Heat transfer
The right Heat Exchanger Design
for good process performance

Heat transfer describes the transfer of energy that occurs when two systems at different temperatures are in contact. In the chemical process industries (CPI), heat-transfer engineering involves the determination of cost, feasibility and size of equipment that is needed to transfer a specific amount of heat from one fluid to another in a given amount of time. Typical heat-transfer equipment includes boilers, heaters, refrigerators and heat exchangers. Since heat exchangers perform the majority of the heat-transfer work in CPI applications, they are the focus of this report.

The relative flows of the two fluids in a heat exchanger can be co-current, counter-current, or a combination of the two. By design, co-current flow can never yield an outlet coolant temperature higher than that of the outlet hot fluid. However, this is not the case for counter-current flow. Using counter-current flow, it is possible to have outlet coolant temperatures approach the inlet temperature of the hot fluid. Thermodynamically, it can be shown that an exchanger arrangement using counter-current flow gives the most efficient heat transfer.

Fouling
Many process streams leave deposits on the heat-exchange surfaces, which impedes heat-transfer efficiency, and often leads engineers to specify heat exchangers that are larger than they actually need. Any fouling deposits must be eventually removed, which calls for periodic heat-exchanger shutdown. The main causes of fouling inside a heat exchanger are water streams that form hard crystalline deposits, and streams whose organic components can form sticky layers of deposits on the surfaces inside the exchanger. Stream composition, flow velocity and wall temperature are the prime variables that impact fouling rates, and both industry and academia continue to invest considerable time and effort to develop more fouling-resistant materials, more-refined methods to estimate fouling rates, and improved techniques for removing fouling that has occurred.

Basic data required
Most heat-exchanger problems ultimately result from faulty or inadequate information at the design stage. For each fluid, the designer needs to know the following:
§ Flowrate at design conditions. This should normally be the maximum flowsheet rate
§ Heat duty. For heating or cooling a single-phase fluid, this may be expressed in terms of the temperature change to be accomplished
§ Process requirements. These include inlet temperature and pressure and allowable pressure drop. If the heating medium is condensing steam, the available pressure at the control valve is needed, not the pressure generated at the powerhouse
§ Fouling nature of the fluid. Using such data, the system designer can decide how much fouling allowance to provide for when sizing the exchanger, and what type of fouling-resistant materials to specify
§ Process-fluid properties. These include the specific heat, viscosity, density and thermal conductivity associated with the anticipated range of operating temperatures and pressures, for both the tube-side and shell-side fluids
§ Expected turndown. This information is required for designing proper control strategies, and should be known to avoid oversizing the exchanger
§ Materials of construction. Some applications stipulate special materials to ensure compatibility with the fluids to be handled by the exchanger
§ Special requirements. Occasionally, applications require special construction requirements, such as removable bundles or double tube sheets

Selecting the right heat-exchanger type
The choice of heat exchanger type directly affects process performance and also influences plant size and layout, the length of pipe runs, and the strength and size of supporting structures. Experts warn that too often, engineers either forego a through analysis of competing heat-exchanger options, or they postpone such an analysis until well into the detailed-design stage. Too often, engineers go immediately to the “workhorse” heat-exchanger design, the standard shell-and-tube exchanger. However, experts note that for many applications, the less-conventional but still well-established heat-exchanger designs, such as plate exchangers, finned-tube designs, and compact exchangers, for example, offer advantages. Even enhancing features within the shell-and-tube exchanger, such as the use of tube inserts, helical baffles, twisted tubes and rod baffles, are routinely ignored. Furthermore the prospects for micro-heat-exchangers are promising. The first step in a thorough selection process is to eliminate those technologies that are clearly unsuitable for a given application. It is not necessary to exhaustively investigate all features of the alternative technologies that are available; in fact, a basic understanding of each one’s benefits and weaknesses will quickly narrow the search. Discussed below are the general strengths of each of the major heat-exchanger types.

Shell-and-tube
This oldest type is still the most pervasive design in use in the CPI, because its rugged and versatile design can accommodate all extremes of process variables, such as pressure, pressure drop, temperature and fluid corrosivity, yet its design also makes it relatively easy to maintain and repair. If constructed in carbon steel, the cost for a basic shell-and-tube unit is relatively low. In the case of difficult or unique process conditions, shell-and-tube exchangers often require the placement of baffles inside the shell, to direct the flow of the shell-side fluid. For example, fluids with viscosity over 5 centipoise (cP) will require helical baffles to be added. The choice of baffle designs, include single-, double- and triple-segmental, and rod and “egg-crate” non-segmental. Recently, a spiral-type baffle has become available, to minimize pressure drop that arises from changes in flow direction. Other types of tube enhancements are also available to improve heat-transfer efficiency. Among these enhancements are: internal and external fins; wavy or otherwise altered tube profiles; and wire inserts, which also reduce fouling. Turbulence promoters have also become popular. One downside of shell-and-tube exchangers is that most of the time, it is not practical to use counter-current flow in a shell-and-tube exchanger. If the flowrate of the tube side fluid is not extremely high, the resulting tube-side velocity will probably be too low to give an acceptable film coefficient or adequate protection against fouling. In this case, it is necessary to employ multiple tube passes (to create changes in flow direction). Most heat-exchanger designs have an even number or tubes, so that both tube-side inlet and outlet piping connections can be made at the same head (elevation).

