Real Separator
Real Separator Guide
Authors: Lucas Rojek & Dharmendra Tiwary
Date: July 2004
Version: 1.0
Program Mode: HYSYS State-State & Dynamics
1
Real Separator
1 Introduction
The HYSYS Separator unit operation normally assumes perfect phase
separation, but it can also be configured to model imperfect separation by
using the HYSYS Real Separator capabilities. The real separator offers the user a
number of advantages:
•
•
Includes carry over so that your model matches your process mass
balance or separator design specifications.
Predicts the effect of feed phase dispersion, feed conditions, vessel
geometry, and inlet/exit devices on carry over.
This document will introduce the concepts needed to use these real separator
features. An example is included to demonstrate a typical real separator
application.
1.1 Modeling Separators
Real World Considerations
In real world separators, separation is not perfect: liquid can become entrained
in the gas phase and each liquid phase may include entrained gas or entrained
droplets of the other liquid phase. Recent years have seen increasing use of
vessel internals (e.g., mesh pads, vane packs, weirs) to reduce the carry over of
entrained liquids or gases.
Carry Over Option
The Carry over
option for steady
state modeling was
introduced in HYSYS
3.1. Carry over in
dynamic models was
added in HYSYS
3.2.
2
Real Separator capabilities like the carry over option were introduced to the HYSYS
separator with the release of HYSYS 3.1. This option can be used to model imperfect
separation in both steady state and dynamic simulation. Gas and liquid carry over
can be specified or calculated. Three different correlations are available for this
purpose.
Real Separator
Vessel Internals
Internals used to reduce carry over can be included in your separator model
with some of the provided carry over correlations. Internals used to reduce the
amount of phase dispersion entering the vessel are termed “inlet devices”.
Internals used to reduce liquid carry over into the outlet gas product are termed
“exit devices”. “Weirs” are used to improve heavy liquid - light liquid separation
in horizontal vessels.
Nozzle Calculations
Included with the carry over correlations are calculation methods for inlet and
outlet nozzle pressure drop. Inlet and outlet devices can be included in these
calculations. The user can also specify pressure drop if the carry-over option is
not in use.
Dynamic Models of Real Separators
Some of these
options (e.g. static
head contribution to
pressure, nozzle
locations, detailed
heat transfer) require
a license for the
HYSYS Fidelity
option.
The dynamic model of a separator must account for changing pressure and
flow due to liquid levels, nozzle pressure drop, and heat effects. As such, vessel
geometry, including internals and nozzle geometry, and heat loss parameters
need to be specified. Modeling imperfect separation with the carry over option and
a specifiable PV work term are also available. Level taps can also be set for
monitoring the relative levels of the different liquid phases. All of these items can be
set up via the Rating tab.
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Real Separator
2 Carry Over Models
2.1 Ideal Separator
By default the HYSYS separator is an ideal separator where complete separation
of different gas and liquid phases is assumed and is calculated based on
thermodynamic equilibrium (i.e., on the Rating tab, C.Over Setup page the
Carry Over Model is set to None.). Inlet and outlet nozzle pressure drops can be
specified on the Design tab, Parameters page. By default these pressure drops
are set to 0. The pressure drop in the vessel itself is assumed to be negligible
relative to the nozzle pressure drops and is set to zero (cannot be changed).
2.2 Specifying Carry Over on a Feed Basis
The Feed Basis model allows you to specify the entrainment of each phase in
the product streams as a fraction of the feed. Where fractions are specified
(non-zero), the product streams exiting the separator will have multiple liquid
and gas phases. For example, if you specify Fraction of Feed as 0.1 for light
liquid in gas, this means that 10 mol% of the light liquid phase in the feed will
be carried over into the gas product leaving the separator. As a result, the gas
product vapor fraction will be less than 1.0 and contain a liquid phase.
There are two checkboxes at the bottom of the Feed Basis page. Checking the
Carry over to zero flow streams checkbox will ensure the specified carry over is
added to the product stream even if it has no flow. Checking the Use PH flash
for product streams checkbox will switch the HYSYS flash for the product
streams from PT to PH; it is only needed if the user encounters flash
inconsistencies with the PT flash. These two checkboxes are also available in
the Product Basis and Correlation Based models.
