HTRI offers a wide range of software solutions for heat transfer equipment used. Exchanger Optimizer simplifies the complicated design process and could.

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1. Software For Design Of Heat Exchangers
2. Heat Transfer Design Software
3. Shell And Tube Exchanger Design

IntroductionThe heat exchanger design equation can be used to calculate the required heat transfer surface area for a variety of specified fluids, inlet and outlet temperatures and types and configurations of heat exchangers, including counterflow or parallel flow. A value is needed for the overall heat transfer coefficient for the given heat exchanger, fluids, and temperatures. Heat exchanger calculations could be made for the required heat transfer area, or the rate of heat transfer for a heat exchanger of given area.

Of average temperature difference is needed. Many heat transfer textbooks have a derivation showing that the log mean temperature difference is the right average temperature to use for heat exchanger calculations. That log mean temperature is defined in terms of the temperature differences as shown in the equation at the right. T Hin and T Hout are the inlet and outlet temperatures of the hot fluid and T Cin and T Cout are the inlet and outlet temperatures of the cold fluid. Those four temperatures are shown in the diagram at the left for a straight tube, two pass shell and tube heat exchanger with the cold fluid as the shell side fluid and the hot fluid as the tube side fluid.Heat Transfer Rate, QHeat exchanger calculations with the heat exchanger design equation require a value for the heat transfer rate, Q, which can be calculated from the known flow rate of one of the fluids, its heat capacity, and the required temperature change. Following is the equation to be used:Q = m H C pH (T Hin – T Hout) = m C C pC (T Cout – T Cin), wherem H = mass flow rate of hot fluid, slugs/hr,C pH = heat capacity of the hot fluid, Btu/slug- oFm C = mass flow rate of cold fluid, slugs/hr,Cp C = heat capacity of the cold fluid, Btu/slug- oF,and the temperatures are as defined in the previous section.The required heat transfer rate can be determined from known flow rate, heat capacity and temperature change for either the hot fluid or the cold fluid. Then either the flow rate of the other fluid for a specified temperature change, or the outlet temperature for known flow rate and inlet temperature can be calculated.

Overall Heat Transfer Coefficient, UThe overall heat transfer coefficient, U, depends on the conductivity through the heat transfer wall separating the two fluids, and the. Convection coefficients on both sides of the heat transfer wall. For a shell and tube heat exchanger, for example, there would be an inside convective coefficient for the tube side fluid and an outside convective coefficient for the shell side fluid. The heat transfer coefficient for a given heat exchanger is often determined empirically by measuring all of the other parameters in the basic heat exchanger equation and calculating U.

Typical ranges of U values for various heat exchanger/fluid combinations are available in textbooks, handbooks and on websites. A sampling is given in the table at the right for shell and tube heat exchangers: SummaryPreliminary heat exchanger design to estimate the required heat exchanger surface area can be done using the basic heat exchanger equation, Q = U A ΔT lm, if values are known or can be estimated for Q, U and ΔT lm. Heat exchanger theory tells us that ΔT lm is the right average temperature difference to use.For example preliminary heat exchanger design calculations, see the article, '.'

For Excel spreadsheet templates that can be downloaded to make preliminary heat exchanger design calculations, see the article: '.' References and Image CreditReferences for Further Information:1. Bengtson, H., an online, continuing course for PDH credit2. And Liu, H., Heat Exchangers: Selection, Rating and Thermal Design, CRC Press, 2002.3. Kuppan, T., Heat Exchanger Design Handbook, CRC Press, 2000.Image Credit:Straight tube, two pass, shell and tube heat exchanger: This post is part of the series: Heat Exchanger Design.

.those that are used in the petrochemical industry which tend to be covered by standards from TEMA, Tubular Exchanger Manufacturers Association (see );.those that are used in the power industry such as feedwater heaters and power plant condensers.Regardless of the type of industry the exchanger is to be used in there are a number of common features (see ).A shell and tube exchanger consists of a number of tubes mounted inside a cylindrical shell. Illustrates a typical unit that may be found in a petrochemical plant. Two fluids can exchange heat, one fluid flows over the outside of the tubes while the second fluid flows through the tubes.

