Types of Evaporative Cooler Pads Compared: Aspen, Rigid Media, Synthetic and More

The pad is where a swamp cooler actually cools. Air passes through a wet medium, water evaporates, and the air leaves cooler and more humid. How completely the pad lets air approach the wet bulb temperature is called saturation efficiency, and it varies a lot by pad type. There are four types in common use: aspen wood fiber, rigid cellulose media, synthetic fiber pads, and thin honeycomb media in portable units. The right one depends less on preference than on what your cooler was built to hold.

Pad types at a glance

Efficiency figures are typical ranges cited by manufacturers and in the ASHRAE Handbook, measured at design face velocity with fully wetted media. Dirty, scaled or partly dry pads perform well below these numbers.

Aspen wood fiber pads

Aspen pads (sometimes called excelsior) are shredded aspen wood held in a plastic net, usually about 1 to 2 in thick. They have been the standard in Southwest coolers for decades for good reasons: aspen wicks water well, holds a lot of it, adds little resistance to airflow, and costs very little. Fresh aspen also tends to give a slightly fresh, woody smell for the first days.

The weakness is durability. Wood fiber compacts and sags, minerals crust on the air-entry side, algae grows on the sunny side, and fibers break down and shed into the pan. As aspen ages, air channels through thin spots and efficiency drops. In hard water areas without good bleed-off, many owners replace aspen at midseason and again at spring startup.

Best for

Existing aspen coolers, budget-conscious owners, anyone willing to change pads every season.

Watch out for

Thin or loose-packed bargain pads that leave gaps; pads not fully seated against the frame let air bypass.

Rigid cellulose media

Rigid media is corrugated cellulose paper treated with stiffening and anti-rot resins, stacked in alternating angled flutes and glued into a block. Munters CELdek is the best known; several manufacturers make similar products. Water is spread evenly across the top by a distribution header and runs down the flutes while air passes through horizontally. The cross-fluted geometry gives a huge wetted surface area per cubic foot, which is why rigid media reaches higher efficiency at the same airflow.

Common thicknesses are 4, 6, 8 and 12 in. Residential rigid-media coolers usually use 8 in. Thicker media raises efficiency and also raises airflow resistance (static pressure), so the blower and motor must be matched to it. Media is designed for a particular range of air velocity across its face; run it too fast and water droplets are carried off into the airstream.

With proper bleed-off and periodic cleaning, rigid media commonly lasts several seasons, often 3 to 5 or more. Neglected media in very hard water can scale up in one or two. For a direct comparison, see aspen vs rigid media.

Synthetic fiber pads

Synthetic pads, often blue, green or white, are made of polyester or blended synthetic fibers and sized to drop into aspen frames. They do not rot, shed little debris, can be rinsed and reused, and typically last several seasons. The tradeoff is that most hold less water than fresh aspen and wet less evenly, so cooling is usually similar to or a little below new aspen. They are a reasonable choice if you dislike the mess of aspen, or for coolers that are hard to reach for frequent pad changes.

Honeycomb and portable cooler media

Portable coolers from brands like Hessaire, Portacool and Honeywell use media cut to fit the unit: thin honeycomb cellulose, rigid media in thinner sections, or synthetic pads. Because thinner media has less contact time, efficiency is often lower than an 8 in rigid pad, which is one reason portables perform best in very dry air. Larger industrial portables may use thicker rigid media. Always buy the pad specified for your model; see the portable cooler buying guide.

What efficiency is worth: a worked example

Try your own climate in the cooling temperature calculator. Higher efficiency also evaporates more water per hour, roughly in proportion to the extra temperature drop; the water usage calculator estimates it.

Frame and conversion considerations

Pads are not interchangeable across cooler designs. Before buying anything other than a like-for-like replacement, check these points:

• Depth. Aspen frames are shallow louvered panels; 8 in rigid media needs a cabinet built for it.
• Water distribution. Aspen uses troughs fed by a spider. Rigid media needs an even header across the full top edge; uneven wetting leaves dry streaks that cut efficiency.
• Airflow resistance. Rigid media adds static pressure. A blower and motor sized for aspen may deliver less air through rigid media unless the pulley or motor is adjusted.
• Sealing. Any gap around rigid media lets air bypass the pad entirely.
• Conversion kits. Some manufacturers offer rigid media conversion kits for specific models. Without one, buying a cooler designed for rigid media is usually more reliable than a homemade retrofit.

If you are shopping for a new unit, the whole-house cooler buying guide covers rigid-media models.

Quality differences within a pad type

Not all pads of the same type perform alike. A few things to look for when buying replacements:

• Aspen density and fiber length. Good aspen pads are evenly packed with long fibers and no thin spots when held up to light. Bargain pads that are loosely packed hold less water and leave gaps sooner as they settle.
• Netting. Intact, UV-resistant netting keeps fibers from shedding into the pan and pump screen.
• Rigid media flute angles and treatment. Common rigid media uses two flute angles (often described as 45 and 15 degrees) that direct water toward the air-entry face. Media sold for evaporative cooling is treated for wet strength and rot resistance; untreated corrugated paper is not a substitute.
• Exact fit. A pad that is 1/2 in too small across the width of a panel opens a bypass strip the full height of the pad.

Some aspen pads are sold pretreated with additives intended to slow scale and algae. They can extend life somewhat, but they do not replace a working bleed-off and regular pad changes. See bleed-off and water quality.

Choosing the right pad

Whatever pad you use, fully wetted, clean media matters more than the label. For change intervals see how often to replace cooler pads, and for the job itself see how to replace swamp cooler pads.