Pressure drop in the separator is specified in the same manner as discussed
previously in section 2.1.
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Real Separator
2.3 Specifying Carry Over on a Product Basis
The Product Basis model allows you to specify the entrainment in the product
streams in terms of mole/mass/volume fraction or flow. For example, if you
specify Frac in Product (Mole Basis) as 0.1 for the light liquid in gas, this means
that the gas product will contain 10 mol% light liquid. If a phase is missing
from the feed stream, checking Use 0.0 as product spec if phase feed flow is
zero allows the separator to continue to calculate the carry over effect (in this
case it ignores any product fraction or flow specification for that phase). The
other two checkboxes function as described in 2.2 above.
Pressure drop in the separator is specified in the same manner as discussed
previously in section 2.1.
2.4 Predicting Carry Over Using Correlations
Note that dimensions
entered on the
Rating! Sizing page
are linked to this
page; you need only
make changes to
one of these pages,
they will be reflected
on the other one.
The Correlation Based model allows you to calculate the expected carry over
based on the configuration of the vessel, the feed conditions and type of
inlet/exit devices installed in the separator. This information is entered on the
Correlation Setup, Dimensions Setup, DP/Nozzle Setup pages.
The Correlation Setup group allows you to select the Correlation Calculation
Type and how you want to apply the correlation. You can apply one correlation
for all of the carry over calculations (Overall Correlation radio button).
Alternately you can select a different correlation for each step in the carry over
calculation sequence:
1. Inlet Calculations
2. Primary Gas-Liquid Separation
3. Primary Liquid-Liquid Separation
4. Exit Device (Secondary Gas-Liquid Separation) Calculations
5
Real Separator
A schematic of these steps is shown in Figure 1.
Figure 1.
Feed
Gas Product
PrimaryGas/
LiquidSeparation
Inlet Calculations
SecondaryGas/
LiquidSeparation
Oil/Gas
Water/Gas
Gas/Oil
Oil distributioninGas
Diameter vsflow
Oil/Gas
Oil distributioninGas
Diameter vs flow
Oil/Gas
Oil distributioninGas
Diameter vsflow
Water distributioninGas
Diameter vsflow
Water/Gas
Water distributioninGas
Diameter vs flow
Water/Gas
Water distributioninGas
Diameter vsflow
Water/Oil
Gas/Water
Oil/Water
PrimaryLiquid/
LiquidSeparation
GasdistributioninOil
Diameter vsflow
Gas/Oil
GasdistributioninOil
Diameter vs flow
Water distributioninOil
Diameter vsflow
Water/Oil
Water distributioninOil
Diameter vs flow
GasdistributioninWater
Diameter vsflow
Gas/Water
Gas distributioninWater
Diameter vs flow
Oil distributioninWater
Diameter vsflow
Oil/Water
Oil distributioninWater
Diameter vs flow
Water Product
Oil Product
Note: Only those parts of the correlation in use that apply to the particular
sub-calculation will be used.
Example: If the Generic correlation is used for the Inlet device and ProSeparator
is used for primary L-L and G-L separation calculations, then the user-supplied
data for the generic inlet calculations (i.e., inlet split and Rossin-Rammler
parameters) will be used to generate the inlet droplet distribution. The
ProSeparation primary separation calculations will then be performed using
these inlet distributions. As ProSeparator correlations will not be used to
calculate the inlet conditions, any ProSeparator inlet setup data is ignored.
Likewise, any critical droplet sizes entered in the Generic correlation will be
ignored as the ProSeparator is being used for the primary separation
calculations.
6
Real Separator
3 Correlation Details
There are three sets of correlations available to calculate carry over: Generic,
Horizontal Vessel, and ProSeparator. After you have selected the type of
correlation, you can click on the View Correlation button to view its
parameters.
3.1 Generic Correlation
The Generic correlation provides a general method for generating the phase
dispersions in the feed and for defining the separation criteria. It is a generic
calculation that ignores vessel geometry.
Inlet Calculations (step 1)
For the inlet calculations the user must specify the percentage of each feed
phase dispersed in each other feed phase and the Rossin-Rammler parameters
(d95 droplet size and Rossin-Rammler index) for each dispersion. RossinRammler parameters are discussed in detail later in this document. Based on
these specifications, the inlet droplet distributions of the dispersed phases are
calculated. For example, the gas phase may have a light and/or heavy liquid
droplet size distribution.
