# Pressure Exchanger

Navigation: Main Page -> Models -> Pressure Changing Models

This model is only available in SysCAD 9.3 Build 137

## 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:

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

In some of these devices, there is no mixing of the two streams (ie. 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:

1. placed downstream of a pump (ie. in series), providing a boost in stream pressure
2. 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 Min Max 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:

$\mathbf{\mathit{Eff=\frac{(Q_{HPo}*P_{HPo})+(Q_{LPo}*P_{LPo})}{(Q_{HPi}*P_{HPi})+(Q_{LPi}*P_{LPi})}}}$
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:

1. PressureExchanger 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.

### Pressure Exchanger Page

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

Tag (Long/short)

Input / Calc

Description/Calculated Variables / Options

### Requirements

On Tick Box This is used to enable or disable the unit. If the unit is disabled, then there is no pressure transfer between the two streams.
Method Set Efficiency User specifies desired efficiency and model calculates outlet pressure of Low Pressure stream.
Set Pressure User specifies desired outlet pressure of Low Pressure stream and model calculates efficiency.
EfficiencyReqd / EffReqd Input Only visible if Method = Set Efficiency. The required efficiency of the energy transfer.
LP.PressureReqd / LP.P_Reqd Input Only visible if Method = Set Pressure. The required outlet pressure of the Low Pressure stream.
Efficiency Calc The actual efficiency of the energy transfer. This may be less than the Efficiency Required if this would result in the Low Pressure stream having a higher outlet pressure than the High Pressure stream inlet.
LP.DemandConnection None No demand connection. The inlet flows are set externally to the model.
General Demand The Low Pressure feed comes from (directly or indirectly) a Feeder with Demand.On selected. The demand logic will aim to match the Low Pressure mass flowrate to the High Pressure mass flowrate.
HP.Leakage Input The mass fraction of the High Pressure stream which is transferred to the Low Pressure stream. For equipment with no direct contact between the streams, set the leakage to zero.
HP.PressureReqd / HP.P_Reqd Input The required outlet pressure of the High Pressure stream.

### Options

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.
TrackFeeds Tick Box Option to produce a warning if the difference between the feed flowrates is greater than a user specified tolerance, see below.
FeedsMaxRelDiff Input The fractional difference allowed between the two feed stream flowrates without a warning being given if TrackFeeds is enabled.

### Results

FeedsRelDiff Calc The fractional difference between the two feed stream flowrates.
Low Pressure Side
LP.TemperatureIn / LP.Ti Calc The inlet temperature of the Low Pressure stream.
LP.TemperatureOut / LP.To Calc The outlet temperature of the Low Pressure stream.
LP.PressureIn / LP.Pi Calc The inlet pressure of the Low Pressure stream.
LP.PressureOut / LP.Po Calc The outlet pressure of the Low Pressure stream.
LP.MassFlowIn / LP.Qmi Calc The mass flowrate of the inlet of the Low Pressure stream.
LP.MassFlowOut / LP.Qmo Calc The mass flowrate of the outlet of the Low Pressure stream.
LP.VolFlowIn / LP.Qvi Calc The volumetric flowrate of the inlet of the Low Pressure stream.
LP.VolFlowOut / LP.Qvo Calc The volumetric flowrate of the outlet of the Low Pressure stream.
High Pressure Side
HP.TemperatureIn / HP.Ti Calc The inlet temperature of the High Pressure stream.
HP.TemperatureOut / HP.To Calc The outlet temperature of the High Pressure stream.
HP.PressureIn / HP.Pi Calc The inlet pressure of the High Pressure stream.
HP.PressureOut / HP.Po Calc The outlet pressure of the High Pressure stream.
HP.MassFlowIn / HP.Qmi Calc The mass flowrate of the inlet of the High Pressure stream.
HP.MassFlowOut / HP.Qmo Calc The mass flowrate of the outlet of the High Pressure stream.
HP.VolFlowIn / HP.Qvi Calc The volumetric flowrate of the inlet of the High Pressure stream.
HP.VolFlowOut / HP.Qvo Calc The volumetric flowrate of the outlet of the High Pressure stream.
Leakage
Leakage.MassFlow / Leakage.Qm Calc The mass flowrate of the leakage of material from the High Pressure stream to the Low Pressure stream.
Leakage.VolFlow / Leakage.Qv Calc The approximate volumetric flowrate of the leakage of material from the High Pressure stream to the Low Pressure stream.

## Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 DLL: Piping2.dll → Units/Links → Piping: Pressure Exchanger or Group: Mass Transfer → Units/Links → Piping: Pressure Exchanger