Transcript PPT - Department of Mechanical Engineering

MEBS 6008
Sea Water Heat Rejection System
Revision 2
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No change to the
following pages of the
power point from
previous version
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Reasons for popularity of sea water cooling in Hong Kong is
mainly due to :
1.
Sea Water is easily available along the coastal regions
2.
The higher first equipment and installation cost are far
outweighted by the lower operating cost in electricity
3.
The shortage of space for installation of air cooled heat rejection
system
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Once through direct sea water heat Rejection System
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Once through indirect sea water heat Rejection System
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Once-through Sea Water Make-up System
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Description of sea water system - 1
Layout and components of seawater plants are varied according to the
specific site conditions such as location, capacity and reliability required,
space limitations and economic considerations.
Some of the seawater intakes are located near bottom of sea walls have
settlement chambers to reduce entry of suspended solids in the circuits.
Flat mesh screens mounted beside intakes to enable primary filtration so as
to enhance screening area and thus reduce the frequency of cleaning.
After primary filtration, seawater will be pumped through pipelines to plant
rooms which are usually located at basement levels of buildings
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Description of sea water system - 2
For buildings which are relatively far from sea water intakes, small pump
houses are situated right beside sea walls that are usually located below
ground level.
Long suction pipelines running across road and pedestrians are common
features for sea water systems in Hong Kong and booster pumps are often
required.
Before entering condensers in plant room, seawater will pass through a
secondary filtration system which are in-line strainers.
Some plants employ drum type manually cleaned strainers and some with
automatic strainers.
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Description of sea water system - 3
Direct Sea Water cooling using tube and shell type condensers are
commonly used.
Indirect sea water cooling using titanium plate type heat exchangers
are also found.
Seawater is usually used to cool and condense refrigerant directly and
then discharged through pipeworks back to sea within plate heat
exchanger.
Fresh-water is cooled by seawater in plate heat exchangers and then
passed to a second stage tube and shell type heat exchanger where
refrigerant is condensed.
Seawater is discharged after passing through plate heat exchangers
while the fresh water re-circulates.
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Description of sea water system - 4
After compression and condensation, refrigerant is passed to
chillers where it absorbs heat from the chilled water.
The low temperature chilled water is then circulated to AHU or
FCU where cool air is required.
Since high frequency of cleaning and maintenance are needed for
some of the components in the circuits, employment of operators
are preferred as technical skill is essential for smooth operation
of the systems.
Therefore, stand-by components such as screens, pumps,
strainers and condensers are commonly adopted.
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Sea Water Characteristics -1
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Sea Water Characteristics -2
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Sea Water Characteristics -3
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Sea Water Characteristics -4
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Sea Water Characteristics -5
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Hong Kong Environment - 1
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Hong Kong Environment - 2
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Hong Kong Environment - 3
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Hong Kong Environment - 4
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Material for Sea Water Equipment - 1
Condensers
Shell and Tube Type
This type of condenser is most commonly used in Hong Kong.
Shell:
The shell is usually made of cast iron or mild steel with or without epoxy coating. The
more expensive shells are made of gunmetal, silicon bronze, aluminium bronze or nickel
alloys.
Tubes:
The tubes are made of the following materials:
70/30 or 90/10 cupro-nickel (more resistant to impingement attack and polluted waters)
Aluminium brass (preferred if relatively higher working temperatures are involved)
Bronze
Stainless Steel
Titanium (very high corrosion resistance)
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Material for Sea Water Equipment - 2
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Material for Sea Water Equipment - 3
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Material for Sea Water Equipment - 4
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Biofouling
Causes
Biofouling may be caused by:
1.
Deposition of large quantities of microbes on surfaces in contact with
the seawater so that formation and development of biofilms on the
surfaces.
2.
The attachment of larvae of invertebrates, mussels and the spores of
marine algae settling and growing on submerged surface.
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Biofouling
Problems caused by Biofouling
1.
Reduction in flow rate
2.
Deterioration of heat transfer rate
3.