Carry Over Calculations (steps 2-4 combined)
Carry over is calculated by assuming that all droplets smaller than a userspecified critical droplet size are carried over.
3.2 Horizontal Vessel Correlations
The Horizontal Vessel correlations were developed for a horizontal three-phase
separator.
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Real Separator
Inlet Calculations (step 1)
For the inlet calculations the Horizontal Vessel correlations calculate the six
types of dispersions in the feed according to an assumed efficiency of a userdefined inlet device, and user-defined dispersion fractions (termed “Inlet Hold
up”; these parameters are found on the Setup!General page of the Horizontal
Vessel correlation view). The droplet distribution of the dispersed phase(s) is
then calculated using user-supplied Rosin-Rammler parameters just as for the
Generic correlation. Please note that the droplet d95 of the liquid-liquid
dispersions (i.e. heavy liquid in light liquid and light liquid in heavy liquid) is
not specified but calculated using the inlet droplet d95 and the densities of the
2 liquid phases.
Primary Separation–Gas-Liquid Separation (step 2)
The primary gas-liquid separation is calculated from the settling velocities for
each liquid (light and heavy) droplet size in the gas phase and the residence
time for the gas in the vessel. A droplet is carried over if the vertical distance
traveled during its residence in the vessel is less than the vertical distance
required to rejoin its bulk phase.
This effectively applies to horizontal vessels.
Primary Separation–Liquid-Liquid Separation (step 3)
The inversion point is
the water fraction at
which the system
changes in
behaviour from a
water-in-oil emulsion
to an oil-in water
emulsion. In many
cases it is observed
to occur at 50-70%
water; however there
is no reliable means
of determining the
actual point and it
must usually be
determined
experimentally.
(adapted from
Section A of HYSYS
Upstream Option
guide)
8
The primary liquid-liquid separation is also calculated using settling velocities
for each droplet of liquid or gas in the liquid phases and residence time for each
liquid phase. The settling velocities are calculated using the GPSA correlations
for all dispersions, except for the water in oil dispersion for which the settling
velocity is calculated by the method of Barnea and Mizrahi. A user defined
liquid phase inversion point is used in the calculation of the appropriate liquid
phase viscosities (i.e. water-in-oil and oil-in-water). A residence time
correction factor can also be applied. A droplet is carried over if the vertical
distance traveled during its residence in the vessel is less than the vertical
distance required to rejoin its bulk phase.
This effectively applies to horizontal vessels.
Exit Device/Secondary Gas-Liquid Separation (step 4)
The secondary separation calculations for the gas phase are defined by a userdefined critical droplet size. The gas loading factor for each device is used to
calculate the size of the exit device.
Real Separator
3.3 ProSeparator Correlations
ProSeparator
correlations are
based on several
SPE papers (see
reference section)
and proprietary
research from the
oil& gas industry.
Maximum droplet
size is determined
with ProSeparator
using empirical
correlations.
Accurate physical
properties of the
fluids (particularly
surface tension) are
very important to this
calculation.
The ProSeparator correlations are rigorous but are limited to calculating liquid
carry over into gas. There are no calculations of liquid-liquid separation or gas
entrainment in the liquid phases (they are set to zero). Light liquid and heavy
liquid entrainments are calculated separately and the total carry over is the sum
of the separate light and heavy liquid carry over calculations.
Inlet Calculations (step 1)
Minimum and maximum droplet diameter are calculated based on inlet flow
conditions (inlet gas flow rate and gas/liquid phase physical properties such as
density and surface tension) and inlet pipe size. The droplet distribution of light
and heavy liquids in the inlet gas are then calculated using a Rossin-Rammler
type distribution. Please note that ProSeparator effectively calculates its own
Rossin-Rammler parameters (droplet diameters), fitting them to match the predetermined minimum and maximum droplet sizes and does not require the
user to specify any of these parameters. The only user input in the inlet
calculations is the ability to limit the amount of phase dispersion calculated.
Primary Separation–Gas-Liquid Separation (step 2)
Critical droplet size is
determined from the
terminal velocity of
the droplets as
calculated from the
inlet gas velocity,
vessel dimensions,
and fluid properties
(liquid & gas density,
gas viscosity).
