THERMAL DESIGN OF STEAM COIL FOR FORCED DRAFT COUNTER TO CROSS FLOW AIR COOLED HEAT EXCHANGER FOR EXTREMELY LOW AMBIENT CONDITION

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1 THERMAL DESIGN OF STEAM COIL FOR FORCED DRAFT COUNTER TO CROSS FLOW AIR COOLED HEAT EXCHANGER FOR EXTREMELY LOW AMBIENT CONDITION Parag Mishra 1 and Manoj Arya 2 1 Department of Mechanical Engineering, AISECT University Bhopal, Madhya Pardesh, India 2 Department of Mechanical Engineering MANIT Bhopal, Madhya Pardesh, India parag.mishra05@gmail.com ABSTRACT Air-cooled heat exchanger will take a serious freezing risk in cold winter, especially at a strong wind speed. Therefore, it is of great concern to study the anti-freezing of dry cooling system. There are several sites, in the world as well as in India, where the ambient temperature reaches below 0 0 C, at this temperature the fluid in heat exchanger freezes. In extremely cold environments, overcooling of the process fluid may cause freezing. If the fluid Freezes in Heat Exchanger, the heat exchanger ceases and it would damage the heat exchanger also. This may lead to tube burst, and hence protection from freezing is required to prevent plugging or damaging the tubes. There are various fluids such as Diethanolamine or Lean DEA which are used in industries like oil Refinery etc. that have serious problems of freezing. So, as to protect the fluid from freezing; we need equipment which maintains the temperature of ambient air & fluid according to their pour point & freezing temperature. For this, we can use the steam coil to raise the temperature of ambient air & fluid, by heating the working fluid. The function of steam coil is that, when the temperature reaches near the pour point of working fluid, the steam coil starts & passes the steam to increase the temperature of fluid up to the designed temperature. Steam Coil is used to heat atmospheric air to the required process temperature by means of saturated steam. By using steam coil with low pressure steam we can prevent the process fluid from freezing in Air Cooled Heat Exchanger. For air or fluid heating, steam is preferred medium for heat transfer throughout industry. Finned tubes are used in making of steam coil applications and this type of coil makes use of the latent heat that is released by the steam when it condenses so it is a very effective way of heating air. Keywords: air-cooled heat exchanger, steam coil, ambient temperature, lean DEA. INTRODUCTION Air-cooled heat exchanger will take a serious freezing risk in cold winter; there are several sites, in the world as well as in India, where the ambient temperature reaches below 0 0 C, at this temperature the fluid in heat exchanger freezes. In extremely cold environments, overcooling of the process fluid may cause freezing. This may lead to tube burst, and hence freeze protection is required to prevent plugging or damage to the tubes. For proper working of heat exchanger, in low ambient temperature or freezing temperature, a steam coil is used. The function of steam coil is that, when the temperature reaches near the pour point of working fluid, the steam coil starts and passes the steam to increase the temperature of fluid up to the designed temperature. By using steam coil with low pressure steam we can prevent the process fluid from freezing in air cooled heat exchanger. In facilities where steam is available a Steam Coil can be a cost effective way to heat ambient air and process fluid. Steam coil is just like a Tube Bundle of Heat Exchanger in which steam is flowing inside the tubes. The steam is coming in steam coil from air pre heater, which is the part of refinery or any other medium such as boiler or any other part of refinery. In steam coil, we flow the steam in tube and by fan we direct the steam to heat exchanger tube bundle for heating the tube bundle and ambient air to avoid the freezing of fluid. The steam