3- Evaluation of the needs of collaborations for cooling prototypes
and for tests.
4- The different measurement systems, (temperatures, pressures, flow-rates,
etc. )
5- AOB
Short Summary:
The operating principles of various relevant cooling systems were presented,
and two families identified:
- fluid circulation systems, based on positive or sub-atmospheric pressure
making use of the specific heat of fluids (systems using binary ice are
in this family).
-evaporative systems employing the vaporization heat of a fluid.
Accent was placed on the fact that, irrespective of the basic system
decided on in detector design, pipes have to be arranged horizontally
and as straight as possible so as to leave roomfor different
alternatives.
Detailed Minutes:
1- Approval of the previous set of minutes:
After comments by R. Gregory and M. Hatch paragraph 6-AOB was rewritten
as follows:
M. Hatch presented Atlas's overall cooling circuit diagram and pointed
out that, in general, CERN (i.e. ST/CV) would be responsible for it up
to the primary exchangers (cold side) in the main cavern. The secondary
circuits, including the heat exchangers, to the detector electronics or
other heat sources, were the entire responsibility of each of the collaborations
responsible for their own detectors. It was up to them to call in outside
assistance.
As representative of the Alice Infrastructure Group, R. Gregory explained
that they will support the cooling systems for the Detector.
Many were interested in the binary ice system and a presentation by
the Atlas Rutherford Group as part of their TDR was planned for the near
future.
2- Overview of cooling systems and usable fluids:
M.Bosteels reviewed the operating principles of the various systems
that could be used for cooling detector electronics.
Leaving aside forced air systems which clearly can no longer cope with
the power dissipation on the scale announced, we can group the systems
under two major families: fluid circulation systems, and those based on
evaporation.
2-1- Fluid circulation system:
Such systems use the specific heat of fluids in circulation to convey
heat from the warm (secondary exchanger) to the cold side (primary exchanger).
Even if the latent heat of fusion is also used, the binary ice system can
also be included under this heading as the circuit design must be based
on the same principles.
Three systems of this type have been explained:
2-1-1- Conventional overpressure system (closed circuit system): Figure
2-1
This system is the best known as it is typically the central heating
system used in homes; as it is a closed system it carries with it certain
problems:
- It must be linked to another pressurized circuit so that it can be
refilled;
- It must withstand pressure differences and so must have an expansion
tank and safety valve.
- Venting is not easy and if there are significant height differences
in the system, each high point must have a bleed.
- For the same reason, draining can be complicated if the pipes are
not straight.
The fluid flows from an atmospheric pressure tank, it is easy to vent
and needs no extra chambers. It allows for some flexibility in circuit
design and the part that is higher than the supply tank goes into sub-atmospheric
pressure when the pump is not running.
The supply tank must hold enough water to fill the entire circuit.
Emptying it presents the same constraints as a conventional pressure
circuit.
2-1-3- Sub-atmospheric pressure system (Leakless): Figure
2-3
Fluid is circulated through an atmospheric pressure tank and the circuit
has an active venting device in the form of a vacuum pump. That part of
the circuit situated between the tank and the bleed is subject to sub-atmospheric
pressure if the pump is of the right size, which has the added bonus that
no fluid is discharged if there is a leak in this part. This permits disconnection
of the various components of the circuit without stopping it.
Its most serious shortcoming is the maximum total pressure loss (theoretically
of 1 bar, 700 mbar in practice) in the "leakless" portion of
the circuit.
There are no venting problems but as with the other two circuits, airlocks
should be avoided for draining.
The tank can be refilled using an automatic filling system..
2-2- Evaporation systems:
Such systems use the latent heat from evaporation of a fluid to transmit
heat and would be particularly appropriate for detectors operating at temperatures
lower than 0°C.
Two types of circuit can be considered:
2-2-1- Conventional evaporative system with compressor and expansion
valve: Figure 2-4
Cold production is based on the principle of compression and expansion
of fluids chosen for their stability characteristics, such as C6F14 or
C5F12, that can be used at sub-atmospheric pressures. This system has a
great heat exchange capacity because it uses the latent heat of evaporation
of a fluid ( in the 90[kJ/kg] range as compared with the 4[kJ/kg.K] of
the specific heat of water) and theoretically can have a 0 temperature
gradient.
Its practicability with large detectors still remains to be shown because
it is hard to set up.
Problems encountered include:
-The length of the circuit in a major detector (~50m) unusual for such
systems.
- The difficulty of regulating fluid evaporation over the entire length
of the exchange pipe and several exchanges mounted in parallel.
- Pressure differences resulting from height differences.
- As the fluid is returned in gaseous state, wider pipes are needed,
which may reduce the advantage in quantities of material that this system
ought to yield, and there is certainly an imbalance in the quantity of
material surrounding the detector.
