|
Laser
Show Systems - System Design
Ultimate
Water Works
Abstract
The supply and regulation of the water to large frame lasers
can pose many problems. This paper discusses a theoretical system designed
to eliminate many common problems. Since this is a theoretical water
supply system, expense and physical size were not considerations in the design. The
design addresses common problems such as pressure drops, condensation and
even power failures with a novel approach.
Introduction
Every laserist who has ever done a gig with water cooled lasers
knows the problems that can occur at the show. Complete loss of
cooling water due to idiot venue staff turning off the supply or
disconnecting hoses to fill a bucket... Careless roadies placing a heavy
case on the drain hose... The dreaded "john effect" that cuts off
the water just a minute before show time necessitating a delay...
Condensation when operating at raves in winter with very cold water
flowing through the system in an overheated venue with thousands of people
dancing... The client pointing to a nearby pond or lake as the source of
cooling water.
Lasers were primarily designed as laboratory equipment to operate in a
controlled environment. In many cases, they are just not able to cope
with the wide fluctuations in water quality, flow and pressure that
occur in mobile applications.
Current
solutions
Most laserists use at least a pressure regulator and a
filter on the water to keep the pressure within a known range and to keep as
much dirt as possible from clogging the cooling system. In many cases
a pump is added to insure that water is pulled from the feeder lines so that
an adequate water supply is available. Some laserists add pressure
tanks to act as a 'ballast' which smoothes pressure fluctuations in the
lasers and insures continuing water to the lasers for a short period of time
in the event the supply goes below critical pressure.
The best present solution to water supply problems is to run the laser(s) on an
industrial chiller. This is a closed loop system with a refrigerator
which provides a constant supply of chilled and pressurized water.
These are typically heavy and expensive machines and are not immune to power
failures. A loss of cooling water to a large frame laser operating at
full power can result in a cracked tube.
Ultimate
Water Works
The ultimate water works would address all of these problems
by being designed specifically for laser applications. It would insure
a continuous supply of cooled and pressurized water. It should also
provide a continuing flow of water and some cooling in the event of a power
failure to insure the tube is not damaged from boiling off the water in the
cooling jacket, or from thermal shock when cooling water flow is re-established.
Looking at the diagram above, we see that these issues are addressed
in a number of way to maximize laser cooling. The system is
essentially closed loop on the laser side [green lines], with the laser
water being cooled by the city water supply [blue lines]. You will
note that all electrical devices are powered by 12 VDC which is provided by
a battery which is charged continually during operation. In the event of a
power failure, the battery would provide sufficient power reserves to
operate the system until the laser had cooled to a safe temperature.
Laser Cooling
section
Starting at the drain from the laser exciter, a quick
connect sends the water to the first device which is a proteaus flow
switch. This is placed in the drain hose so as to detect any failure
that may occur in the laser system itself such as a disconnected hose or a
major leak. The signal from the flow switch goes to the interlock and
alarm system [orange lines] such that if the flow falls below the pre-set
value, the interlocks are opened and an alarm is given..
The second device encountered is a water temperature detector which measure
the temperature of the drain water to insure the laser is receiving adequate
cooling. The temperature detector would be a pre-set type that would
send an alarm and open the interlock if the drain water temperature went
above the set point.
The next section is a set of three radiators and fans designed to be able to
shed 5 KW of heat . These operate at all times but do not play a major
role in laser cooling under normal circumstances. In the event of a
power failure or city water failure, these radiators and fans allow the closed loop
to continue cooling the laser water slowly so as to prevent thermal shock to
the tube.
After the radiator and fans assembly, the water encounters two water to
water heat exchangers. These are compact devices that are cooled by
the city water supply and designed to shed 5 KW each. This first
section sheds 10 KW of heat before the reservoir.
The reservoir holds the supply of cooling water for the laser system and has
an overflow in case of emergencies. For even more bullet-proof
operation, a detector could be fitted to the overflow to detect water and
open the interlock and/or give an alarm.
The reservoir is equipped with a quick connect on the output side.
This can be uncoupled and connected to the 12V air compressor for blowing the
water out of the lines after the show.
After the reservoir is a strainer to remove any gross particulates in the
water. A 12V, 3 gallon per minute, self-priming, RV type pump with
it's own small pressure tank pressurizes the water in the laser cooling
loop.
Immediately after the pump is a pressure switch to detect that the pump is
functioning. It is a normally open type which is closed when the pressure
is above 15 PSI. It is also connected to the interlock and alarm
system.
Next is a check valve. This device only allows the water to flow in
one direction. It is required for the pressure tank which follows so
that the tank remains charged and the water does not flow back through the
pump in the event of a pump or power failure.
The pressure tank is a device with an air bladder in the top part of the
tank. When pumped full of water, the air bladder is compressed.
When the flow fails, the air in the bladder expands forcing the water out of
the tank. Typically pressure tanks have a 'draw down' of half their
volume, In other words, a fully charged 6 gallon tank will allow 3
galloons of water to flow before it is depleted. This is about enough
to provide one minute of cooled water to the laser in the event of a catastrophic
failure.
After the pressure tank, a set of quick connects allows for optional
additional cooling. These could be connected to additional radiators
and fans if desired.
The water then floes through four additional water to water heat
exchangers. This is where the majority of the cooling takes place as
they provide 20 KW of cooling.
