Basics of Heat Transfer

Heat, a measure of thermal energy, can be transferred from one point to another. Without the use of external agency of work/energy, heat flows from the point of higher temperature to one of lower temperature. Heat transfer takes place in three main ways: conduction, convection and radiation. The process of conduction in thermal dynamics is dependent on the factors such as heat transfer coefficient of the material, area, thickness through which the heat is transferred and the change in temperature. In the simplest of terms, the discipline of heat transfer is concerned with only two things: temperature, and the flow of heat. Temperature represents the amount of thermal energy available, whereas heat flow represents the movement of thermal energy from place to place. On a microscopic scale, thermal energy is related to the kinetic energy of molecules. The greater a material’s temperature, the greater is the thermal agitation of its constituent molecules (manifested both in linear motion and vibrational modes). It is natural for regions containing greater molecular kinetic energy to pass this energy to regions with less kinetic energy. Several material properties serve to modulate the heat transferred between two regions at differing temperatures. Examples include thermal conductivities, specific heats, material densities, fluid velocities, fluid viscosities, surface emissivities, and more.

The heat content, Q, of an object depends upon its specific heat, c, and its mass, m. The heat transfer is the measurement of the thermal energy transferred when an object having a defined specific heat and mass undergoes a defined temperature change.

Heat transfer = (mass)×(specific heat)×(temperature change)
Q=mcΔT, where Q = heat content in Joules, m = mass, c = specific heat, J/g C, T = temperature, and ΔT = change in temperature.

Heat Transfer Devices

 Heat sink: a heat sink is a component that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems or the radiator in a car.

 Heat pipe: a heat pipe is another heat-transfer device that combines thermal conductivity and phase transition to efficiently transfer heat between two solid interfaces.

Heat exchanger: A heat exchanger is a device used to transfer heat between a solid object and a fluid, or between two or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. Heat exchangers are widely used in refrigeration, air conditioning, space heating, power generation, and chemical processing. One common example of a heat exchanger is a car’s radiator, in which the hot coolant fluid is cooled by the flow of air over the radiator’s surface. Common types of heat exchanger flows include parallel flow, counter flow, and cross flow. In parallel flow, both fluids move in the same direction while transferring heat; in counter flow, the fluids move in opposite directions; and in cross flow, the fluids move at right angles to each other. Common constructions for heat exchanger include shell and tube, double pipe, extruded finned pipe, spiral fin pipe, u-tube, and stacked plate. The basic component of a heat exchanger can be viewed as a tube with one fluid running through it and another fluid flowing by on the outside. There are thus three heat transfer operations that need to be described:
• Convective heat transfer from fluid to the inner wall of the tube,
• Conductive heat transfer through the tube wall, and
• Convective heat transfer from the outer tube wall to the outside fluid.

Heat exchangers are typically classified according to flow arrangement and type of construction. The simplest heat exchanger is one for which the hot and cold fluids move in the same or opposite directions in a concentric tube (or double-pipe) construction. Alternatively, the fluids may be in cross flow (perpendicular to each other). The two configurations differ according to whether the fluid moving over the tubes is unmixed or mixed. Some of heat exchanger types in use are summarized as:
• Shell and tube heat exchanger
• Plate heat exchangers
• Plate and shell heat exchanger
• Adiabatic wheel heat exchanger
• Plate fin heat exchanger
• Pillow plate heat exchanger
• Fluid heat exchangers
• Waste heat recovery units
• Dynamic scraped surface heat exchanger
• Phase-change heat exchangers
• Direct contact heat exchangers
• Micro-channel heat exchangers

Heat Exchanger Efficiency

The science of thermodynamics is universally concerned with the relationship between heat and temperature, and energy and work. Though thermodynamics is by no means simple enough to explain in brief terms, one of its main tenets essentially states that 100% efficiency is unattainable in the real world. As a result of this problem, the right manufacturing and selection of heat exchangers is fundamentally important to increase thermal efficiency to its maximum potential and save on energy costs. Choosing the right heat exchanger can be a daunting task, because there is no single and best solution. Heat exchanger efficiency is calculated by comparing between the real and perfect performance. Though perfect performance is calculable, it is unfeasible in the real world, because of the thermodynamic limitation that states nothing can be 100% efficient. The real performance of heat exchangers, however, can be optimized to achieve maximum efficiency by optimizing copious amounts of data for every application. For every heat exchanger and its operation conditions, there exists a solution that minimizes the amount of entropy, while maximizing the amount of heat transfer. Part of the optimization process requires
• the selection of a thermally conductive material with the desired properties
• a corrosion-resistant material must retain elasticity when dealing with high temperatures and pressure, and continuous contractions and expansions
• size, weight, and cost limitations must then also be accounted for, which requires adjustments to complex flow patterns