Primary separation is based on critical droplet size; however, the critical droplet
size is not user-specified but calculated based on the gas velocity through the
vessel.
Exit Device/Secondary Gas-Liquid Separation (step 4)
Secondary separations accomplished using exit devices (e.g., demisting pad)
are calculated by device specific correlations. The user can choose from vane
pack or mesh pad devices. There are two different calculation methods
available for each type of device.
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Real Separator
3.4 Rossin-Rammler Parameters/Distributions
3.4.1 Particle Size Analysis
In order to properly analyse the particle size distribution in a system, the
engineer must characterize these particles by collecting particle size
measurements. Where such data is not available the engineer can resort to
known typical values for the system in question. As a last resort the default
values provided in HYSYS can be used but great care should then be taken in
interpreting the results.
Aspentech’s
Process Manuals
provide a great
overview on particle
analysis; more
information and
further references
can be found there.
The complete topic of particle size analysis has been discussed by Allen (1981),
and this work is an excellent starting point to explore the subject in greater
depth than can be provided in this overview. A recent summary of the range of
most modern techniques available for the characterisation of particles in
liquids has been given by Svarovsky (1990).
3.4.2 Particle Size Distributions
Particle size distributions (PSD) can be expressed as discrete statistics (e.g.
histograms) or as continuous mathematical functions: Gaussian (classic bellcurve) and Rossin-Rammler distributions are just 2 examples of the latter.
These mathematical functions usually have 2 paramaters: 1 related to “central
tendency” (e.g. mean , median , or mode) and 1 related to the amount of
“spread” (e.g. standard deviation).
For more information
on particle size
distributions and
distribution functions
please refer to the
Process Manuals
Mini Manual 1 (Gas
& Particle
Properties).
10
It must be emphasized that the use of continuous size distribution functions to
represent experimental data is almost always a compromise, since measured
data rarely fit the models exactly. However, distribution functions have the
advantage that they enable the comparison of a large amount of data using a
few basic parameters. An important feature is the ability to represent the size
distribution in cumulative form as a straight line by means of a scaling that is
constructed so as to linearise the cumulative size distribution function.
Real Separator
Rossin-Rammler distributions are defined by:
F = exp(-d/dm)x)
(1)
Where:
F = fraction of droplets larger than d
dm is related to d95
x = RR index
d95 = 95% of droplets are smaller than this diameter for the specified dispersion
RR Index = exponent used in the RR equation (also known as the “spread
parameter”)
* mode = is the commonly occurring diameter (peak of the histogram /
frequency curve), as compared to other different measures of central tendency
such as mean or median diameters.
** spread is a measure of degree of deviation from the central tendency; its
value is characteristic of the substance / system being considered
Another way of expressing this is:
ln(F) = (-d/dm)x
or
ln (ln(1/F)) = B + x ln(d)
(2)
Where:
B = constant = ln (1/dm)
Therefore plotting ln(ln(1/F) against ln(d) can be used to calculate the R-R
parameters. Should such a plot not yield a straight line, it would follow that the
particle distribution cannot be adequately described by the Rossin-Rammler
function.
For more information on particle size analysis and particle size distributions
please refer to the Aspentech Process Manuals.
3.4.3 Modified Rossin-Rammler Distributions used in HYSYS
Real Separator
HYSYS Real Separator correlations ask the user for d95 data rather than dm.
d95 represents the diameter for which 95% of droplets are smaller than this
diameter.
Note: if required dm can be relatively simply calculated from d95 as follows:
dm = d95 / (-ln(1 - 0.95))^(1/x)
(3)
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Real Separator
4 Pressure Drop Calculations
The Horizontal Vessel correlations also include pressure drop methods based
on momentum loss. If a Correlation Based carry over model is selected, these
methods can be selected on the DP/Nozzles Setup page for the calculation of
inlet and exit nozzle pressure drop. If no method is selected then the userspecified pressure drops (Design!Parameters page) are used instead.
