Horizontal Shell and Tube Heat Exchanger

Table of Contents:

Nomenclature. Pg. 3 Introduction and Background Pg. 4
Experimental Methodology Equipment and Apparatus Pg. 6 Experimental Procedures Pg. 7 Results Pg. 8 Analysis and Discussion……………………………………………………………………..Pg. 11 Summary and Conclusions Pg. 12 References Pg. 13 Appendices Pg. 14

Nomenclature Symbol | Term | Units | A | Heat transfer surface area for the tubes | Inches2 (in2) | Cp | Heat Capacity | J/(mol*K) | F | Correction Factor | __ | | Heat | W | c | Cold Side Heat Duty | W | H | Hot Side Heat Duty | W | Shell Side | Hot Side | __ | T | Temperature | Celsius | ∆T | Change in Temperature | Celsius | Tube Side | Cold Side | __ | ∆Tlm | Log mean temperature difference | Kelvin (K) | U | Heat Transfer Coefficient | W/(K*in2) | V | Volume | L | ṁH | Hot water flow rate | L/min | ṁC | Cold water flow rate | L/min |

1.0 Introduction and Background
A heat exchanger is a device designed to efficiently transfer thermal energy from one fluid to another fluid, which can be a liquid or a gas [1]. These fluids do not mix or come into direct contact with each other. Even though all heat exchangers do the same job of passing heat from fluid to fluid, there are various types that work in many different ways.
The two most common types of heat exchangers are the shell-and-tube and plate/fin. In shell-and-tube heat exchangers, one fluid flows through a set of metal tubes while the second fluid passes through a sealed shell that surrounds them. The two fluids can flow in the same direction, parallel flow, in opposite directions, counter flow, or at right angles, cross flow. Plate/fin heat exchangers have a lot of thin metal plates or fins with a large surface area causing the heat to exchange more rapidly [2].
Shell-and-tube heat exchangers are the most versatile heat exchangers, made from a variety of metal and nonmetal materials and range in size from 1 ft2 to 106 ft2 surface area [1]. Shell-and-tube heat exchangers are designed for practically any operating conditions or capacity, ranging from no pressure to ultra-high pressures and from low to high temperatures. Different materials of construction are the only limitation to temperature and pressure differences between the fluids [1]. They can be designed for various operating conditions such as erosion, corrosion, toxicity, radioactivity, and vibration.
A shell-and-tube heat exchanger has tubes running through a cylindrical shell, parallel to the shell. While one fluid runs through the inside of the tubes, the other runs along the outside. This allows for heat transfer without mixing of the fluids [1]. As the cold fluid flows within the tubes, heat is transferred from the hot fluid to the cold, creating a temperature change. The more tube-side passes there are within the shell, the more efficient the heat transfer [1].
Shell-and-tube heat exchangers are widely used in industry. They can be used as process heat exchangers in petroleum-refining and chemical industries; as steam generators, condensers, boiler feed water heaters, and oil coolers in power plants; as condensers and evaporators in air-conditioning and refrigeration processes; in waste-heat recovery applications; and in environmental controls.
A small shell-and-tube heat exchanger was used during this lab. The effects of increasing the tube-side flow rate and shell-side flow rate on the heat duties of cold and hot sides were analyzed, including the heat transfer coefficient. The effects of different cold side flow rates while maintaining the hot side flow rate on the outlet temperatures of the both streams were analyzed. Similarly, the cold side flow rate was maintained with varying hot side flow rates to determine the effects of these conditions on the outlet temperatures of the streams. The results of both of these conditions were recorded and used to assess the heat duties of the hot and cold side as well as the heat transfer coefficient.

2.0 Experimental Methodology 3.1 Equipment and Apparatus Figure 2. Chiller
Figure 2. Chiller
Pump
Pump
Heat reservoir
Heat reservoir
Small shell heat exchanger
Small shell heat exchanger
Cold flow out
Cold flow out
Cold flow valve
Cold flow valve
Cold flow in
Cold flow in
Hot flow valve
Hot flow valve
Thermocouple
Thermocouple
Hot flow in
Hot flow in
Hot flow out
Hot flow out
Figure 1. Heat Exchanger
Figure 1. Heat Exchanger
Chiller
Chiller
Water
storage tank

