Economic Calculations

Monitor™ economic calculations answer common questions about the cost of fouling, the benefits to be gained from cleaning and help to develop an optimum cleaning strategy.

Cleaning Economics Reports

Monitor™ Cleaning Economics calculations determine the effects of cleaning exchangers in the Network.  These are shown in tabular form  and include:

• the cost of removing exchangers for cleaning

• the savings that could be made by cleaning selected exchangers

• the optimum cleaning cycles for each exchanger and for user-defined groups of exchangers.

In addition, one of the standard spreadsheet reports is the Fouling Cost Summary which shows the additional costs that have been incurred over a range of cases for the furnace to make up the duty lost. 

Economic Data Required

The Cleaning Economic calculations require the following data:

Globally:

• The Fuel Cost and the Furnace Efficiency are used when generating all economic reports. 

• The Length of Plant Run is only used in the optimum cleaning cycle report. 

• Fuel Cost expressed in $ per unit of duty.  The default value is the Solomon value for fuel oil.

• Furnace Efficiency. The fuel cost is divided by this value to obtain the total cost of the fuel.

• Length of Plant Run expressed as number of days between plant shutdowns.  This is used in the optimum cleaning cycle calculation.

For each exchanger:

• Date Last Cleaned. This is used in the optimum cleaning cycle report.

• Days to Clean. The number of days that an exchanger will be out of service when it is removed for cleaning.  This is used in the optimum cleaning cycle report.

• Cleaning Cost. The fixed costs (man time, cleaning materials, etc.) associated with removing and cleaning an exchanger.  This does not include additional costs incurred by the reduced Network duty.  This cost is used in the optimum cleaning cycle report.

• Clean Fouling Factor for the exchanger when returned to service after cleaning.  This is used in the Cleaning Economics calculation and the optimum cleaning cycle report.

How Cleaning Economics are Calculated

Fouling factors are first calculated for a selected Case.  These are used to calculate temperatures for all exchangers and at the furnace inlet.

Cost of removing exchangers for cleaning

The Network is solved and the furnace run-up temperature calculated with each exchanger in turn bypassed.  This determines the effect of removing the exchangers for cleaning in terms of lost enthalpy at the furnace inlet, also expressed as $/day. The program also calculates the amount by which the throughput would have to be reduced in order to maintain the furnace inlet temperature at its previous level.

Savings from cleaning

The fouling factor of each exchanger in turn is then set to its clean value and the Network is solved to obtain the increase in duty that this would produce at the furnace inlet.  This is shown as extra throughput which could be achieved and $/day.

Optimum cleaning cycles

The above results are then used, along with fixed costs, to calculate the optimum cleaning cycles for each exchanger.  The optimum cycle is that which minimises the annualised cost of fouling. By default, each exchanger is cleaned in turn.  Combinations of exchangers to be cleaned in addition to individual exchangers are defined in the User Defined Cleaning Economics Combinations Window.

[ Automated Running ] [ Data Input ] [ Database Storage ] [ Economics ] [ Fluid Definition ] [ Fouling Calculations ] [ Import ] [ Normalisation ] [ Output ] [ PFD ] [ Data Reconciliation ] [ Splitter Ratios ] [ Thermodynamics ]

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