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.