Overall heat exchanger efficiency is a combination of following efficiencies.

 Thermodynamic Efficiency: is entirely dependent on terminal temperatures and nothing else: = (t2 – t1) / (T1 – t2), where T1 is the temperature of the hot fluid at inlet, t1 is the temperature of the cold fluid at inlet, and t2 is the temperature of the cold fluid at outlet. We can calculate the effectiveness of a heat exchanger depends on of what type it is like counter current or co-current and can define which type of heat exchanger is more efficient on basis of design. For more heat recovery counter current HE is preferred over co current. However in some cases parallel design also preferred (isothermal heat transfer, fast heat transfer). For calculating effectiveness we can use-

counter current = (1-exp(-N(1-C))/(1-C(exp(-N(1-C))
co-current = (1-exp(-N(1+C))/(1+C)
where C= Cmin/Cmax
as if McCc < MhCh then C=McCc/MhCh
(Mc mass flow rate of cold fluid, Cc heat capacity of cold fluid and so Mh and Ch for hot fluid)
N=UA/Cmin
U= overall heat transfer coefficient
A= heat transfer area

Temperature Transfer Efficiency: Temperature transfer efficiency of a heat recovery unit can be expressed as:
Mt = (t2 – t1) / (t3 – t1), where
μt = temperature transfer efficiency
t1 = temperature in outside make-up air before the heat exchanger (C)
t2 = temperature in outside make-up air after the heat exchanger (C)
t3 = temperature in outlet air before the heat exchanger (C)

 Moisture Transfer Efficiency: Moisture transfer efficiency of an heat recovery unit can be expressed as:
μm = (x2- x1) / (x3- x1), where
μm= moisture transfer efficiency
x1= moisture outside make-up air before the heat exchanger (kg/kg)
x2= moisture outside make-up air after the heat exchanger (kg/kg)
x3= moisture outlet air before the heat exchanger (kg/kg)

 Enthalpy Transfer efficiency: Enthalpy transfer efficiency of an heat recovery unit can be expressed as:
Me = (h2 – h1) / (h3 – h1), where
μe = enthalpy transfer efficiency
h1 = enthalpy outside make-up air before the heat exchanger (kJ/kg)
h2 = enthalpy outside make-up air after the heat exchanger (kJ/kg)
h3 = enthalpy outlet air before the heat exchanger (kJ/kg)

Efficiency Improvement

There is a constant battle in any industry to be able to increase performance and production, while keeping costs at a minimum. A big part of being able to keep costs low is the ability to operate at a high level of efficiency. And, when it comes to efficiency, we want to look at all the equipment we use in order to find areas where we can get better. For example, there are ways to improve heat exchanger performance and get more for our money from these valuable pieces of equipment. The performance of a heat exchanger is evaluated on the basis of its efficiency. The more efficient a heat exchanger is, the more value it offers consumers. The efficiency of heat exchangers may not worry much if the only heat exchangers are exposed to be the ones in home air conditioner and refrigerator. However, in case of heat exchangers use in automobiles or in factories, then the efficiency of heat exchangers can have serious financial implications. A significant amount of savings can be made simply by increasing the efficiency of a heat exchanger by a minimal percentage. In the current global economic atmosphere, cutting costs even by the smallest percentage can make a massive difference at the end of the month or the year.