Plate exchangers
These are predominantly used in the food industry, because they are extremely easy to clean. For applications with sufficient allowable pressure drop, plate exchangers will remain strong competitors to shell-and-tube designs. Specific designs include plate-and frame, plate-in-shell and plate-fin types. Plate-fin units are most easily suited for multi-streaming — that is, the contacting of more than one hot stream and one cold stream in a single unit. Such a design can effectively contain a whole heat-exchanger network within the body of a single exchanger. And, the use of welded compartments allows the use of plate exchangers at higher pressures, although this will reduce the unit’s ease of cleaning.

Compact heat exchangers
Predominant in cryogenic applications, compact heat exchangers offer a high area-to-volume ratio and often combine several streams in one unit. A major limitation is the relative difficulty of cleaning. With air as the coolant stream, they are widely as automotive coolers and condensers. A key attraction is that they offer extremely small liquid passages (down to 1-mm dia.). When assessing heat-exchanger options for a given application, the following steps should be carried out, to eliminate designs that are not suitable for a given application:
§ Compare temperature and pressure limitations of the exchanger type against required duty
§ Check the required material of construction against the range of materials that can be used for the type of exchanger
§ Evaluate the suitability of the exchanger type for hazards associated with the fluids being processed (such as toxicity, flammability or mixing hazard)
§ Determine if the exchanger is likely to be subject to fouling. If so, decide what cleaning mechanism is to be typically used, and check the suitability of the exchanger candidate for this type of cleaning
§ Verify the availability of maintenance personnel at the proposed location. If specially skilled or trained workers are not readily available, eliminate any technologies that require special attention

Thermal design
Sizing a heat exchanger for a specific process application is often referred to as thermal design. This is an inherently trial-and-error process and must be accurate before the unit’s mechanical design can begin. First, an exchanger configuration is proposed and then rated — meaning that its estimated performance is computed and the results are compared to the process requirements. If calculations indicate that the trial configuration is inadequate, the specifications must be modified, and the rating must be repeated. Assuming that the basic data has already been assembled, the analysis generally follows these four steps:
§ Compute heat duty: The required heat duty expresses the amount of heat that must be transferred through the exchanger. It depends on the fluid that needs to be heated or cooled and is calculated from its flowrate, specific heat, and the inlet and desired-outlet temperatures.
§ Compute mean temperature difference: The mean temperature difference is the driving force for heat transfer and directly affects the required heat transfer surface area. In practice, it is estimated through a logarithmic relationship that depends on the flow arrangement and is referred to as the log-mean temperature difference (LMTD).
§ Design a trial exchanger configuration: This step involves a number of decisions. In the case of a shell-and-tube exchanger, for example, a designer must decide: which fluid to put in the tubes; whether to use fixed tubes or a removable bundle; what tube size, thickness and materials to specify; the number of tubes and tube passes (changes in direction) to specify; and what baffle considerations will be necessary.
§ Compute heat-transfer coefficients and pressure drop: These calculations must be determined for all fluids in question, and must take anticipated fouling into consideration.
§ If the final step does not satisfy the process requirements, the designer must return to Steps 3 and 4 before moving on to the setting the mechanical specifications for the exchanger.

Process simulations
The greatest impact of the today’s advanced computer technology has been the widespread availability of complex process-simulation programs, which, in principle, can be used to simulate entire plant designs. This ability has allowed engineers to study, in detail, numerous process alternatives without capital expenditures. Simulations are particularly valuable during heat-exchanger assessment, design and specification. However, complex designs such as that of a shell-and-tube exchanger will require a close interaction with a knowledgeable engineer, who usually must assess many proposed changes of variables using during the simulation modeling, before obtaining a valid design.

Heat-exchanger networks
A typical CPI plant has a number of streams that must be heated and a number of streams that must be cooled. Economy of production requires that heat be interchanged (or recovered) between the process streams, with minimal heat supplied from outside sources, and minimal heat lost irretrievably to sinks, such as rivers. In recent years, design engineers have focused their efforts on exploring many design variations that will allow for the most advantageous pairing of streams, respecting temperatures and flow quantities, to arrive at the optimal design. <<
Source: Dechema