It is the second week of November, the wet bulb finally drops below 26 degrees at nine at night, and you have a six hour window to lay down the base on the beginner trail before the weather turns again. Everything on the mountain is ready except one thing: the compressor room cannot hold pressure with all forty guns open, so you are running thirty.

That is the whole business. Snow gets made in windows, the windows are short, and the size of your window is set by how much air you can put on the hill. Compressed air for snowmaking is not a support utility at a ski area the way it is in a machine shop. It is the production line.

Why a resort's air demand looks nothing like a factory's

A factory buys a compressor for a load that repeats every day for twenty years. A ski area buys one for a load that runs flat out, at 100 percent, for maybe eight hundred hours a year, in the cold, at altitude, and then sits idle for seven months.

Four things follow from that, and they drive every decision in the plant room:

• The load is enormous and it is all or nothing. Published figures from resort plants run into the tens of thousands of CFM. Roundtop in Pennsylvania has been described as running ten compressors for a combined output around 30,750 CFM, and Tahoe-area resorts in the same range near 28,500 CFM. Even a small community hill with twenty guns is looking at a few thousand CFM.
• Cost per CFM is the whole economics. Because the machines run at full load whenever they run at all, power is the dominant line item. A rough industry rule of thumb puts a lubricated screw at about 5 SCFM per horsepower at 100 psig. Every point of efficiency you give up shows up on the November and December bills.
• Altitude derates you. Thin air means less mass per stroke. If your plant sits at 8,000 feet, the nameplate CFM on a machine rated at sea level is not what you are going to get. We covered the math in our writeup on air compressor altitude derating, and it matters more here than almost anywhere else.
• Everything happens below freezing. Condensate that would drain harmlessly in a Georgia plant turns into an ice plug in a nucleator nozzle at 2 a.m. on a ridge.

What the air is actually doing at the gun

Snow guns fall into two families, and they put very different demands on the plant.

Air and water guns use compressed air for two jobs at once. A small nucleation stream flash-expands, drops in temperature and creates the ice nuclei that water droplets freeze onto. A larger stream atomizes and throws the water. These guns are cheap, light, easy to move, and they make snow in marginal temperatures better than anything else. They also eat air.

Fan guns use an electric fan to atomize and throw, and compressed air only for nucleation. Air demand per gun drops by a large multiple. In exchange you are paying for the fan motor, a heavier machine, and more money per gun up front.

Most mountains run a mix, and the mix determines the plant. The ratios are worth understanding because they explain why two resorts with the same trail count can have wildly different compressor rooms:

Function	Typical air to water ratio (SCFM per GPM)	What it means for the plant
Nucleation only	Often above 20:1 on the nucleator stream itself	Small absolute volume, high pressure sensitivity
High efficiency air and water gun, whole gun	Under 5:1	Moderate volume per gun, many guns
Older or low efficiency air and water gun	Well above 5:1	This is where legacy plants bleed money
Fan gun	Nucleation air only	Smallest air draw per gun, biggest electrical draw

Pressure matters as much as volume. Nucleation generally works somewhere between 60 and 120 psi, and most snowmakers operate in the 70 to 100 psi band. Too low and the expansion does not drop the temperature enough to nucleate. Too high and you can freeze the nozzle solid. That band is narrow enough that a plant which sags 15 psi when every gun opens is not just inefficient, it is producing worse snow.

Sizing the plant

Do not size from horsepower or from what the neighbor mountain bought. Size from the guns.

1. Build a gun inventory with its actual SCFM draw at operating pressure. Manufacturer curves, not marketing numbers. Note which guns are on which trails.
2. Define your realistic maximum simultaneous load. Not every gun on the mountain at once. The number you actually open on the coldest night when you are pushing to open terrain.
3. Add the distribution loss. Miles of buried pipe up a mountain is a real pressure drop, and it is not small. Our pipe sizing guide covers the math, though a resort loop deserves an engineered design rather than a rule of thumb.
4. Derate for altitude and for intake temperature. Cold intake air is actually denser and helps you, which is one of the few breaks the mountain gives you, but altitude takes more than the cold gives back at most western resorts.
5. Add headroom for one machine being down. A compressor failing on December 18 is the difference between opening for the holidays and not.

A demand analysis with data logging on the existing plant is worth paying for before you spend capital. Our note on what a compressed air audit actually is explains what you should expect to get out of one.

Screw or centrifugal

This is the argument in every plant room on every mountain.

A rotary screw is the practical choice for most hills. It handles the seasonal start and stop, it tolerates being shut down for seven months, the service is straightforward, parts are everywhere, and the capital cost per CFM at the sizes most resorts need is far lower. For anything under a few thousand CFM it is usually the only sensible answer.

Centrifugal machines come into the conversation at large resorts with very high continuous volume. They are oil free on the air path, which removes the carryover risk entirely, and at the top of the size range they are more efficient per CFM. They also cost a great deal more up front and want a more sophisticated maintenance program. Some large resorts have moved that direction specifically to eliminate oil carryover and gain efficiency at scale.

