Heat Ex Fouling 2

[zenapp]

Hi I have a problem with a fouling heat exchanger.

The fouling has happened over the last 3 months. Please can you confirm my assumption that the more fouled the heat exchanger the hotter the temperature of the hot water will need to be to transfer into the product?

In the diagram the three generators are CHPs producing electric and heat then there are two boilers (duty standby) then a feed product heat ex then four vessel maintenance heat ex

Is this correct? Assuming the heat conductivity reduces with the fouling.

Thanks

ZEN

 

[pmover]

follow the fundamental rules: Q = UA LMTD and Q = m x Cp x dT

an increase in fouling of an exchanger results in a reduction in U. so, to meet the needs of Q, the LMTD needs to increase since the A does not change.

suggest investigating the source of fouling and then apply a remedy.

hope this helps. Good Luck!
-pmover

 

[macmet]

As pmover said, if you have fouling in your HX you should be looking to find that source and remove it. Otherwise, you will be constantly increasing the temperature and will face reduce capacity.

I've worked with HX before, and although I don't know what your exact application is, I would have been concerned if I had issues come up in a 3 month time period. IMO increasing your temperature is not the solution to your problem.

 

[racookpe1978]

Fouling also tends to reduce flow rate since the tubes (outside or inside) are rougher and covered with crud. That changes the "Time" value ...

In worse-case conditions, flow stops completely in the lowest-flow tubes and so Area also goes down.

 

[zenapp]

Thanks for all your replies,

racookpe1978 perhaps you can help me out some more with a few questions relating to pump flow

If I have a pipe transferring from a boiler to a heat exchanger.

The pipe is 100meters long, the water at the beginning of the 100m is at 90C exactly. At the end of the pipe the water is at 88C exactly just before heat ex

The hot water pump is moving water at lets say 10m3/hr and we double its flow to 20m3/hr.

I assume at the higher flow the temperature at the end of the pipe is higher.

My understanding is that the U Value of the pipework and insulation is constant. Also that energy cannot dissapear it can only transfer. I understand that the pump must be adding energy to the system increasing the flow. What I cant understand is what that energy is made up of and how it is distributed, I understand the pump will have a certain efficiency and there will be losses but with regard to the temperature increase at the end of the line is this all related to the extra thermal input at the pump or is the higher flow driving an efficiency somewhere?

How would we calculate the difference in loss between the two points across the pipe?

There must be limitations to the effectiveness of increasing flow, im assuming the relationship has rules?

In terms of heat loss in the pipe we want to minimise this but then at the heat exchanger, we want to maximise the heat transferred out of the hot water, achieving the highest possible drop. So.. the questions is - Are there rules also for this for example a curve that allows calculation of optimal flow. Im assuming low flow causes issues and high flow does too. Is this right? Would too higher flow drop the transfer at the exchanger?

I hope my questions make sense.

Thanks for your help.