coil is widely used for heating the fluid and air in industry. We use this steam coil in heat exchanger which is a part of Refinery, by which we protect the fluid from freezing and for proper functioning of Heat Exchanger. Steam flows inside the tube while air passes over the finned tubes. Finned tubes are almost always used in steam coil applications and this type of coil makes use of the latent heat that is released by the steam when it condenses so it is a very effective way of heating air. The selection of coil construction and materials is a multistep process that must take a number of factors into consideration. Types of steam based on pressure Low Pressure Steam (2.5 bar -4.5 bar) Intermediate Pressure Steam (4.5 bar -20 bar) High Pressure Steam (Above 20 bar) In this case the inlet pressure of steam is 2.5kgf/cm 2 A. In steam coil for heating the process fluid, we want a low pressure steam (2.5 bar -4.5 bar), which we will report to the client. This is a responsibility of client to provide this steam for proper functioning of Air cooled heat exchanger. For this type of heating process, we use low pressure steam as guided by API 661 & IBR. If we take medium pressure steam or high pressure steam for heating the fluid, the dimensions of steam coil like; length, tube thickness, number of tubes etc will increase & hence cost of steam coil will increase & there is no need of high or medium pressure steam for heating the process fluid. Low pressure steam is enough capable for heating the process fluid. We always take the dimension of steam coil, 5682

2 as +-500mm of Air cooled heat exchanger. After leaving the steam from steam coil, steam will become condensate & will be used further for steam formation. Steam will be generated from low pressure boiler. In this Air cooled heat exchanger, single row steam coil is used, because as per API guidelines maximum 2 rows are allowed. For proper working of heat exchanger, in low ambient temperature or freezing temperature, a steam coil is used. Problems with heat exchangers in low-temperature environments Heat Exchanger is designed on the basis of hot fluid temperature, cold fluid temperature & ambient temperature, but in practical sense, the ambient temperature changes throughout the year. In extremely cold environments, overcooling of the process fluid may cause freezing. This may lead to tube burst, and hence freeze protection is required to prevent plugging or damage to the tubes. If the fluid Freezes in Heat Exchanger, the heat exchanger ceases and it would damage the heat exchanger also. Here, we are designing the steam coil for Air Cooled Heat Exchanger, for heating the process fluid. Particularly in this design analysis extreme ambient temperature in winters is -28 o C. In this case, the steam is hot fluid and air is acts as a cold fluid. The steam coil design is accordingly at -28 o C ambient temperature. Tube pass arrangement The Tube Pass arrangement of this forced draft counter to cross flow heat exchanger is given below: Tube Pass Mean Metal Temperatures Mean tube metal temperature in each tubepass, (Deg C) Tuberow Tubepass Inside Outside Radial Figure-1. Tube pass draft. Tube Pass 1276 mm Outer Wall Transverse Diameter Thickness Pitch Name Type (mm) (mm) (mm) 1 TubeType1 High-finned Wall Number Clearance Number Row of Tubes Tube Type (mm) Row of Tubes Longitudinal Fin Pitch Height (mm) (mm) Wall Clearance Tube Type (mm) Bundle Information Bundle width mm Number of tube rows 2 Number of tubes 22 Minimum wall clearance Left mm Right mm Number of tubes per pass Tubepass # 1: TubeType TubeType Figure-2. Rating horizontal air-cooled heat exchanger. 5683

