Heat exchanger

Author: Alex Valdes ^[2015]

Stewards: Jian Gong, and Fengqi You

Introduction

Heat exchangers are necessary process units that are part of any detailed process flow diagram. Process streams commonly interact through heat exchangers in order to save money on heating and cooling utilities. Furthermore, the surface area of the heat exchanger is proportional to the amount of heat that can be transferred and is the most indicative cost component of a heat exchanger. Therefore, all of the commercial simulators include models for heaters, coolers, heat exchangers, fired heaters,and air coolers (Towler and Sinnott, 2013). Typically, the only inputs necessary for heat exchanger models to converge are properly specified inlet streams (flow rate, temperature, pressure, composition), the pressure drop of flow pathways, and the outlet temperatures or the duty.

Aspen HYSYS V8.0

Model simulators such as HYSYS are extremely useful for engineers to quickly estimate capital costs and utility requirements.

Model Equations

HYSYS uses the following equations for adiabatic steady state heat exchangers:

$Q=Nps(Hps,in-Hps,out)$ (heat transferred from process stream) (1)

$Q=Nus(Hus,out-Hus,in)$ (heat transferred to utility stream) (2)

$Q=UAF(dTavg)$ (rate of heat transfer; actually defining U) (3)

There a few common problems that arise when using heat exchangers in these programs. When heat exchangers are used with streams that go to earlier stages of the process, an information loop occurs and the program is less likely to converge. Many times the process design requires a later process stream, such as the bottoms of a distillation column, to heat an earlier stream, such as the feed to the same column (Towler and Sinnott, 2012). Another problem arises if the specifications of the heat exchanger are impossible, but the model still converges with physically unreasonable results - such as a temperature cross. To avoid these problems, it is good practice to use utility heaters and coolers instead of heat exchangers to get an idea of the required heat load and parameters of an exchanger. The heaters and coolers are also useful for obtaining initial guesses of outlet temperatures and pressure drops. After that information is obtained, the designer will have a much better chance of simulating a heat exchanger that will converge with meaningful results.

Tutorial

Input Properties

• Open Aspen HYSYS and create a New Case under File menu.
• Create a component list by adding all components present in the process.
• Select a thermodynamic fluid package that is applicable to the process (see Property Package article for more details on options)

Enter Simulation

• In the model palette, there are a few options for heating/cooling units. Use a Heater or a Cooler to change the temperature of one process stream using a utility. Use a Heat Exchanger for two process streams exchanging heat, thereby changing the temperature of each. The following steps are for a Heat Exchanger, but the steps are similar for a Heater or Cooler.
• Click on the exchanger in the Flowsheet window. In the Design Tab under Connections, create Tube and Shell Side Inlet and Outlet streams (four streams). Choosing which fluid goes tube side and which goes shell depends on many factors, but some rules of thumb include putting high pressure and/or corrosive fluids tube side [2].
  
• To get a feel for how HYSYS interprets the situation, open the Specs section still in the Design Tab (image to the right). It can be seen that HYSYS needs five pieces of information (Degrees of Freedom =5) about the inlet and outlet streams. If the process is overspecified, HYSYS should give a message indicating the type of error, or may simply say no solution. After understanding the information in this table, enter four inlet parameters and one outlet parameter (the temperature of one of the outlets).
• At this point, HYSYS should say that there is an unknown Delta P. Specify (guess) the inlet and outlet pressures, the exchanger should then converge. Good initial guesses for pressure drop are between 0.3-0.7 bar or 30-70 kPa [1]

Customizing Heat Exchanger

• Once the exchanger has converged and the desired outlet temperature of one of the process streams has been achieved, various parameters can modified and more detailed analyses on the exchanger can be looked at.
• Under the Design Tab, under Parameters, choose one of the five potential heat exchanger models (most common choice is rigorous shell and tube).

Troubleshooting note: sometimes the rigorous heat exchanger model won't converge at first, even if the system is properly specified. Sometimes when this happens, switching to Simple End Point model and then switching back to the rigorous model will make it converge.

• At this point, utilize a neat feature of HYSYS in the Rigorous Model Section of the Parameters page: Size Rigorous Shell&Tube. Clicking on this will have HYSYS automatically run cases to determine optimal parameters of pressure drop and heat exchanger area (there is also an option to automatically run this feature). Once this feature converges, very useful information about the required overall heat transfer coefficient, pressure drop, heat transfer area, and other physical parameters is readily viewable under the Rating Tab.
• Also under the Rating tab, physical features of the heat exchanger such as the number of shell/tube passes can be modified (real heat exchangers typically have more than one or two tube passes per shell). These features of course change the necessary heat transfer coefficient (favorably), pressure drop (unfavorably), and unspecified outlet stream conditions.

References

1. G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.
2. Sloley. "Shell-and-Tube Heat Exchanger: Pick the Right Side". Chemical Processing. October, 2013