Issues regarding cooling water consumption and cooling water outlet temperature
Thread Content
A shell-and-tube heat exchanger is used to cool compressed air with a flow rate of 300 Nm3/min (operating pressure of 0.7 MPaG) using cooling water at 32°C; the air temperature is to be reduced from 140°C to 40°C. It is necessary to calculate the amount of cooling water required. My question is that the cooling water outlet temperature is not specified; or rather, how is the cooling water outlet temperature generally determined? Process specified? We don’t have process specifications here. . Is it confirmed that when used in a certain factory, the entire factory has set values for the cooling water return temperature and pressure drop? It would be easier if the factory has a set return water temperature. What if not? In other words, how is the setting for the normal cooling water outlet temperature determined? The cooling water outlet temperature can be determined based on heat balance, making it easy to calculate the amount of cooling water required. PS: The questions are a bit messy; please forgive me.
A temperature rise of 8 degrees is generally chosen for cooling water design.
Is this temperature rise related to the cooling water inlet? For example, if the cooling water inlet temperature is 22°C, should the outlet temperature still have an increase of 8°C? What is the basis for this temperature increase of around 8℃? Why not 12 or some other number?
This is a complete heat exchange calculation. The information you provided and the questions you raised are indeed quite messy. After cooling the compressed air to 40 degrees, the temperature at the cooling water outlet depends on whether the cold fluid (cooling water) and the hot fluid (compressed air) flow in counterflow or co-current. Theoretically, if it is flowing downstream, the outlet temperature of the cooling water must be less than or equal to 40 degrees. If it is in reverse flow, the outlet temperature of the cooling water can be very high, even above 140 degrees (turning into steam). These are all extreme conditions. In fact, the design of heat exchangers aims to achieve the best and most economical outcome. It is impossible for us to design a very large heat exchanger that uses as little cooling water as possible to cool the compressed air. Therefore, during design, certain parameters can be set according to the circumstances in order to conduct calculations and achieve the most ideal design. For shell-and-tube heat exchangers, it is first necessary to determine which fluid will flow through the tube side and which through the shell side, to check whether the flow velocities are appropriate, to estimate the heat transfer coefficient, and to determine the temperature difference in order to calculate the required area of the heat exchanger. Based on my experience, generally in a counterflow configuration, an outlet temperature of 60 degrees for the cooling water should not be a problem; in a coflow configuration, it is advisable to keep the temperature at least below 38 degrees. For reference.
There is likely no similar design for an air compressor aftercooler available upstairs. Put it this way: the outlet temperature should be determined based on the requirements of the processes downstream of the cooling water. If there is no waste heat recovery system and the water simply returns to the cooling tower for reuse, then an outlet temperature of around 40 degrees is recommended, with a maximum of 42 degrees. For conventional shell-and-tube heat exchangers, you can design them with an outlet water temperature of 38–40 degrees. For shell-and-tube heat exchangers with high-efficiency heat transfer elements, such as those with internal finning, I generally design them for a temperature range of 40 to 42 degrees. During actual operation, the temperature difference between the cooling water inlet and outlet is higher than the designed value, typically ranging from 10 to 15 degrees. This is why the early Economic and Trade Commission listed them as **the first batch of water-saving products**. Finally, once the gas-side load and the temperature difference on the water side are determined, you can calculate the water flow rate.