3 Design of steam coil for forced draft counter to cross flow air cooled heat exchanger for extreme ambient condition - under extremely low ambient condition (-28 o C) In this case, we design the steam coil for Air Cooled Heat Exchanger, for heating the process fluid. We know that the extreme ambient temperature in winters is - 28 o C. The most important parameter while taking into consideration for designing Air Cooled Heat Exchanger is permissible /minimum tube skin temperature and approach temperature. In this case, the steam is hot fluid & air is acts as a cold fluid. Process parameters for design of steam coil for extreme ambient temperature i.e. at -28 o C, the process parameters are: a) Flow rate of hot fluid (steam) = (1000- kg/h) b) Flow rate of cold fluid (air) = (1000-kg/h) c) Inlet temperature of hot fluid (steam) = o C d) Outlet temperature of hot fluid (steam) = o C e) Inlet temperature of cold fluid (air) = -28 o C f) Allowable pressure drop of hot fluid = kgf/cm 2 g) Inlet pressure of hot fluid = 2.5kgf/cm 2 h) Altitude (a) (b) Figure-3. Approach temperature of counter to cross flow heat exchanger. From output summary 1.1 and 1.2 we come to know the following parameters of steam coil at low ambient temperature i.e. -28 o C Minimum tube skin temperature = o C Hot fluid (steam) enters in ACHE = t hin = o C Hot fluid (steam) leaves the ACHE = t hout = o C Cold fluid (air) enters in ACHE = t cin = - 28 o Cold fluid (air) leaves the ACHE = t cout = -1 o C Change in hot fluid (lean dea) temperature, Δt h = t hin- t hout = =1.05 o C Change in cold fluid (air) temperature, Δt c = t cout- t cin = -1-(-28) = 27 o C Approach Temperature = ᶿ m1 = t hout- t cin = (-28) = o C Approach Temperature = ᶿ m2 = t hin - t cout = (-1) = o C There are two different approach temperatures, but in counter to cross flow heat exchanger, we consider the approach temperature = ᶿ m1 = t hout - t cin = (-28) = o C In the designing of steam coil the most important parameter is tube skin temperature. The minimum tube skin temperature of steam coil as per API 661 guidelines is pour point of fluid (0 O C) plus API margin (9 o C), Therefore, Minimum tube skin temperature = pour point of steam (water) + 9 o C API Margin = 0 o C+ 9 o C = 9 o C But, in this case minimum tube skin of hot fluid (steam) = o C. Hence, this is far safer limits as per guidelines. Hence design of steam coil is safe and up to the mark. 5684

4 Table-1. Output summary obtained. Output Summary Process Conditions Outside Tubeside Fluid name Steam Fluid condition Sens. Gas Cond. Vapor Total flow rate (1000-kg/hr) Weight fraction vapor, In/Out Temperature, In/Out ( o C) Skin temperature, Min/Max ( o C) Pressure, Inlet/Outlet (kgf/cm 2 A) Pressure drop, Total/Allow (mmh 2 O) (kgf/cm 2 ) Midpoint velocity (m/s) In/Out (m/s) e-2 Heat transfer safety factor (--) 1 1 Fouling (m 2 -hr-c/kcal) Exchanger Performance Outside film coef (kcal/m 2 -hr-c) Actual U (kcal/m 2 -hr-c) Tubeside film coef (kcal/m 2 -hr-c) Required U (kcal/m 2 -hr-c) Clean coef (kcal/m 2 -hr-c) Area (m 2 ) Hot regime Cond. Vapor Overdesign (%) 5.25 Cold regime Sens. Gas Tube Geometry EMTD ( o C) Tube type High-finned Duty (MM kcal/hr) Tube OD (mm) Bundle Airside Pressure Drop, % Ground clearance 0.00 Fan guard 1.54 Fan ring 5.71 Fan area blockage Steam coil analysis for optimal temperature By analyzing the data, when the ambient temperature is 38 o C, the minimum tube skin temperature is o C and when we go to another extreme i.e. at -28 o C, the minimum tube skin temperature is o C. and by examining the API 661 guidelines, we come to know, that for designing of Air cooled heat exchanger, the permissible tube skin temperature is pour point of the fluid (lean dea) and 9 o C of API margin, i.e. for lean dea the permissible tube skin temperature =17 o C, but when the ambient temperature falls to -28 o C, the tube skin temperature goes on o C, so we cannot afford the temperature of tube skin below 17 o C as per guidelines. In this case we are analyzing, at what temperature we start the steam coil, because if the fluid in the heat exchanger freezes, more steam and more auxiliary power (fan, motor, pump) is required to defreeze the fluid and it will also affect the performance of heat exchanger. So, we have to find the optimum temperature to start the steam coil, for optimum use of steam and auxiliary power without affecting the performance of heat exchanger. In this case, we conduct the run test of steam coil on various temperatures. 5685

