Recently, controversy has been growing over the question of optimal compressed air pressure for snowmaking. Until a few years ago, conventional wisdom was that air pressure should be kept at 85 to 100 psi. Then, some designers introduced 150 psi air, insisting that higher snowmaking efficiency and lower friction rates could be realized. Others split the difference and designed systems for 125 psi. Now, the Dendrite group advocates a reduction of air pressure to 40 to 70 psi.
Just what is the role of compressed air in snowmaking and what are the implications on power consumption and snowgun performance of increasing or reducing air pressure?
First, it must be understood that compressed air aids the production of snow in three major ways: 1) atomization, 2) expansion cooling/nucleation and 3) droplet acceleration/propulsion.
Atomization is the process in which a stream of water is broken up into droplets. In a conventional air/water gun, this is achieved by mixing the water with compressed air that is expanding through a nozzle section. The expansion of the compressed air creates very high velocities that “blasts” the water to form droplets.
Expansion cooling/nucleation occurs as the compressed air passes through the nozzle section and expands. When air is compressed it naturally heats up and, if left alone, would cool down to approximately the ambient temperature when expanded. In snowmaking systems, the air is cooled before exiting the compressor station, so that when it expands through a snowmaking nozzle it gets extremely cold. This helps cool the snowmaking water and forms ice nuclei for the water droplets to freeze around.
Droplet acceleration/propulsion is accomplished in conventional snowguns by means of rapidly moving compressed air expanding through the nozzle. The fast moving air stream imparts much of its momentum to the water droplets, thereby picking up the slower moving water droplets and propelling them high into the air.
What Air Pressure Does
How does compressed air pressure affect these processes?
Research into compressed air atomization has indicated that compressed air forms droplets by creating sharp waves on the surface of the water stream as it blows by. These waves travel a short distance until they are swept up into the rapidly moving air stream. The faster the air is moving, the more effective it is at breaking up the water.
Curiously enough, the speed of the compressed air in a snowgun is not increased if the pressure is increased (providing the air pressure is above 25 psi). This is because the air is already moving at the speed of sound through the nozzle exit; it can go no faster than that. On the other hand, if the pressure is increased, the air inside the snowgun becomes more dense. This helps break up the water, just as swinging a heavier axe makes it easier to split wood. Therefore, the higher the compressed air pressure, the more effective it will be at breaking up the water and, in general, the smaller the droplets will be produced.
As to the expansion cooling/nucleation process the higher the air pressure is, the more it will cool when expanded to ambient pressure. This should enhance the snowmaking process and does, but to a limited extent. However, even during marginal snowmaking conditions, expansion cooling contributes only a very minor portion (roughly three percent of the cooling required to freeze a water droplet). The more dominant influence of expansion cooling is on nucleation rates, as the lower temperatures that will result from the expansion of higher compressed air pressures will immediately freeze larger particles to act as ice seed nuclei. Depending on the design of the snowgun, this may or may not improve performance.
Droplet acceleration/propulsion in snowmaking is also affected by air pressure. As mentioned previously, increasing the air pressure does not increase the velocity of the air inside the snowgun, because it is limited to the speed of sound. However, the higher density resulting from increased air pressure can improve the propulsion rate — the best analogy for this is a heavier bat hitting a ball farther — as a result, the droplets will leave the snowgun a little faster. The effect of this will depend on the snowgun design, but in general, above a certain pressure (60 to 70 psi for many guns) the effect is minimal because the droplets quickly decelerate to their terminal velocity.
How Much Does Air Pressure Cost?
In all three of these aspects, higher compressed air pressure presents some potential benefit to the snowmaking process. However, as will be shown below, there is a substantial expense to higher compressed air pressures. If a snowgun is designed to achieve the desired atomization/expansion cooling/droplet acceleration without resorting to higher air pressures, it will be far more energy efficient.
When testing a snowgun design, it may be determined that increasing the air pressure improves the visual performance of the gun. Whether it increases the efficiency of the gun depends on how much energy is required to produce the higher air pressure. A snowgun operating at high air pressures may have a low air/water ratio but be less efficient than at standard pressures if more energy is consumed in producing the high-pressure air. Conversely, a low pressure snowgun operating at a worse air/water ratio may be more efficient if the low pressure air requires less energy to produce.
To compare the requirements for producing various pressures of compressed air, operating levels of specific centrifugal compressors were obtained from Ingersoll-Rand. Power consumption numbers depend on a variety of factors including the machine selected, the inlet conditions, and the temperature of the cooling water. The averages indicated below are given for representative low elevation (1000-foot elevation), 35° inlet air temp and 50° cooling water.
