Rigorous Heat Exchanger - Technical Information

The Rigorous Heat Exchanger unit operation carries out a performance rating of an existing shell and tube heat exchanger handling single phase, condensing or vaporizing streams. Vapor-Liquid and Vapor-Liquid-Liquid phase equilibria are supported.

The exchanger may have one or more shells, arranged in series, parallel, or both. All shells have identical geometry.

Starting with PRO/II version 7.0, a new HTRI interface has been integrated into the HXRIG model. This allows solving a rigorous heat exchanger using HTRI’s XIST exchanger model. The interface requires the user to have an additional licensing agreement with HTRI. Contact your PRO/II representative for details.

Calculation Type

Two basic simulation modes are offered: Predictive or Specified. The mode is determined by the data options chosen for Calculation Type in the main Rigorous Heat Exchanger Window.

Either: specify the area of the unit and its fouling resistance. PRO/II rates the unit and predicts both product stream temperatures and the unit’s duty

Or: specify the duty of the exchanger or one of the product stream temperatures. PRO/II calculates:

1. the area required to achieve the duty with the given fouling factor

2. the fouling factor required to achieve the duty with the given area

Attaching to a Column

If one side of the exchanger is attached to a column, the duty can be calculated either by the column or the heat exchanger. It is typical to allow the column to calculate the duty, since this considerably enhances the column convergence. In that case, select Calculation Type "Rating(Predictive)" and specify the desired column internal stream to use in the Attach to Column Window.

Minimum Data Requirements

Geometry

The minimum required geometric data to enter is the heat transfer area. Enter this on the main Rigorous Heat Exchanger Window Window as one of the following:

Number of tubes per shell
Area per shell
Shell Inside Diameter

PRO/II calculates the other two from tube count. If the user enters more than one, PRO/II checks to ensure the data are consistent.

Product Phases

If either of the sides has more than one product, the Product Phases must be allocated among the product streams. Use the Feeds and Products Data Window to perform this allocation.

Stream Properties

The rigorous heat transfer calculations performed in this unit need transport properties. Define these in the Thermodynamic Data Window.

Defaults

PRO/II uses a series of Defaults for most geometric data that are not entered by the user.

Calculation Method

For details about correlations used and references, refer to the PRO/II Reference Manual,

Volume 2, section 2.5.3.

Duty

Regardless of the calculation type choosen, the following relationship is met:

Q(duty) = U * area * LMTD

The calculations are iterative and normally fast. However, in rare cases of need, convergence may be enhanced by entering an initial estimate for the overall coefficient in the Film Coefficient Data Window.

Heat Transfer

The overall heat transfer coefficient is calculated from shell and tube-side film heat transfer coefficients, fouling resistances and tube material conductivity. Shell-side calculations include allowances for shell cross-flow variations due to such factors as baffle leakage. Tube-side coefficients are calculated by one of three equations, depending on the values of the Reynolds and Graetz numbers.

Area

A tube count derives the relationship between number of tubes, surface area and shell diameter. Users may specify plain or finned tubes.

LMTD

When phase changes occur within a heat exchanger, the conventional LMTD derived from the terminal temperature differences is not accurate enough to calculate the temperature driving force. The log mean temperature difference used in the Rigorous Heat Exchanger model is a weighted mean value calculated from Zones Analysis.

Pressure Drop

Pressure drops are calculated rigorously and take into account nozzle, return losses and leakages. Two correlations are available, Bell-Delaware and Stream Analysis. Stream Analysis is an iterative method whereas Bell-Delaware is a global method. Because Stream Analysis has not been tested as much as Bell-Delaware on multi-phase systems, you are recommended to restrict the use of Stream Analysis to single phase exchangers.

Tuning the Unit to Match Plant Data

It is simple to match the results of the simulation to measured plant data. PRO/II allows over-riding or adjusting various parameters in the Film Coefficient Data Window and the Pressure Drop Data Window. Available options include the following::

$image\EDIT03.gif$ Impose film coefficients in place of the calculated ones.

$image\EDIT03.gif$ Apply adjustment factors to input or calculated film coefficients.

$image\EDIT03.gif$ Apply an adjustment factor to the calculated overall coefficient.

$image\EDIT03.gif$ Impose pressure drops in place of the calculated ones.

$image\EDIT03.gif$ Apply an adjustment factor to the calculated pressure drops.

Feeds and Products

Each side of the Rigorous Heat Exchanger may have any number of feed streams. If there is more than one feed stream to a side, the feeds to that side are mixed and flashed at the lowest feed stream pressure.

A side must have either process stream feed(s) or be attached to a distillation column. Only one side of an exchanger can be attached to a column.

Each side can have up to four process product streams, with each stream being of a different phase. The possible phases are: vapor; liquid; decanted water (or second liquid phase), mixed (a mixture of liquid and vapor); and solids. The phases are defined on the Product Phases Window. No phase can be used in more than one product stream. For example, a mixed product cannot coexist with a vapor or with a liquid product. A mixed phase includes vapor and liquid).