The fluids can be single or two phase and can flow in a parallel or a cross/counter flow arrangement.Front Header—this is where the fluid enters the tubeside of the exchanger. It is sometimes referred to as the Stationary Header.Rear Header—this is where the tubeside fluid leaves the exchanger or where it is returned to the front header in exchangers with multiple tubeside passes.Tube bundle—this comprises of the tubes, tube sheets, baffles and tie rods etc. To hold the bundle together.Shell—this contains the tube bundle.The remainder of this section concentrates on exchangers that are covered by the TEMA Standard. Fixed Tubesheet Exchanger (L, M, and N Type Rear Headers)In a fixed tubesheet exchanger, the tubesheet is welded to the shell. This results in a simple and economical construction and the tube bores can be cleaned mechanically or chemically. However, the outside surfaces of the tubes are inaccessible except to chemical cleaning.If large temperature differences exist between the shell and tube materials, it may be necessary to incorporate an expansion bellows in the shell, to eliminate excessive stresses caused by expansion. Such bellows are often a source of weakness and failure in operation.

In circumstances where the consequences of failure are particularly grave U-Tube or Floating Header units are normally used.This is the cheapest of all removable bundle designs, but is generally slightly more expensive than a fixed tubesheet design at low pressures. Floating Head Exchanger (P, S, T and W Type Rear Headers)In this type of exchanger the tubesheet at the Rear Header end is not welded to the shell but allowed to move or float.

The tubesheet at the Front Header (tube side fluid inlet end) is of a larger diameter than the shell and is sealed in a similar manner to that used in the fixed tubesheet design. The tubesheet at the rear header end of the shell is of slightly smaller diameter than the shell, allowing the bundle to be pulled through the shell. The use of a floating head means that thermal expansion can be allowed for and the tube bundle can be removed for cleaning. There are several rear header types that can be used but the S-Type Rear Head is the most popular. A floating head exchanger is suitable for the rigorous duties associated with high temperatures and pressures but is more expensive (typically of order of 25% for carbon steel construction) than the equivalent fixed tubesheet exchanger.Considering each header and shell type in turn.

L-Type rear headerThis type of header is for use with fixed tubesheets only, since the tubesheet is welded to the shell and access to the outside of the tubes is not possible. The main advantages of this type of header are that access can be gained to the inside of the tubes without having to remove any pipework and the bundle to shell clearances are small.

The main disadvantage is that a bellows or an expansion roll are required to allow for large thermal expansions and this limits the permitted operating temperature and pressure. U-tubeThis is the cheapest of all removable bundle designs, but is generally slightly more expensive than a fixed tubesheet design at low pressures. However, it permits unlimited thermal expansion, allows the bundle to be removed to clean the outside of the tubes, has the tightest bundle to shell clearances and is the simplest design. A disadvantage of the U-tube design is that it cannot normally have pure counterflow unless an F-Type Shell is used. Also, U-tube designs are limited to even numbers of tube passes. Figure 5. Baffle arrangements.The center-to-center distance between baffles is called the baffle-pitch and this can be adjusted to vary the crossflow velocity. In practice the baffle pitch is not normally greater than a distance equal to the inside diameter of the shell or closer than a distance equal to one-fifth the diameter or 50.8 mm (2 in) whichever is greater.

In order to allow the fluid to flow backwards and forwards across the tubes part of the baffle is cut away. The height of this part is referred to as the baffle-cut and is measured as a percentage of the shell diameter, e.g., 25 per cent baffle-cut. The size of the baffle-cut (or baffle window) needs to be considered along with the baffle pitch. It is normal to size the baffle-cut and baffle pitch to approximately equalize the velocities through the window and in crossflow, respectively.There are two main types of baffle which give longitudinal flow.