It may be concluded that before selecting the technique, full-scale
tests must therefore be run.
2-2-2- Evaporative system with fluid circulation: Figure
2-5
This system is in fact based on a combination of two principles because
it operates on the circulation of fluorocarbons (such as C4F10) through
a primary exchanger and partial evaporation in a secondary exchanger. Temperature
is function of pressure and return pipe has to be precisely calculated.
Its advantage is simplicity because there is no expansion valve requiring
regulation but its effectiveness and implementation need to be tested because
there is no known application.
In conclusion, and generally speaking, the use of liquids in pipework
requires a certain level of care in design Figure
2-8 , irrespective of the system opted for, the airlocks Figure
2-6 in the pipes can always be a source of problems if not disaster.
Incorrect calculations on the exchangers mounted in parallel can produce
the same situation.
In the same way the fact that the same routes are used as power cables
acts as a considerable limitation on the choice of cooling system Figure
2-7 .
Among the other problems raised is the question of potential vibration
in the structure that can occur from turbulent flow through piping. Particular
attention was also paid to the minimum exchange surface needed for heat
transfer between the inner wall of the exchanger pipe and the fluid. The
minimum diameter of the pipes would in the long run also be more governed
by the surface area than by load losses.
Finally, it should not be forgotten that the more complex a system
is the dearer it gets.
The situation with regard to fluids is much clearer:
-For ambient temperature systems, water is obviously the best fluid
and its characteristics are known; what remains to be decided is its quality
and that of its additives as function of the materials used in the circuit.
- For systems operating at -20°C , depending on the type of cooling
system opted for, we can choose between water (or binary ice) with antifreeze
additives (glycol, methanol, ethanol and acetone) and the fluorocarbons.
If the characteristics (specific heat, density, viscosity) of the latter
are available from producers, the same is not true for water and antifreeze
in the same temperature range. Tests with glycol and methanol have been
made at the DPNC University of Geneva and RAL Rutherford and are available
(for viscosity). The same tests are due to be run with acetone at RAL.
If we wish to take systematic measurements (and pay for them), V.Vacek
has contacts with 2 institutes (Imperial College, London and Aristotle
University, Thessaloniki) which are equipped for the purpose.
The greatest part of the work to be done is on the corrosion of pipes,
because we are talking of wall thickness of just a few dozen microns that
must be capable of lasting for at least 10 years.
This subject will be dealt with at a future meeting but it is already
clear that a very serious study and test programme will have to be set
in motion.
3- Evaluation of the needs of collaborations for cooling prototypes
and for tests:
The message still has not gone the rounds of the collaborations since
no figures have been given.
To choose a unit, the following parameters must be determined:
-Dissipated power
-Operating pressure
-Control temperature
-Permissible temperature gradient
-Degree of precision
-Fluid used
-Source of energy (refrigerator, chilled water, lake water, etc.)
It is important to recall that use of the CERN drinking water supplies
for cooling is not permitted.
M.Bosteels presented a module-based system with electronic regulation
produced by his section. It can be used under pressure or in leakless mode,
with a refrigerator or using chilled water. Its power can reach 2kW.
Prices vary from 4 kCHF to 7 kCHF depending on the version.
4- The various measurement systems, (temperatures, pressures, flow-rates,
etc. ):
This subject is very broad and it is not within the scope of the CWG
to study each technology and each type of measurement. The object of the
exercise is rather that those with practical experience in a measurement
field and with a specific type of equipment should share it so as to avoid
everyone is repeating the same mistakes. For example, a Pelton (Badge Meter
PWP) turbine flow meter showed serious sealant shortcomings when it had
to operate at -15°C.
Or some measurement devices cannot withstand the magnetic fields reigning
in the detectors.
Moreover, many teams are developing applications using LABView and
it would be interesting to share experience.
The main areas of measurement related to cooling are the following
(non-exhaustive list):
- Some people are beginning to raise the question of power cable dissipation
which could seriously affect temperature regulation in the detectors. More
data are needed before a discussion can be embarked on.
- There is a strongly felt need for an interactive data base concerned
with everything connected with heat exchange. It is a very broad subject
and requires an enormous amount of work once we have the Web site.
-Some people are also interested in technological systems connected
with cooling screens.
-It is proposed to enquire into all incidents connected with cooling
that have occurred in the LEP experiments.
The next meeting would be held on 19/02/97, Building 40
Room RA10 from 10 a.m. to 12 noon.
Agenda:
-Approval of the previous set of minutes.
-Presentation of needs and work already done on cooling in ALICE, ATLAS
and CMS (detectors and power racks).
-H.P.Gugerli from Advanced Ceramics Corporation will present their
last product in thermal management for electronic packaging
-T.Niinikoski will make a presentation of an 8 channel precision thermometry
system.
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