5 Table-2. Output summary HTRI Company. Output Summary Process Data Airside Tubeside Fluid name Steam Fluid condition Sens. Gas Cond. Vapor Total flow rate (1000-kg/hr) Weight fraction vapor, In/Out (--) Temperature, In/Out ( o C) Skin temperature, Min/Max ( o C) Wall temperature, Min/Max ( o C) Pressure, In/Out (kgf/cm 2 A) Pressure drop, Total/Allowed (mmh 2 O) (kgf/cm 2 ) Pressure Drop, A-frame reflux section (kgf/cm 2 ) Velocity - Midpoint (m/s) In/Out (m/s) e-2 Film coefficient, Bare/Extended (kcal/m 2 -hr-c) Mole fraction inert (--) Heat transfer safety factor (--) 1 1 Fouling resistance (m 2 -hr-c/kcal) Overall Performance Data Overall coef, Design/Clean/Actual (kcal/m 2 -hr-c) / / Heat duty, Calculated/Specified (MM kcal/hr) / Effective mean temperature difference ( o C) Table-3. Temperature readings obtained. S. NO Temperature ( o C) Minimum tube skin temperature ( o C)

6 We always have to maintain the approach temperature for proper design and performance of Air Cooled Heat Exchanger. So, by the run test of steam coil, we come to know that at -1 o C temperature, the minimum tube skin temperature is o C, and by API guidelines we know that the minimum tube skin temperature is 17 o C. So, we start the steam coil at -1 o C that is when the ambient temperature reaches -1 o C, we start steam coil, at this temperature, the fluid in the heat exchanger starts to freeze. We start the steam coil before fluid starts freezing in the heat exchanger. With the help of steam coil we maintain the approach temperature, tube skin temperature and performance of air cooled heat exchanger. Objectives a) The main objective of this thesis is that at low ambient temperature, the fluid does not freeze in the heat exchanger as; it would damage the heat exchanger. b) To maintain and enhance the performance of Heat Exchanger. c) Power saving is achieved, as we start steam coil when the fluid just starts to freeze. d) To give the thermal design & procedure of steam coil for air cooled heat exchanger at normal ambient temperature e) To discuss the importance of steam coil in Air Cooled Heat Exchanger for low ambient temperature. f) To discuss the importance of steam as a heating medium in air cooled heat exchanger. g) To discuss about protection of heat exchanger under freezing temperature. This is the thermal design of steam coil for protecting the fluid from freezing and maintaining the performance of Air cooled heat exchanger. During heat exchanging process, steam loses its heat from o C to o C and cold fluid (air) gains the heat from -28 o Cto -1 o C. As we know, the pour point of steam is 0 o C and considering API margin, the minimum tube skin temperature is 9 o C. From results 12.3 and at -28 o C ambient temperature, the permissible tube skin temperature is o C, which is far greater than the permissible tube skin temperature (17 o C), So, the design and performance of steam coil for Forced Draft Counter to Cross Flow Air Cooled Heat Exchanger is safe and hence the fluid in the heat exchanger does not freeze; hence there is not a problem in design. Steam coil will maintain the Heat exchanger approach temperature and heat exchanger tube skin temperature without compromising the performance of heat exchanger, And then a question arises that at what temperature we start the steam coil for heating the fluid to achieve maximum auxiliary power saving, energy saving and lower use of steam. 5687

