1. Basic Math and Heat Transfer Knowledge
2. HTRI Software
3. Have a Computer
4. Have a Desire
1.How do you design a heat exchanger?
Designing a Heat Exchanger is performed through step-wise sequence, the first step being understanding the process. The second step is to generate physical properties via Aspen hysys and put them in HTRI. The third step is specifiying shell, Baffles, Tube geometries and charachteristics in HTRI. Finally, the software is run and users change the geomatry to meet pressure drop criteria and overdesign factor.
2.What is the design criteria needed for heat exchanger?
Heat Exchangers are typically designed based on TEMA and HTRI software. In TEMA several factors are stated as the design criteria but overdesign factor, pressure drop, velocity, vibration-free are the most important ones.
3.What are exchanger baffles used for?
Baffles are installed inside the shellside to serve two functions: first and foremost, to improve thermal heat transfer by changing the direction of fluid. Also they are used to tackle vibration-related problems.
4.What is impingement plate on heat exchanger?
An impingement plate, or other means to protect the tube bundle against impinging fluids,
shall be provided for all shell side inlet nozzle(s), unless the product of p V 2 in the inlet
nozzle does not exceed the limits stated in TEMA.
You become familiar with the enviroment of HTRI software
In this part you get to know how to select the best TEMA type for a shell and tube heat exchanger
The head types used in shell and tube heat exchanger construction are described in the following section. The TEMA stationery and rear-head designations are shown against each type head where applicable.
Bolted channel ( TEMA A and L)
The bolted channel, shown in the following figure consists of a cylindrical barrel, or channel, which has a flange at both ends. Removal of the flat cover provides access to the tube ends in situ, without having to unbolt the flanges connecting the external piping to the channel nozzle.
Welded channel (TEMA C and N)
Welded channel, shown in the following figure is similar to the bolted type and the cylinderical barrel or channel has a flange at one end, which is bolted to a flat cover, but the other end is welded to the stationary tubesheet. Again, removal of the flat cover provides access to the tube ends, in situ, without having to unbolt the flanges connecting the external piping to the channel nozzle. The welded channel is cheaper than the bolted channel because there is only one flange instead of two , and this type is often selected for high pressure/lethal services where it is desirable to minimise the number of external joints.
Bonnet (TEMA B and M )
The bonnet-type channel, shown in the following figure consists of a cylindrical barrel with a bonnet welded-on at one end and a flange at the other . Bonnet-type heads are cheaper than the bolted or welded types and after removal provide unristricted access to the tube ends. However, this is achieved only after unbolting the flanges connecting the external piping to the channel nozzles.
Bolted-type channels are usually fitted at the rear end of fixed tubesheet exchangers when there are no nozzles there. This type of head is generally used if cleaning of the inside of the tubes is expected to be infrequent.
In this time you reach an understanding to Select proper Orientation
In this part you are taught how to select tube length, tube OD and tube orientation
The selection of tube diameter is a compromise taking into account the fouling nature of the fluids, the space available and the cost. Tubes of 19.05 and 25.4 mm outside diameter are the most widely used, but small units with clean fluids may use tubes as small as 6.35 mm outside diameter, and units handling heavy tars may use tubes 50.8mm outside diameter.Large reactors may use tubes 76.3 mm outside diameter.
The tube-wall thickness must be checked against the internal and external pressure seperately, or the maximum pressure differential across the wall. However in many cases the pressure is not the governing factor in determining the wall thickness . For example , a steel tube , 19.05 mm outside diameter , 2.11 mm thick, at 350C is suitable for internal and external design pressure of 200 bar and 115 bar, respectively, within code rules, which is adaquate for many applications. Except when pressure governs, tube-wall thickness is selected on the basis of (a) providing an adequate margin against corrosion , (b) resistence to flow-induced vibration , (c) axial strength particularely in fixed tubesheet exchangers , (d) standardisation for the stocking of spares and (e) cost.
The following table shows typical wall thicknisses for various tube diameters and metals.
For a given surface area the cheapest exchanger is one which has a small shell diameter and a long tube length, consistent with the space and handling facilities at site and in the fabricator’s shop. The incentive, therefore, is to make exchangers as long as possible, limited only by the tube length available from tube suppliers , but practical limitations usually control. Irrespective of the exchanger type, there must be available at site an unristricted length of about twice the tube length in order that a tube bundle or tube may be withdrawn and replaced. Tube lengths of 2438,3658,4877,6096 and 7315 mm are often regarded as standard for both straight and U-tubes, but other length may be used.
It is customary practice to arrange the tube pitch ( center-center distance ) such that it is not less than 1.25 times the outside diameter of the tubes . in certain applications involving clean fluids and small tubes, say 12.7 mm outside diameter and less, the pitch diameter ratio is sometimes reduced to 1.2 . The following guidline lists typical tube pitches for various tube diameter and the pitch arrangements are shown in the following figure . Selection of pitch angle is as follows:
For a given pitch/diametre ratio and shell inside diameter, about 15% more tubes can be accomodated for 30 and 60 pitch angles compared with 45 and 90. To achieve compactness the incentive is to use 30 and 60 pitch angles, which are satisfactory for clean services but these patterns are not permitted when external mechanical cleaning is required. To provide adaquate mechanical cleaning lanes 45 and 90 pitch angles must be used with a minimum gap between tubes of 6.35 mm. In turbulant flow the 90 pitch angle has superior heat transfer and pressure loss characteristics to the 45 pitch angle; in laminar flow the 45 pitch angle is superior. A further requirement for the tube layout of 45 and 90 pitch is that the cleaning lane should be continuous throughout the bundle.
In this part you will learn how to determine tube passes
Each traverse of the tube-side fluid from one end of the exchanger to the other is termed as a pass.Tube-side passes have thermal significance in that by changing the number of passes , the thermal designer is able to change the fluid velocity. Depending on process conditions, the number of passes required may be only one or as many as sixteen.
In this minute the instructor Know how to select a proper baffle type with suitable baffle spacing
Cross-type baffle have thermal significance in that the shell-side fluid is made to flow to and across the bundle from one end of the exchanger to the other. By increasing or decreasing the distance between adjacent baffles, the thermal designer is able, within limits, to change the fluid velocity. Equally important the baffle are spaced to support the tubes adaquately to prevent sag and flow-induced vibration. The following figure shows three common cross-type baffles : segmental, double segmental and triple segmental.Segmental baffles are the most common, but where a design is governed by shell side pressure loss , it may be reduced appreciably by restoring to double- or triple-segmental baffles.
In this minute you become familiar with impingement plate concept and its types
To protect the tubes below the shell-side inlet nozzle from damage due to solid particles or liquid droplets entrained in the shell side fluid, an impingement plate might be required. The plate is about 6 mm thick, flat or curved with dimention slightly greater than the nozzle bore. The following figures show different types of impingment plate and its criteria based on TEMA standard.
Tie rod and spacers
A number of tie rods and spacers hold the bundle together and locate the baffles in their correct positions. Tie rods are circular metal rods, usually 9.5-15.5 mm diameter, which are screwed into stationary tubesheet and extend the length of the bundle up to the last baffle, where they are secured by lock nuts. All the rods have the spacer tubes fitted over them, each spacer being tube or pipe with an inside diameter slightly greater than that of the tie-rod diameter and a length equal to the required baffle spacing.
In this part you will learn the followings:
1. Know how to deal pressure drop issues.
2. Know how to play with heat exchanger mechanical features such as tube length, shell ID, baffle spacing to reach an optimum design of shell and tube heat exchanger.