Thermocompressor

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Related Links: Thermocompressor TVR Example

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.

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

The model operates in a Flash Train where the steam line follows Secondary Steam feed through to Product. Separately General Demand can be used on the Primary Steam Feed.

Diagram

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
Label	Required /Optional	Input /Output	Min	Max	Description
Primary	Required	Input	1	1	The High Pressure steam feed to the unit.
Secondary	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:

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

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

Key Performance Parameters

P_p - Primary (motive) Pressure

P_s - Secondary (suction) Pressure

P_c - Discharge Pressure

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

where:

h_c - Product steam Enthalpy

h_p - Primary (HP) steam Enthalpy

h_s - Secondary (LP) steam Enthalpy

[math]\displaystyle{ \eta }[/math] - Thermocompressor Efficiency

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

The user can specify the desired entrainment ratio and the model can set the flow of Primary (HP) steam to achieve this if the user enables the Demand functionality.

Alternatively, the user can specify the desired discharge pressure.

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 or discharge pressure;
Superheats or partially condenses the discharge steam in order to make maintain the energy balance (adiabatic process).

Full Steam 1D Model

This model is based on the one dimensional analysis which:

Does not depend on any explicit assumptions about the dimensions and the internal geometry of the thermocompressor;
Requires as few user specified parameters as possible.
Can be "tuned" to manufacturer's data.
Is based on the properties of steam, rather than an ideal gas approximation.

Excess energy is assumed to be present as kinetic energy in the discharge stream which is dissipated downstream of the Thermocompressor.

Demand Connection

If the Entrainment Ratio or Discharge Pressure method is chosen, then the user can choose from one of three options for the demand connection. This applies to either the Primary (HP) Inlet or Secondary (LP) Inlet, based on user selection.

None (Calc Only): The required flow will be calculated but no warning will be given if it is not met. No demand is set in the inlet pipe.
None (Manual): The required flow will be calculated and a warning will be given if it is not met. No demand is set in the inlet pipe. It is expected the user will configure another controller to set the required flow (e.g. Set Tag Controller, PID Controller or General Controller).
General Demand: The required flow will be calculated and a warning will be given if it is not met. The demand is set in the inlet pipe.

Assumptions, Limitations and comments

The secondary pressure is always less than the primary pressure.
The discharge pressure is less than the primary pressure and greater than the secondary pressure.
There is an envelope of pressures and flow rates within which the device operates.
The Entrainment ratio is restricted to a reasonable range. The default range is between 10 and 400%. The user can change these limits.

References

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.
Shutte & Koerting. Performance Data on Jet Compressors. Bulletin 4F Supp.
Hisham El-Dessouky, Hisham Ettouney, Imad Alatiqi, Ghada Al-Nuwaibit. Evaluation of Steam Ejectors. Chemical Engineering and Processing (2002) 551-561.
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:

Thermocompressor tab - contains the main configuration information relating to the unit.
QPrimary - Available from Build 139. Optional tab, visible if ShowQPrimary is enabled. This and subsequent tab pages, e.g. QPrimary.. and Sp, shows the properties of the primary steam (high pressure) stream. The tags in the QPrimary tab are valid even when the ShowQPrimary option is not selected.
QSecondary - Available from Build 139. Optional tab, visible if ShowQSecondary is enabled. This and subsequent tab pages, e.g. QSecondary.. and Sp, shows the properties of the secondary steam (low pressure) stream. The tags in the QSecondary tab are valid even when the ShowQSecondary option is not selected.
QProd - Optional tab, visible if ShowQProd is enabled. This and subsequent tab pages, e.g. QProd.. and Sp, shows the properties of the discharge stream. The tags in the QProd tab are valid even when the ShowQProd option is not selected.
Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
Links tab, contains a summary table for all the input and output streams.
Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.