Fouling Calculations

Fouling Calculations are the main simulation calculations in The Monitor™ computer program .  On a case-by-case basis, the program determines the heat, material and pressure balances over all the units in the network and the fouling coefficient for each exchanger.

Network Balance Calculations

Solving the Network balances involves calculating the heat, mass and pressure balances on all the exchangers and other unit operations.  These balances use unit feed stream data along with user specifications to calculate product stream information.  The calculations iterate until the network has converged for stream temperatures, flows and pressures. If Data Reconciliation has been run for a case, the reconciled data will automatically be used in Fouling Calculations.

Exchanger Balances

The flow rate and temperature of each feed to an exchanger are known.  One exit temperature must be specified.  The other exit temperature is determined by heat balance. The calculated exit temperature may depend on the pressure so the pressure and heat balances must be solved together.  The exchanger pressure drop is calculated from the flow rate, temperatures, fluid properties and mechanical data.  Instead of using the calculated pressure drop, you may enter pressure values for exchanger exit streams.

Actual Coefficient (U_actual)

The Actual Heat Transfer Coefficient (sometimes called the Dirty Heat Transfer Coefficient) is determined for each exchanger for each case from the equation:

Q = U_actual.A.MTD

where:

Q = duty, calculated from heat balance
U_actual = actual heat transfer coefficient
A = total exchanger area specified
MTD = Mean Temperature Difference

MTD is calculated from the Logarithmic Mean Temperature Difference (LMTD) which, for a counter-current heat exchanger, is defined by:

                

where

 

The calculation of LMTD assumes that the exchanger is single pass.  However, most real exchangers have more than one pass.  The MTD is a modified value for the LMTD which takes into account the number of shell and tube passes. MTD is determined by:

where: F_T = LMTD correction factor.

For a single shell and tube pass exchanger, F_T is always 1.0. Correction Factors for multi-pass exchangers are functions of the four exchanger stream temperatures and the number of shell passes in the unit.  The more shell passes, the higher the value of F_T.  They are calculated from methods available in open literature.

Clean Coefficient (U_clean)

The clean heat transfer coefficient value is a function of Reynolds and Prandtl numbers, calculated from fluid properties and exchanger configuration, and tube wall thermal conductivity.

Fouling Resistance

The difference between the actual value and the clean value determines the extent of fouling in the exchanger. The fouling resistance (Rf ) is determined as:

R_f = 1/U_actual – 1/U_clean

Results

After the calculation, the results are written back into the database.  You may examine all results in the Results Output Tabular Report. You may view results for individual units or streams using their Shortcut Menus.

Specifying Fouling Factors

If you later want to investigate the effect of different fouling factors on the furnace inlet temperature, use the Specify Fouling calculation. This calculation allows you to modify fouling factors for any or all exchangers and determine exchanger exit temperatures.  The results are shown in the Specify Fouling Report which shows the calculation history, exchanger exit temperatures and the furnace inlet temperatures.

Monitor™  output capabilities are extensive.  

• You can export the PFD drawing to PowerPoint so that you can use it in a presentation. 
• You can get results tables displayed directly on the screen.
• You can have a summary of Reconciliation, Fouling and NFIT runs for a range of cases.
• You can plot results from a range of cases to view trends.

Tabular Output

You can get the results of the current case displayed as a table directly on the screen.  

You can export these results to Word or to a Text file. 

