LNG Heat Exchanger Heat Leak
The LNG Heat Exchanger model now supports a heat leak option. Basically, the user specifies the heat leak as fraction of the total duty of all the hot-side cells. A positive value represents heat leaked into the hot-side cells from the surroundings. Conversely, a negative value represents heat leaked out of the hot-side cells to the external environment. The heat leak is not applied directly to cells that do not have a performance specification, since the duties of such cells are varied to accomplish the overall exchanger heat balance. The heat leak is intended to be a minor but significant contributor to overall exchanger duty, and as such should be limited to a small fraction of the total duty.
The heat leak feature does not model a particular physical configuration; but a good way to understand it is to visualize an exchanger with an outer shell composed entirely of hot side cells completely surrounding the cold cells. In this visualization, only the hot-side cells are in contact with the ambient surroundings. Therefore, heat leaked into or out of the exchanger adds to or detracts from the duty of the hot-side cells. This change in duty is carried through all the calculations and affects the amount of energy exchanged to the cold-side cells. It is fully integrated into all the zone analysis calculations as well.
The heat leak entry appears in the main Data Entry Window of the LNG heat exchanger. Simply enter a value for the heat leak as a fraction of total exchanger hot-side duty. Typical values fall in the range of -0.05 to +0.05 (i.e., + or -5%). By default, the heat leak is 0.0
This feature is fully supported through key words as well as through the PRO/II Graphical User Interface. The feature also functions properly in both "Batch mode" and "Run Only" mode. To define a heat leak in key words, a new OPERATION statement has been added to the LNG HX input. The syntax of this new statement is described below.
OPERATION HLEAK( frac ) = value
The keyword OPERation is required as the first entry to identify the statement.
HLEAK( frac ) = value
The keyword HLEAK is required to activate the heat leak option.
frac This qualifier is optional. It is used to declare the basis upon which the numerical value fof heat leak is specified. Available options are:
FRAC Indicates the heat leak is supplied as a fraction of total hot-cell duty. This is the default basis when no qualifier is specified.
PCT Indicates the heat leak is supplied as a percentage of total hot-cell duty.
value This is the amount of heat leak. Depending upon the absence or presence of the PCT qualifier (see "frac" above), it is expressed either as a fraction or a percentage of the total duty of all hot cells that have performance specifications. A positive value indicates heat leaked into the hot cells; a negative value leaks heat out of the hot cells to the environment. Typical values fall in the range of -0.05 to +0.05 (i.e., + or -5%). In all cases, regardless of the basis for input, the value is constrained to the range -0.5 > value > +0.5 after it is converted to a fraction.