Pressure Exchanger

Navigation: Models ➔ Pressure Changing Models ➔ Pressure Exchanger

General Description

The pressure exchanger unit model can be used to model energy recovery devices such as those used in reverse osmosis plants to recover energy from high pressure brine streams.

The unit has two feeds, one at low and one at high pressure, and two products, one at low and one at high pressure. The total flows of the two feeds are usually very similar. The unit acts like a pump for the low pressure stream, increasing its pressure.

Diagram

The diagram shows the default drawing of the Pressure Exchanger, 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.

Uses

The model can be used to model a wide variety of physical devices including:

Transfer of hydraulic power in a high pressure stream to a pump via a rotating shaft (e.g. Pelton wheel and Francis turbines), also known as a reverse pump.
Use of hydraulic power in a high pressure stream to drive a pump (e.g. Turbocharger).
Direct transfer of hydraulic power in a high pressure stream to hydraulic power in a low pressure stream (e.g. PX).

In some of these devices, there is no mixing of the two streams (i.e. similar to a heat exchanger), while in other cases, such as the PX, a small amount of mixing can occur. The model includes a leakage parameter which can be set to zero if modelling the type of device where no mixing occurs.

Within a flowsheet, the Pressure Exchanger can be:

placed downstream of a pump (i.e. in series), providing a boost in stream pressure
placed in parallel to a pump, providing an equivalent boost in stream pressure as the pump, but for a fraction of the flow

Inputs and Outputs

Label	Required /Optional	Input /Output	No. of Connections		Description
Label	Required /Optional	Input /Output	Min	Max	Description
LP In	Required	Input	1	1	The Low Pressure feed to the unit.
LP Out	Required	Output	1	1	The Low Pressure outlet. This corresponds to the Low Pressure feed.
HP In	Required	Input	1	1	The High Pressure feed to the unit.
HP Out	Required	Output	1	1	The High Pressure outlet. This corresponds to the High Pressure feed.

Behaviour when Model is OFF

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

The 'LP In' stream will flow straight out of 'LP Out' with no change in pressure, temperature or composition;
The 'HP In' stream will flow straight out of 'HP Out' with no change in pressure, temperature or composition;
No pressure exchange will occur between the Low Pressure and High Pressure streams.

So basically, the unit will be 'bypassed' without the user having to change any connections.

Model Theory

The efficiency of the unit is based on the transfer of energy from the High Pressure (HP) stream to the Low Pressure (LP) stream:

[math]\displaystyle{ \mathbf{\mathit{Eff=\cfrac{(Q_{HPo}*P_{HPo})+(Q_{LPo}*P_{LPo})}{(Q_{HPi}*P_{HPi})+(Q_{LPi}*P_{LPi})}}} }[/math]

where

Q - volumetric flowrate of given stream

P - pressure of given stream

Subscripts:

LPi - inlet conditions of Low Pressure stream

HPi - inlet conditions of High Pressure stream

LPo - outlet conditions of Low Pressure stream

HPo - outlet conditions of High Pressure stream

The inlet conditions are known and the user can specify the outlet pressure of the High Pressure stream (usually atmospheric). The user then has the option to either specify the efficiency of the unit or the outlet pressure of the Low Pressure stream, and SysCAD will calculate the alternate parameter.

Data Sections

The default access window consists of several sections:

PressureExchanger tab - contains the main configuration information relating to the unit.
QLPFeed - Available from Build 139. Optional tab, visible if ShowQLPFeed is enabled. This and subsequent tab pages, e.g. QLPFeed.. and Sp, shows the properties of the low-pressure feed stream. The tags in the QLPFeed tab are valid even when the ShowQLPFeed option is not selected.
QHPFeed - Available from Build 139. Optional tab, visible if ShowQHPFeed is enabled. This and subsequent tab pages, e.g. QHPFeed.. and Sp, shows the properties of the high-pressure feed stream. The tags in the QHPFeed tab are valid even when the ShowQHPFeed option is not selected.
QLPProd - Available from Build 139. Optional tab, visible if ShowQLPProd is enabled. This and subsequent tab pages, e.g. QLPProd.. and Sp, shows the properties of the low-pressure product stream. The tags in the QLPProd tab are valid even when the ShowQLPProd option is not selected.
QHPProd - Available from Build 139. Optional tab, visible if ShowQHPProd is enabled. This and subsequent tab pages, e.g. QHPProd.. and Sp, shows the properties of the high-pressure product stream. The tags in the QHPProd tab are valid even when the ShowQHPProd 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.