- Basics of Shell & Tube Heat Exchangers
- Heat Exchanger Design Guide - 1st Edition
- Static Equipment: Understanding Heat Exchangers
One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. There are several thermal design features that are to be taken into account when designing the tubes in the shell and tube heat exchangers.
Process liquid or gas cooling Process or refrigerant vapour or steam condensing Process liquid, steam or refrigerant evaporation Process heat removal and preheating of feed water Thermal energy conservation efforts, heat recovery Compressor, turbine and engine cooling, oil and jacket water Hydraulic and lube oil cooling. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult.
To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities.
However, there are many limitations for this, including the space available at the site where it is going to be used and the need to ensure that there are tubes available in lengths that are twice the required length so that the tubes can be withdrawn and replaced. In many cases, plate heat exchangers are oversized simply to reduce the pressure drop for the secondary fluid.
Basics of Shell & Tube Heat Exchangers
On existing plant, an indication of actual load may be obtained if the flow and return temperatures and the pumping rate are known. Shell and tube heat exchangers and plate heat exchangers are typical examples of flow type applications. Therefore, when determining the steam consumption for these applications, Equation 2.
The start-up load may be ignored if it occurs rarely, or if the time taken to reach full-load output is not too important. Heat exchangers are more often sized on the full running load, with the possible addition of safety factors. Heat losses are rarely taken into account with these flow type applications, as they are significantly less than the full running load. Shell and tube heat exchangers are usually lagged to prevent heat loss, and to prevent possible injury to personnel.
Plate heat exchangers tend to be more compact and have a much smaller surface area exposed to the ambient air, in relation to the size of the unit.
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A heating calorifier is designed to operate at full-load with steam at 2. A plate heat exchanger consists of a series of thin corrugated metal plates between which a number of channels are formed, with the primary and secondary fluids flowing through alternate channels. Heat transfer takes place from the primary fluid steam to the secondary process fluid in adjacent channels across the plate. Figure 2.
A corrugated pattern of ridges increases the rigidity of the plates and provides greater support against differential pressures. This pattern also creates turbulent flow in the channels, improving heat transfer efficiency, which tends to make the plate heat exchanger more compact than a traditional shell and tube heat exchanger. The promotion of turbulent flow also eliminates the presence of stagnant areas and thus reduces fouling. The plates will usually be coated on the primary side, in order to promote the dropwise condensation of steam. The steam heat exchanger market was dominated in the past by the shell and tube heat exchanger, whilst plate heat exchangers have often been favoured in the food processing industry and used water heating.
However, recent design advances mean that plate heat exchangers are now equally suited to steam heating applications. A plate heat exchanger may permit both the condensing and sub-cooling of condensate within a single unit. If the condensate is drained to an atmospheric receiver, by reducing the condensate temperature, the amount of flash steam lost to the atmosphere through the receiver vent is also reduced. This can eliminate the need for a separate sub-cooler or flash steam recovery system.
Although a nominal heat transfer area may theoretically be calculated using Equation 2. In a gasketed plate heat exchanger the plates are clamped together in a frame, and a thin gasket usually a synthetic polymer seals each plate around the edge. Tightening bolts fitted between the plates are used to compress the plate pack between the frame plate and the pressure plate. This design allows easy dismantling of the unit for cleaning, and allows the capacity of the unit to be modified by the simple addition or removal of plates.
The use of gaskets gives a degree of flexibility to the plate pack, offering some resistance to thermal fatigue and sudden pressure variations. This makes some types of gasketed plate heat exchanger an ideal choice as a steam heater for instantaneous hot water supply, where the plates will be exposed to a certain amount of thermal cycling.
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The limitation in the use of the gasketed plate heat exchanger lies in the operating temperature range of the gaskets, which places a restriction on the steam pressure that may be used on these units. In a brazed plate heat exchanger all the plates are brazed together normally using copper or nickel in a vacuum furnace. It is a development of the gasketed plate heat exchanger, and was developed to provide more resistance to higher pressures and temperatures at a relatively low cost.
However, unlike the gasketed unit, the brazed plate heat exchanger cannot be dismantled.
