0262 1762/06 © 2006 Elsevier Ltd. All rights reserved WORLD PUMPS August 200622

According to current climateforecasts, the next 25 to 50years will see the guaranteed-

snow boundary in skiing areas recedefrom 1200 to 1500 m above sea-level(Figure. 1). Without snow, though,winter sports are impossible.Consequently, more and more ski-liftoperators in the Alps are installingsnow-making systems. The basictechnology was developed in the USAin the 1950s. Nowadays, 90% of all skiresorts have taken to producingartificial snow. More than 1500 snow-making systems are in operation aroundthe world, and roughly 500 of them arelocated in Europe.

Snow forms

Natural snow forms when a mass ofmoist, warm air encounters a cold layer(stratum) of air and cools down as aresult. The air cooled this way is no

longer able to hold the moisture thatwas in it before. The moistureprecipitates. If the surroundingtemperature is extremely low, theprecipitating water will begin tocrystallize. If particulate matter (dust) ispresent, crystallization will be triggeredmore quickly. The crystals descendthrough a number of different air stratawith different moisture levels andtemperatures and gradually assume thekind of shape we know as snow (Figure. 2).

Artificial snow

The generation of snow by mechanicalmeans copies the process that was justdescribed. There are, however, certaindifferences: the temperature of thewater used for making snow must beabove 0°C. Otherwise, it would freezeon its way from the water tank to thesnow gun. This means that the crystalnuclei (= seed crystals) needed forconverting the water into snow firsthave to be artificially generated byatomizing jets of pure water in spraynozzles mounted on the snow gun. Inthe short time between the droplets'ejection from the nozzles and theirlanding on the ground, they have toturn into crystals of snow. That, inturn, requires rapid super-cooling.Snow-making works all the better thelower the ambient temperature risesabove -3°C (27°F), snow-makingoptimal conditions for making snow.The snow should be as 'dry' and light aspossible in order to achieve thefavourable physical characteristics ofhigh air permeability, low thermal

conductivity and low icing tendency,temperature and humidity. If thetemperature is above -5°C (21°F), onlyvery little water can be passed througha given snow gun, and if the tem-becomes uneconomical. Cold, dry airprovides optimal conditions formaking snow. The snow should be as"dry" and light as possible in order toachieve the favourable physicalcharacteristics of high air permeability,low thermal conductivity and lowicing tendency.

Snow-makingsystems High-pressure snow guns orsnow lancesSnow lances are often used on steep,poorly accessible slopes. A centralcompressor station and a system ofsupply lines provide compressed air andhigh-pressure water to the individuallances, i.e., the hydrants only haveconnections for air and water - noelectric power is needed. The water isatomised as it emerges from the waternozzle. Compressed air is blown intothe atomised water. As the air emergesfrom the air nozzle, it expands and coolspractically instantly. The resultant coldshock produces ice nuclei (or seednuclei) around which crystals of snowcan form. Snow lances stand as much as10 metres high in order to give theprecipitation enough time to formsnow crystals before it falls to theground. The disadvantage of lances isthat wind can keep the snow fromlanding where the ski run operatorwants to have it.

Pumps and valves forsnow generation Skiing is becoming ever-more popular; unfortunately as the planet becomes warmer andsnow-boundaries retreat ever higher from sea-level, finding suitable places to ski isbecoming harder. However, one solution is to create artificial snow through high-pressurepumps, but this process has its own problems. Here Arnold Ofner and Christoph P Pauly ofKSB present some considerations in the art of successful snow making.

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Figure 1. Globalwarming is costingthe Alps its snow.

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Low-pressure blowerBlower-type snow-making machineshave their own compressors forproducing the required com-pressed air.Consequently, they need both high-pressure water and electricity fromtheir hydrants, but no compressed air.The pressurised water and thecompressed air are mixed together in asprayer. As the mixture emerges fromthe sprayer, the pressurised air expandsand cools both the water and thesurrounding air. This produces icecrystals (nuclei). At the same time,atomised water is ejected from a ring ofnozzles surrounding the nucleatornozzle to help the ice crystals turn intosnow crystals. A large fan blows themixture high up into the air tomaximise the time available for snowcrystals to form. Most propeller-typesnow guns are mobile models that arehauled to the desired connecting pointon the slope by helicopter or snowcatvehicle. Subterranean hydrants at theconnecting points allow their pinpointpositioning to get the snow where it isneeded.

Snow-making

The water needed for generating snowis taken from open waters, a reservoir(storage pond) or even the localdrinking water supply system. Anywater taken directly from the latter, aswell as groundwater, would be toowarm for snow-making purposes andtherefore has to be cooled downbeforehand. Consequently, it is mucheasier to draw water from a storagepond, where the water cools down byitself (the colder the weather, thebetter) and significantly reduces theoverall energy requirement. Also,

storage ponds can serve as retentionbasins for surface runoff, hence helpingto prevent erosion. A typical snow-making system consumes somewherebetween 0.25 and 2.8 kWh per squaremetre of artificially snow-covered skirun (30 cm deep), depending on thesize and design of the system. A largesystem can consume 500,000 kWh ormore in a season. That is roughly thesame as the annual power consumptionof a year-round artificial ice rink.

