Dry Cooler, Cooling Tower or Chiller: Choose by Dry- and Wet-Bulb

This 13-page guide answers the question engineers and designers ask us most often: what to install for heat rejection, a dry cooler, a wet cooling tower or a chiller. Brand, price and fashion have little to do with it. The answer comes down to 2 numbers: the dry-bulb and the wet-bulb temperature at the installation site. Compare them with the fluid temperature your process needs, and the right class of equipment becomes obvious. You will not overpay for a chiller where a cooling tower is enough, and you will not fight water treatment for years where a dry unit would have done. The PDF is in Russian; the key points are below in English.

What is inside the guide?

• The two thermometers, dry-bulb and wet-bulb, and why they decide the choice.

• Dry cooler, open wet cooling tower and closed cooling tower: when to choose each, strengths and limits.

• Chillers, vapour-compression or absorption: when mechanical cooling cannot be avoided.

• A 3-step selection algorithm by temperature and a comparison matrix.

• Three typical mistakes that cost millions of rubles.

Key takeaways

Wet-bulb is always below dry-bulb. Dry-bulb is the ordinary air temperature, and it sets the limit for a dry cooler: a passive unit cannot cool fluid below the air. Wet-bulb is the temperature air reaches when water evaporates into it, sometimes 8 to 12 °C below dry-bulb. Wet cooling towers work on evaporation, so they cool fluid noticeably deeper than a dry cooler, at the price of water, water treatment and contact with outdoor air.

A dry cooler fits when the process accepts 5 to 10 °C above dry-bulb. Typical duties are oil, recirculating water from hot processes and data-center free-cooling loops. If the process accepts +50 °C, a dry cooler will handle it. On a +30 °C day, though, the realistic outlet temperature is +40 to +45 °C. In return you get a sealed glycol loop with no water consumption, no water treatment station and no Legionella risk. Adiabatic panels pre-cool the inlet air on hot days and give a margin for peak days.

An open cooling tower has the lowest CAPEX of all heat rejection options. It cools water close to wet-bulb, in practice to 3 to 5 °C above it. The downside: hundreds of thousands of cubic metres of outdoor air with dust, pollen and poplar fluff pass through the water, and the basin water warms to about 30 °C, a good environment for biological growth. A water treatment station is mandatory: a guide figure is RUB 300,000 to 400,000 per station plus about RUB 100,000 per year for chemicals.

A closed cooling tower gives tower temperatures with dry-cooler cleanliness. Process fluid flows through a sealed coil that is sprayed with water and blown with air from outside. The fluid stays clean and is cooled almost to wet-bulb. Without a chiller, the power draw of this scheme is about 10 times lower than a chiller scheme. It also works for liquid cooling of data centers, where the coil rejects heat directly. CAPEX is higher than for an open tower, and the external spray circuit still needs water treatment, though a simpler one.

Below wet-bulb, only a chiller will do. The practical line is roughly wet-bulb plus 3 °C. A vapour-compression chiller pays for cooling with electricity; an absorption chiller runs on heat: gas, steam or hot water. You get any temperature and a stable setpoint all year, but the highest CAPEX and energy use. The cooling tower or dry cooler stays in the scheme, now as the heat rejection unit of the chiller itself.

The rule in one line. The closer the required temperature is to wet-bulb, the more expensive the unit. Go below wet-bulb, and you cannot do without mechanical cooling.

The most expensive mistake is a chiller where a cooling tower would have been enough. If the process accepts a temperature above wet-bulb, a chiller means unnecessary capital cost and a much higher electricity bill for years. The other two mistakes: a dry cooler where the process needs a temperature close to wet-bulb, and an open tower where the loop should have stayed clean.

The guide's summary matrix (CAPEX and energy use on a relative scale; ₽ is the most affordable):

For choosing a whole plant cooling system, with project goals and a 10-year cost of ownership, see the separate guide Choosing a plant cooling system: 9 questions, 6 schemes, 1 algorithm.

Who is it for?

• Designers selecting heat rejection for a process or a data center.

• Chief engineers who are offered a chiller in a quote when the process may only need a cooling tower.

FAQ

Can a dry cooler reach cooling tower temperatures?

No. A dry unit cannot cool fluid below the outdoor air temperature. Trying to squeeze tower temperatures out of it inflates it to the size of a football pitch, and it still falls short in hot weather. A setpoint below dry-bulb belongs to a cooling tower.

When should an open tower be replaced with a closed one?

When the process fluid is expensive or sensitive, the heat exchangers cannot tolerate dirt, or fouling of the loop is unacceptable. Otherwise the CAPEX saved on an open scheme turns into fouling, downtime and chemical costs.

Related articles

• Cooling tower vs dry cooler: which to choose for equipment cooling

• How to choose an industrial cooling tower: open, closed or hybrid

• How much water does a cooling tower use, and how to cut make-up

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