If you are on screws, and most mountains are, a few things earn their keep:

• Variable speed on at least one machine. Your load swings with temperature and with how many guns the crew has open. A fixed speed bank with nothing trimming the top is wasteful. See our breakdown of control modes if you are deciding how to sequence.
• A real sequencer. Several machines running part loaded is the classic way to waste a hundred horsepower without noticing.
• Heat recovery. You are dumping enormous heat in a building that is cold and often needs to heat a shop or a maintenance bay. Our writeup on compressed air heat recovery covers what is realistic to capture.
• The CAGI data sheet on anything you are quoted. At the hours a resort plant runs, a small difference in specific power is a large number. See how to read a CAGI data sheet.

Cooling and drying, where most plants fail

Here is the part that is specific to this industry and it catches new plants every time.

Air leaving a screw compressor can be well over 180 degrees F. Hot air arriving at a snow gun does not make snow. It has to come down to close to ambient before it reaches the hill, and published guidance around snowmaking talks about getting discharge air down near 36 degrees F at the gun. That happens through aftercooling at the plant and then through the run up the mountain in a buried or exposed line, which is doing a lot of the cooling for you whether you planned it or not.

And that is exactly the problem. Every degree the air cools in the pipe is water coming out of it. A line that leaves the plant at 100 degrees and arrives at the gun at 20 degrees has condensed a great deal of liquid somewhere along the way, and that liquid freezes at the first restriction it finds, which is usually a nucleator orifice.

What a working plant has:

• A properly sized aftercooler on every machine, so most of the water drops out where you can drain it rather than on the mountain. This is the single most important piece of equipment in a snowmaking plant after the compressor itself, and we go through sizing, air cooled versus water cooled, and the separator and drain behind it in aftercoolers for snowmaking.
• A separator and drains at the plant, and drains at every low point on the hill line. These need to be checked, because a frozen or seized drain in this climate is not a slow problem.
• Enough drying that the pressure dew point is below the coldest pipe temperature on the mountain. This is the part plants skimp on and then spend the season thawing nozzles. Our guide to pressure dew point explains why the dryer rating and the pipe temperature have to be compared directly.
• Filtration ahead of the hill line, because a nucleator orifice is small and unforgiving.

Oil carryover deserves its own line. On a lubricated screw, a tired separator element puts oil into the air, oil coats the inside of the hill line, oil ends up in the nucleator, and oil ends up in the snow. It plugs nozzles, it is an environmental problem on a watershed, and it is the single best argument for staying on top of separator element service. Our post on tracking down oil in the lines walks the diagnosis in order.

The mistakes we see at ski areas specifically

• Sizing to the gun list instead of the operating plan. Plants get built for every gun open simultaneously, which never happens, and the capital gets spent in the wrong place.
• Ignoring altitude on a replacement machine. A like for like swap by nameplate CFM at 9,000 feet quietly leaves you short.
• Treating the seven month shutdown as free. Water sitting in a receiver from April to October rusts it. Our guides to layup and winterizing and tank rust both apply directly.
• Doing all the service in November. The plant should be serviced, tested and run in September, not the week you need it.
• Leaks. A resort air system is miles of pipe with hundreds of connections, much of it outdoors and thermally cycling. Leak rates at ski areas are often awful and nobody checks in July. See leak detection.
• Undersized hill piping. Adding guns without upsizing the main is how a plant with enough CFM still cannot hold 90 psi at the top.

Frequently Asked Questions

How much CFM does a snowmaking system need?

It depends entirely on gun type and count, not on trail acreage. Work from manufacturer SCFM curves at your operating pressure, multiply by the number of guns you actually run at once, then add distribution loss and altitude derate. Large destination resorts run in the tens of thousands of CFM. A twenty gun community hill running efficient guns may be in the low thousands.

What pressure do snow guns need?

Nucleation generally works between about 60 and 120 psi, with most operations running 70 to 100 psi. The important part is holding that pressure at the top of the hill with every gun open, which is a piping and storage question as much as a compressor question.

Should a ski area buy rotary screw or centrifugal compressors?

Rotary screw for most mountains. The capital cost per CFM is far lower, the seasonal start and stop is easy on them, and service is simple. Centrifugal makes sense at very large continuous volumes where the efficiency gain and oil free air path justify the price and the maintenance program.

Why do our nucleators keep freezing up?

Almost always water in the air. Air cools as it travels up the mountain, condenses, and the liquid freezes at the first small orifice it reaches. Fix it at the plant with adequate aftercooling, separation, drains at every low point, and a dryer rated below the coldest pipe temperature on the hill.

Does altitude really change how much compressor we need?

Yes, and at a western resort it is a large effect. Less dense intake air means less mass flow per revolution, so a machine rated at sea level delivers noticeably less at 8,000 or 9,000 feet. Cold intake air recovers a little of it, but not all of it. Size with the derate applied.

Where to start

If you are replacing a machine or adding guns, start with a demand analysis on the plant you have, not a catalog. Know your real simultaneous load, your real pressure at the top of the hill, and your real leak rate before you spend money. Then size the compressor with the altitude derate applied and build the cooling and drying to match the coldest pipe on the mountain.

Get those three right and compressed air for snowmaking stops being the thing that decides whether you open on time. Browse our rotary screw air compressors, or send us your gun list and elevation and we will work the numbers with you.