Increase erosion and corrosion caused by cavitation around fouling
build-up
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Bio-fouling
Preventive Measures – 1
Screening
•
Use of mechanical screening or filtration of the sea water to remove
large size matured organisms
•
Seaweeds, debris and large-size matured organisms can be kept out of
heat exchangers by traditional mesh screens followed by secondary
screens.
•
However, such screening systems cannot prevent the passage of small
particles.
•
Spawn, mussels and barnacles enter the cooling water intakes and
attach to the walls of the water boxes.
•
The steady stream of relatively slow-moving water supplies food and
oxygen for their development so these organisms grow to adult size
and multiply.
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Control of Bio-fouling
Chemical Treatment
•
Application of chlorination and/ or biocides to kill
small size organisms such as spawn, mussels and
barnacles.
•
Chlorination by:
- Dosing of chlorine gas (0.1 to 2 ppm)
- Adding sodium hypochlorite solution
- Using electro-chlorinator to produce
hypochlorite ions by electrolysis of sea water
•
Application of biocides to prevent growth of small
size organisms
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Bio-fouling
Preventive Measures – 3
Flow Control & Thermal Soak
•
Maintaining a proper flow velocity to inhibit settlement of
fouling organisms
•
Velocity greater than 1.5 m/s can inhibit settling of various
fouling organisms
•
Application of thermal soak to kill fouling organisms.
Recycling of water through condenser until the water
temperature reach a level that organisms cannot tolerate.
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Bio-fouling
Preventive Measures – 4
Mechanical Cleaning - 1
Brushes
•
Traditional manual cleaning method of pushing long handled
brushes through condenser tubes
High pressure water gun
•
New method includes the use of high-pressure water gun to force
nylon brushes down the tubes to scrape scale and force debris out
the outlet end.
•
Nylon brushes are shuttled through the individual tubes by
reversing the cooling water flow in the system.
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Bio-fouling
Preventive Measures – 4
Mechanical Cleaning - 2
Sponge Balls Method
•
Automatic online mechanical cleaning of condenser tubes by
frequently wiping of the water side surfaces of the condenser tubes
while the unit is in operation.
•
Sponge balls are re-circulated constantly from the inlet to outlet water
boxes.
•
Since these sponge balls are slightly larger in diameter than the tubes,
they are compressed and clean the surface as they travel the length of
the tube.
•
The balls are caught in a screening device installed directly in the
pipeline.
•
A collection unit would reroute them back to the condenser inlet ball
injection nozzles.
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Corrosion Control
Causes of corrosion in sea water system
Corrosion arises in the waterside heat exchangers using seawater as the
cooling medium as a result of: 1.
Impingement Corrosion - fairly high velocity of flow. Impingement
attack is usually 1ocalized, e.g. at inlet ends or partial obstruction,
2.
Sand erosion,
3.
Pitting - the effects of polluted water,
4.
Galvanic corrosion - between tube plate and tubes,
5.
Crevice attack underneath deposits and corrosion induced by bio-fouling
countermeasures,
6.
Failures by stress corrosion or corrosion fatigue cracking.
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Seawater corrosion in the Hong Kong harbour
•
The fresh water discharged from the Pearl River greatly influences the
hydrology of the western side of Hong Kong. The salinity of the water
will vary with the seasons and fresh water flow over a wide area and
the salinity will vary daily with the tides in a narrow region.
•
The city discharges into the harbour minimally treated wastes which
include sewage effluents and industrial effluents. Parts of the harbour
are land-locked that cause these effluents not efficiently dispersed.
•
The in-takes of many seawater handling systems are positioned close to
the sea-bed or are affected by reclamation. Thus suspended solids and
debris drawn into the in-takes can cause erosion.
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Corrosion
Approaches to solve
1.
Design and Construction Features
2.
Employing different preventive measures
3.
Selection of appropriate tube materials
4.
Correct operation and maintenance of plant
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Corrosion Control
Design and Construction Features
1.
Proper selection of seawater intake location
2.
Provision of efficient screening and filtration system
3.
Proper design of condenser to eliminate local turbulence
4.
Maintaining adequate flow can reduce the possibility of inlet
impingement, attack due to deposition and foreign body partial
blockage
5.