Tube insertsThese are normally wire wound inserts or twisted tapes. They are normally used with medium to high viscosity fluids to improve heat transfer by increasing turbulence. There is also some evidence that they reduce fouling. In order to use these most effectively the exchanger should be designed for their use. This usually entails increasing the shell diameter, reducing the tube length and the number of tubeside passes in order to allow for the increased pressure loss characteristics of the devices.Consider any and every safety and reliability aspect and allocate fluids accordingly.

Software For Design Of Heat Exchangers

Shell selectionE-type shells are the most common. If a single tube pass is used and provided there are more than three baffles, then near counter-current flow is achieved. If two or more tube passes are used, then it is not possible to obtain pure countercurrent flow and the log mean temperature difference must be corrected to allow for combined cocurrent and countercurrent flow using an F-factor.G-type shells and H shells are normally specified only for horizontal thermosyphon reboilers. J shells and X-type shells should be selected if the allowable DP cannot be accommodated in a reasonable E-type design. For services requiring multiple shells with removable bundles, F-type shells can offer significant savings and should always be considered provided they are not prohibited by customer specifications. Front header selectionThe A-type front header is the standard for dirty tubeside fluids and the B-type is the standard for clean tubeside fluids. The A-type is also preferred by many operators regardless of the cleanliness of the tubeside fluid in case access to the tubes is required.

Do not use other types unless the following considerations apply.A C-type head with removable shell should be considered for hazardous tubeside fluids, heavy bundles or services requiring frequent shellside cleaning. The N-type head is used when hazardous fluids are on the tubeside. A D-type head or a B-type head welded to the tubesheet is used for high pressure applications. Y-type heads are only normally used for single tube-pass exchangers when they are installed in line with a pipeline. Rear header selectionFor normal service a Fixed Header (L, M, N-types) can be used provided that there is no overstressing due to differential expansion and the shellside will not require mechanical cleaning.

If thermal expansion is likely a fixed header with a bellows can be used provided that the shellside fluid is not hazardous, the shellside pressure does not exceed 35 bar (500 psia) and the shellside will not require mechanical cleaning.A U-tube unit can be used to overcome thermal expansion problems and allow the bundle to be removed for cleaning. However, countercurrent flow can only be achieved by using an F-type shell and mechanical cleaning of the tubeside can be difficult.An S-type floating head should be used when thermal expansion needs to be allowed for and access to both sides of the exchanger is required from cleaning. Other rear head types would not normally be considered except for the special cases. Thermal DesignThe thermal design of a shell and tube exchanger is an iterative process which is normally carried out using computer programs from organizations such as the Heat transfer and Fluid Flow Service (HTFS) or Heat Transfer Research Incorporated (HTRI).

Heat Transfer Design Software

However, it is important that the engineer understands the logic behind the calculation. In order to calculate the heat transfer coefficients and pressure drops, initial decisions must be made on the sides the fluids are allocated, the front and rear header type, shell type, baffle type, tube diameter and tube layout. The tube length, shell diameter, baffle pitch and number of tube passes are also selected and these are normally the main items that are altered during each iteration in order to maximize the overall heat transfer within specified allowable pressure drops.The main steps in the calculation are given below together with calculation methods in the open literature. Mechanical DesignThe mechanical design of a shell and tube heat exchanger provides information on items such as shell thickness, flange thickness, etc. These are calculated using a pressure vessel design code such as the Boiler and Pressure Vessel code from ASME (American Society of Mechanical Engineers) and the British Master Pressure Vessel Standard, BS 5500. ASME is the most commonly used code for heat exchangers and is in 11 sections.

Shell And Tube Exchanger Design

Section VIII (Confined Pressure Vessels) of the code is the most applicable to heat exchangers but Sections II—Materials and Section V—Non Destructive Testing are also relevant.Both ASME and BS5500 are widely used and accepted throughout the world but some countries insist that their own national codes are used. In order to try and simplify this the International Standards Organization is now attempting to develop a new internationally recognized code but it is likely to be a some time before this is accepted.