Transcript PowerPoint 簡報 - University of Hong Kong

MEBS6008 Environmental Services II
http://www.hku.hk/mech/msc-courses/MEBS6008/index.html
Thermal Storage Systems - Two
Department of Mechanical Engineering
The University of Hong Kong
1
Content
Ice storage and chilled water storage systems
Typical ice storage and chilled water storage systems are as follows:Ice storage
Static Ice Production Systems
Ice-on-coil, internal-melt ice storage system
Ice-on-coil, external-melt ice storage system
Encapsulated ice storage system
Dynamic Ice Production Systems
Ice-harvesting ice storage system
Ice slurry system
Chilled water storage
Stratified chilled water storage system
2
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
3
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Liquid coolant to absorb/ store heat energy in thermal storage systems
Brine is a salt solution or glycol solution
Used as a heat-transfer medium.
Freezing point < water (depends on the concentration of salt or glycol)
Consider ethylene glycol and propylene glycol for brine (both are colorless liquids).
Inhibitors must be added to ethylene and propylene glycols to prevent metal corrosion.
Brine is used as freezing point depressants to lower the freezing point of water
Freezing point : ethylene glycol solution at 25 percent by mass drops to -12.2°C
Freezing point : propylene glycol solution at 25 percent by mass drops to -9.4°C.
Therefore, ethylene glycol solution is preferred.
Ethylene glycol solution with 25-30% ethylene glycol circulates inside tubes at -4.4°C.
4
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
ICE-ON-COIL, INTERNAL-MELT ICE
STORAGE SYSTEM
System - 1
Brine flowing inside coils to make ice and to
melt ice in the water that surrounds the coil.
5
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
A chilled water or brine-incorporated ice
storage system consists of
Chillers,
Ice storage tanks,
Chiller pumps,
Building pumps,
Controls,
Piping,
AHUs
etc
6
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Chillers
Centrifugal, screw, & reciprocating
chillers
Selection based on the size of the
plant and types of condenser (watercooled, air cooled, or evaporative cool).
If daytime maximum temperature minus
nighttime off-peak hours ≥ 12oC , aircooled chillers more efficient than watercooled ones.
7
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Ice Storage Tank
Make use of many closely packed storage tanks
connected in parallel => more flexible especially for
retro-fit projects.
Ice is produced, or charged, in multiple storage
tanks.
There are closely spaced multi-circuited
polyethylene or plastic tubes surrounded by water.
Plastic tubes occupy about 1/10 of the tank volume.
Another 1/10 is left empty to accommodate the
expansion of ice during ice making.
8
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Ice Storage Tank
 The water surrounding the tubes freezes into
ice up to a thickness of about 12.7 mm.
 During ice burning, melted water separates
the tube and ice.
 Brine typically leaves the storage tank at 1.1°C.
 Water has a thermal conductivity 0.61 W/m
°C than much lower than ice 2.25 W/m °C=>
capacity is dominated by the rate of ice
burning.
 During ice burning, brine returns from AHU
at 7.8°C or higher.
 This brine melts the ice on the outer
surface of the tubes and is thus cooled to 1.1
to 2.2°C.
9
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Example : Demand-limited partial-storage
strategy
For summer cooling, the daily 24-h operating cycle
can be divided into three periods: off-peak,
direct cooling and on-peak.
Off-peak: Ice is charged to reduce energy costs.
On-peak: Ice is burned to reduce the demand
charge. One chiller is operated.
10
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
OFF-PEAK
1) Ice making :Charge ice tanks
2) Direct cooling: Chiller(s) for night load
Ethylene glycol solution
1.1°C
Tanks sensors :
100 percent charged =>
terminate ice-making
90 percent ice inventory => icemaking restart
Chiller Pump : High speed (ice making)
1) Higher flow rate
2) Pressure drop in evaporator & coils in ice storage tanks.
-6°C
Both Chillers
Load limit = 100%
11
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
DIRECT COOLING
The start of AHUs & before peak period starts.
