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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.

Water works diagram

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.

Control system block diagram

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.

 

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