The fatigue failure is supporting plate normally avoided by
suitable design of spacing
6.
Stress corrosion cracking by reducing residual stresses at the
rolled-in portion of tube through special heat treatment.
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Corrosion Control
Employing different preventive measures
1.
Nylon inserts for protection of the inlet tube ends.
2.
Addition of ferrous ions by dosing ferrous sulphate or iron
corrosion products. This promote the formation of protective
firm on the condenser tubes and reduce the harmful effect of
any residual chlorine ( as low as 0.2 ppm may be very
harmful).
3.
Provision of cathodic protection to the tube inlets, the tube
plates and water boxes using sacrificial zinc or iron.
4.
High molecular weight inhibitor for pre-treating tubes for
protecting tube surface.
5.
Application of artificial protective films on the tube surface.
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Corrosion Control
Selection of appropriate tube materials
Condenser tube material :
•
Aluminium brass
•
90/10 Cu/Ni
•
70/30 Cu/Ni
•
Stainless steel condenser tubes (suffers crevice and stress corrosion
attacks)
•
Titanium tubes or plates (high cost)
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Corrosion Control
Correct operation and maintenance of plant
Cleaning and Maintenance
•
Proper periodic cleaning and maintenance is important
•
To avoid substantial accumulation of deposits and stagnant water
condition
•
To follow the correct procedure
•
To be carried out during period of down time.
Tube cleaning
•
Manually
•
By flow driven brushes
•
Re-circulating balls system.
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SEA WATER TREATMENT BY ELECTROCHLORINATOR - 1
•
Electro-chlorinator designed for production & injection of on-site
sodium hypochlorite solution.
•
Such system shall take water under pressure upstream of the main
sea water pump (usually in pump chamber) and after passing
through the electro-chlorinator, inject the solution back into the
pump inlet side.
•
The operation of the electro-chlorinator shall be interlocked to work
or stop as the seawater pumps.
•
Duplicate electro-chlorinator plant shall be required to ensure
continued treatment when one unit is 'off-line' for maintenance.
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SEA WATER TREATMENT BY ELECTROCHLORINATOR - 2
•
The electrodes shall be made of high corrosion resistant material, in
particular, anodes shall be made of uranium substrate coated with
protective oxide.
•
Integral safety facilities in the electrolytic cells such as water flow
switch and cell voltage imbalance detector shall be provided to prevent
build-up of hydrogen gas by-product.
•
Vent and other necessary facilities shall be provided to dilute and
disperse the hydrogen gas from the degas tank (note to FSD’s
requirements).
•
An adequate electrical supply point shall be made available to the
equipment plant room.
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SEA WATER TREATMENT BY BIOCIDES
General Approach
•
The biocides used shall inhibit the growth of marine organisms
throughout the system and inhibit the formation of scale and slime by
acting as an efficient dispersant.
•
This shall be achieved by the appropriate biocide treatment being
carried out at a regular interval such that the whole system shall
contain a 6-ppm concentration of biocide solution for approximate 1
hour during treatment is sufficient to kill all organisms.
•
Continuous dosages shall not be applied at lower ppm levels lest they
allow the organisms to build up immunity.
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SEA WATER TREATMENT BY BIOCIDES
Properties of Biocides
The biocide for sea water condenser cooling systems shall have the following
characteristics: •
•
•
•
•
•
•
Strong biocidal properties
Dispersing effect on deposits
Filming properties providing a protective film to internal of pipe
work
pH value of 6.8
Non-flammable
Easily application by chemical metering pump with or without
dilution
Being sufficiently bio-degradable to avoid pollution (impact life of
marine organism) and acceptable to the Environmental Protection
Department
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SEA WATER TREATMENT BY BIOCIDES
System Sizing and Design - 1
• The advice of approved water treatment specialist shall be sought on the
dosages required, together with calculation, for a specified systems water
volume.
• In some cases where fixed sea water pumped quantities apply, metering
pumps can be set for a particular input rate to achieve the appropriate
dosage and duration. In such cases it is usually possible to pump biocide
direct from its supply container.