Refrigeration load
> both chillers’
capacity=>
discharge from ice
Direct cooling mode:
Both chillers operates to supply
chilled water. Chiller pumps at low
speeds
Direct cooling with ice-burning mode:
Both chillers are turned on. Chiller pumps
at low speeds
12
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
ON-PEAK
1) Ice-burning : chiller at
demand limit mode
1.1°C
2) Ice-burning without chiller
mode
0°C
Both pumps during ice burning at low speed
Greater head :
pressure drop of
AHU & coil in ice
tanks
Chiller 1 or 2 lead/lag
sequence. Load limit of
chiller.
13
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
14
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
The oldest type of ice storage system.
Chillers,
evaporating coils,
storage tanks,
condenser,
heat exchanger,
refrigerant pumps,
chilled water pumps,
air system controls,
piping &
Schematic diagram of ice-on-coil, external melt ice storage system
fitting.
15
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Chilled Water Supply (CWS)
Ice melts, => water supply at 1.1-3.3°C
Refrigerant
HCFC-22 is currently used.(HFC??)
Compressor
Capacity < 2400 ton h => reciprocating
compressor
Other capacity => Screw compressor
Compressor suction temperature on ice
building:
-5.6 °C to -4.5°C
Condenser
Use evaporatively cooled
condenser (a higher system
energy efficiency ratio)
16
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Ice Builders
Large, well-insulated steel tanks containing many
serpentine (彎彎曲曲的) coils (steel pipes of 25-30mm
diameter).
Refrigerant-filled coils submerged in water and as
evaporators.
Ice builds up on outer surface of coils/tube banks :
25 - 64 mm thick
Stored ice occupies only about ½ tank volume
Prevention of blockage in water circulation paths:
1) Good spacing of steel tubes to prevent the built-up of ice
cylinders which bridge each other;
2) Baffle plates to guide the water flow (which is also a secondary
heat-transfer surface between refrigerant and water)
17
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Heat exchanger
Reasons of use:
Isolate storage tank brine system
from the chilled water system
connected to the AHUs.
Because the chilled water system in a
multistory building is always under a
static head at lower levels
Storage tank
It contains refrigerant coils at a lower level or
on grade because of weight.
An electric probe senses water level in the tank
to determine amount of ice stored in the tank.
Omission of storage tank:
To supply chilled water directly to
the storage tanks and pressurize the
tanks.(a corresponding increase in
brine temperature of about 1.7°C).
18
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Refrigerant feed
Refrigerant feed
Advantages
Disadvantages
Direct expansion:
Simple and No
refrigeration pump is
required
15 to 20 percent of
the coil surface is
used for superheat
and is not available
for ice build-up
Uses the pressure difference
between the receiver at the
high-pressure side and the
suction pressure to force the
refrigerant to flow through
the ice builder.
Liquid overfeed:
It uses a refrigerant pump to
feed ice-builder coils about 3
times the evaporation rate
they need
The liquid refrigerant Refrigeration pump
wets the inner surface is required
of the ice-builder coils,
=> a higher heattransfer coefficient
than direct expansion
19
ENCAPSULATED ICE STORAGE SYSTEMS
20
ENCAPSULATED ICE STORAGE SYSTEMS
An encapsulated ice
storage system consists of:
chillers,
steel tank,
encapsulated containers,
pumps,
Charging: Secondary coolant is circulated
through the tank.
Discharging : Returned warm coolant from
AHUs circulated through tank => ice melted.
Direct cooling: Chillers => direct cooling at a
coolant temperature from 2 to 6°C.
air system controls,
piping,
and accessories.
21
ENCAPSULATED ICE STORAGE SYSTEMS
Chillers and storage tanks are
usually connected in series
When partial storage is used,
two arrangements are possible:
1) chiller upstream or
1) chiller downstream
Chiller upstream
Chiller downstream
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ENCAPSULATED ICE STORAGE SYSTEMS
Chilled water returned from AHUs at 8°C
is first cooled in the chiller to 4°C, and
then it enters the storage tank and is
cooled down to 1°C.
4°C
8°C
1°C
Advantage:
Chilled water cooled at the chiller is at a
higher temperature => a higher COP at
the chiller.