Water storage tank

Cooling radiator Cooling radiator Lab View
Lab View
Figure 3. Lab View Program Referring to Figure 1, there are two inlet streams and two outlet streams for the small shell-and-tube heat exchanger. These streams are specifically labeled as hot flow in, hot flow out, cold flow in, and cold flow out. The heat reservoir increased the temperature of the shell side as it passed through, while the cooling radiator decreased the temperature of the shell side as it passed through. If there is not enough water available the flow control valve/Lab View will regulate the flow rate as necessary. The pump on the platform is for the hot side flow, while the pump contained in the chiller is for the cold side flow. The hot flow in and cold flow in are both regulated by the hot flow and cold flow valves respectively. Temperature sensors, labeled as thermocouples, are located at the specific colored stars. The yellow, green, orange, and red stars represent the cold in, cold out, hot in, hot out temperature thermocouple readings, respectively. These thermocouples measured the temperature of both cold and hot inlet and outlet streams. The actual heat exchanging process occurred where the two inlet streams came into close contact with each other in the small shell heat exchanger. In order to record all of the necessary data a program called Lab View was used, which can be seen in Figure 3. This program shows percentage of opening of the valves, flow rates of both hot and cold streams, and temperatures of all inlet and outlet streams. 3.2 Experimental Procedure
In order to start the heat transfer process the hot and the cold-water valves were opened to let the temperatures reach steady state. Steady state was reached when the temperature values changed by less than .5 degrees Celsius after two minutes. While steady state was being reached, the hot water valve was set at 50% open and the cold-water valve was set at 10% open. After this, the hot water valve was kept at 50% open while the cold-water valve was varied between 10%, 30%, 60%, and 100% open. The temperature of the hot and cold water entering and leaving the heat exchanger was measured with the varying cold water valve openings until each combination had reached steady state. Time was measured to determine how long it took to reach steady state at each combination.
Furthermore, the cold-water valve was kept steady at 50% open, while the hot water valve was set to be 10%, 30%, 50%, and 70% open, respectively. Time to reach steady states and flow rates were also recorded. During this process the cold-water temperature was set at 20 degrees Celsius by the chiller, while the heater maintained a constant energy level but varied in temperature.

3.0 Results Tables 1-3 present data found in the experimentation. Further explanation of the trends presented in this section will be discussed in detail in the Discussion section of this lab report.

Table 1. Corresponding steady-state temperatures in degrees Celsius and flow rates in liters per minute at different valve positions for tube and shell-side flows. | Run | Temp In Cold (OC) | Temp In Hot (OC) | Temp outCold (OC) | Temp out Hot (OC) | Flow rate Cold (L/min) | Flow rate hot (L/min) | Objective 150%H | 10%C | 20.2 | 47.39 | 30.5 | 41.18 | 1.26 | 5.7 | | 30%C | 20.14 | 41.18 | 24.96 | 35.41 | 2.83 | 5.63 | | 60%C | 20.17 | 39.38 | 23.5 | 33.11 | 4.18 | 5.55 | | 100%C | 20.26 | 38.54 | 23.17 | 32.47 | 4.78 | 5.61 | Objective 250% C | 10%H | 20.19 | 47.6 | 23.66 | 33.27 | 3.87 | 2.15 | | 30%H | 20.17 | 42.08 | 23.75 | 33.56 | 3.87 | 3.84 | | 50%H | 20.33 | 40.01 | 23.96 | 33.73 | 3.815 | 5.4 | | 70%H | 20.44 | 38.83 | 24.0 | 33.58 | 3.87 | 6.47 |

While the shell-side valve was kept constant and the tube-side was increased, the flow rate of the cold side increased while the other flow rate stayed the same. While the tube-side was held constant and the shell-side was increased, the flow rate of the hot side increased. | Run | Heat Duty (Cold): C [W] | Heat Duty (Hot): H [W] | Efficiency: [%] | Coefficient of Heat Transfer [W/(K*in2)] | Objective1(50% H) | 10% C | 905 | -2468 | 36.7 | 1.00 | | 30% C | 951 | -2513 | 37.8 | 1.23 | | 60% C | 979 | -2427 | 40.3 | 1.42 | | 100% C | 970 | -2375 | 40.8 | 1.47 |

Table 2. Corresponding heat duties, efficiencies, and coefficients of heat transfer for a constant shell side valve position.

Table 3. Corresponding heat duties, efficiencies, and coefficients of heat transfer for a constant tube side valve position. | Run | Heat Duty (Cold): C [W] | Heat Duty (Hot): H [W] | Efficiency: [%] | Coefficient of Heat Transfer [W/(K*in2)] | Objective 2(50% C) | 10% H | 936 | -2148 | 43.6 | 1.09 | | 30% H | 966 | -2281 | 42.3 | 1.28 | | 50% H | 966 | -2365 | 40.8 | 1.37 | | 70% H | 961 | -2369 | 40.6 | 1.43 |

Figure 4. This plot shows the relationship between the flow rates of the cold stream in liters per minute to the heat transfer coefficient while the hot side was held constant.

Figure 5. This plot shows the relationship between the flow rates of the hot stream in liters per minute to the heat transfer coefficient while the cold side was held constant.