The effects of operating variables: There are a few operating variables that are worth noting when it comes to improving heat exchanger’s performance. One such variable is operating pressure, which refers to the pressure differential between the suction and discharge of each stream within heat exchanger. If there are deposits present, the pressure differential can be affected, which results in inadequate flow and a less efficient heat exchanger. Operating temperature is another variable to take into consideration. It’s important to monitor both the inlet and outlet temperature, in order to prevent the fluids from condensing and coating the internal components of heat exchanger, which, again, can result in decreased efficiency. The effects of nature and the properties of the heat exchanger are also important. For example, a heat exchanger that is designed to handle cooling water shouldn’t be used for a hydrocarbon application, as the exchanger would be unlikely to stand up to the job, based on the materials of construction within the unit. Monitoring surface thickness, and steady sampling and analyzing for metals is important for process personnel to do in order to get the most efficiency from a heat exchanger. If the operating variables are monitored and controlled closely, it will help keep your heat exchanger not only running for a longer period of time without breaking down, but also help your heat exchanger run at its maximum level of efficiency.

 The advantages of compact heat exchangers: Compact heat exchangers are becoming more popular for a number of applications, and for major industries and businesses all over the world. They can bring huge potential savings over their more traditional shell-and-tubes counterparts. Here are a few of the cost-savings benefits that more efficient, compact heat exchangers can bring. Energy savings is one area where compact heat exchangers can be beneficial. A compact heat exchanger, for example, can be up to five times more efficient than a shell-and-tubes heat exchanger. More energy is put back to use which would have been wasted in older, less thermally efficient heat exchangers. Compact heat exchangers also offer energy savings when it comes to fuel consumption. They use less fuel to power themselves, which also means they operate with lower emissions. Compact heat exchangers also operate with less maintenance. A major reason for this is that compact heat exchangers, specifically, spiral heat exchangers, run with a highly turbulent flow, allowing them to essentially self-clean, lowering cleaning costs. They are, therefore, excellent for heavy-fouling duties. Compact heat exchangers can also help to operate at a higher level of production, which is always a good thing. One way they can do this is by “debottlenecking” in relation to heating or cooling.

In addition, the reduction in physical space required by compact heat exchangers makes a significant difference. The physical space that would have otherwise been used by a heat exchanger can be used to increase production without having to reinvent the wheel and invest in expensive construction on the plant. Finally, compact heat exchangers can help with cost-savings simply because their initial costs are much lower than similar shell-and-tube models. For one thing, fewer materials are needed for heat exchangers with a smaller footprint, and, if more expensive materials are required for tougher jobs, those costs are subsequently lowered as well. In addition to the purchase price being lower, installation costs are also much lower in a compact heat exchanger. Finally, switching to a compact heat exchanger means that we can increase production while still using the same utility systems already have in place. Clearly, getting as much efficiency as possible out of heat exchanger is extremely important, and will save money and help increase production.

 Follow installation guidelines: Increasing the efficiency of the heat exchanger begins with getting the installation of the device absolutely on spot. If we have bought the heat exchanger from a reputable manufacturer, we are sure to have received a set of installation guidelines from the makers of the device. These guidelines should not be overlooked under any circumstances. Failure to follow the guidelines will not only compromise the efficiency of the heat exchanger, but may also hamper the basic operation of the device. More often than not, the most efficient way of installing the heat exchanger is by keeping the fluids flowing in a counter-current arrangement. When it comes to air cooled heat exchangers, make sure that no part of the core is blocked. The slightest of obstructions will compromise the cooling capacity.

  Changing the flow rate: Whenever technicians come in for routine inspection and repair work, we can ask them to check if the flow of the fluids in both the primary and the secondary side of the heat exchanger has the correct velocity. Increase flow rates enhance the capacity of the heat exchanger to transfer heat. It is advised not to excessively increase the velocity, as it adds on to the mass and makes it more difficult for the energy to be removed.

  Removing corrosion: If there is any corrosion on any region of the heat exchanger, then it should be removed immediately in order the device not to become inefficient. Sometimes the corrosion becomes so bad that we need to have the tube plates replaced. Make sure to carry out all the necessary removals and replacements for the sake of a smoother operating heat exchanger. A perfectly installed and well maintained heat exchanger is bound to remain efficient for extended periods of time.

  Effects of heat exchanger operating pressure: The pressure differential between the suction and discharge of each fluid stream is the main driving force of that stream. The pressure differential is affected by fluid flow rates, pipe surface friction, number of heat exchanger passes, bulk density and viscosity. Deposits, if present, reduce the available surface area and increase the pressure differential, thus resulting in inadequate flow. If a pressure difference is noticed, the system should undergo troubleshooting to identify the cause.

