It is possible that the fluid being heated may leave the exchanger at a higher temperature than the exit temperature of the heating fluid. This is the most efficient design because of higher temperature differential over length of the exchanger. There can be more than one method of heat transfer in a heat exchanger.
Heat transfer will occur using one or more modes of transfer, conduction, convection, or radiation.
Proper implementation of heat exchangers in multi-process systems, like oil refineries, requires a consideration of the network of heat flows on a systems level. Sophisticated software is available to aid the designer in this process. Fouling mitigation is also a consideration of design and can include the consideration of various technologies, velocities, bypasses for cleaning individual HX during the operation, and the incorporation of spare heat exchangers.
Similarly, software is available for managing heat exchanger fouling. Based on process conditions and component selection, some software packages can predict the rate at which heat exchangers are likely to experience fouling. Software packages are also available to monitor fouling by examining heat exchanger performance over time. Estimates of the costs of cleaning heat exchangers versus the economic benefits in terms of reduced energy use are also calculated.
Cooling ponds may be used to allow warm water to naturally cool through evaporative loss to the atmosphere. The water in the pond can then be recirculated into the plant as cooling water. These ponds may offer secondary recreational purposes such as fishing, boating, or swimming. Make up water is required to account for evaporative losses. A large amount of land is required for this option. Direct venting of steam may reduce the need for process water cooling but this option ignores the primary reasons for cooling which is to improve system efficiency and conserve process quality water and results in additional make up water and water treatment chemicals.
This option is generally not used except in start-up, emergency venting and shut down operations.
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Heat Exchangers require regular maintenance to operate at high efficiency and usually require a rigorous overhaul schedule. Much of this effort is aimed at countering the effects of fouling, wherein solids like foreign particles or precipitates accumulate on heat exchanger surfaces, inhibiting heat transfer and restricting fluid flow. Chemical additives can also prevent the precipitation of particles and may be a cost effective means of fouling prevention.
Overhauls can range from simple preventative maintenance activities i. This down time should also be taken into consideration when sizing the heat exchangers and designing the process network.
Department of Energy Handbooks / Manuals
Many heat exchangers operate at high pressures and temperatures or with hazardous fluids and adequate operating procedures must be followed to avoid personnel risks and system outages. New equipment designs and any repairs should comply with applicable codes. Many heat exchanger designs are available in numerous materials and can be customized for specific applications as well as standard designs that are available with minimal lead time at lower costs.
Several benefits of using heat exchangers are listed below:. Air to air heat exchanger for waste heat recovery This study reviews how a food processing facility used a heat exchanger to recover waste heat from a process and used it for heating process make-up air. Seeking to control odor from its roasting operation, the facility installed an efficient new Regenerative Thermal Oxidizer RTO. To further reduce operating costs, the company sought to recover waste heat from the RTO to preheat in-coming air.
To do this, they contracted a design consultancy to analyze and design the HX solution. The critical design factors for this project were the air flow rate, temperature of the airstream, permissible pressure drop of the application and the desired heat to be transferred to the heat exchanger. A secondary plate type heat exchanger was chosen because of its versatility and rugged, yet cleanable plates.
It has a relatively low pressure drop, a small foot print and low capital cost, making it the most economical option for this application. The consultancy analyzed the application data using heat exchanger performance modeling software. With this software they performed a boundary layer analysis and adjusted the plate thickness and spacing of the heat exchanger to maximize performance.
The RTO exhaust heat was used to preheat 3. This hot air mixes without side air to provide The secondary heat exchanger transfers approximately 1. Build-up of dirt deposits, or fouling, on the metal surfaces of petrochemical plant heat exchangers is a major economic and environmental problem worldwide.
Estimates have been made of fouling costs due primarily to wasted energy through excess fuel burn that are as high as 0. Many millions of tons of carbon emissions are the result of this inefficiency. This case study examines the use of fouling prediction software by French oil company Total.
This software, developed by an industrial design consultancy in conjunction with major oil companies, aims to reduce or even eliminate crude oil fouling in pre-heat train heat exchangers. In , Total experienced heavy fouling in its preheat train, soon after revamping the refinery to improve efficiency.
This led to a significant throughput reduction as the furnace bottlenecked. These were implemented, solving the problem and restoring normal system operation.
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Home Resources Heat Exchangers. Common flow-deflecting features include: Segmental baffles staggered perpendicular barriers, each blocking a fraction of the shell side; see Figure 1 , Disk and donut baffles — staggered circular and annular barriers force the shell-side flow alternately away and towards the axis of the shell Helical baffles - angled to promote a spiralling flow around the shell side Rod baffles - grids of rods, usually perpendicular to the shell axis.
Two other categories of heat exchanger exist, however: Open Flow - one fluid is contained and the other fluid is not. A cooling tower is used to cool water as it is sprayed into a cooling air stream. The air and water do not mix but heat is exchanged by the evaporative process. The cooled water is then collected and returned to the plant8. Other heat exchangers of this type include rotating wheel regenerative and spray columns.
Note that if the two fluids do not separate, then the device is referred to as heater or a cooler.
A typical conduction problem in its analogous electrical form is given in the following example, where the "electrical" Fourier equation may be written as follows. A composite protective wall is formed of a 1 in. Calculate the thermal resistance of each layer of the wall and the heat transfer rate per unit area heat flux through the composite structure. Heat transfer across a rectangular solid is the most direct application of Fourier's law.
Heat transfer across a pipe or heat exchanger tube wall is more complicated to evaluate. Across a cylindrical wall, the heat transfer surface area is continually increasing or decreasing. Figure 3 is a cross-sectional view of a pipe constructed of a homogeneous material. The surface area A for transferring heat through the pipe neglecting the pipe ends is directly proportional to the radius r of the pipe and the length L of the pipe. As the radius increases from the inner wall to the outer wall, the heat transfer area increases. The development of an equation evaluating heat transfer through an object with cylindrical geometry begins with Fourier's law Equation From the discussion above, it is seen that no simple expression for area is accurate.
Neither the area of the inner surface nor the area of the outer surface alone can be used in the equation.
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For a problem involving cylindrical geometry, it is necessary to define a log mean cross-sectional area A lm. This expression for log mean area can be inserted into Equation , allowing us to calculate the heat transfer rate for cylindrical geometries. A stainless steel pipe with a length of 35 ft has an inner diameter of 0. Calculate the heat flux at the outer surface of the pipe.
A 10 ft length of pipe with an inner radius of 1 in and an outer radius of 1.
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Find the interior surface temperature. The evaluation of heat transfer through a cylindrical wall can be extended to include a composite body composed of several concentric, cylindrical layers, as shown in Figure 4. Engineering Library. Pumps Valves. Structural Calculators. Relevant Textbooks.