Disadvantage:
Usable portion of the total storage
capacity is reduced because of the lower
storage tank discharge temperature
Chiller upstream
23
ENCAPSULATED ICE STORAGE SYSTEMS
Chilled water returned from AHU at 8°C is
often first cooled in the storage tank to 4°C,
and then it enters the chiller and is cooled
down to 1°C.
8°C
Disadvantage:
COP of the chiller is lower,
4°C
1°C
Advantage:
The usable portion of the total storage
capacity of the ice storage tanks is increased.
Chiller downstream
24
ENCAPSULATED ICE STORAGE SYSTEMS
High-density polyethylene containers, filled with de-ionized
water, are immersed in a secondary coolant (ethylene glycol
solution) in a tank.
Two types of encapsulated ice containers :
1)Dimpled spheres of 100mm diameter
2)Rectangular containers :35x300x750 mm.
The Containers can withstand the pressure of expansion during
freezing.
Containers are put or stacked in a way allowing free circulation
of fluid and discouraging short-circuit of fluid flow
25
ENCAPSULATED ICE STORAGE SYSTEMS
Storage Tank - Open, non-pressurized type
It needs a barrier to keep the frozen containers
submerged into the coolant.
Ice-charging inventory in the storage tank is measured
based on the displacement of water in the tank when
the ice is formed inside the encapsulated containers.
A static pressure transducer is often used to detect
the water level in the storage tank.
Storage Tank- Pressurized type
Expansion of the frozen containers forces the
secondary coolant overflowing into a separate inventory
tank, and its water level is measured.
26
ENCAPSULATED ICE STORAGE SYSTEMS
Chiller priority control-chiller upstream.
When the system refrigeration load is less than the
chiller capacity, the chilled water bypasses the storage
tanks completely (Pink line).
When refrigeration load > the chiller capacity => chiller
leaving temperature> set point, the control system
diverts part of the chilled water flow through the
storage tanks (blue line).
Storage priority
Chiller upstream
It requires refrigeration load prediction algorithm to
forecast the chiller cooling needed each day.
Increasing the chilled water leaving setpoint to limit the
chiller capacity
Most of or all the refrigeration load is by ice storage.
27
ICE-HARVESTING ICE STORAGE SYSTEMS
28
ICE-HARVESTING ICE STORAGE SYSTEMS
An ice-harvesting ice storage
system consists of:
chillers,
an ice harvester,
storage tank,
air system controls,
piping,
and accessories.
29
ICE-HARVESTING ICE STORAGE SYSTEMS
Equipment
Ice is produced in a harvester, which is
separate from the storage tank where ice is
stored.
The evaporator of the chiller is a vertical
plate heat exchanger mounted above a water
/ ice storage tank.
Low-pressure liquid refrigerant is forced
through the inner part of the plate heat
exchanger is vaporized => refrigeration
effect.
30
ICE-HARVESTING ICE STORAGE SYSTEMS
Ice Making or Charging
A chilled aqueous is :
pumped from the storage tank.
is distributed over the outer surface of the
evaporator plates at a temperature equal to or
slightly above 0oC.
flows downward along the outer surface of the plate
in a thin film.
Water is cooled and then frozen into ice sheets
approximately 5 to 7.5 mm thick.
Ice is formed in 20 to 30 min.
31
ICE-HARVESTING ICE STORAGE SYSTEMS
Ice harvesting
Ice is harvested,in the form of flakes or chunks and falls
into the storage tank below.
Periodically, hot gas is introduced into 1/4 evaporator
plates by reversing the refrigerant flow.
Ice is harvested within 20 to 40s (plate evaporator acts
as condenser).
Ice accumulates about 60 percent of storage tank volume.
The ice flakes are around 150 mm by 150 mm by 6 mm
But melting of the ice during the harvesting process
decreases the amount of ice harvested and adds an
incremental refrigeration load to the system.
32
ICE-HARVESTING ICE STORAGE SYSTEMS
Other considerations
Successfully used in load shifting and load
leveling to reduce electric demand and
energy cost.