7 Table-4. Results for thermal design of steam coil for air cooled heat exchanger under extremely low ambient condition i.e. (-28 o C) Final Results Process Conditions Outside Tubeside Fluid name Steam Fluid condition Sens. Gas Cond. Vapor Total flow rate (1000-kg/hr) Weight fraction vapor, In/Out Temperature, In/Out ( o C) Skin temperature, Min/Max ( o C) Pressure, Inlet/Outlet (kgf/cm 2 A) Pressure drop, Total/Allow (mmh 2 O) (kgf/cm 2 ) Midpoint velocity (m/s) In/Out (m/s) e-2 Heat transfer safety factor (--) 1 1 Fouling (m 2 -hr-c/kcal) Exchanger Performance Outside film coef (kcal/m 2 -hr-c) Actual U (kcal/m 2 -hr-c) Tubeside film coef (kcal/m 2 -hr-c) Required U (kcal/m 2 -hr-c) Clean coef (kcal/m 2 -hr-c) Area (m 2 ) Hot regime Cond. Vapor Overdesign (%) 5.25 Cold regime Sens. Gas Tube Geometry EMTD ( o C) Tube type High-finned Duty (MM kcal/hr) Tube OD (mm) Bundle Airside Pressure Drop, % Ground clearance 0.00 Fan guard 1.54 Fan ring 5.71 Fan area blockage

8 Table-5. Rating air cooled heat exchanger. Final Results Process Data Airside Tubeside Fluid name Steam Fluid condition Sens. Gas Cond. Vapor Total flow rate (1000-kg/hr) Weight fraction vapor, In/Out (--) Temperature, In/Out ( o C) Skin temperature, Min/Max ( o C) Wall temperature, Min/Max ( o C) Pressure, In/Out (kgf/cm 2 A) Pressure drop, Total/Allowed (mmh 2 O) (kgf/cm 2 ) Pressure Drop, A-frame reflux section (kgf/cm 2 ) Velocity - Midpoint (m/s) In/Out (m/s) e-2 Film coefficient, Bare/Extended (kcal/m 2 -hr-c) Mole fraction inert (--) Heat transfer safety factor (--) 1 1 Fouling resistance (m 2 -hr-c/kcal) Overall Performance Data Overall coef, Design/Clean/Actual (kcal/m 2 -hr-c) / / Heat duty, Calculated/Specified (MM kcal/hr) / Effective mean temperature difference ( o C) After that we did the analysis that at what temperature we start the steam coil. From Research paper, Computational thermal design of forced draft counters to cross flow Air Cooled heat exchanger at normal ambient temperature i.e. at 38 o C and from research paper Performance & Design Analysis of forced Draft counter to cross flow Air cooled heat exchanger at extremely low ambient temperature i.e. at -28 o C, when ambient temperature is 38 o C, the minimum tube skin temperature is o C and when we went to another extreme condition i.e. -28 o C, the minimum tube skin temperature is o C. So we cannot afford the temperature of tube skin below 17 o C as per guidelines. We found the optimum temperature to start the steam coil, for optimum use of steam and auxiliary power and for without affecting the performance of heat exchanger. We started the steam coil at different ambient temperatures and performed the run test of steam coil and tabulated the data corresponding to the tube skin temperature. After the run test of steam coil, at -1 o C ambient temperature, the minimum tube skin temperature is o C, and by API guidelines the minimum tube skin temperature is 17 o C. So, we start the steam coil at -1 o C, i.e. when the ambient temperature reaches -1 o C, we start steam coil. At this temperature, the fluid in the heat exchanger start to freeze, but before the fluid start to freeze we turn on the steam coil. CONCLUSIONS In the thermal design of steam coil for Counter to cross flow Air Cooled Heat Exchanger, we come to know that the tube skin temperature of steam coil at- 28 o C is o C, which is far greater than the permissible tube skin temperature (17 o C). So, the thermal design of steam 5689

9 coil for Forced Draft Counter to Cross Flow Air Cooled Heat Exchanger is safe and hence the fluid in the heat exchanger does not freeze, hence there is not a problem in design. We found the optimum temperature to start the steam coil, for optimum use of steam and auxiliary power and without affecting the performance of heat exchanger. We start the run test of steam coil at different temperatures. So, by the run test of steam coil, we concluded that at -1 o C temperature, the minimum tube skin temperature is o C, and by API guidelines we come to know that the minimum tube skin temperature is 17 o C, so we start