12
Real Separator
5 Tutorial Examples
13
14
Process Overview
Real Separator
Real Separator
Workshop
Process Description
In this workshop, a 3-phase Separator is used to separate an oil/water/gas
mixture. Entrained liquids in the gas product have been identified as a potential
process issue. The HYSYS Real Separator will be used to account for liquid
entrainment in the model.
Carryover of liquids can be troublesome, especially if the gas is then passed
through a turbine/compressor where liquid droplets can cause major damage
to the internals of the machine. We will determine if a demisting pad is
appropriate to prevent carryover and how to size it appropriately.
Build an Ideal Separator
1. Open the SS Real Separator.hsc case.
2. Create a stream called Water, and specify its temperature and pressure to
be the same as To LP Sep Clone with a flowrate of 4000 kg/h.
3. Add a Mixer and provide the following information:
In this cell…
Connections
Name
Inlets
Outlet
Parameters
Automatic Pressure Assignment
Enter…
MIX-100
To LP Sep Clone
Water
Feed
Set Outlet to Lowest Inlet
4. Add a 3-phase Separator and specify it with the following information:
In this cell…
Connections
Name
Inlets
Vapour
Light Liquid
Heavy Liquid
Enter…
V-101
Feed
Vapour
LLiquid
HLiquid
15
Real Separator
5. Open the separator unit operation and select the Worksheet tab.
What is the vapour fraction and molar flow of the product stream?
Vapour
______________________
Light Liquid ______________________
Heavy Liquid ______________________
Add Carryover Effects
Let us say that we know (from a plant mass balance or as a design assumption)
that approximately 800 kg/h of liquid is entrained in the vapour stream. How do
we specify this in our model and ensure an accurate mass balance?
1. Select the Rating tab. Click on the C.Over Setup page to bring up the
carryover models, and choose Product Basis as the active model.
2. Enter the entrainment data. Select Specification By: Flow and choose Basis
= Mass. Enter 800 kg/h for Light liquid in gas.
3. Examine the product streams and the C.Over Results page and compare to
the ideal separation case.
What is the vapour fraction of the vapour product stream? ______________
What is the rate of liquid carryover (kgmole/h)? ________________________
16
Real Separator
Using the Carryover Correlations
The Setup and Results
views will be different
depending on which
correlation is used.
Refer to page 6 for a
detailed description of
each correlation and its
required parameters.
As an alternative to specifying the carryover, we can use correlations to predict
the carryover:
1. Return to the C.Over Setup page and change the model selection to
Correlation Based. For steps 2 – 4 select the appropriate radio button.
2. Correlation Setup (radio button):
a) Select Overall Correlation and choose the “ProSeparator” correlation.
b) Click the View Correlation button to enter inlet and separation
parameters.
In this case, the Inlet setup page can be left as is. The ProSeparator
correlations will calculate the inlet dispersion without the need for
further information.
Since we do not have an exit device, we need to set this for the
ProSeparator correlation: select the Vap. Exit Device page; select Mesh
Pad; enter thickness = 0.0.
Close the View Correlation window.
3. Dimensions Setup (radio button): Enter the vessel dimensions as length
8.0 m, diameter 3.0 m, light liquid level 1.5 m.
Vessel dimensions can
also be entered on the
Sizing page of the Rating
tab. Data on these two
pages is linked.
4. DP / Nozzle Setup (radio button): Enter the following values for nozzle
location (this is the horizontal or radial distance from the feed location):
Feed 0.0 m, Vapour 6.0 m. Keep the default values for nozzle diameter and
height.
17
Real Separator
18
Real Separator
Analyze the Results
There are several pages where useful results are displayed:
a) Open the Worksheet tab.
What is the vapour fraction in the Vapour stream?
___________
b) Open the Rating tab and select the C.Over Results page. To view the
carryover details, click the View Dispersion Results button. You should
see results similar to this:
We need to eliminate all droplets larger than 50 microns (0.05
mm). Do we need an exit device to do secondary separation? _____
Open the Rating tab and select the C.Over Setup page. Click the View
Correlation button and open the Results tab.
Adding a Secondary Separation Device
1. Open the Rating tab and select the C.Over Setup page.
2. Click the View Correlation button and open the Setup tab.
3. Select the Vap. Exit Device page; select Mesh Pad and enter a thickness of
150.0 mm.