It is an open system. More water
treatment is required than in an ice-on-coil,
internal-melt ice storage system
Evaporator plates must be located above the
storage tank, ice-harvesting systems need
more headroom
33
SLURRY ICE SYSTEM
34
SLURRY ICE SYSTEM
Slurry ice
It is a suspension of very small ice crystals in
a liquid.
The binary ice fluid contains latent energy in
the form of ice minute crystals (sizes various
from 1/10 – 1/100 mm)
It changes from the frozen state to the
liquid state when heat is absorbed.
This phase change is instantaneous (much
faster than normal ice melting).
The slurry liquid is pumpable.
35
SLURRY ICE SYSTEM
Slurry ice
The working fluid’s liquid state consists of a solvent
(water) and a solute such as glycol, ethanol, or calcium
carbonate.
The initial solute concentration varies from 2% to over
10% by mass.
The solute depresses the freezing point of the solvent.
The freezing point of an aqueous solutions calcium
magnesium acetate decrease with increase in
concentrations.
8% calcium magnesium acetate
The fraction of ice in a given system can be estimated by
knowing :
1)The initial solute concentration and
2)The freezing point characteristic of the working fluid.
36
SLURRY ICE SYSTEM
Advantages of Slurry ice storage system
The Slurry-Ice system is a dynamic type ice storage system which offers
the pumpable characteristic advantage over any other type of dynamic
systems. and the icy slurry can be pumped
Slurry-Ice is a very versatile cooling medium: The handling characteristics,
and cooling capacities can match any application by adjusting the percentage
of ice concentration.
Slurry-Ice does not suffer from the static type disadvantages of ice
bridging and ice insulation effects.
Slurry-Ice instantly melts to meet varying cooling load. Hence, ensuring
steady and accurate system leaving temperature control.
37
SLURRY ICE SYSTEM
Comparison on cooling capacity
Conventional chilled water systems
The enthalphy difference of water between 6 and 12oC => a cooling
capacity of 30 kJ/kg.
Slurry ice Cooling System
The latent heat carried by ice particles in the water adds great cooling
capacity to the flow.
The cooling capacity of slurry ice operation at 0/13 oC with ice fraction
of 20% equals to 144 kJ/kg.
The use of slurry ice significantly decreases the volumetric flow
requirements.
38
SLURRY ICE SYSTEM
39
SLURRY ICE SYSTEM
Compressor/Condenser
It supplies refrigerant to the evaporator.
Ice Slurry Generator (evaporator)
Water & freeze depressant mixture to produce a pumpable
ice slurry.
Options of Generators are as follows: Supercooled Slurry Ice Generator
Scraper Type Slurry Generator
Ejector System
Vacuum Type Slurry Ice Generator
Falling Film Type Slurry Ice Machine
40
SLURRY ICE SYSTEM
Falling Film Type Slurry Ice Machine
The internal falling film process is based on
supercooling the solution which is disturbed by a
spinning rod in order to overcome the formation of
solid ice and prevent it sticking to the inner surface of
the pipe.
Once the solution is supercooled and disturbed, it
forms microscopic fine binary ice crystals which are
collected at the bottom of the vessel.
The ice concentration and capacity control can be
adjusted by either controlling suction pressure,
solution flow rates or both.
Low friction between the rod and the tube wall and the
slurry ice solution acts like a lubricant.
41
SLURRY ICE SYSTEM
Insulated Ice Storage Tank
It separates ice manufacturing from ice usage.
The tank contains a glycol/water solution which is
converted to an ice slurry in the ice slurry
generator.
The slurry melts as the stored ice absorbs the
heat of the cooling load.
42
SLURRY ICE SYSTEM
Plate Heat Exchanger
It separates the storage tank from cooling
equipment.
It prevents cross contamination between the
ice- melting loop & cooling equipment.
Heat Exchangers - Variation of Heat Transfer Coefficient with Ice Fraction.
An increasing ice fraction reduces the overall heat
transfer co-efficient.
A 17-20% reduction in the heat transfer co-efficient
can be expected when the ice fraction was increased
from 0 % to 15%.
This reduction can be explained by the fact that slurry
ice reduces turbulence in the liquid.
