LNG Heat Exchanger - Technical Information
The LNG (Liquified Natural Gas) Heat Exchanger, also called a "Cold Box", simulates the exchange of heat between any number of hot and cold streams. It is used for cryogenic cooling in the natural gas and air separation industries and can achieve close temperature approaches.
The exchanger is divided into cells representing the individual cross-flow elements. Cells are designated as "Hot", where the streams are cooled or as "Cold" where they are heated. The exchanger must contain at least one hot cell and one cold cell.
Calculation Method
The performance specification on the cell of an LNG heat exchanger unit can be an outlet temperature or exchanger duty. At least one cell must not have a specification. Other specifications, such as outlet phase, stream temperature approaches or minimum internal temperature approach (MITA) can be imposed by using a controller.
The calculation does not consider the exchanger configuration. The cells with specified duty or outlet temperature can be calculated directly and the remaining cells are then calculated by simple heat balance. The products from all unspecified cells leave the exchanger at the same temperature. For each cell, the following relationship is met:
Q(duty) = (Hout - Hin)
When phase changes occur within the LNG heat exchanger, PRO/II can perform a Zones Analysis to locate and report any internal temperature pinches or crossovers. See the Zones Analysis Window for more details. In this case, the UA and LMTD are calculated using the composite hot and cold streams.
Each cell in the LNG heat exchanger unit may use a VLE or VLLE method to determine the individual phase compositions. See VLE Model and VLLE Model for more details.
Feeds and Products
Each cell in the LNG heat exchanger unit can have any number of feed streams. The inlet pressure is taken to be the lowest pressure of all the feed streams.
Each cell can have up to four process product streams, with each stream containing a different phase. The possible phases are vapor, liquid, decanted water/second liquid phase, a mixture of liquid and vapor, and solids. If there are multiple product streams leaving the cell, the phase condition for each stream must be specified using the Product Phases Window.
LNG Heat Exchanger Main Window
LNG Heat Exchanger - Cell Data Window