the steam coil at -1 o C, that is when the ambient temperature reaches -1 o C. And finally we know that at -1 0 C we have to start the steam coil to maintain the performance of heat exchanger and doing the thermal analysis of forced draft counter to cross flow Air cooled heat exchanger we know that the tube skin temperature at - 1 o C is o C, which is nearly greater than the permissible tube skin temperature (17 o C), So, the design and performance of Forced Draft Counter to Cross Flow Air Cooled Heat Exchanger is safe. OUTCOME These are expected outcomes: a) By the use of steam coil we maintain the Heat exchanger approach temperature and heat exchanger tube skin temperature without compromising the performance of heat exchanger b) Steam coil, heats the heat exchanger fluid and ambient air, so the fluid in the exchanger does not freezes. c) Performance of Air cooled heat exchanger is increased. 4. More power saved. d) At extremely low ambient temperature the fluid in the heat exchanger freezes. For this a steam coil was required for heating the fluid, so that the tube skin temperature is maintained at 17 o C. So we designed a steam coil to prevent the fluid from freezing without affecting the performance of air cooled counter to cross flow heat exchanger. REFERENCES Parag Mishra, Dr Manoj Arya Performance & Design Analysis of forced Draft counter to cross flow Air cooled heat exchanger at extremely low ambient temperature i.e. at -28 o C. International Journal on Emerging Technologies. 7(02): Parag Mishra Dr Manoj Arya Computational thermal design of forced draft counters to cross flow Air Cooled heat exchanger at normal ambient temperature i.e. at 38 o C. International Journal of Current Research (IJCR). 8(08): Parag Mishra, Dr Manoj Arya A Review of Literature on Steam Coil for Air Cooled Heat Exchanger. International Journal of Innovations in Engineering and Technology (IJIET). 7(1): Parag Mishra, Dr Manoj Arya A Review of Literature on Thermal Design of Forced Draft Counter to Cross flow Air Cooled heat exchanger. International Journal of Engineering Sciences & Research Technology. 5(4): Rohit S. Andhare, Amir Shooshtari, Serguei V. Dessiatoun, Michael M. Ohadi Heat transfer and pressure drop characteristics of a flat plate manifold microchannel heat exchanger in counter flow configuration. Journal of Applied Thermal Engineering. 96: Haitao Hu, Xiaomin Weng, Dawei Zhuang, Guoliang Ding, Zhancheng Lai, Xudong Xu Heat transfer and pressure drop characteristics of wet air flow in metal foam under dehumidifying conditions Applied Thermal Engineering. 93: Parag Mishra, Dr Manoj Arya Auxiliary Power Saving In Air Cooled Heat Exchanger by Fans. International Journal of Enhanced Research in Science Technology & Engineering (IJERSTE). 4(11): Parag Mishra, Dr Manoj Arya Auxiliary Power Saving In Air Cooled Steam Condenser by Pumps Condenser (A Heat Exchanger Used in Steam Power Plant). International Journal of Enhanced Research in Science Technology & Engineering (IJRASET). 3(11): Alireza Vali, Gaoming Ge, Robert W. Besant, Carey J. Simonson Numerical modeling of fluid flow and coupled heat and mass transfer in a counter-cross-flow parallel-plate liquid-to-air membrane energy exchanger. International Journal of Heat and Mass Transfer.89: Parag Mishra Dr Manoj Arya Review of Literature on Air Cooled Steam Condenser (A Heat Exchanger Used in Steam Power Plant). International Journal of Research in Aeronautical & Mechanical Engineering (IJRAME). 3(10): 1-8. Parag Mishra, Dr Manoj Arya A Review of Literature on Air Cooled Heat Exchanger. International Journal of Latest Trends in Engineering & Technology (IJLTET). 5(4): Rafat AL-Waked, Mohammad Shakir Nasif, Graham Morrison, Masud Behnia CFD simulation of air to air enthalpy heat exchanger: Variable membrane moisture resistance. Journal Applied Thermal Engineering. 84:

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