Knowledge/Systems/How evaporative cooling actually works
How evaporative cooling actually works
An evaporative cooler can only cool the air by the gap between dry bulb and wet bulb on the day, the wet bulb depression. That gap is the limit, and no amount of money buys past it. The cooler gets a share of it, typically 70 to 85 per cent, and rigid cellulose pads run 70 to over 95.
- Supply air
- Dry bulb minus a share of the gapthe AS 2913 equation
- Typical efficiency
- 70 to 85%as declared by the maker
- Rigid cellulose pads
- 70 to 95%+depends on thickness and air speed
- Governing standard
- AS/NZS 2913the only test standard there is
The equation, in plain terms
The formula from AS 2913 looks worse than it is. Take the gap between the outside dry bulb and the outside wet bulb. Multiply it by the efficiency, as a fraction of 100. Take that off the outside dry bulb and you've got your supply air temperature.
Written the way the standard has it, that's to = ti - (e/100 x (ti - twi)). In that, to is the supply air, ti the outside dry bulb, twi the outside wet bulb, and e the efficiency in per cent.
The gap is the wet bulb depression, and it's the whole story. It's the most cooling the air can possibly give you that day. Efficiency just tells you what share of it the machine captures.
So on a Melbourne day at 35°C dry bulb and 18°C wet bulb, you've got 17 degrees of depression to play with. At 80 per cent efficiency that's about 13.6 degrees of cooling, giving supply air around 21°C. Good day for it.
Why a humid day kills it
Take the same 35°C with a wet bulb of 26°C, which happens here in a northerly before a change. Depression is now 9 degrees. At the same 80 per cent efficiency you get 7.2 degrees of cooling, so supply air around 28°C, and you're adding moisture to the house while you do it.
Nothing is wrong with the unit. This is the limit of the physics, and it's the single most useful thing you can explain to a customer who thinks their cooler has failed. It hasn't. The weather changed.
It's also why the honest answer in a hot, humid climate is refrigerated, and why on a dry inland Victorian summer evaporative is genuinely hard to beat on running cost.
Pads, and what they're actually made of
| Pad type | What it is | Replacement |
|---|---|---|
| Rigid cellulose, Chillcel or Celdek type | Treated (impregnated) corrugated cellulose paper sheets, with the flutes laid at two different angles | Every 7 or 8 years |
| Aspen or woodwool | A pack of thin shredded wood slivers, 3 to 5 cm thick | Every 3 to 4 years |
New pad smell is normal and it isn't a fault. The fix is the controller's pad flush function, and telling the customer it's coming saves you the phone call.
What the standard doesn't cover
AS/NZS 2913:2000 is the only standard in Australia for testing evaporative air conditioners. It rates airflow, evaporation efficiency, sound power and how much power they draw.
- It doesn't test how much water the unit uses
- There's no energy rating for evaporative equipment
- It doesn't cover indirect or two stage evaporative units at all
Worth knowing when a customer asks you to compare star ratings between an evaporative cooler and a split system. You can't, because one of them doesn't have any.
Weighing up what to put in front of a customer?
Most of the hard calls aren't technical. They're about what you can stand behind when the customer rings you in February.
Kevin's been having that conversation for thirty years. Ask him.
Who this is for: small and medium HVAC contractors in Victoria. If you're a homeowner after a rebate or a repair, you need a licensed contractor, not us, and yours can point you the right way.
Related
- Sizing an evaporative cooler on air changes per hour20 to 40 air changes an hour is the engineering range and 30 is the trade shortcut. Both are sourced. Then the 80 Pa check that decides your outlet count.
- How an indirect gas ducted heater worksIndirect means the flue gases never mix with the air you breathe. How it holds outlet temperature, what shuts it down, and the gas pressures it needs.
- Can you reuse the ducts on a gas to reverse cycle swap?The honest version. No engineering document covers reusing the ducts, so here's what's checked, what's our best judgement, and the seven things that go wrong.
- Hydronic heating, and what the rental rule does to itSteady radiant warmth, slow to respond. And under the 2027 rental rule, a failed gas boiler points to reverse cycle rather than an air to water heat pump.
Review date: 1 April 2027. Figures in this area change without much notice and this page carries some that will go stale. Every number above is dated and sourced so you can check it yourself. If you find something out of date here, tell us and we will fix it.