Thermal Unit Operation (ChEg3113) Lecture 5- Heat Exchanger Design Instructor: Mr. Tedla Yeshitila (M.Sc.)
Today Review Heat exchanger design vs rating of heat exchanger Heat exchanger general design procedure Overall heat transfer coefficient Fouling Double pipe heat exchanger design
Radiator Review Most common heat exchanger types: Tubular heat exchanger (double pipe) Shell and tube heat exchanger Plate and frame (plate) HX Regenerative type Condenser Evaporator Vaporizer
The double pipe heat exchanger is the simplest type of heat exchanger, while the shell-and-tube is the most widely used type of exchanger in the chemical process industries. Heat-exchanger calculations can be divided into two distinct categories, namely, thermal and hydraulic calculations on the one hand and mechanical design calculations on the other. Thermal and hydraulic calculations are made to determine heat-transfer rates and pressure drops needed for equipment sizing. Mechanical design calculations are concerned with detailed equipment specifications, and include considerations such as stress and tube vibration analyses. First we will see thermal calculations, then hydraulic calculations, but we will not performs mechanical design calculations which mostly done using software.
Heat-exchanger problems may also be categorized as rating problems or design problems. Rating of heat exchanger In rating of heat exchanger, the objective is to determine the performance of heat exchanger when the heat transfer area and construction details are known. In a rating problem, one must determine whether a given, fully specified exchanger will perform a given heat-transfer duty satisfactorily. It is evaluation of thermodynamic performance of fully specified heat exchanger. It is immaterial whether the exchanger physically exists or whether it is specified only on paper.
Heat exchanger design: The principal objective in design of heat exchanger design is to determine the surface area required for specific duty (rate of heat exchanger) using temperature difference available. In a design problem, one must determine the specifications for a heat exchanger that will handle a given heat-transfer duty. A rating calculation is generally an integral part of a design calculation. However, a rating problem also arises when it is desired to use an existing exchanger in a new or modified application. Selection of heat exchanger: The objective of selection of heat exchanger is to choose the appropriate and suitable heat exchanger from a standard unit from manufacturer catalogue.
The basic steps in typical design procedures are: 1. Define duty (use heat and mass balance to define the rate of heat and mass transfer) 2. Collect physical properties data (μ,ρ ) 3. To decide on the type of heat exchanger to be used 4. Select a trial value for average heat transfer coefficient (U) 5. Calculate the mean temperature difference 6. Calculate the heat transfer area 7. Decide in heat exchanger layout 8. Calculate the individual heat transfer coefficients 9. Calculate the overall heat transfer coefficients, compare with trial and iterate 10. Calculate ΔP; if not good choose different layout. 11. Optimize the design, repeat steps 4-10 as necessary to determine the cheapest heat exchanger that will satisfy the duty.
Overall heat transfer coefficient An essential, and often the most uncertain, part of any heat exchanger analysis is determination of the overall heat transfer coefficient. Overall heat transfer coefficient is reciprocal of the overall resistance to heat transfer. This coefficient is defined in terms of the total thermal resistance to heat transfer between two fluids. The coefficient determined by accounting for conduction and convection resistances between fluids separated by composite plane and cylindrical walls, respectively. For a wall separating two fluid streams, the overall heat transfer coefficient may be expressed as: 1 UA = 1 = 1 = 1 + R U c A c U h A h (ha) w + 1 c (ha) h
If the surface is not known, U can be obtained independent of Fourier equation from the two film coefficient. => 1 U = 1 h c + R w + 1 h h where c and h refer to the cold and hot fluids, respectively. The wall conduction term in above equation may often be neglected i.e. the pipe wall resistance is neglected since a thin wall (small thickness )of large thermal conductivity is generally used. 1 U = 1 h c + 1 h h => U = h ch h h c +h h The magnitude of the individual coefficients will depend on the nature of heat transfer process, on the physical properties of the fluid, on the flowrates and on the physical arrangement of heat transfer surface.
