (typically 100 nodes).
These nodes are then coupled with appropriate heat transfer analytical expressions. This method can be used for machinery or equipment thermal structure identification, thermal design, cooling system optimisation, heat generation estimation and enclosure ventilation system design.