Figure 4 shows that an increase in the flow rates of the cold stream while the hot stream remained constant corresponds to an increase in the heat transfer coefficient. Figure 5 shows that an increase in the flow rates of the hot stream while the cold stream remained constant also corresponds to an increase in the heat transfer coefficient. 4.0 Analysis and Discussion of Results
During the first trial, the shell-side valve position remained constant while the tube-side value position varied. From the data obtained during the experiment, the heat duties, efficiencies, and coefficients of heat transfer were calculated and can be seen in Table 2 of the Results section. Sample calculations for this objective can be seen in Appendix A.1. Two trends were easily observable after performing these calculations. First, as the flow rate of the tube-side increased, the efficiency of heat transfer increased. This is due to the fact that more cold fluid was traveling through the tube per unit time; therefore, the cold fluid was able to absorb more heat. This trend also applied to the coefficient of heat transfer, which also increased as a function of increasing tube-side flow rate. The correlation between these values can be seen in Figure 4 of the Results section as well.
For the second trial, the tube-side valve position was held constant while the shell-side valve position was altered. As with the first trial, the experimental data for this trial was used to calculate the heat duties, efficiencies, and coefficients of heat transfer. Sample calculations for this experiment can be seen in Appendix A.2 of this report, and the values of the corresponding calculations can be seen in Table 3 of the Results section. When increasing the flow rate of the hot liquid (shell-side), the coefficient of heat transfer increased as well. This was due to the fact that there was an increase in hot liquid passing through the small shell-and-tube heat exchanger per second; therefore, more heat could be transferred. The correlation for this trend can be seen in Figure 5 of the Results section. On the contrary, the efficiency of heat transfer decreased with an increase in the hot liquid flow rate. This seems to contradict what was expected; however, a closer examination of the shell-and-tube heat exchanger will make this point more clear.
The cold fluid flows through the inner portion and the hot liquid flows through the outer portion of the shell-and-tube heat exchanger. When the hot valve position was held constant in the first trial, the heat lost by the hot liquid was able to equilibrate with the surroundings. This means that any change in temperature would be due to adjusting the tube-side valve position. In the second trial, the cold liquid flow rate was held constant. Since these tubes are located on the inner portion of the heat exchanger, it was only able to equilibrate within the system. When the hot liquid flow rate increased, both the cold liquid and the surroundings gained heat. With that being said, the efficiencies of heat transfer were very low. This was due to the large exterior surface area of the exchanger and small areas of contact between the hot and cold liquids. The large exterior surface area on the exchanger caused heat to be lost to the surroundings. 5.0 Summary and Conclusion
The first technical objective was to evaluate the effect of tube-side flow rate at 10%, 30%, 60%, 100% and the overall heat transfer coefficient of the heat exchanger. The second technical objective was similar to the first; however, this time the shell-side flow rates were varying while the tube-side flow rate remained constant at 50%. From experimental results and analysis it can be seen that for both objectives, varying the flow rates of either the shell-side or tube-side caused changes in efficiency and coefficient of heat transfer.
Upon further analysis of the first objective, trends started to form between tube-side flow rates, efficiency, and coefficient of heat transfer. These trends showed a positive linear relationship between the cold flow and the heat transfer coefficient. As the flow rate increased so did the heat transfer coefficient. The same relationship was noticed with the hot flow and the heat transfer coefficient. As the flow rate increased in both cases, the ability for the cold water to absorb heat from the hot water, through the shell, increased and vice versa. This was because there was simply more cold water for the heat to dissipate into.
When analyzing the results of the varying shell-side flow rates, different results can be seen. Increasing the flow rate of the shell-side produced an increase in coefficient of heat transfer, which was a similar trend to that of the varying tube-side flow rate part of this experiment. On the other hand, increasing the flow rate of the shell-side did not increase efficiency. This was due to heat loss from the hot water to the surroundings as well as the cold water because of the shell-side being the outer portion of the heat exchanger.
To obtain maximum efficiency in a small or large-scale heat exchange process it is imperative that shell-side flow rate is held constant, while the tube-side varies. This will produce maximum efficiency because less heat is lost to the surroundings (heat exchanger). One solution to make varying the shell-side flow rates more efficient would be to use a more insulated heat exchanger. In an industrial environment, this could reduce heat loss and be as effective as using varying tube-side flow rates. Another method to increase the overall efficiency would be to have cold fluid flow through the shell-side, thus reducing the heat loss to the surroundings. 6.0 References [1] Shah, Ramesh K., and Dusan P. Sekulic ́. "Fundamentals Of Heat Exchanger Design." (n.d.): n. pag. Http://engbag.org. Web. 5 Mar. 2015. [2] Basic Construction of Shell and Tube Heat Exchangers. Rep. Alfalaval, n.d. Web. 26 Feb. 2015. <local.alfalaval.com/en-us/key-technologies/heat-transfer/shell-and-tube-heat-exchangers/process-industrial/Documents/TEMA%20basics%20of%20construction%20-%2007.10.pdf>. [3] Thome, John R. "Construction of Shell and Tube Heat Exchangers. “Engineering Data Book III. N.p.: Wolverine Tube, 2004. 32-39. Print.