Thermocompressor

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The Thermocompressor model is only available in SysCAD 9.3 Build 137

General Description

A Thermocompressor is a form of steam ejector in which a flow of higher pressure primary steam is used to entrain lower pressure secondary steam. The discharge stream is at an intermediate pressure between the high and low pressure steam flows. This potentially enables recovery of some of the energy in the low grade steam.

The Thermocompressor model in SysCAD is based on data sheets from the Shutte and Koerting company.

Diagram

Thermocompressor 1.png

The diagram shows the default drawing of the Thermocompressor, with the required connecting streams. The unit will not operate unless ALL of the above streams are connected.

The physical location of the connections is not important, the user may connect the streams to any position on the drawing.

Inputs and Outputs

Label Required
/Optional
Input
/Output
No. of Connections Description
Min Max
Primary In Required Input 1 1 The High Pressure steam feed to the unit.
Secondary In Required Input 1 1 The Low Pressure steam feed to the unit.
Discharge Required Output 1 1 The discharge from the unit. This contains the steam at the Intermediate pressure.

Behaviour when Model is OFF

If the user disables the unit, by un-ticking the On tick box, then the following actions occur:

  • If the unit is set up in 'Demand' mode, then the demand for the high pressure steam will be set to 0;
  • Otherwise, the unit will perform all calculations on the high and low pressure steams and produce an intermediate pressure steam product.

So basically, turning the unit 'Off' only has an effect if the high pressure steam is in 'Demand' mode.

Model Theory

The options for predicting the behaviour of a Thermocompressor are:

  1. Simple heat balance model with empirical corrections derived from the data; or
  2. A simple 1 dimensional model with fitted parameters from manufacturers data.

A large database, extracted from data curves produced by the Shutte and Koerting company, was used to fit a heat balance model and a simple 1D model.

Key Performance Parameters

Pp - Primary (motive) Pressure

Ps - Secondary (suction) Pressure

Pc - Discharge Pressure

[math]\mathbf{\mathit{\omega}}[/math] - Entrainment ratio = Mass of Secondary Steam/Mass of Primary Steam

Er - Expansion ratio = Primary Steam Pressure/Secondary Steam Pressure

Cr - Compression ratio = Discharge Pressure/Secondary Steam Pressure

Energy Balance Mode

The Energy Balance mode assumes that the flow through the Thermocompressor is adiabatic.

In this mode the user chooses the efficiency of the Thermocompressor and the enthalpy of the product steam is calculated using the following equation:

[math] \mathbf{\mathrm{h_c = h_s+\frac{\eta(h_p-h_s)}{1+\omega}}}[/math]

where:

hc - Product steam Enthalpy
hp - Primary (HP) steam Enthalpy
hs - Secondary (LP) steam Enthalpy
[math]\eta[/math] - Thermocompressor Efficiency

Assuming that the product steam is saturated, then the discharge pressure, Pc, can be calculated.

The entrainment ratio may also be selected if the user enables the Demand functionality for the Primary (HP) steam.


1D Model based on S&K Data

This model is based on the one dimensional analysis which:

  • Is based on Shutte & Koerting data curves;
  • Does not depend on any explicit assumptions about the dimensions and the internal geometry of the thermocompressor;
  • Requires at the most 1 user specified parameter - The entrainment ratio, which may be selected if the user enables the Demand functionality for the Primary (HP) steam.


Assumptions, Limitations and comments

  1. The secondary pressure is always less than the primary pressure.
  2. The discharge pressure is always saturated and is less than the primary pressure and greater than the secondary pressure.
  3. There is an envelope of pressures and flow rates within which the device operates.
  4. The Entrainment ratio is restricted to between 10 and 400%.

