There are some standard opertaing procedure for start up and shut down of a heat exchangers. Check the fourth set of interview questions on heat exchanger's.
1. What is the use of blind tubes in a heat exchanger?
In order to improve the efficiency of a heat exchanger, the bypass gap between the outer tube rows and the shell should be minimized or blocked. This is done with blind tubes . This will force the medium to flow across through the tube rows.
2. What is the use of sealing strip in a heat exchanger?
With floating head heat exchanger's which have a larger gap in the shell due to the conditional construction, sealing strips are used to block the outer gap.
3. Which product should be to the tube or shell side?
Through the tubes: the medium with higher pressure, corrosive media and products with high fouling tendency.
On the shell side: viscous products.
4. How are the different expansions accommodated?
- Through an expansion joint in the shell of fixed tube sheet bundles.
- Through a U-tube construction with free tube expansion.
- With a floating head for the free expansion.
5. Which Heat Exchanger Types Can Be Cleaned?
Tube-side cleaning: TEMA-types, AEL and NEN, with tube diameters >20 mm
Shell-side cleaning: Removable bundle with square pitch: TEMA-types, CEU, CES, AES
6. What is the requirement of providing lifting devices in a heat exchanger as per TEMA?
Channels, bonnets, and covers which weigh over 60 lbs. (27.2 Kg) are to be provided with lifting lugs, rings or tapped holes for eyebolts.
Unless otherwise specified, these lifting devices are designed to lift only the component to which they are directly attached. Lugs for lifting the complete unit are not normally provided. When lifting lugs or trunnion's are required by the purchaser to lift the complete unit, the device must be adequately designed.
7. What is the space requirements to be considered while locating the position for a heat exchanger in a layout plan?
Clearance for dismantling has to be considered.
For straight tube exchangers fitted with removable bundles, provide sufficient clearance at the stationary head end to permit removal of the bundle from the shell and provide adequate space beyond the rear head to permit removal of the shell cover and/or floating head cover.
For fixed tubesheet exchangers, provide sufficient clearance at one end to permit withdrawal and replacement of the tubes, and enough space beyond the head at the opposite end to permit removal of the bonnet or channel cover.
For U-tube heat exchangers, provide sufficient clearance at the stationary head end to permit withdrawal of the tube bundle, or at the opposite end to permit removal of the shell.
8. Where to refer for the design and operating conditions of a heat exchanger?
The design and operating conditions will be in the design data sheet of the heat exchanger. But easily we can found them from the Name Plate of the heat exchanger.
Equipment must not be operated at conditions which exceed those specified on the nameplate(s).
9. What are the START-UP operation requirements as per TEMA?
Most exchangers with removable tube bundles may be placed in service by first establishing circulation of the cold medium, followed by the gradual introduction of the hot medium. During start-up all vent valves should be opened and left open until all passages have been purged of air and are completely filled with fluid.
For fixed tubesheet exchangers, fluids must be introduced in a manner to minimize differential expansion between the shell and tubes.
10. What are the SHUT-DOWN operation requirements as per TEMA?
For exchangers with removable bundles, the units may be shut down by first gradually stopping the flow of the hot medium and then stopping the flow of the cold medium. If it is necessary to stop the flow of cold medium, the circulation of hot medium through the exchanger should also be stopped.
For fixed tubesheet exchangers, the unit must be shut down in a manner to minimize differential expansion between shell and tubes.
when shutting down the system, all units should be drained completely when there is the possibility of freezing or corrosion damage. To guard against water hammer, condensate should be drained from steam heaters and similar apparatus during start-up or shut-down. To reduce water retention after drainage, the tube side of water cooled exchangers should be blown out with air.
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