Pumps

Snow-making systems have a low-pressure section and a high-pressuresection. The low-pressure sectioncomprises the water extraction,filtration and flow-meteringequipment. If the water is taken from alower point, submersible motor orborehole pumps are needed. The typeof pump actually employed depends onthe local terrain and, hence, therequired head, as well as on howcontaminated the water is. If the watercomes from a stream, it is likely to becontaminated with sand and gravel.Dry-installed process pumps (Figure 6)are practice-proven for watercatchment under suction headconditions (Figure. 5). Since all of thepipes are normally of ample size, so thatthe pressure stays below 16 bar, soft-seated, symmetric butterfly valves aremostly used.

The high-pressure section naturallyincludes the multistage pumps andtheir accompanying minimum flow,shut-off and drain valves. The high-pressure valves have nominaldiameters ranging up to 250 mm, andthe system pressures are around 100bar. Indeed, the pump dischargepressure cannot be any higher due tothe hydrants and other fieldequipment. Consequently, shut-offvalves and gate valves made of steeland belonging to pressure class PN 100are used here most frequently.

High pressure

Before we can calculate the required flow rate of the pump or

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Figure 2. Snow crystal.

Figures 3. High-pressure snow guns or snow lances.

Figure 4. Low-pressure snow gun.

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pumps to be used in a snow makingsystem, we first have to determinehow much snow will be needed. The answer to that question is theproduct of the run's surface area, insquare metres, and the targeted depth of snow.

Good skiing requires a snow depth ofsome 30 to 40 cm. Artificial snow asa specific gravity of 400 to 500kg/m3. With water weighing approx.1,000 kg/m3 the water/snow factorfigures to between 2.0 and 2.5.Dividing the required volume ofsnow [m3] by the water/snow factor,we arrive at the required watervolume [m3]. Next, we have todecide how long it should take toapply the initial snow cover.Normally, it will take somewherebetween 70 and 100 hours. Now,dividing the required water volume[m3] by the time [h] needed for

applying the initial snow cover, wearrive at the requisite flow rate[m3/h]. Finally, the discharge head[hreq] the high-pressure pump isobtained by adding the geodetichead [hgeo] and the pipe friction(head) losses [hpipe] the requiredsnowing pressure [hsnow] at thehighest point (150 to 200 m).

Example:

• Area of ski run requiring snow: 10 ha

• Duration of initial application: 75 h

• Initially applied snow depth: 0.35 m

• Water/artificial-snow factor: 2.25

• Required flow rate: 207 m3/h [60 l/sec]

• Geodetic head: 400 m

• Required snowing pressure: 153 m

• Pipe diameter: DN 200

• Pipe length: 1800 m

Example calculation of fluid warm-upin a pump when producing the totalflow rate and discharge head with onepump/two pumps (discharge head: 580 m).

• Pipe material: cast iron with cementmortar lining

• Coefficient of pipe roughness, k: 0.1 mm

• Pipe friction losses acc. to Moody:[60 l/sec] 30 m

• Required discharge head: 583 m

The pump input power figuresaccording to the following well-knownequation:

Preq = 460 kW

Required motor rating: = 500 kW

Heat problems

Due to the rough treatment that skiruns have to contend with, today'soperators cannot avoid addingartificial snow at practically anyoutdoor temperature. Unsteadyweather conditions and fluctuatingrelative humidity levels call forappropriately adjusted snow-gunoutputs. The warmer the weather,the less water that can be turnedinto snow. Frequently, this meansthat the pumps will have to rununder low flow conditions.

The resultant loss of efficiency tends –undesirably – to warm up the pumpedmedium. In a large pump, the heatgain can be as much as 3 °C, and thatcan be quite disadvantageous whenthe outdoor temperature is hoveringaround the freezing point.Consequently, it is often a good idea toconnect several pumps in parallel, soeach individual pump can be keptoperating at or near its best efficiencypoint. Moreover, several pumps canprovide more total throughputcapacity than can a single pump.

The required total pump inputpower remains unchanged. Anotheraspect that speaks in favour ofequipping a snow-making systemwith several high-pressure pumps isthe resultant gain in operatingreliability. Even if one pump unit

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Figure 5 . Water crib with suction head conditions.

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should fail, the system can at leastremain in operation, if not at fulloutput. Considering how the naturalsnow season is becoming shorter andshorter, major economic damage canresult from the outage of a singlehigh-pressure pump. The multiple-pump option also keeps thetemperature rise much lower duringlow flow operation. With regard tothe higher investment cost,however, parallel operation can only

be recommended for systems with atotal throughput of more than 40litres per second. In principle, thegain in temperature can also becountered by use of a speed controlsystem.

Summary

Due to ongoing climate change, nowinter sports region can get along

without snow-making equipment.Experience gathered over the pastfew years shows that it is always agood idea to include enough pumppower to cover future developments,too. This will ensure that there isadequate pump input power availableshould the number of snow-makingunits be increased. Thus, whenselecting the pump hydraulic systems,care should always be taken to makesure that the units will be able to copewith low flow conditions. In thatconnection, speed control presentsitself as a good solution. The very firstspeed-controlled pumps for snow-making systems were installed byKSB in Austria in the early 1980s. ■

CONTACTKSB AktiengesellschaftJohann-Klein-Strausse 9D-67225 FrankenthalGermanyTel: +49 6233 860Fax: +49 6233 863401Email: info@ksb.comwww.ksb.com

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Figure 6. Dry-installed process pumps (Etanorm).