Simple Heat Exchanger - Technical Information
The simple heat exchanger unit operation is used to heat or cool a process stream, or exchange heat between two streams. The heat exchanger unit can have one or two sides. One-sided units exchange energy between a process stream and a theoretically infinite source or sink. Two-sided units exchange heat between two process streams, or exchange heat between a process stream and a utility stream. Additionally in the two-sided type, either side may also be Attached to a Tray of a Distillation Column, to exchange heat with a column internal tray stream.
The exchanger calculations can be based on either Countercurrent (default mode) or Cocurrent flows. See Heat Exchanger Configuration for more details.
Calculation Method
The Performance Specification of a heat exchanger unit can be an outlet temperature, outlet phase, exchanger duty, stream temperature approaches, minimum internal temperature approach (MITA), or overall U and Area.
Most of the specifications require a single pass calculation, in which the specified side outlet conditions can be calculated directly and the other side is then calculated by simple heat balance. For several of the temperature approaches, MITA, and overall U and Area specifications, an iterative procedure is required. If U and area are not specified, they are determined from the solution conditions. For all the exchanger specifications, the following relationship is met:
Q(duty) = U * area * LMTD = (Hout - Hin)cold
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 suggested that the column be allowed to calculate the duty, since the column convergence is considerably enhanced in PRO/II.
When phase changes occur within the heat exchanger, the conventional LMTD derived from the terminal temperature differences is not accurate to calculate the temperature driving force. PRO/II can perform a Zones Analysis in these cases to locate the internal temperature pinches and calculate the duty-weighted LMTD for more accurate results.
Feed and Product streams
The heat exchanger unit can have any number of process feed streams for both Hot and Cold sides. Water, air, and refrigeration medium can be used as a Cold Utility Stream, while steam or any other heating medium can be selected as a Hot Utility Stream.
Each heat exchanger side 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. See Product Phases for more details.
Related Topics