Rating Heat Exchangers 1 1 Rating Heat Exchangers © 2004 AspenTech - All Rights Reserved. 11 Rating Heat Exchangers EA1000.32.02 2 Rating Heat Exchangers 2 Workshop A heat exchanger is a vessel that transfers heat energy from one process stream to another. A common physical configuration for heat exchangers is a shell and tube exchanger, where a bundle of tubes sits inside a shell. There is no mixing of fluid between the shell and the tubes. Learning Objectives In this workshop, you will learn how to: • Use the Heat Exchanger Dynamic Rating Method in HYSYS for heat exchanger design • Determine if an existing heat exchanger will meet the process specifications Prerequisites Before beginning this workshop, you need to: • know how to install and converge simple Heat Exchangers • understand the principles of Heat Exchanger design Process Overview 4 Rating Heat Exchangers 4 Modelling Heat Exchangers In this workshop, we will examine a gas to gas heat exchanger from a Refrigerated Gas Plant. Heat exchangers are modelled in HYSYS using one of three configurations: • Shell and Tube • Cooler/Heater • Liquified Natural Gas (LNG) exchanger The Cooler/Heater operations are single-sided unit operations where only one process stream passes through the operation. The LNG Exchanger allows for multiple (more than two) process streams. A shell and tube heat exchanger is a two-sided unit operation that permits two process streams to exchange heat. In this module, a shell and tube exchanger of given dimensions will be rated to see if it will meet the requirements of the process. Heat Exchanger Calculations The calculations performed by the Heat Exchanger are based on energy balances for the hot and cold fluids. The following general relation defines the heat balance of an exchanger. where: M = Fluid mass flow rate H = Enthalpy Q leak = Heat Leak Q loss = Heat Loss The Balance Error is a Heat Exchanger Specification which, for most applications, will equal zero. The subscripts “hot” and “cold” designate the hot and cold fluids, while “in” and “out” refer to the inlet and outlet. (1) M cold H out H in –() cold Q leak – M hot H in H out –() hot Q loss – BalanceError=– Rating Heat Exchangers 5 5 The Heat Exchanger duty may also be defined in terms of the overall heat transfer coefficient, the area available for heat exchange and the log mean temperature difference: where: U= Overall heat transfer coefficient A= Surface area available for heat transfer LMTD = Log mean temperature difference F t = LMTD correction factor Log Mean Temperature Difference (LMTD) The LMTD is calculated in terms of the temperature approaches (terminal temperature differences) in the exchanger using the following equation: where: The LMTD can be either terminal or weighted. This means that it can be calculate over the exchanger as a whole (terminal) or over sections of the exchanger (weighted). The need for this type of calculation is shown on the next page. (2) (3) QUALMTD()F t M hot H( in H out ) hot – Q loss M cold H out H in –() cold Q leak –=–== LMTD ∆T 1 ∆T 2 – Ln ∆T 1 ∆T 2 ⁄() = ∆T 1 T hot,out T cold,in –= ∆T 2 T hot,in T cold,out –= 6 Rating Heat Exchangers 6 The following plot is a heat loss curve for a single phase stream. It compares the temperatures of the process streams with the heat flow over the entire length of the exchanger. For single phase streams, these plots are linear. The following curve represents a superheated vapour being cooled and then condensed. Note that it is not linear because of the condensation that takes places inside the exchanger. Figure 1 Figure 2 Rating Heat Exchangers 7 7 If the LMTD is calculated using the hot fluid temperatures at points A and C, the result would be incorrect because the heat transfer is not constant over the length of the exchanger. To calculate the weighted LMTD: 1. Break the heat loss curve into regions at point B. 2. Calculate the terminal LMTD for each region. 3. Sum all of the LMTDs to find the overall LMTD. HYSYS will do this automatically if the Heat Exchanger model is chosen as Weighted. Therefore, if condensation or vaporization is expected to occur in the exchanger, it is important that Weighted is chosen as the model. Heat Exchanger Specifications As with all other unit operations in HYSYS, the Heat Exchanger is assumed to adequately meet the process requirements. There are several choices for specifications for the heat exchanger. The choices are given here: • Temperature. The temperature of any stream attached to the Heat Exchanger. The hot or cold inlet equilibrium temperature may also be defined. The temperature difference between the inlet and outlet between any two streams attached to the Heat Exchanger can also be specified. • Minimum Approach. The minimum temperature difference between the hot and cold stream at any point in the exchanger, i.e. not necessarily at the inlet or outlet. • UA. The overall UA can also be specified. This specification can be used to rate existing exchangers. • LMTD. The overall log mean temperature difference. • Pressure Drops. The pressure drops on both the shell and tube sides on the exchanger are important specifications that should not be ignored. If the pressure drops are not known HYSYS may be able to estimate them. Care must be taken when choosing specifications because it is possible to select specifications that are either infeasible or impractical. This may result in a Heat Exchanger that will not solve. Typical specifications for most heat exchangers are Pressure Drops, and one of either, Temperature, Minimum Approach, Duty, or UA. 8 Rating Heat Exchangers 8 Specifications are added on the Specs page of the Heat Exchanger Property view. Enough specifications must be added to ensure that the Degrees of Freedom equals 0. Heat Exchanger Performance A summary of the Heat Exchanger’s performance can be viewed on the Details page of the Performance tab: Heat exchangers are sometimes compared on the basis of UA values, i.e., for a fixed surface area, what is the amount of heat (duty) that can be exchanged? 