Effects of nature and properties of heat exchanger: Regarding the properties and nature of the heat exchanger, process personnel must pay particular attention to the chemical relationship between the heat exchanger materials of construction and the chemical nature of the fluid stream in transit. For example, process personnel would be ill advised to use a heat exchanger designed to handle cooling water for a hydrocarbon application, as the materials of construction would likely not stand up to the conditions of the application. To ensure a long service life, process personnel should have a firm understanding of material properties and their corresponding effects at varying conditions. Further, process personnel should take special care in the operation and maintenance of the heat exchanger.

Towards Efficient Heat Exchangers

There are a wide range of heat exchanger problems which may cause poor performance, or in some cases cause the exchanger to stop working all together. Much of the time heat exchanger performance issues can be fixed with a variety of simple solutions which will ensure that process plant can continue to maximize its performance. However, in some instances the most common heat exchanger problems can be a lot harder to resolve, leading to ever increasing operating costs and high capital costs to implement a suitable solution. Some of the most common heat exchanger problems for many process plants include the following:

  • Vibration issues
    • Exchanger leakage
    • Increasing exchanger energy consumption
    • Pass Partition bypassing (thermal leakage)
    • Air cooler air recirculation
    • Fouling

All of the above problems can contribute to an under-performing exchanger, the main issue is determining the critical problem (or combination) that needs to be resolved in order to improve performance. An audited approach to analyzing the given heat exchangers needs to be adopted, as one particular problem can be closely linked to another problem on the list; acting as a chain reaction. A lot of exchanger leakage can be found to come from flow distribution issues. If the flow through the exchanger is not uniform, then high flow velocities can cause an additional problem, vibration. This vibration can increase the effect of erosion in exchangers which then leads to frequent leakage of exchangers creating problematic maintenance and associated costs.  Closely linked to flow distribution problems is the high energy consumption in terms of utilities that can be created. With varying flow velocities occurring there will be tubes or areas on the shell side where poorer heat transfer is taking place compared to other areas inside the heat exchanger. Operationally fans and pumps are usually increased in power to overcome the poor heat transfer, but such technologies can greatly improve the heat transfer efficiency, and as a result decrease the demand of cooling or heating energy needed for a given duty. Traditional engineering practices such as site visits, taking measurements and examining the exchanger in operation also play a part in determining pass partition bypassing through thermal infrared readings, resolving an unidentified problem with a simple solution. Onsite audits can also determine air recirculation problems, taking into account the exchangers surrounding environment which also can have an impact, i.e. high winds, nearby exhaust gas etc. Finally the problem of fouling can come in many forms listed below but it is essential to determine the type of fouling and it’s mechanism to offer a solution. Different types of fouling can be described as the following;

  • Crystallization
    • Decomposition
    • Polymerization and or oxidation
    • Settlement of sludge, rust or dust particles
    • Biological deposits
    • Corrosion

Conclusion

The complexities of heat exchanger R&D and continuous new technologies make it an esoteric field and, as a result, it’s essential to contact an expert, like Fluid Dynamics, in the field to reap the benefits of choosing the right heat exchanger. Investing time, money, and effort into finding the right heat exchanger can save countless hours and plentiful future costs, so it will be worth the initial investment to make. The complexities of heat exchanger design and selection are innumerable and include leakage, cross contamination, corrosion, cleaning, maintenance, capital and running costs, durability, layout, baffle size and number, tube thickness, diameter, and length, stream flow rates, inlet and outlet temperatures, operating pressure, pressure consistency, replacement costs, load fatigue, stresses from heat and pressure on structural components, condensation, stream properties, size constraints, material type, and fouling. Although many manufacturers of heat exchangers believe they’ve fully optimized their products, technological advances will make the process never-ending. As a result, forethought regarding future expansions and upgrades must be given when installing heat exchangers. Finding the balance between the initial and running cost of a heat exchanger can feel overwhelming. Given the inherent mathematical and mechanical problems associated with choosing the right heat exchanger, it is exceedingly necessary to consult a professional.

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