5 replies to this topic

[Ronnie]

I'm having some difficult predicting the fouling on the heat exchanges. At this plant we use water to cool HF, and the inlet temperatures of the gas are unknown, however the outlet temperatures are recoreded. To predicts the maxiumum ammount of time untill next cleaning is required, i've taken the max q possible from out water flow rates (i have both inlet and outlet T), and compared it with the q value of the current water flow rate. This shows a steady increasing ratio showing fouling, due to the higher flow rate of water required. However, sometimes the plant runs the flow rate high and lowers outlet temperature gas temperature,below required, causing my graph to look like there is fouling, when in reality they're just over cooling.
I wish to be able to compare the ratio of q and incorporate the current outlet temperature of the gas with what it should be. eg. If the water flow rate is 694 (max) and the outlet is 110 F when it should be 115, then there is not a lot of fouling... but if there is 694 flow rate and the outlet is 120 F, then the precooler needs to be cleaned. How can i put this into numbers... I've tried ratio of q ^ d(115-current) but it doesn't correct the way i want it to... Anyone know of a good alebraic way to calculate "fouling".

[joerd]

If you have an idea of the HF properties and flow rate, you can estimate an inlet temperature from the calculated duty, because the duty on the process side should equal the duty on the water side. You can then compute an overall heat transfer coefficient U = Q / (A*LMTD). The decrease of heat transfer coefficient over time (or U*A over time, because the heat transfer area will be constant) will indicate fouling.

[Ronnie]

I have used this method previously, but I am having troubles setting the limit. When I look at just the water, I can say "ok 694 is the max flow rate, and if the gas is coming out higher than desired (115) then its time for cleaning". What value of U should i say is too low?

[sgkim]

I have used this method previously, but I am having troubles setting the limit. When I look at just the water, I can say "ok 694 is the max flow rate, and if the gas is coming out higher than desired (115) then its time for cleaning". What value of U should i say is too low?

I fully agree with Ronnie's comment. But I tried to calculate the fouling resistance in the cooling water side from the data got by changing the CW flow rate. Refer to the file attached.

sgkim

Attached Files

•  Estimation_of_Fouling_Factor..xls   68.5KB   113 downloads

[Ronnie]

thats for the excel, real interesting

My next question is... how do i detrmine which value of U is too low, which will not let me reach my desired outlet temperature of HF? I have all data except mass flow rate of gas.

[joerd]

Obviously it's up to you to decide what reduction in U is still acceptable. There is going to be some limit that you run into when the heat exchanger is fouling. Right now, the fouling effect (reduction in heat transfer coefficient) is countered by increasing the water flow rate (I think). So, at some point, the water flow rate is at a maximum, and the duty of the exchanger can no longer be improved by increasing the water flow rate. You need to clean the exchanger before you reach that point, or accept (temporary) lower duty (i.e. higher process temperature or reduction in throughput).