1. Open the HYSYS case, Gas-Gas.hsc on the disk that was supplied with this module. 2. Double-click the Gas-Gas heat exchanger, and answer the following questions. Figure 3 What is the UA value of the Gas-Gas Exchanger?_________________________ What is the resulting minimum approach temperature if the UA is fixed at 15 000 kJ/C-h (8000 BTU/F-Hr)? _______________________________________ What are the temperatures of streams Gas to Chiller and Sales Gas?______________________________ and _____________________________ Typically, heat exchangers are solved using delta T minimum approach and UA target values. Rating Heat Exchangers 9 9 Heat Exchanger Rating The Rating option can be chosen by selecting Dynamic Rating from the Heat Exchanger Model drop-down menu on the Parameters page on the Design tab. Delete the Delta P on both the tube and shell side. This is because with this type of model the required information must be specified elsewhere. Dynamic Rating Model The physical design specifications of an exchanger must be supplied on the Sizing page of the Rating tab. 1. Firstly, specify the TEMA type to match the desired conditions. The radio button selection in the Sizing Data group will dictate the type of information shown at any given moment. Each parameter will be defined later on in this module. Figure 4 10 Rating Heat Exchangers 10 The radio buttons in the Sizing Data group include: • Overall. Required information about the entire exchanger. Most of the information entered here is used only in dynamic simulations. • Shell. Required information concerning the shell side of the exchanger. • Tube. Required information concerning the tube side of the exchanger. The TEMA Type is selected as part of the Overall sizing data. There are three drop down lists which allow you to specify the geometry of the front end stationary head type, the shell type and the rear end head type for the exchanger. The following tables provide brief descriptions for each designated TEMA Type letter. Drawings of the various TEMA types can be found on page 11-4 of Perry’s Chemical Engineers Handbook, Sixth Edition. TEMA - Front End Stationary Head Types TEMA – Shell Types TEMA Type Description A Channel and Removable Cover B Bonnet (Integral Cover) C Channel Integral with TubeSheet and Removable Cover (removable tube bundle only) N Channel Integral with TubeSheet and Removable Cover D Special High Pressure Closure TEMA Type Description E One Pass Shell F Two Pass Shell with Longitudinal Baffle G Split Flow H Double Split Flow J Divided Flow K Kettle Type Reboiler X Cross Flow [...]... correlations for heat transfer coefficients and pressure drop These correlations are suitable for approximate results in most cases but may not be valid for every exchanger For more accuracy, a rigorous model may be required Please contact your Hyprotech representative for a list of available third party heat exchanger packages that are compatible with HYSYS through OLE Extensibility 12 Rating Heat Exchangers. . .Rating Heat Exchangers 11 TEMA - Rear End Head Types TEMA Type Description L Fixed TubeSheet like ‘A’ Stationary Head M Fixed TubeSheet like ‘B’ Stationary Head N Fixed TubeSheet like ‘N’ Stationary Head P Outside Packed Floating Head S Floating Head with Backing Device T Pull Through Floating Head U U-Tube Bundle W Externally Sealed Floating TubeSheet Rating Parameters Brief... determined the overall heat transfer coefficient, U Tube Wall Cp, and Tube Wall Density Two physical properties of the tube material, used only in dynamics If you want HYSYS to use general correlations to determine the shell and tube side pressure drops and heat transfer coefficients, select the Detailed model on the Parameters page This will allow HYSYS to calculate the desired terms The Rating model in... exchanger, used only in dynamic simulations TEMA Described earlier Shell Side Required Information • • Shell Diameter Can be specified or calculated from inputted geometry Number of Tubes per Shell 11 12 Rating Heat Exchangers • • • • • • • Tube Pitch The shortest centre to centre distance between 2 tubes Tube Layout Angle A choice between four different configurations Shell Fouling The fouling factor on the... the Simulation You are asked to find a heat exchanger that will serve as the Gas-Gas exchanger However, since you are on a very strict budget, you can only consider used equipment A heat exchanger has been found in the surplus supply of a nearby plant If the critical process parameter is to maintain a Sales Gas temperature of at least 10 °C (50 °F), can this heat exchanger be used for the Gas-Gas service?... What will the temperature of the Sales Gas be after 6 months of service? Will this exchanger be adequate after 6 months of service? Save your case! 13 14 Rating Heat Exchangers Challenge Why was the Recycle needed in this Flowsheet? For an interesting challenge, disconnect the recycle operation and stream 1 Connect the stream LTS Vap in place of stream 1 What... mm Baffle Type = Double Baffle Orientation = Vertical Baffle Cut (% Area) = 15% Baffle spacing = 100 mm All other parameters are the HYSYS default values Use the Dynamic Rating mode to determine if the exchanger is suitable; on the Rating tab, Parameters page, use the Detailed Model in HYSYS What is the temperature of the Sales Gas using this exchanger? _ Previous experience has shown you that... Floating Head S Floating Head with Backing Device T Pull Through Floating Head U U-Tube Bundle W Externally Sealed Floating TubeSheet Rating Parameters Brief explanations are provided below for each Simple Rating parameter The parameters are categorized according to the radio buttons in the Sizing Data group box Some of these parameters are only available when the model on the parameters page is selected . Rating Heat Exchangers 1 1 Rating Heat Exchangers © 2004 AspenTech - All Rights Reserved. 11 Rating Heat Exchangers EA1000.32.02 2 Rating Heat Exchangers 2 Workshop A. converge simple Heat Exchangers • understand the principles of Heat Exchanger design Process Overview 4 Rating Heat Exchangers 4 Modelling Heat Exchangers In