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Monitor is a trademark of Nalco Company, Naperville, Illinois, U.S.A.

Frequently Asked Questions

 

Question Index

Question 1: We use the formula Q=U.A.∆TLM and plot the U value for each exchanger against time. We calculate clean U values at the beginning of a run. This gives us a reference to compare U values as the exchangers foul.  We use this process to determine which exchangers to clean during shut downs. How does that compare with Monitor™ methodology?

Question 2: If I install the Monitor™ program, do I also need to have a simulator to provide thermophysical fluid properties?

Question 3: How can I generate a condensation curve to check the fluid data I have entered?

 

Question 1

We use the formula Q=UADTLM and plot the U value for each exchanger against time. We calculate clean U values at the beginning of a run. This gives us a reference to compare U values as the exchangers foul.  We use this process to determine which exchangers to clean during shut downs. How does that compare with Monitor™ methodology?

Answer

Using the formula Q=U.A.Ft.LMTD to calculate the heat transfer coefficient is a simple way of studying the performance of a single exchanger. As long as process conditions do not change over the period of the study, it gives an accurate indication of the trend in fouling resistance. As the fouling resistance increases, you observe that the heat transfer coefficient decreases.

However, where process conditions change it may not tell the whole story. In fact, it can be very inaccurate. In some circumstances the U measured in this way can increase even though the fouling is increasing – as discussed further below.

The Monitor™ procedure and the one you follow are similar. The Monitor™ procedure does it this way:

  • Calculates the duty Q based on the flowrate, specific heat and DT on one side.
  • Calculates the LMTD and correction factor Ft from the four temperatures.
  • Knowing the area A, calculates the Actual (dirty) U using the equation
            Q=U.A.Ft.LMTD.
  • Calculates the Clean U from flowrates, physical properties and temperatures of the two fluids.
  • Calculates the fouling resistance Rf by comparing the Actual U with the Clean U using the equation
            Rf = 1/Uactual – 1/Uclean

The difference between the Monitor™ method and your method is in the calculation of the Clean U. You assume that it is the same as the Actual U after a plant shutdown. The Monitor™ method calculates it from first principles using fluid rates and properties at each measurement occasion – daily or weekly.

Many factors can affect the Clean U. In a normal refinery, the crude feed source – and therefore composition and properties – changes frequently. In addition, market needs will require hot product stream properties and flowrates to change regularly. As you know, flowrates and physical properties affect Reynolds number and Prandtl number and hence film heat transfer coefficient and therefore the Clean U.

Variations in temperatures also affect physical properties. In systems where condensing and vaporisation can occur, these effects can be most marked. In addition, exchangers in networks are influenced by changing conditions upstream and, in a counter-flow system, downstream.

In some circumstances the Actual U can increase even though the fouling is increasing. Using a single exchanger as an example, the curves, which are produced from a Monitor™ simulation, shows the effects of two parameters varying together. Changes in fluid property and flowrate cause the Clean U to increase more than the fouling. The result is that the Actual U increases even though the fouling resistance increases too.

This demonstrates that changes in conditions can mask the true rate of fouling. This is handled by the Monitor™ Normalisation procedure. Very briefly: this procedure removes the effects of the variations of the other parameters and displays the changes that are due only to fouling. The Monitor™ procedure calculates the fouling factors in each case and then uses them with the flowrates and properties of a Base case to calculate normalised values: heat transfer coefficients and furnace inlet temperatures (NFIT). The curves of Normalised U and NFIT show the correct downward trend:

Question 2

If I install the Monitor™ program, do I also need to have a simulator to provide thermophysical fluid properties?

Answer

The short answer to this question is: "No"

The longer answer is: "Nooooo." 

The reason is that Monitor™ thermodynamics and fluid definitions are built into the program. Whether your fluid is single phase, multiphase, with or without lightends, separate water or hydrogen, Monitor™ thermodynamics and component properties can handle it. Have a look at the Feature page on Fluid Properties for more details.  What is more important is that you do not have to pay for a copy of another program which you might otherwise never use.

Question 3

How can I generate a condensation curve to check the fluid data I have entered?

Answer

A condensation curve is a curve of liquid fraction against temperature.  If you have a feed stream that you know may condense or boil in the network, it is a good idea to check the data you have entered by generating condensation and other similar curves. 

How to do it

Set up a network with just one unit: a cooler.

Define a feed to the unit. You will need to use ASTM data. 

Set the feed's temperature to the lowest temperature in the range you are interested in and set the temperature of the cooler to the same.

Allocate a tag to the feed temperature.

Create an Excel input data template with one row by exporting this case.

Using Excel functions, add rows to this spreadsheet, each with a different feed temperature. Each row has to have a different date but the actual dates don't matter. In this example there are 31 rows from -50oC to 560oC in increments of 20oC.

Re-import the data in this spreadsheet and run the fouling calculations for the range of cases.

 

Define a new spreadsheet report with the duty of the cooler and the temperature and liquid fraction of the stream. Export the results to this report.

The Stream worksheet shows the stream temperature and the stream Lfrac for each case. The Cooler worksheet shows the duty required to cool the feed from the case temperature to the temperature of the initial case.

The Data worksheet has the raw results which you can manipulate as you wish.  In this example we have plotted LFrac against temperature.