Fouling factor During normal heat exchanger operation, surfaces are often subject to fouling by fluid impurities, rust formation, or other reactions between the fluid and the wall material. The subsequent deposition of a film or scale on the surface can greatly increase the resistance to heat transfer between the fluids. This effect can be treated by introducing an additional thermal resistance in the above equation, termed the fouling factor, R f. Most process and service fluids will foul the heat transfer surface in an exchanger to a greater or lesser extent. Fouling impairs or weaken heat exchanger. Its value depends on the operating temperature, fluid velocity, and length of service of the heat exchanger.
It is very difficult to predict value of fouling factors. In addition, we know that fins are often added to surfaces exposed to either or both fluids and that, by increasing the surface area, they reduce the overall resistance to heat transfer. Accordingly, with inclusion of surface fouling and fin (extended surface) effects, the overall heat transfer coefficient is modified as follows: 1 UA = 1 + R f,c + R (η o ha) c (η o A) w + R f,h 1 + c (η o A) h (η o ha) h The quantity η o in the above equation is called the overall surface efficiency or temperature effectiveness of a finned surface.
Although representative fouling factors are listed in Table below, the factor is a variable during heat exchanger operation (increasing from zero for a clean surface, as deposits accumulate on the surface).
Overall surface efficiency defined such that, for the hot or cold surface without fouling, the heat transfer rate is: q = η o ha(t w T ) where Tw is the base surface temperature and A is the total (fin plus exposed base) surface area. η o = 1 A f A (1 η f) where A f is the entire fin surface area and η f is the efficiency of a single fin. Also, one of the convection coefficients is often much smaller than the other and hence dominates determination of the overall coefficient. For example, if one of the fluids is a gas and the other is a liquid or a liquid vapor mixture experiencing boiling or condensation, the gasside convection coefficient is much smaller. It is in such situations that fins are used to enhance gas-side convection.
For the unfinned, tubular heat exchangers 1 UA = 1 + R f,c + ln (D 2/D 1) (ha) c (A) c 2ΠkL + R f,h (A) h + 1 (ha) h Where D 1 is inner diameter and D 2 is outer diameter Therefore, overall heat transfer coefficient can be determined from knowledge of the hot and cold fluid convection coefficients and fouling factors and from appropriate geometric parameters.
Chapter 4 Design of Double Pipe Heat Exchanger A simple double-pipe exchanger consists of two pairs of concentric pipes arranged as shown in below. Such a configuration is called a hairpin (U-shaped) because it is arranged in two legs. The principal parts are two sets of concentric pipes, two connecting Tees, and a return head and a return bend. The inner pipe is supported within the outer pipe by packing glands. And the fluid enters the inner pipe through threaded connection located outside the exchanger.
Chapter 4 Design of Double Pipe Heat Exchanger Also, nozzles may be provided on the inner-pipe (tube) side as well as on the annulus side to facilitate connection to process piping. The Tees have nozzle or screwed connections attached to them to permit the entry and exit of annulus fluid which crosses from one leg to other through the return head. The two length of the inner pipe are connected by a return bend which is usually exposed and does not provide effective heat transfer surface. The maximum nozzle size is generally equal to the size of the outer heat-exchanger pipe. The two fluids that are transferring heat flow in the inner and outer pipes, respectively. The outer pipe may be insulated to minimize heat transfer to or from the environment.
Chapter 4 Design of Double Pipe Heat Exchanger Batteries of hairpins connected in series or in series-parallel arrangements are commonly employed to provide adequate surface area for heat transfer. Simple double-pipe exchangers are commercially available with outer-pipe sizes ranging from 2 to 8 in. and inner pipes from 3/4 to 6 in. Double-pipe exchangers are commonly used in applications involving relatively low flow rates and high temperatures or pressures, for which they are well suited. Other advantages include low installation cost, ease of maintenance, and flexibility. The double pipe heat exchanger is extremely useful because it can be assembled in any pipe-fitting shop from standard parts and provides inexpensive heat transfer surface.
At the end of this class: You will be able to understand the difference between designing, rating and selection of rating You will be able to use know the basic procedure in designing of heat exchanger You will be familiarize with overall heat transfer coefficient and fouling considerations during calculations
End of lecture -5