09-04-16 Topic — Selection of fouling coefficients and area margins in thermal calculations (prize for participation)

Analysis of the impact of heat exchanger fouling thermal resistance on design: http://info.178b2b.com/2007554.htm Let’s take a look together, and we look forward to even more interesting responses below!

Let’s start with the easier ones to talk about. 1. The area margin is generally set at 10~20%. 2. Refer to the manual on fouling thermal resistance; of course, Chen Minheng’s book \"Principles of Chemical Engineering\" on page 282 also provides information on the thermal resistance values for various types of water, water vapor, air, tar, fuel oil, and \"organic substances\". Dirt accumulation occurs gradually; therefore, as time of use increases, the heat transfer coefficient decreases. If the application is for heating, then as usage duration grows, the energy consumption of the heat source increases. Sometimes, it is necessary to choose between regular descaling or replacing the heat exchange tubes, depending on the degree of scaling. 3. The selection of environmental conditions should generally follow those under the most severe conditions. Generally, in processes where strict requirements are placed on the outlet temperature of heat exchangers, automatic control is implemented; thus, choosing harsh environmental conditions does not have any adverse effects on the process, although it does increase the manufacturing costs of the equipment. Everyone is welcome to point out any mistakes.

For the issue of design margin, a percentage range of 15 to 20 is generally adopted, though this can vary depending on the circumstances. More experienced engineers might choose a smaller value when considering costs based on past examples. In actual operation, the requirements of the process should still be met, as this margin is intended to account for various unstable factors. In cases where there are no previous examples and the design isn’t entirely reliable, a higher margin of 20% might be necessary. As for the selection of heat transfer resistance, it’s relatively easier to determine it in situations involving cooling gases or heat exchange between gases, since the proportion of heat resistance in such cases isn’t very large, so minor errors aren’t a problem. However, in cases of night-time heat exchange or situations involving phase changes, the proportion of fouling-induced resistance becomes significant, making it crucial to determine this value accurately. If the heat resistance value is set too high, it will result in lower fluid flow rates both at the start of operation and over time, which accelerates fouling formation and is therefore detrimental. Low flow rates can also lead to corrosion due to fouling accumulation. As for the worst-case conditions, I believe it’s sufficient to use the average annual maximum temperature; the same applies to humidity. These conditions aren’t actually the worst possible, as there are certainly a few days each year when temperatures exceed these values, but adjustments can be made to handle such situations, and the system will still perform well. After all, it’s important not to design the system in a way that fails to meet the process requirements under severe conditions (such as in summer). The above are my personal opinions

The heat exchange area margin shall be no less than 10%. For the thermal resistance of scale, we use 6.45*10^-5 m2℃/W per layer.

Hehe, after reading everyone’s discussions, it seems that this is indeed the way most of us think about thermal calculations these days. I thought the same way before attending the lectures, and no one considered it to be wrong. I’d like to talk to you about an issue that affects the manufacturing process. If the value of the fouling coefficient is set too high, is it possible that this will result in cost wastage without any improvement in the performance of the process? Please think carefully and engage in active discussion on this topic

Building on the discussion from the 6th floor, I think that if a higher fouling coefficient is used, the heat exchanger will become larger accordingly, while the flow velocity will decrease as well. The reduced flow velocity makes it easier for the heat exchanger to become fouled, resulting in a decline in its heat exchange efficiency; moreover, increased fouling leads to a greater pressure drop. In many operating conditions, the amount of fouling is not fixed; it is related to various factors such as flow rate and temperature. If an excessive amount of fouling is assumed, it is likely to result in cost wastage without any improvement in process performance.

Different heat exchanger configurations require different fouling coefficients; for example, the fouling coefficient of water in a plate heat exchanger is 1/10 of that in a tubular heat exchanger.

In our calculations, we usually use a dirt coefficient that is slightly higher than the recommended value; as a result, the calculated area is also larger. The safety factor is generally around 1.15

Our company only manufactures spiral plate heat exchangers; the fouling coefficient used is based on that of shell-and-tube heat exchangers, and it is set at a relatively high value. This is probably because the equipment is supplied to coking plants, where higher fouling coefficients are expected.