References

  1. WeiXiong Chen, Ming Liu, DaoTong Chong, JunJie Yan, Adrienne Blair Little, Yann Bartosiewicz. A 1D model to predict ejector performance at critical and subcritical operational regimes. International Journal of Refrigeration 36 (2013) 1750-1761.
  2. Shutte & Koerting. Performance Data on Jet Compressors. Bulletin 4F Supp.
  3. Hisham El-Dessouky, Hisham Ettouney, Imad Alatiqi, Ghada Al-Nuwaibit. Evaluation of Steam Ejectors. Chemical Engineering and Processing (2002) 551-561.
  4. Yveline Marnier Antonio, Christelle Périlhon, Georges Descombes, Claude Chacoux. Thermodynamic Modelling of an Ejector with Compressible Flow by a One Dimensional Approach. Entropy 2012, 14, 599-613.

Data Sections

The default access window consists of several sections:

  1. Thermocompressor tab - contains the main configuration information relating to the unit.
  2. Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  3. Links tab, contains a summary table for all the input and output streams.
  4. Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Thermocompressor Page

Class: Thermocompressor - The first tab page in the access window will have this name.

Tag (Long/short)

Input / Calc

Description/Calculated Variables / Options

Common First Data Section

Requirements

On Tick Box This is used to enable or disable the unit. If the unit is disabled and the unit is configured to act in 'Demand' mode, then the Primary Steam flow to the unit will be set at the minimum flow.
Method Energy Balance User specifies desired efficiency and model calculates outlet pressure of Low Pressure stream.
1D Model - S&K Data User specifies desired outlet pressure of Low Pressure stream and model calculates efficiency.
EfficiencyReqd / EffReqd Input Only visible if Method = Energy Balance. The user specifies the required efficiency of energy transfer from the High Pressure steam to the product steam.
Entrainment.Method User Value The user specifies the desired entrainment ratio of High pressure to Low Pressure steam. The High pressure steam MUST be set as a 'Demand' connection, i.e. the Thermocompressor will calculate the required amount of High pressure steam = Low Pressure Steam/Entrainment Ratio and send this value to the High pressure steam connection.
Calculated The model will calculate the entrainment ratio of High pressure to Low pressure steam. In this case the flow of High pressure steam is set elsewhere in the project.
EntrainmentRatioReqd / EntRatio_Reqd Input Only visible if Entrainment.Method = User Value. The unit will then calculate the amount of Primary (HP) steam required to satisfy the user defined entrainment ratio:
Primary Steam = Secondary (LP) steam / Entrainment Ratio.
The allowable range for this variable is 10 to 400% (Shutte & Koerting range).
Prm.DemandConnection Stand Alone No demand connection. The High Pressure steam flow is set externally to the model.
General Demand The High Pressure steam feed comes from (directly or indirectly) a Feeder with Demand.On selected. The demand logic will aim to match the High Pressure steam mass flowrate to the Thermocompressor requirements.

Results

EntrainmentRatio / EntRatio Calc This is the entrainment ratio of the LP steam with the HP steam, = LP steam mass/HP steam mass.
If the HP steam is in 'Demand' mode, then this value should be equal to the Entrainment Ratio specified by the user.
ExpansionRatio / ExpRatio Calc This is = HP steam pressure/LP steam pressure.
CompressionRatio / CompRatio Calc This is = Discharge steam pressure/LP steam pressure.
IdealWork Calc The maximum amount of work that the HP steam can produce, i.e. if Efficiency = 100%.
ActualWork Calc The actual amount of work available from the HP steam.
GridDisplay Tick Box This is a global selection, if this is selected, then where available, variables will display in a table format, if not selected, they will be listed in separate groups as a single list.

Feed and Product Conditions

The following values will be displayed for the Primary, Secondary and Discharge streams. If the 'GridDisplay'box is ticked, then they will be displayed in a grid, otherwise the variables for each stream will be displayed together in a group.
Temperature / T Calc The Temperature of each stream.
Pressure / P Calc The Pressure of each stream.
MassFlow / Qm Calc The mass flowrate of each stream.
VapourFraction / Vf Calc The Vapour fraction in each stream.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 

DLL:

HeatExchange.dll

Units/Links

Heat Transfer: Thermocompressor

or

Group:

Energy Transfer

Units/Links

Heat Transfer: Thermocompressor

See Project Configuration for more information on adding models to the configuration file.


Insert into Project

 

Insert Unit

Heat Transfer

Thermocompressor

See Insert Unit for general information on inserting units.