Determination of heat exchanger design conditions
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The operating pressure on the shell side of the heat exchanger is 1.0 MPa, the operating temperature is 180/90, and the fluid is hot water. The operating pressure for the tube side is 1.0 MPa, and the operating temperature is 0/50°C; the medium used is methanol. A design pressure of 1.6 MPa is adopted for both sides. Is it reasonable to set the design temperature at 200°C for the shell side and 100°C for the tube side? What is the relationship between operating pressure and design pressure? Is there any other way to describe it besides the fact that the design pressure is higher than the operating pressure? Is it also classified based on nominal pressure?
Regarding the selection of design pressure, one can refer to the HG20584 five-in-one standard; the design pressure should be higher than the maximum operating pressure. It seems there is no distinction based on nominal pressure
The design pressure is generally taken as about 1.1 times the maximum operating pressure, while the design temperature is usually set about 15 degrees higher than the maximum operating temperature. Of course, heat exchangers are also designed based on nominal pressure, and generally, the standard construction drawings for heat exchangers are prepared using nominal pressure as a reference.
The design temperature is a parameter that determines the allowable stress of the selected material under operating conditions; it represents the maximum temperature that the material’s surface must not exceed. A value slightly higher than the operating temperature is sufficient. The design pressure should be equal to or greater than the highest operating pressure; when a safety valve is installed, this value should be 1.05 to 1.1 times the operating pressure. With a burst disc in place, the value is even higher, but it still does not reach 1.6 MPa. Using 1.6 MPa is too conservative.
The nominal pressure can be used as the design pressure, but an increase in the design pressure may lead to greater wall thicknesses for the cylinder, as well as for the flanges and tube sheets. Considering economic feasibility, I believe this is not a good approach.
The design pressure should be determined based on different operating conditions; there should be a distinction between internal pressure and vacuum. As for the design temperature, it should be determined by considering the wall temperature of the casing and pipes, along with a certain margin. The designer in this case has adopted a rather conservative approach, which naturally results in a higher cost.
The design pressure is generally set at 1.05 to 1.1 times the operating pressure. foreigners tend to be more conservative in this regard and choose a higher value; I believe this is due to cultural factors among foreigners. Perhaps the high cost of labor there makes it easier to standardize designs across different specifications, allowing for more replication of existing designs. Another factor is that costs do not have a particularly significant impact in such cases.
1. The design pressure should be based on the value specified above; it is generally taken as 1.1 times the maximum operating pressure. As for the nominal pressure, it is used to select the standard flanges for the equipment, and it should not be confused with the design pressure. 2. The heat exchangers designed using nominal pressure, as you mentioned, are generic designations used by manufacturers to reduce the number of different models available. They merely help in selecting the appropriate equipment (indicating the maximum operating pressure at various working temperatures). Reason: Once the product selection is finalized, the manufacturer will carry out a secondary design based solely on the parameters provided by the user; it is not possible to use such high safety factors throughout, so the thickness of the materials must be reduced where necessary (especially the thickness of the heat exchange tubes and tube sheets).