Parts of shell and tube heat exchanger as following below:
Shell is usually made of steel and alloys. Dimension and wall thickness of shell is set by TEMA standard in inch and millimeter unit ranging from 6 inches (152 mm) to 100 inches (2540 mm) for shell diameter and 1/8 inch (3.2 mm) up to ½ inch (12.7 mm) for thickness. While material that is used for shell specification is recommended refer to ASTM standard. For the shell cover, minimum thickness is equal to thickness of shell.
Figure 1 shows mechanical arrangement of shell and tube heat exchanger. Figure 1(a) shows arrangement of one shell and tube single-pass, while Figure 1(b) to Figure 1(d) shows arrangement of one shell and two tube-passes. Figure 1(b) is arrangement for U-tube type. Figure 1(c) for fixed tubesheet and Figure 1(d) for floating tubesheet.
|Figure 1: Mechanical Arrangement of Tube and Shell Heat Exchanger|
Characteristics of tube in heat exchanger have major effect on heat transfer between working fluid. These characteristics include dimensions, materials, and arrangements. The dimension of tube is available in inch unit. In TEMA standards, outside diameter of tube is limited ranging from ¼ inch (6.4 mm) up to 2 inch (50.8 mm). Wall thickness of tube is related to thermal resistance on the side of tube wall, thicker wall thickness means that tube has higher thermal resistance and has lower quality to heat transfer. Tube materials are available for manufacture of tube is usually in the form of carbon steel and its alloys, nickel and its alloys, and aluminum and its alloys.
Baffle serves as partition to direct fluid flow inside shell and increase flow rate or make the flow becomes turbulent. The presence of turbulence will increase heat transfer coefficient so it will increase the rate of heat transfer. Nevertheless, installation of baffle also increases pressure drop of fluid flow. Baffle is mounted on the tie rod.
In TEMA standards, distance between one baffle to other baffle is set and it is called as baffle spacing. The distance between one baffle to one another is made same, if not possible then be taken to nearest distance is adjacent section with the edge of shell, while other distance is made same distance between one baffle to other baffle.
The closest distance between baffle is limited 1/5 diameter of the shell. This aims to avoid too high pressure drop, while the furthest distance is usually not greater than diameter of shell, it aims to avoid occurrence of vibration on the pipe. In case of vibration, then tube that is mounted on tubesheet will also vibrate and potentially to create leak.
- Tie Rod
Tie rod is a rod with certain diameter that serves as baffle support. Tie rod attached to the tubesheet with screw mechanism, while at the other end is equipped with couple bolt and screw to hold baffles keep arriving at his position. TEMA standards set dimension diameter tie rod as a function of inside diameter of shell, the larger inside diameter of shell, diameter of tie rod will aso greater and number of baffles will increase because of the greater size and weight of baffle.
Tubesheet function is as place of tube installation. The arrangement of installed tubes can be shaped triangular, rotated triangular, square, and rotated square. Tubes are usually installed not symmetrical. Asymmetry is found in section adjacent to nozzle, a place where fluid flow into shell. At this place, there is not tube to aims to minimize contraction that occurs in tubes as an effect of fluid flow enters shell.
- Tube Pitch
In TEMA standards is arranged that the shortest distance is 1 - 1/4 times from center point of tube. The shortest distance between two holes is usually called as clearance. The advantages of square pitch form are the ease of maintenance mechanic because there is a regular clearance position to form horizontal and vertical line and also has low pressure drop because there is not blocking fluid flow.
If higher rate of heat transfer is desired, tube pitch type can choose triangular pitch type. In this type, fluid can not flow smoothly because it was blocked by tubes in front of it, causing turbulence and pressure drop becomes large. Mechanical maintenance (cleaning crust on outside tube) of triangular pitch type is more difficult because scrubber brush can not pass clearance easily. The existence of triangular tubes arrangement blocks the scrubber brush.