Chemical Engineering Interview Questions and Answers
Question - 51 : - When An Expansion Is Joint Needed On The Shell Side Of A Shell And Tube Heat Exchanger?
Answer - 51 : -
A fixed tube sheet exchanger does not have provision for expansion of the tubing when there is a difference in metal temperature between the shell and tubing. When this temperature difference reaches a certain point, an expansion joint in the shell is required to relieve the stress. It takes a much lower metal temperature difference when the tube metal temperature is hotter than the shell metal temperature to require an expansion joint. Typically, an all steel exchanger can take a maximum of approximately 40-0F metal temperature difference when the tube side is the hottest. When the shell side is the hottest, the maximum is typically 150 0F. Usually if an expansion joint is required, it is because the maximum allowable tube Compressive stress has been exceeded. According to the TEMA procedure for evaluating this stress, the compressive stress is a strong function of the unsupported tube span.
Question - 52 : - What Kind Of Concerns Is Associated With Temperature Pinch Points In Condensers?
Answer - 52 : -
Be extra careful when condensers are designed with a small pinch point. A pinch point is the smallest temperature difference on a temperature vs heat content plot that shows both streams. If the actual pressure is less than the process design operating pressure, there can be a significant loss of heat transfer. This is especially true of fluids that have a relative flat vapor pressure plot like ammonia or propane. For example: If an ammonia condenser is designed for 247 PSIA operating pressure and the actual pressure is 5 PSI less and the pinch point is 8 0F, there can be a 16% drop in heat transfer.
Question - 53 : - What Factors Go Into Designing The Vapor Space Of Kettle Type Reboiler?
Answer - 53 : -
The size of the kettle is determined by several factors. One factor is to provide enough space to slow the vapor velocity down enough for nearly all the liquid droplets to fall back down by gravity to the boiling surface. The amount of entrainment separation to design for depends on the nature of the vapor destination. A distillation tower with a large disengaging space, low tower efficiency, and high reflux rate does not require as much kettle vapor space as normal.
Normally the vapor outlet is centered over the bundle. Then the vapor comes from two different directions as it approaches the outlet nozzle. Only in rare cases are these two vapor streams equal in quantity. A simplification that has been extensively used is to assume the highest vapor flow is 60% of the total. In one case, where this would cause an undersized vapor space is when there is a much larger temperature difference at one end of the kettle then the other. The minimum height of the vapor space is typically 8 inches.
Question - 54 : - Is There A Quick Rule-of-thumb To Estimate A Gas Side Heat-transfer Rate Inside The Tubes Of A Shell And Tube Heat Exchanger?
Answer - 54 : -
If you need to estimate a gas heat transfer rate or see if a program is getting a reasonable gas rate, use the following: h = 75 X Sq. Root(Op. pressure/100) The operating pressure is expressed as absolute. This is for inside the tubes. The rate will be lower for the shell side or if there is more than one exchanger.
Question - 55 : - Are There Any Alternatives To Scraping A Shell And Tube If A Capacity Increase Will Make The Pressure Drop Across The Exchanger Too Large?
Answer - 55 : -
When an increase in capacity will cause excessive pressure drop, you may not have to junk the heat exchangers. A relatively inexpensive alteration is to reduce the number of tube passes. Other possibilities are arranging the exchangers in parallel or using lowfins or other special tubing.
Question - 56 : - What Is A Good Method Of Minimizing Shell Side Pressure Drop In A Shell And Tube Exchanger?
Answer - 56 : -
When shell pressure drop is critical and impingement protection is required, use rods or tube protectors in top rows instead of a plate. These create less pressure drop and better distribution than an impingement plate. An impengement plate causes an abrupt 90-degree turn of the shell stream, which causes extra pressure drop.
Question - 57 : - What Are Some Good Strategies For Curing Tube Vibration In Shell And Tube Exchangers?
Answer - 57 : -
Most flow-induced vibration occurs with the tubes that pass through the baffle window of the inlet zone. The unsupported lengths in the end zones are normally longer than, those in the rest of the bundle. For 3/4 inch tubes, the unsupported length can be 4 to 5 feet. The cure for removable bundles, where the vibration is not severe, is to stiffen the bundle. This can be done by inserting metal slats or rods between the tubes. Normally this only needs to be done with the first few tube rows. Another solution is to add a shell nozzle opposite the inlet to cut the inlet fluid velocity in half. For non-removable bundles, this is the best solution. Adding a distributor belt on the shell would be a very good solution if it were not so expensive.
Question - 58 : - What Are Some Of The Consequences Of An Undersized Kettle Type Reboiler?
Answer - 58 : -
The effect will be a decrease in the boiling coefficient. A boiling coefficient depends on a nucleate boiling component and a two-phase component that depends on the recirculation rate. An undersized kettle will not have enough space at the sides of the bundle for good recirculation. Another effect is high entrainment or even a two-phase mixture going back to the tower.
Question - 59 : - Are Some Heat Transfer Services More Prone To Tube Vibration That Others For A Shell And Tube Exchanger?
Answer - 59 : -
Bundle vibration can cause leaks due to tubes being cut at the baffle holes or tubes being loosened at the tubesheet joint. There are services that are more likely to cause bundle vibration than others are. The most likely service to cause vibration is a single-phase gas operating at a pressure of 100 to 300 PSI. This is especially true if the baffle spacing is greater than 18 inches and single segmental.
Question - 60 : - Is There A Difference In Mtd (mean Temperature Difference) Between "e" And "j" (divided Flow) Type Shell And Tube Heat Exchangers?
Answer - 60 : -
Divided flow (shell type J) does not have the same correction as the usual flow pattern (shell type E). Thermal design program make this correction factor mistake. True, there is very little difference at correction factors above 0.90. However, there is a difference at lower values. For example, Equal outlet temperatures Shell type "E" correction Fn = 0.805 Shell type "J" correction Fn = 0.775 Cold outlet 5F higher than hot outlet Shell type "E" correction Fn = 0.765 Shell type "J" correction Fn = 0.65 Contact us if you do not have MTD correction factor charts for divided flow. TEMA has one chart for a single shell but it gives high values for the above examples and it is hard to read in this range.