Input Data Reprint To enable you to check your data, Monitor™ results include it all at the start of the main output. The network data include: connectivity  mechanical data  Case data include: stream temperatures, pressures, flowrates and properties exchanger temperatures mixer temperatures splitter ratios
Reconciliation Monitor™ calculations use measured plant temperatures and flowrates to determine heat exchanger fouling factors. The Data Reconciliation calculation identifies inconsistencies in the input data and enables you to obtain a more consistent set of data for the fouling calculation. Reconciliation output shows the duties calculated from the supplied flowrates and temperatures for each side of target exchanger and the differences between them. It also shows the differences between the target and calculated temperatures of the  products of target mixers. The same data are then shown after the reconciliation has been completed. The table shows by how much feed stream temperatures and flowrates and splitter ratios have been changed to achieve a reconciled data set.
Fouling After a reprint of the input data, the fouling report shows tabular results for each type of heat exchanger in turn, followed by results for all other types of unit operation in turn. Finally, the temperatures, pressures, flows and properties are presented for all the streams in the Network.  Heat exchanger results include: Exchanger duties, clean and dirty coefficients Tube and shell velocities and Reynold's Numbers Mixer temperatures and duties Stream temperatures, rates and properties.
Normalised Output Normalisation removes the effect of the changes in external parameters, such as crude and product variations, to determine a true picture of the degradation of network performance due to fouling alone. The calculated fouling factors from each case are superimposed onto the feeds from a selected base case and the resultant temperatures reported. Normalisation output includes: fouling resistances used exchanger exit temperatures run-up stream temperature
Cleaning Economics Cleaning Economics calculations determine the effect of cleaning exchangers in the Network and an optimum cleaning cycle for each exchanger. Cleaning output includes: The cost of removing each  exchanger as the additional fuel cost/day to make up the duty and as the loss in throughput if the duty was not made up by the furnace. The increase in duty of the complete Network after each exchanger has been cleaned expressed as a reduced furnace cost to maintain current operation and an increased throughput which could be achieved if furnace duty was not reduced. The Optimum Cleaning Cycle period which provides the lowest annualised cost of fouling. The savings shown are compared with the cost of cleaning once at the end of the plant run.
Splitter Optimisation Splitter Optimisation determines splitter product ratios which maximise the heat recovery of the Network. This minimises the required furnace duty for the Network. Optimisation output includes: Initial and final enthalpy and temperature of the chosen run-up stream. all exchanger inlet and outlet temperatures and flowrates at the optimum splitter setting.
Calculation Histories This report summarises the Reconciliation and/or Fouling and/or NFIT results for all cases within a range you select. It is particularly useful for troubleshooting data errors.   The report here shows a range of Data Reconciliation results, including some failed cases. You can choose to display all cases or just failed cases.
Plotted Output
For monitoring over a time period, by far the most meaningful form of output is that presented in graphical format. There are a number of standard, predefined reports. You may also define your own reports to include the parameters, units and/or streams that you require. The data for the selected report are written to a text file which is then opened automatically in Excel. A plot is created for each set of data in the spreadsheet. You may then modify and save the workbook or copy plots or data into other applications. The standard reports are:
Reconciliation This report compares results for data reconciliation over the range of Cases. It shows: the initial duty imbalance for target exchangers and the initial and final duties on the shell and tube sides for each exchanger. changes in flowrate and temperature of feed streams.
Heat Transfer This report contains heat transfer data for all the exchangers in the Network and any normalised furnace inlet temperatures. For each exchanger there are plots for: duty fouling U-values .. clean, actual and normalised tube velocity shell velocity effectiveness
Fouling Cost This report shows the additional costs incurred when a furnace makes up the duty lost because of fouling.  The spreadsheet plots the following on separate worksheets: The normalised furnace inlet temperature. The Cost/day of the additional furnace duty required to make up for the network duty loss caused by the current level of fouling. The cumulative additional furnace duty required since the start of the specified period.  The cumulative fuel cost of the exchanger fouling since the start of the specified period.
Stream Temperatures This report shows the temperatures of all streams in the Network.  Feed streams, product streams and internal streams are presented separately and are plotted on separate worksheets. Compare this plot of the actual furnace inlet stream temperatures with the normalised plot abov.  This shows the benefit of normalisation to give a more meaningful presentation of the true effect of fouling.
Weight and Volume Flow Rates These two reports show the weight and volume flowrates of all streams in the Network. Feed streams, product streams and internal streams are presented separately and are plotted on separate worksheets.
	Weight Flowrates
	Volume Flowrates
Pressures The pressure at the inlet and outlet of each exchanger is listed on this report along with pressure drops. The plots for each exchanger are on separate worksheets and show: Shell side inlet and outlet pressures Shell side pressure drops Tube side inlet and outlet pressures Tube side pressure drops
User Defined Spreadsheet Outputs If none of the standard spreadsheet reports meets your requirements, you may define your own reports. You may specify a Exchanger duty, U (actual and clean), fouling factor, velocities, Re, temperatures, MTD, LMTD and Ft. Pump duty, heater and cooler temperatures and splitter ratios. Stream temperatures, pressures, flowrate and liquid fraction.