Calculation of the pumping capacity of a vacuum system
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This post was last edited by huazig on 2022-2-14 at 14:26. Dear colleagues, regarding the calculation of the gas extraction rate in vacuum systems, the formula provided in the \"Chemical Process Design Manual\" is as follows: Gas extraction rate of the vacuum system: Q = Q1 + Q2 + Q3. Where Q represents the total amount of gas extracted per hour in kg/h; Q1 is the amount of gas generated during the operation of the vacuum system per hour in kg/h; Q2 is the amount of gas released from the vacuum system per hour in kg/h; and Q3 is the amount of gas that leaks from the vacuum system per hour in kg/h. Among these, Q3 is the easiest to determine, so it will not be discussed here. The gas release rates of the Q2 vacuum chamber and vacuum components are generally negligible under improved process conditions, so they are not discussed here either. Q1 I used to calculate it this way: the theoretical amount of gas that actually exists (or the amount of non-condensable gas) under the temperature and pressure at the inlet of the vacuum system, based on the process medium ； If this value is not available, a process gas content of 3–5% is artificially added based on Q3. However, I came across online a method for calculating the volatility of process media based on Raoult’s law: The amount of vapor of the process material, W_material, is given by W_material = W_air * (M_material * P_material) / (M_air * P_air). The pressure in the vacuum system is P = P_material + P_air, where P_materiali = P * Xi. Here, W_material represents the amount of vapor of the process material in kg/h; W_air represents the amount of air that leaks out, also in kg/h; M_material is the average molecular weight of the material’s vapor; M_air is the molecular weight of air; P_material is the partial pressure of the material’s vapor; and P_air is the partial pressure of air ； P* is the saturated vapor pressure of the component ； Xi is the molar concentration of the component in the liquid phase. I calculated using the cases from the literature and the formula mentioned above: Case 1: Vacuum pressure: 9 kPa; Vacuum temperature: 18°C; Vacuum volume: 35 m3; Process fluids: acetic acid, chloroacetic acid, water (liquid-phase mole composition: 83.5% acetic acid, 3.23% chloroacetic acid, 13.27% water). According to theoretical calculations, there is no non-condensable gas in the process, so the total gas removal rate = amount of volatile gases + amount of leakage gas = 18.2 + 6.14 = 24.32 kg/h. Case 2: Vacuum pressure: 9 kPa; Vacuum temperature: 18°C; Vacuum volume: 35 m3; Process fluids: The same as in Case 1, with an additional small amount of non-condensable gases, namely acetic acid, chloroacetic acid, water, CH4, and H2 (liquid-phase mole composition: 83.5% acetic acid, 3.23% chloroacetic acid, 13.25% water, 0.01% CH4, 0.01% H2). Theoretical calculations show that there is a small amount of non-condensable gases—CH4, H2, water, and acetic acid. The amount of these non-condensable gases depends on the total flow rate of the process fluids, and it is likely to be much less than 6.14 kg/h in Case 1. Therefore, does the total amount of process gases in Case 2 still need to include the 6.14 kg/h calculated using the method from Case 1? This confuses me greatly; please advise, fellow netizens!
Your calculations consider the vacuuming process in isolation... Process balances need to be taken into account... Besides that, Q3 is the most uncertain factor; the rest are actually the easiest to determine. When you said that Q3 was clear, I knew you were working on a math problem rather than doing process design calculations… Based on the math problem you described, I think since the amount of medium has been increased based on Case 1, it’s natural to simply add it together……
Could you explain how Q1, Q2, and Q3 are calculated specifically? Seeking advice humbly: handshake
As a math problem, you’ve already done the calculations above... If it’s for process design calculations, then first you need to describe the process. You need to understand that in most processes, the vacuum is not created by vacuum pumps; rather, it arises from the design of the process itself. The vacuum pump is simply needed to maintain this vacuum level, and this applies to most processes such as evaporation and distillation. From a process design perspective, once it’s clear what problems can occur in the absence of vacuum, each of those problems can be treated as a parameter. Only then does the mathematical calculation process come into play!