The data indicate that, in most size ranges, the power requirements for low pressure are highly dependent on the machine selected. Machines are available that provide good efficiency when operated between pressures of 80 and 150 psi, or 50 to 70 psi, but it is difficult to find a machine that can be efficiently operated between 50 and 100 psi. This means that when evaluating compressors operating in the 60 to 75 psi range, the power consumption depends on whether a higher pressure machine operating at the low point of its range is selected, or whether a low pressure machine operating at the top of its range is selected. The data is included below:
| Pressure | Power Consumption | % Power Change From 100 PSI |
|---|---|---|
| 150 psi | 23.4 BHP/100 cfm | +13.0% |
| 125 psi | 22.3 BHP/100 cfm | +7.7% |
| 100 psi | 20.7 BHP/100 cfm | — |
| 70 psi | 19.0 BHP/100 cfm | −8.2% |
| 70 psi* | 16.8 BHP/100 cfm | −18.8% |
| 50 psi* | 15.2 BHP/100 cfm | −26.5% |
To put these figures in a more familiar form, equivalent air/water ratios were calculated using these power figures at the various pressure ranges. This table indicates at what air/water ratios snowguns operating at different air pressure ranges use the same amount of power:
| 150 psi | 100 psi | 70 psi | 70 psi* | 50 psi* |
|---|---|---|---|---|
| 12.4 | 14.0 | 15.3 | 17.3 | 19.0 |
| 10.6 | 12.0 | 13.0 | 14.8 | 16.3 |
| 8.8 | 10.0 | 10.9 | 12.3 | 13.6 |
| 7.1 | 8.0 | 8.7 | 9.9 | 10.9 |
| 5.3 | 6.0 | 6.5 | 7.4 | 8.2 |
| 3.5 | 4.0 | 4.4 | 4.9 | 5.4 |
Using this table, it can be determined that a snowgun running at a 10:1 air/water ratio at 100 psi uses the same amount of power as a snowgun running at an 8.8:1 air/water ratio at 150 psi.
How About Air Piping Friction?
The other implication of air pressure on snowmaking operations involves the friction loss that will be experienced in the air piping. When air is compressed, it takes up less volume. Therefore, at the same flow rate, a higher pressure air flow will travel more slowly through a pipe than a lower pressure air flow. This results in a lower pressure drop at higher air pressures as indicated in the tables below:
| 150 psi | 100 psi | 70 psi | 50 psi | |
|---|---|---|---|---|
| 12″ | .047 | .067 | .091 | .119 |
| 10″ | .114 | .164 | .222 | .290 |
| 8″ | .366 | .526 | .712 | .932 |
| 150 psi | 100 psi | 70 psi | 50 psi | |
|---|---|---|---|---|
| 10″ | .030 | .043 | .058 | .076 |
| 8″ | .095 | .136 | .184 | .241 |
| 6″ | .380 | .552 | .748 | .979 |
From these tables it can be seen that changing the system air pressure from the conventional 100 psi will have the following effect in existing pipelines:
- Change from 100 to 150 psi = decrease friction by 30%
- Change from 100 to 70 psi = decrease friction by 38%
- Change from 100 to 50 psi = decrease friction by 78%
The impact of these friction rates will depend on each piping system, but in general are not as dramatic as it might seem. This is because:
- In many existing snowmaking systems, friction losses can be substantially reduced by looping the flow from multiple lines and replacing limited sections of undersized pipe that act as a bottleneck.
- The friction tables show that when increasing the pressure from 100 to 150 psi, the decrease in friction is not enough to warrant a drop in the pipe size. (For example, with 10,000 cfm, the loss in a 10 inch line at 100 psi is .164 psi/100 feet; with an 8 inch line at 150 psi the loss is .386 psi/100 feet, or 135 percent greater.
- Similary, when decreasing the pressure from 100 psi to 70 or 50 psi, increasing the pipe sizing by one diameter compensates for the increased friction loss (i.e., with 10,000 cfm in a 10 inch line at 100 psi, the loss is .164 psi/100 feet; with a 12 inch line at 50 psi, the loss is .119 psi/100 feet or 27 percent less). In most cases, the strategic placement of an air trunk line can upgrade the pipe network to use lower air pressures.
In general, the difference in friction rates is not large enough to drive the decision process on which air pressure to use in an existing snowmaking system. This would be analogous to selecting a car solely because it used the same size tires as the previous model.
Is There a “Best” Pressure?
This review of the role of compressed air in snowmaking and the potential impact that changing the air pressure will have on efficiency indicates that higher air pressure has the potential to aid the snowmaking process, but at a price of increased energy consumption. The optimal air pressure depends on the specifics of the snowgun design, so no blanket generalizations can be made on the “best” air pressure for snowmaking.
The table with equivalent air/water ratios for different compressed air conditions can assist in the evaluation of varying air pressures. This table is very important as it provides operators with a yardstick for comparing different operating pressures based on actual compressor performance characteristics.
As to piping friction losses, higher pressure lowers pipe friction, but not to a major extent. The best procedure is to evaluate snowguns to determine the optimal operating pressure and then the specific piping network to determine if modifications are necessary, given the operating pressure selected.