43
SLURRY ICE SYSTEM
Load Control Pump and Valve
For provide the flow and control the supply temperature to the
load.
A minimum velocity must be maintained for ice fractions between
0.1 and 0.25.
Below this velocity=> the pressure gradient increases when the
velocity is decreased=> Phase separation, with ice floating to the
top of the pipe, causes the effective liquid flow cross-section to
decrease.
A number of pressure drop experiments conducted at slurry ice
velocities over 3m/s and ice fractions in excess of 20% in
straight tubes with internal diameters of 25, 51 and 76 mm
indicate no difference in pressure gradient between water and
slurry ice.
At at slurry ice velocities < 1 m/s, the loop pressure drop was
slightly above the pressure drop of the water.
44
STRATIFIED CHILLED WATER STORAGE SYSTEMS
45
STRATIFIED CHILLED WATER STORAGE SYSTEMS
A large storage tank to store
chilled water (4-7oC).
To shift the load to the offpeak hours and reduces the
energy cost.
The chilled water incorporated
storage system consist of
Chillers,
A cylindrical storage tank,
Pumps,
Piping,
Accessories
etc
Chilled Water Storage System
46
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Chilled water in the storage tank is stratified
into three regions
The stored cooling capacity depends
1)Temperature difference between water
return from AHUs and stored chilled water
2) Amount of water stored.
The larger the storage tank, the lower the
capital cost per unit stored volume.
It was found that a chilled water storage
system is economical for large capacity
(storage capacity exceeds 7000 kWh).
47
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Charging
1. Filling the storage tank
with chilled water from
the chiller
2. The warmer return
chilled water from AHU
is extracted from the
storage tank and
pumped to the chiller
11-16oC
4-7oC
48
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Discharging
Chilled water from the
storage tank is supplied to
the terminal units such as
air handling units.
At the same time, the
warmer return chilled water
from the coils fills the tank
with an aid of storage water
pumps.
5-7oC,
49
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Figure of merit (FOM)
It is used to indicate the loss of cooling capacity of the stored chilled water
during the charging and discharging processes.
50
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Figure of merit (FOM)
The smaller the difference between outlet temperature of
stored chilled water during discharging and the inlet
temperature of stored chilled water during charging, the
higher the FOM.
The smaller the losses of cooling capacity during chilled
water storage, the greater the value of FOM.
Well-designed storage tanks have
FOM = or > 90% for daily complete charge/discharge cycles
FOM : 80 % and 90% for partial charge/discharge cycles.
51
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Storage Tanks
Stratified tank, Membrane tank and Empty tank
The stratified tank is the simplest and most efficient method.
Stratified tanks are widely used in chilled water storage installations.
A membrane tank is a storage tank in which a membrane separates the
colder stored chilled water and warmer return water.
An empty tank is a storage tank in which walls are used to separate the
colder and warmer chilled water.
Compared with membrane tanks and empty tanks, stratified tanks have
the advantages of simpler construction and control, greater storage
capacity, and lower cost.
It was found that there is no significant difference in FOM between
stratified tanks and membrane tanks or empty tanks.
52
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Storage Tanks
Chilled water storage tanks are usually flatbottomed vertical cylinders.
A cylindrical tank has a lower surface-to-volume
ratio than a rectangular tank.
Large cylindrical tanks typically have a heightto-diameter ratio of 0.25 to 0.35.
Steel is the commonly used material for abovegrade tanks, and concrete is widely used for
underground tanks.
All outdoor above-grade structures should have
a minimum of 50mm thick external insulation
layer spray-on polyurethane foam, a vapor
barrier, and a highly reflective top coating.
53
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Storage Tanks
Stratified tanks rely on the buoyancy of
warmer return chilled water, which is lighter
than colder chilled water, to separate these two
chilled waters during charging and discharging.
Diffusers are used to lower entering and leaving
water velocity to prevent mixing (<0.3 m/s).
Colder stored chilled water is always charged
from the bottom diffusers which is also for
discharge.
The warmer return chilled water is introduced
to and withdrawn from the tank through the
top lateral diffusers.
Field measurements shows that stratified tanks
have a figure of merit between 0.85 and 0.92.