First of all, I should clarify that I am from a process engineering field. The issues I bring up are intended to be discussed from a process engineering perspective, with the goal of understanding the pumping capacity of vacuum systems. I am not an expert in mechanical pumps or vacuum pump manufacturers, and I am not interested in the design of vacuum systems! After spending some time researching and discussing this issue with colleagues, it has been resolved. In keeping with the principle of seeing things through to the end, and not wanting to leave an incomplete post on our HaiChuan forum, I am sharing the results of my research below. Feel free to engage in objective and polite discussions if you have any questions! Vacuum system pumping capacity: Q = Q1 + Q2 + Q3. Q1: The amount of gas generated during the operation of the vacuum system. This refers to the amount of gas produced by the process medium under vacuum conditions; theoretically, Q1 consists of two components: 1) Non-condensable gases: These are the non-condensable gases that actually exist in the process, assuming no air leakage ； 2) Condensable vapor: refers to the condensable gases that result from the evaporation of the process liquid due to air leakage. The formula I provided earlier: W_material = W_air * (M_material * P_material) / (M_air * P_air) is used to calculate the second part, namely the condensable vapor, for Q1. Regarding the question I was previously struggling with about whether to stack them or not, the answer is that they should not be stacked. But as for the reasons behind this and the precautions when doing the calculations by hand, I don’t want to go into detail – it’s not easy to explain clearly. What I want to say here is that, in engineering design, as part of the process engineering field when determining the required pumping volume, the simplest and most accurate method for calculation is to input the amount of air that leaks in, as calculated as Q3, into the software used for vacuum simulation. By selecting the appropriate property parameters, the simulation software will calculate the actual total pumping volume, which includes the amount of air that leaks in (Q3), as well as the gas generated by the process medium (including non-condensable gases and condensable vapors). I would like to remind everyone that, whether or not there is non-condensable gas in your process medium before air is introduced, you still need to perform these calculations. Once air is introduced, it disrupts the existing balance, and more of the process medium will be carried away! Q2: Gas emission from vacuum chambers and vacuum components (from the \"Vacuum Design Manual\", P780), which includes three aspects: gas emission from the surfaces of materials in the vacuum system, gas that penetrates through the chamber walls from the atmosphere, and gas emitted as a result of evaporation/sublimation/decomposition of vacuum materials. I think this is something that the process engineering department doesn’t need to worry about; it should be the responsibility of the vacuum pump manufacturers ； Q3: Vacuum system leakage rate – This is covered in many books; there are roughly 4 calculation methods, which will not be detailed here. If there are any unclear points, please include more details!
I said it clearly on the 5th floor……
The original poster is really thoughtful; few people take the time to discuss issues in such a clear manner! After reading it, I would like to add and explain the following points for reference: 1) Q2: The outgassing rate of the vacuum system in kg/h. For high vacuums where pressures are on the order of 10^-something Pa, it’s necessary to consider the material’s outgassing rate. However, for pressures at the Pa or hPa level, this factor generally doesn’t need to be taken into account in the chemical industry ； 2) Q3: Vacuum system leakage rates are discussed in plenty in the manuals; most of those values are based on experiences from the last century and have been used ever since, so they are likely to be considerably higher than the actual values ； In practice, by accumulating more, we should be able to gain some experience of our own ； Most manuals seem to draw from the HEI standards regarding condenser vessels and leakage rate curves ； 3) Q1: The amount of gas generated during the operation of the vacuum system changes constantly as the inlet pressure and temperature vary during processes such as evaporation, distillation, and drying. The value we calculate at the design pressure and temperature should be the most stringent one ； As the pressure decreases, the non-condensable gases and substances with high saturated vapor pressures in the process material are largely released during this process ； The amount of gas generated by the process medium is likely to be the most difficult to calculate, especially when the process medium is a mixture of various substances, some of which are miscible while others are not. Manual calculations involve approximate estimations, whereas software can simplify this process ； At a given pressure and temperature, the amount of gas generated by the process medium is closely related to the amount of non-condensable gas; as the original poster mentioned, at the design pressure, P = P_non-condensable + P_condensable ； As I understand it, P_non-condensable = P_leakage + P_release_of_non-condensable_gases. Under normal circumstances, after a certain period of evacuation, the partial pressure of the non-condensable gases released by the process medium at the design pressure can be ignored and is not taken into account ； The leak flow rate remains constant when the pressure in the container is below 50 kPa abs; it is an estimated value, and as a result, the process condensable gases calculated based on this value are also not very accurate.
Thank you for your patient additional explanations; my understanding has improved further. Seeing that you mentioned the air leakage rate remains basically constant at 50 kPa shows that you have conducted in-depth research on this topic. thumbs up
According to theoretical calculations, there is no process-related non-condensable gas; therefore, the total pumping volume = amount of volatile gases + amount of leakage gas = 18.2 + 6.14 = 24.32 kg/h. Questioner: Is the amount of volatile gases really 18.2? ?
This is mentioned quite often in various books; there are roughly 4 calculation methods. What are these calculation methods? Could the original poster please explain it? Thank you.