Misting System Theory: A Deep Dive
High-pressure misting systems work on a thermodynamic principle called evaporative cooling. An evaporative cooling system is one in which total heat neither increases nor decreases. Here’s a (slightly) technical overview:
Definition: Evaporative cooling happens when a liquid (such as water) evaporates by absorbing heat from its surroundings and, in the process, reduces the ambient temperature. Evaporative cooling involves the phase change of a liquid to a vapour.
Mechanism: For a liquid to change into vapour, it needs energy (this is called the latent heat of vaporisation). This energy is taken from the liquid itself and its immediate surroundings. As the liquid evaporates it takes this heat and the surrounding air cools down.
Total Heat: In evaporative cooling, the total heat (of the liquid and its surroundings) neither increases nor decreases. There is no forcible addition or removal of heat, like with an air conditioner. Heat transfers from the surroundings to the liquid by the well-understood principles of convection, conduction and radiation.
A high-pressure misting system, then, provides a dramatic cooling effect simply by creating billions of tiny water droplets.
Let’s dig in deep. We have three aspects to talk about:
- The misting system
- Air
- Water.
The Misting System
Our misting systems use high-pressure water, around 50 bar (~750 psi), to atomise water effectively. Atomisation is a mechanical process where liquid (in this case, water) is physically broken down into billions of tiny droplets.
A high-pressure misting system includes a low-noise pump, a water supply, and a distribution network of nozzles connected by piping.
Its purpose is to deliver billions of tiny water droplets to the outdoor space to be cooled.
Pump
The pump is specially designed for misting. We use a low-noise positive displacement pump that delivers water at the high pressure necessary to activate the nozzles. Our pumps include additional features such as filtration, overload protection, no-flow protection, timers, and pressure relief. All these are important for trouble-free safe operation.
Nozzles
The nozzles are small yet complex. They include a precision orifice jet, typically 150 µm, a disposable microfilter, and a spring-loaded no-drip valve. When properly pressurised, each nozzle atomises a flow of about 40 ml of water per minute into billions of tiny water droplets, each averaging 5 µm in diameter (one-tenth the diameter of a human hair).
The diameter of each water droplet is crucial. It is the big difference between a high-pressure system, a low-pressure system, and cheap systems that rely on mains pressure (~3 bar, or 45 psi) straight out of the tap.
Distribution
The distribution network is simply one or more high-pressure piping runs that interconnect nozzles to the pump. There can be quite a distance between the pump and the distribution network because the combination of high pressure and low flow rates means pressure drop is not an issue. It’s normal to space nozzles 600 to 750 mm apart, but this can and should change if site conditions require.
Now, let’s talk about air.
Air
We learned in school that air is a mixture of two gases – nitrogen and oxygen – with much smaller amounts of other gases.
Moisture Content
One of those ‘small amount’ gases is water vapour; air at 25°C can hold up to 24 ml of water vapour per cubic metre. This is very temperature dependent – air at 35°C, for example, can hold up to 41 ml, and air at 15°C holds just 13 ml.
So – air can hold a lot of water as invisible water vapour. However, we have yet to talk about relative humidity.
Relative Humidity (RH)
Relative humidity is a measure of how much water vapour is in the air – air at 25°C and 50% humidity, for example, holds just 12 ml of water per cubic metre and can absorb another 12 ml without condensation forming.
Wet Bulb Temperature
Have you seen how, as the night air cools, water condenses on windows, walls, and anything left outside? This is because as air cools, its moisture-holding capacity diminishes. Once the air drops to its wet bulb temperature (where relative humidity is 100%), that water vapour condenses out to liquid water. You may even see frost and ice form if it’s really cold.
The wet bulb temperature is the temperature at which air can’t hold any more water vapour. Here are some examples:
- Air at 25°C, 75% humidity: 21°C wet bulb
- Air at 25°C, 50% humidity: 18°C wet bulb
- Air at 25°C, 25% humidity: 14°C wet bulb.
These wet bulb temperatures are achievable with a properly designed misting system.
Specific Heat
Air has a property called specific heat, and it’s measured at 1.18 kJ/m³.°C. This tells us the amount of heat, as in kilojoules, that’s needed to elevate (or reduce) one cubic metre of air by 1°C. So, if we have a way to remove 1.18 kJ from one cubic metre of air, its temperature will be reduced by 1°C.
Okay? Let’s move to water.
Water
Water is magical stuff. Not only is it abundant on this piece of rock we call Earth, but it is also the most effective coolant available to humankind (except for a couple of exotic, non-natural materials).
Liquid water requires a massive 2,260 MJ/m³ to transform into water vapour, even without a temperature change.
And this is the information we need to put this all together.
Assembling Our Misting System
We know that heat is neither added nor removed by our misting system.
So if our air at 25°C and 50% RH can absorb 12 ml of water, and we add water in the form of water droplets, that water will take heat from the surrounding air and transform from liquid water droplets to water vapour.
We know that for our 25°C 50% RH air, the lowest temperature we can achieve without condensation is 18°C.
We also know that each nozzle discharges 40 ml of water per minute in the form of 600 billion water droplets (!).
Cooling Effect
This means:
- Each nozzle can theoretically cool over 8m³ of air to 18°C every minute.
- Each nozzle generates a massive cooling effect of 1.2 kW (1 kW is the same as 1 kJ per second).
Energy Use
Air conditioner manufacturers quote their efficiency as “coefficient of performance”, or COP. This is simply a ratio of the heat removed divided by the energy consumed. A modern inverter air conditioner can achieve a maximum COP of around 6, which means that for 2 kW of consumed energy (typical for a domestic room air conditioner), you get 12 kW of cooling.
So. What about our misting system?
Our smallest pump consumes 60 W (0.06 kW, or 0.06 kJ/sec) to supply 6 nozzles with high-pressure water.
At 1.2 kW per nozzle, this cooling effect is 7 kW. And for just 60 W of energy consumed – that’s an equivalent COP of 120.
Now, that’s energy efficiency.
Droplet size
Tiny water droplets mean your high-pressure misting system won’t wet you or the area around you. Here’s why.
Back in 1851, a scientist called George Gabriel Stokes derived an expression, now known as Stokes’ law, for the frictional force exerted on spherical objects. It tells us how fast a water droplet will fall due to gravity.
We expect that smaller water droplets fall slower because of the friction from air molecules. Ever driven through fog? Fog is pretty much the same as our misting system droplets; they are so small they just hang in the air, almost unaffected by gravity.
As per Stokes' Law, we calculate that our 5 µm water droplets fall at 0.7 mm/ sec. In perfectly still air and with the nozzles installed at the recommended 2.4 metre height, they take just under an hour (57 minutes) to hit the ground. More than enough time to evaporate, don’t you think?
Let’s follow that rabbit hole for a minute because droplet size is important for another reason.
The droplets are spherical due to the effects of water’s surface tension, and we can calculate the total surface area of all those droplets generated every minute per nozzle.
It works out to be 48 square metres (!).
Try this thought experiment. Imagine you take 40 ml of water and spread it evenly in an area of 6 x 8 metres (48 square metres), about the size of a double garage. How long would it take to dry? Less than 57 minutes, for sure.
This is why a high-pressure system won’t wet surfaces – the evaporation process is complete within seconds of discharge, and the cooling is immediate.
Nice.
Closing Thoughts
A high-pressure misting system:
- Is a fully engineered and integrated system.
- Cools outside air to its wet bulb temperature which, in some instances, can be down to single-figure cool.
- Is incredibly energy-efficient, far beyond anything achievable with an air conditioner.
- Doesn’t get anything wet.