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SEA WATER TREATMENT BY BIOCIDES
System Sizing and Design - 2
• Where variable pumping flows exist, the metering pump/pumps
concerned shall have the facility to automatically vary the amounts of
chemicals pumped in to correspond in proportion with the actual sea
water flow.
• This can be achieved by metering pumps which respond to the reading
of a flow rate sensor inserted into the main sea water pipe.
• Variable pumping rates shall be required where a number of pumps are
'stage' controlled or where individual pumps are being 'speed' controlled.
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SEA WATER TREATMENT BY BIOCIDES
System Sizing and Design - 3
•
Sets of sea water pumps can be supplied with fixed biocide input
from one metering pump per main sea water pump with inlet
before the pump and as near as possible to the primary inlet sea
water gate valve.
•
Such metering pumps would be controlled from the individual
pump circuit such that they only operate when their respective
pump is operated.
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SEA WATER TREATMENT BY BIOCIDES
System Sizing and Design - 4
•
Sets of pumps can also be supplied with a single source
variable duty metering pump controlled from a flow
metering device placed after the main header outlet.
•
The chemical injection shall be distributed to all pump
inlets via solenoid valves with each controlled to open
when their respective main sea water pump is switched on.
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Problems encountered in Sea Water Heat Rejection System
•
Rapid blockage of primary screens and strainers by
floating or semi-floating objects (especially plastics bags)
and various marine foulants.
•
Other frequently found problem are corrosion of screens
and filtration elements of strainers along welded lines.
•
Corrosion of seawater valves, erosion corrosion of pump
impellers and inlet attack of condenser tubes.
•
Guidance for effective usage of preventive measures is
imperative for sea water cooled systems in Hong Kong.
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Sea Water System and Equipment
Maintenance -1
•
Frequent cleaning and maintenance of screens, strainers, and condensers are
necessary for sea water cooled systems in Hong Kong, especially during the
peak operation period of summer.
•
Frequent cleaning and maintenance are needed as a consequence of rubbish
and heavily polluted harbor waters by untreated sewage, the peaks of marine
fouling growth, and the directions of wind and sea water current during
different seasons.
•
Maintenance works and frequency depend on the types of preventive
measures and equipment used, sea water quality and etc.
•
Furthermore, different components of the plant may also require different
frequencies of cleaning in resume efficient operation.
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Sea Water System and Equipment
Maintenance - 2
•
Experienced personnel in plants can determine suitable times for
maintenance by observation of equipment performance.
•
Abnormal increase in pressure drop across condenser inlets and outlets,
strainers and screens may indicate extensive fouling or blockage in
condensers and cleaning may be necessary.
•
However, some of the plants carry out cleaning or maintenance regularly in
order to maintain continuous good efficiency.
•
The interval of cleaning may also be determined according to past
experience.
•
Mesh screens (primary screening) require more frequent cleaning as large
sized objects such as plastic bags are trapped at the screens (may be up to
three to four times a day during the summer).
•
Anti-foulant is often dosed after accumulation of marine foulants taken
place at the well exposed meshes.
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Remark: meaningless to remember HK$/kWh figures
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You would learn more by working out alternative
solution(if applicable), identifying missing
information in question, pointing out errors in
solution, etc.
Please e-mail to me in case you have any queries
on these examples and the lecture notes before
meeting with me on 13 May 2005 7:30pm student
canteen (informal meeting for Q & A only)
Check the sea water file later as I may have further
information to you after the Q&A meeting.
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BASIC DESIGN TECHNIQUES
Selecting fans (or other related mechanical equipment) and designing air distribution
systems + minimization of sound transmitted from different components to the
occupied spaces :
Step 1- Design the air distribution system to minimize flow resistance and turbulence.
High flow resistance increases the required fan pressure, which results in higher noise
being generated by the fan.
Turbulence increases the flow noise generated by duct fittings and dampers in the air
distribution system(especially at low frequencies).
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Turbulence increases the flow noise generated by duct fittings and dampers in the air distribution
system(especially at low frequencies).
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