A second 6 gallon pressure tank provides another minute of reserve water
cooling in the event of a failure. This is followed by a cartridge
filer with disposable elements and a pressure regulator with gauge.
Part of the function of the pressure regulator is to 'throttle' the amount
of water flowing thought the system to insure that the pressure tanks remain
charged.
The temperature of the water supply to the laser is measured at the shut off
valve before the quick connect that connects the laser to the system.
By keeping this temperature a few degrees above the dew point, condensation
within the laser and the exciter is prevented. This temperature
detector is also connected to the interlock and alarm system so that if the
water goes above the pre-set point, the laser is shut down. This
detector also controls a solenoid valve which will be discussed later.
City Water
Section
The cooling of the laser is ultimately provided by the city
water as in conventional laser cooling. In this case, the city water
is isolated from the laser water to minimize problems from interruptions or pressure
fluctuations in the city water. This type of isolation is also
desirable in situations when less than optimal water
sources such as rivers and lakes must be used.
The city water enters the system and encounters a water hammer
arrestor. This is a device that is designed to minimize shocks, such
as those produced by solenoid values on dishwashers or spring loaded toilet
valves, in the water flow. Note that to be effective, the water hammer
arrestor must be facing the direction in which shocks will propagate and the
water must flow at a right angle to this direction.
A strainer is provided to remove any gross particulates in the water.
A 12V, 3 gallon per minute, self-priming, RV type pump with it's own small
pressure tank pressurizes the water in the city cooling section. If
the city water pressure is high enough, this need not be used. In the
case of inadequate pressure, or water that needs to be pulled from a lake,
the pump can be used.
Immediately after the pump is a pressure switch to detect that the pump is
functioning. It is a normally open type which is closed when the pressure
is above 15 PSI. It is also connected to the interlock and alarm
system [red line] in conjunction with a overpressure switch on the output
side of the city water system to detect if the flow is blocked.
Next is a check valve. This device only allows the water to flow in
one direction. It is required for the pressure tank which follows so
that the tank remains charged and the water does not flow back through the
pump in the event of a pump or power failure.
After the pressure tank, there is a cartridge type water filter with
disposable elements and a pressure regulator with meter.
From here, the water flows freely through the first and third pair of water
to water heat exchangers encounter by the laser water to provide approximately
20 KW of cooling.
City water flow through the middle set of of water to water heat exchangers
is controlled by the solenoid valve. This opens when the temperature
of the laser cooling water raises above a pre-set point to provide
additional cooling. A second water hammer arrestor is provided to minimize
water shocks to the system cased by the operation of the solenoid
valve. Note that a small diameter bypass tube is provided so that
there is always some small amount of cooling to prevent thermal shocks when
the additional 10 KW of cooling is brought on-line. This set of heat
exchangers is also placed in the middle of the three pairs to further
minimize thermal shocks. Naturally this solenoid based system will not
provide the fine temperature control that a chiller would provide, but it
would be sufficient to keep the laser water within the desired parameters.
After the water has passed through the heat exchangers, it heads for the
drain via the overpressure switch mentioned earlier [to detect a blocked
drain] and through a flow gauge. This would be a simple sight glass
and spring type to indicate that sufficient water is flowing thought the
system.
NOTE: The extensive use of quick connects throughout the system allows
for the flexibility to connect the laser directly to the city water if
desired, or to reconfigure the system in the event that a critical component
fails.
Control
System
The water works would need a control system that also
operates from 12 VDC. This system would do all of the temperature and
flow detection and control the laser interlocks and alarms.
The block diagram above gives an idea of how the control
system might work. Note that there are two interesting features.
Since the laser is not directly cooled by city water, small drops in
pressure or flow can be tolerated on the city water side. There is
nothing more annoying than to have the water interlock on your later trip
moments before the show due to a few seconds of sag in pressure and then
have to wait two minutes with an anxious client breathing down your neck
while the laser re-starts. In this proposed system, a user adjustable
short delay timer on the city water detection circuit will allow short
duration city water failures to be ignored.
The alarm system also incorporates both an audible alarm that can be switched
off, and a strobe light. At corporate shows, the last thing you need is a shrieking
water alarm while the big cheese is making an important speech.
Conversely in high noise situations such as a rave where you are wearing ear
plugs, you will never hear the alarm. The strobe light allows for an
effective visual alarm as long as the water system is within visual range of
yourself or one of your staff.
Summary
The water system described above address many issues with
water supply for lasers. It provides detectors with alarms in the
event of flow failure or over temperature of the laser water. It would
be more compact and lighter weight than a conventional chiller since the
cooling is provided by city water in a closed loop configuration. It
provides 2 minutes of cooled water at 3 GPM in the event of city water failure
and radiator cooling for the laser water after that to bring the laser down
to a safe temperature and avoid thermal shock to the laser tube. The
battery operation means that cooling can continue even in the event of a
power failure and total loss of city water.
The control system would give you detailed information on the flow and temperature
of the water to the laser as well as providing audible and visual alarms in
the event of a failure.
DISCLAIMER:
Some of the information in the Backstage area is provided by the persons or
companies named on the relevant page(s). Laser F/X does NOT endorse or
recommend any products/services and is NOT responsible for the technical
accuracy of the information provided. We provide this information as a
service to laserists using the Backstage area.
[
Introduction - System
Design - Scanning Systems
- Support
Equipment - Laser
Graphics - Show
Production - Pinouts ]
|