Heat Exchanger Design Guide - 1st Edition
If cleaning is required it must be either back-flushed or chemically cleaned. It also means that these units come in a standard range of sizes, consequently oversizing is common. While the brazed heat exchanger has a more robust design than the gasketed type, it is also more prone to thermal fatigue due to its more rigid construction. Any sudden or frequent changes in temperature and load should therefore be avoided, and greater attention should be paid to the control on the steam side to avoid thermal stress.
Brazed heat exchangers are more suitable and primarily used for applications where temperature variations are slow, such as in space heating. They may also successfully be used with secondary fluids which expand gradually, such as thermal oil. In a welded plate heat exchanger the plate pack is held together by welded seams between the plates. The use of laser welding techniques allows the plate pack to be more flexible than a brazed plate pack, enabling the welded unit to be more resistant to pressure pulsation and thermal cycling.
The high temperature and pressure operating limits of the welded unit mean that these heat exchangers normally have a higher specification, and are more suited to heavy duty process industry applications. They are often used where a high pressure or temperature performance is required, or when viscous media such as oil and other hydrocarbons are to be heated. The shell and tube heat exchanger is probably the most common method of providing indirect heat exchange in industrial process applications. A shell and tube heat exchanger consists of a bundle of tubes enclosed in a cylindrical shell.
The ends of the tubes are fitted into tube sheets, which separate the primary and the secondary fluids. Where condensing steam is used as the heating medium, the heat exchanger is usually horizontal with condensation taking place inside the tubes. Sub-cooling may also be used as a means to recover some extra heat from the condensate in the heat exchanger.
However, if the degree of sub-cooling required is relatively large it is often more convenient to use a separate condensate cooler. A common design for a steam to water non-storage calorifier is shown in Figure 2. It is said to have two tube passes because the steam inlet and outlet connections are at the same end of the exchanger, so that the tube-side fluid passes the length of the unit twice.
A pass partition also called a partition plate or a feather plate divides up the exchanger header, so that the tube-side fluid is diverted down the U-tube bundle rather than straight through the header. This is a comparatively simple and inexpensive design because only one tube sheet is required, but it is limited in use to relatively clean fluids as the tubes are more difficult to clean. Note; it is more difficult to replace a tube with these types of heat exchanger. Baffles are usually provided in the shell, to direct the shell-side fluid stream across the tubes, improving the rate of heat transfer, and to support the tubes.
As mentioned in Module 2.
For this reason, control valves and heat exchangers will often be found to be sized on full-load plus the usual safety factors. With systems that shut down at night and weekends, secondary water temperature can be low at start-up on a cold winter morning, and condensing rates in heating calorifiers will be higher than the full-load condition.
Consequently, pressure in the steam space may be considerably below the pressure at which the heat exchanger normally operates, until the secondary inlet temperature rises to its design figure. From a thermal viewpoint, this may not pose a problem - the system simply takes longer to heat up.
Static Equipment: Understanding Heat Exchangers
However, if the designer has not taken this situation into consideration, an inadequate steam trapping and condensate removal system can cause condensate to accumulate in the steam space. Buildings - A practical, subjective method to estimate a heating load is to look at the building itself. Calculations can be complicated, involving factors such as the number of air changes and heat transfer rates through cavity walls, windows and roofs.
A practical way to establish steam consumption for an existing installation is to use an accurate reliable steam flowmeter. The steam load at the design condition can be estimated simply by the ratio of the temperature differences:. Hot water storage calorifiers are designed to raise the temperature of the entire contents from cold to the storage temperature within a specified period.
The mean rate at which steam is condensed during the heat up or recovery period can be calculated using Equation 2. This mean value can be used to size the control valve. The pressure in the coil will drop significantly, with the net effect of reducing the capacity of the steam trapping device.
If the trapping device is wrongly sized or selected, condensate may back up into the coil, reducing its ability to transfer heat and achieve the required heat up time. Waterhammer may result, causing severe noise and mechanical stresses to the coil. However, if condensate is not allowed to back up into the coil the system should still maintain the correct heat up time.
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The solution is to ensure proper condensate drainage.