Is there anyone expert in heat transfer calculations? A major issue regarding the calculation of heat exchange area!
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Use water as a medium to cool the flue gas. The heat exchange area is calculated using the heat transfer temperature and pressure method; in this calculation process, there are 4 parameters in total: flue gas inlet temperature, flue gas outlet temperature, water inlet temperature, and water outlet temperature. Among these 4 variables, the flue gas inlet temperature and the flue gas outlet temperature are given; the flue gas is cooled from 200°C to 140°C. The water inlet temperature and water outlet temperature are unknown; typically, the value of the water inlet temperature is assumed, and the water outlet temperature is calculated based on the heat transfer amount. This assumed method has a major problem! In other words, the heat exchange area is highly sensitive to the inlet water temperature; a change of 10°C in the inlet water temperature can cause a change of nearly 20% in the heat transfer temperature and pressure, which in turn leads to significant changes in the heat exchange area. In the actual calculation process, the inlet water temperature is selected within the range of 100°C to 160°C; different values for this parameter can result in a three-fold difference in the final heat exchange area! Dear experts, how can this situation be resolved? Is it necessary to assume an inlet water temperature in order to calculate the outlet water temperature? Is it possible to calculate the inlet water temperature using other methods, rather than relying on assumptions?
The inlet water temperature is a design parameter provided by Party A; it is possible to meet the heat exchange requirements under the most severe conditions； I suspect there’s an issue with your heat exchanger calculations, although I have no evidence!
Can water with an inlet temperature of 160°C cool flue gas to 140°C? It is recommended that you verify the temperature range of the cooling water inlet again.
I understand that the water temperature is determined by another system, not by the flue gas heat exchange system. The water here in the original post is circulated, right? The temperature of the inlet and outlet water is determined based on the requirements of other systems.
Water is in a cycle: in the flue gas cooler, water flows through the tube side while flue gas flows through the shell side; after being heated, the water proceeds to the next stage to release heat before circulating back again. The key issue now is that there are two unknowns regarding water: the temperature at the high-temperature end and the temperature at the low-temperature end; these two unknowns cannot be determined entirely based on thermal equilibrium
It’s a rough flowchart; the key issue at present is that the water temperature cannot be determined. Based on heat balance calculations, there are two unknowns (the temperature of the high-temperature working fluid and that of the low-temperature working fluid). One of these must be assumed first in order to calculate the other. However, the heat exchange area is also related to the temperature of the working fluids, so using assumptions for calculations results in large errors
Fellow, I had the same question as you regarding MGGH before
This post was last edited by arpcd on 2019-5-16 at 19:43. The simple flowchart drawn by the original poster illustrates the typical application of a MGGH (tubular GGH) in flue gas desulfurization. There are numerous problems that arise when GGHs are used in practice, which is why MGGHs are now gaining some market presence. Here is a simple approach for the original poster: it actually involves some basic engineering design parameters related to heat exchangers; for more detailed information, it’s best to consult the manufacturer of the MGGH directly. 1. First, it is necessary to clarify one concept: the heat load in the flue gas cooling section, Qc, is equal to the heat load in the flue gas reheating section, Qh. 2. According to the workflow diagram of the MGGH, the owner needs to identify the temperature parameters; there are six such parameters for the flue gas, four of which are independent variables (usually, the flue gas exit temperature is not an independent variable, while the other three can be considered independent variables). For hot water, there are only two parameters – the inlet temperature of the hot water in the cooling section corresponds to the outlet temperature of the hot water in the reheating section, which is what the owner refers to as the temperature of the low-temperature working fluid, t2. The outlet temperature of the hot water in the cooling section corresponds to the inlet temperature of the hot water in the reheating section, which is what the owner refers to as the temperature of the high-temperature working fluid, t1. It is possible to specify only one working fluid temperature; two temperatures cannot be specified. In other words, there is only one independent variable for the hot water temperature, and this is very important. For example, in your cooling section, if you specify that the flue gas temperature should be reduced from 170 to 120, resulting in a temperature drop of 50°C, then given the flue gas flow rate, the hot water circulation rate, and the inlet temperature of the hot water at t2, the outlet temperature of the hot water at t1 is determined. This is because, according to the heat balance principle, the heat released by the flue gas equals the heat absorbed by the hot water; the same principle applies to the reheating section as well. Whether it is a cooler or a heater, for a single-stage heat exchanger, from the perspective of pure counterflow heat transfer, the temperature difference between the high-temperature side and the low-temperature side should be kept approximately the same. For heat exchangers, the temperature difference between the hot and cold sides should not be too large; in the field of GGHs, this value generally should not exceed 10°C. For the cooling section, the temperature of the flue gas at the inlet T1 minus the temperature of the hot water at the outlet t1 is approximately equal to the temperature of the flue gas at the outlet T2 minus the temperature of the hot water at the inlet t2. For the reheating section, the temperature of the hot water at the inlet t1 minus the temperature of the flue gas at the outlet T3 is approximately equal to the temperature of the hot water at the outlet t2 minus the temperature of the hot water at the inlet T4. 3. Taking an example, based on the principles mentioned above, the parameters for an MGGH in a 550MW coal-fired power plant are as follows: The design parameters provided by the supplier to the owner show that, after taking into account the thermal losses in the equipment and pipelines as well as the efficiency of the heat exchangers, there is a difference of about 3°C in the temperature of the working fluid between the cooler and the reheater. In general engineering design, MGGH systems require the installation of steam pipes to compensate for heat losses (especially in cold winters in the north). Essentially, this is a mathematical problem involving the solution of a system of equations; this system has many variables, and its solutions are not unique. It is necessary to make assumptions based on engineering experience in order to determine the appropriate solution – and that is what design entails. . . The widespread use of MGGH at present for \"whitening\" in flue gas wet desulfurization is actually a costly and labor-intensive practice. . . . . As for the issue of the large heat exchange area that concerns the original poster, it’s easy to see from the table that if T2 is set at 116°C, then for the cooling section, the temperature difference at the cold end is only 4°C. In the example where T2 is set at 100°C, the corresponding temperature difference is 20°C; in that case, the area required for heat exchange is only 1/5 of yours. When it comes to cost estimates, the price of such a heat exchanger would be extremely high. These heat exchangers are huge, with areas in the tens of thousands of square meters, and since your heat exchange area is many times larger than theirs, how is that even possible? ? Be careful. . .