What effect does this have on the carryover? __________________
19
Real Separator
Exercise 1
It is expected that the inlet hydrocarbon flow to the separator may vary by up to
25%. Anticipating that the separator may not be able to handle this increased
flow, the engineer decides to model the new conditions in the separator and
design a demister pad to remove the larger droplets.
1. Increase the flowrate of the To LP Sep Clone stream by 25%.
2. Select the C.Over Results page, then click the View Dispersion Results
button.
What is the Total Carryover with no mesh? With 150mm of mesh?
_______________________________________________________
What is the removal efficiency of 50 micron droplets?
_______________________________________________________
Based on this predicted dispersion, the engineer decides to install a thicker
mesh pad. How would you suggest the engineer use HYSYS to determine the
correct thickness?
Perform the analysis yourself; how thick should the mesh pad be?
_______________________________________________________
Now what is the vapour fraction of the Vapour product stream?
________________________________________________________
20
Real Separator
Exercise 2
Connect the real separator into the two-stage compression loop to replace the
ideal separator that is currently in use. Keep the Water feed stream connected.
Is the real separator still capable of stopping 50 micron drops reaching the
compressor suction?
Carryover in Dynamic Models
Please open sample case “Dynamic Real Separator.hsc”. This case is based on
the one you have been working on, but dynamic specifications, controllers and
strip charts have been added as needed.
Specifically, the following changes were made to the model:
1. Valves were added to all boundary streams (e.g. Feed0 and VLV-100 were
connected to the Feed stream).
2. Pressure-flow specifications were set on all boundary streams (you will find
these specifications on the Dynamics tab of each boundary stream, e.g.
Feed0 has a pressure specification of 30.05 kPa).
3. Dynamic specifications were set on the separator:
All dynamic specifications used in this example or the separator were
already entered on the Rating tab.
a. Sizing & carry over data were left the same.
b. Heat loss left at none
c. Level taps and PV Work term options were not used
4. Strip charts were created for 2 sets of variables (open the databook tabs
titled Variables to see the list of variables and Stripcharts to view the strip
chart configurations):
The Vessel Conditions strip chart tracks vessel pressure, temperature, and
liquid level. The Carry Over strip chart monitors liquid phase flow out of
the vapour nozzle, as well as inlet flow rate to the vessel.
5. Finally controllers were added to the alternate sample case called
Controlled Dynamic Real Separator.hsc.
21
Real Separator
Demonstration
1.
2.
3.
4.
Open Dynamic Real Separator.hsc.
Click on the strip charts to bring them to the foreground.
Click the Dynamic Mode button.
Start the Integrator. When the liquid carryover flow achieves a steady value,
stop the integrator.
5. Change the position of VLV-100 to 25% open. Re-start the integrator. When
the liquid carryover flow achieves a steady value stop the integrator.
6. Change the position of VLV-100 to 75% open. Re-start the integrator. When
the liquid carryover flow achieves a steady value stop the integrator.
Is the mesh pad thick enough to account for all process conditions?
_________________________________________________________________
A thick pad creates more pressure drop; are there other mitigations to
consider?________________________________________________________
7. Open Controlled Dynamic Real Separator.hsc; repeat the same exercise.
What effect does controlling the liquid level have?
________________________________________________________________
6 Reference
Horizontal Vessel:
GPSA, Vol 1, 10th Ed., January 1990.
Separation Mechanism of Liquid-Liquid Dispersions in a deep-layer Gravity
Settler, E. Barnea and J Mizrahi, Trans. Instn. Chem. Engrs, 1975, Vol 53.
Droplet size spectra generated in turbulent pipe flow of dilute liquid-liquid
dispersions, A J Karabelas, AIChE, 1978, vol. 24, No. 2, pages 170-181.
ProSeparator Model:
Society of Petroleum Engineers papers:
SPE36647 – Separator Design and Operation: Tools for Transferring "Best
Practise"
SPE21506 – Proseparator – a novel separator/scrubber design program
22
Real Separator
Rossin-Rammler Distributions:
Aspen Process Manuals – Mini Manual 1: Gas & Particle Properties; Part 8 –
Particle Size
General References:
Aspen Process Manuals – Gas Cleaning Manual:
Vol 1 – Introduction
Vol 2 – Demisting
Vol 10 – Applied Technology
23