54
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Vertical temperature profiles
Formed during charging or discharging in
stratified tanks at various time intervals.
Illustrated on a height-temperature diagram at
the beginning, the middle, and near the end of
the charging process.
In the middle of the charging process along the
vertical height of storage tank, chilled water is
divided into three regions:
1. Bottom colder-and-heavier stored chilled water
2. Thermocline
3. Top warmer-and lighter return chilled water.
55
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Thermocline
A thermocline is a stratified region in
which there is a steep temperature
gradient. The water temperature often
varies from 6 to 16°C.
The thermocline separates the colder
stored chilled water from the warmer
return chilled water.
The thinner the thermocline, the smaller
the mixing loss.
The layout and configuration of diffusers
in a stratified tank have a significant
effect on the formation of the thermocline.
56
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Mixing
The inlet temperature of chilled water should be controlled within a
narrow band (say + or - 1°C) during charging to avoid additional mixing.
Mixing at at the start of the charging and discharging processes.
Mixing at formation of the thermocline, and at the inlet side of the
thermocline after the thermocline has been formed.
Mixing near the inlet diffuser (Incoming chilled water initially forms a
thin layer of gravity current that slowly pushes the chilled water
originally in the tank out of the way so that mixing only occurs at the
front of the gravity current).
57
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Mixing
Inlet of the top diffusers : upward or horizontal.
Bottom diffusers : downward and slots spreading
120°.
The cross-sectional inlet area of the branch pipe
leading to the diffuser = or > total area of the
diffuser openings in that branch.
Inlet and outlet streams must be kept at
sufficiently low velocities, so that buoyancy forces
predominate over inertia forces
Large stratified tanks usually
incorporate linear diffusers.
58
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Mixing on the inlet side of thermocline depends on the inlet
Reynolds number Rei and Froude number Fri.
The inlet Reynolds number is closely related to the inlet velocity and is
defined as
Re i 
q
vw
Where
q
= Volume flow rate per unit diffuser length (m3/s.m)
vw
= Kinematic viscosity of water (m2/s)
When Rei < 850, loss due to mixing and loss of cooling capacity during
discharge can be significantly reduced.
59
STRATIFIED CHILLED WATER STORAGE SYSTEMS
The inlet Froude number Fri is defined as
Fri 
q
 3 (i   a ) 
 ghi

i


0 .5
Where
q
= Volume flow rate per unit diffuser length (m3/s.m)
g
= Acceleration of gravity (m/s2)
hi
= Inlet opening height (m)
ρ
i
= Density of inlet water (kg/m3)
ρ
a
= Density of ambient water (kg/m3)
60
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Inlet opening height
hi indicates the inlet opening height m.
For the bottom diffusers, inlet opening height hi
indicates the vertical distance between the tank floor
and the top of the opening of the diffuser.
Stratification diffusers must be designed and
constructed to produce and maintain stratification at
the maximum flow through storage.
Designers typically select a diffuser dimension to
create an inlet Froude number of 1.0 or less (streams
at low velocities => buoyancy force predominate inertia
force).
61
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Self-balancing
It should be achieved on evenly distribution of the flow by:
The piping design should be symmetric.
1.
Branch pipes should be equal in length.
2.
Flow splitters should be added at the appropriate points.
3.
Pipe diameter reduction should be combined with the flow splitter.
4.
Long-radius elbows should be used.
62
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Storage Tank Insulation
Exposed tank surfaces should be insulated to maintain
the temperature differential in the tank.
Insulation is especially important for smaller storage
tanks (high surface area to stored volume ratio).
Heat transfer between the stored water and the tank
contact surfaces (including divider walls) is a primary
source of capacity loss.
63
A Comparison – From Some Papers
1. Internal melt ice-on-coil systems are the most commonly used type of ice
storage technology in commercial applications.
2. External melt and ice harvesting systems are more common in industrial
applications, although they can also be applied in commercial buildings and
district cooling systems.
3. Encapsulated ice systems are also suitable for many commercial applications.
4. Ice slurry systems have not been widely used in commercial applications.
See also the attached table.
64
Question and Answer