Cooling Tower in Design: Webinar Slides on Selection and Checks

These are the slides of my talk "Cooling tower in design" at the webinar "Industrial ventilation and cooling" on 26 August 2026. Fifteen minutes on the four areas where designers make mistakes most often: the design point, the choice of tower type, water and winter, pumps and siting. All of it can be checked on paper in an hour, or on site over two seasons. The talk is for design engineers, lead engineers and owner's engineers; behind it is the practice of Cold Energy (NCT), with more than 350 installed cooling towers and membership in CTI (Cooling Technology Institute). The PDF is in Russian; the key points are below in English.

What is inside the slides?

• Why a cooling tower at all: once-through, dry cooling and a recirculating loop in numbers.

• Design point: two sources of the wet-bulb temperature, a table for six cities, SP 60 siting corrections.

• What to send the manufacturer with the brief: six items.

• Three tower types, open, closed-circuit and dry: leaving water temperature, make-up, capital cost.

• How the cooling tower sets the chiller COP, and when the chiller leaves the scheme.

• A cooling tower for an absorption chiller, and two modes of a closed-circuit tower: plume abatement and winter without spray.

• A worked water balance and where capacity goes by the second year.

• Open-circuit pumps: head and NPSH.

• Recirculation, site plan and eight final questions.

The handout for the talk: Cooling tower design checklist: 56 checks.

Key takeaways

A recirculating loop needs almost forty times less fresh water. A 1,000 m³/h loop cooled by 10 K needs 1,000 m³/h of fresh water once-through. With a cooling tower it is 17 m³/h of evaporation plus blowdown, about 25 m³/h of make-up. That is why the tower is on site.

The cooling limit is the wet bulb, not the thermometer outside. By evaporating 1 % of the flow, the tower cools the other 99 % by about 5.5 K. Design wet bulb per the VNII VODGEO cooling tower manual (Table 7) at 1 % exceedance: Moscow 20.8 °C, St. Petersburg 20.1, Kazan 18.7, Krasnodar 21.6, Yekaterinburg 18.8, Novosibirsk 19.3 °C. The records there end in the late 1980s, so check the figures against current climatology.

Siting moves the design point by 6 K. Under SP 60.13330.2020 (Russian HVAC code), clause 8.15, the Moscow table value of 20.8 °C becomes 22.3 °C in shade, 23.8 °C in sun and 26.8 °C on a roof next to walls. Six degrees of wet bulb means a different unit in size and price, so the location is chosen before selection.

The three tower types differ by multiples. Open: leaving water at wet bulb + 3 to 5 K, make-up about 1.8 % of flow at a 10 K range, baseline capital cost. Closed-circuit: wet bulb + 5 to 8 K, make-up only for spraying the coil, capital cost ×1.8 to 2.5. Dry: dry bulb + 5 to 10 K, zero make-up, capital cost ×1.5 to 3 and a larger footprint.

The cooling tower sets the chiller COP. An air-cooled chiller condenses at about 42 to 45 °C, with a dry cooler at 38 to 43 °C, with a wet cooling tower at 30 to 34 °C, which is 8 to 12 K lower. Each degree of condensing temperature is 2 to 3 % of compressor power; ten degrees are 20 to 30 % of electricity, every hour the machine runs. If the process is fine with 30 to 40 °C, a closed-circuit tower removes the chiller altogether: 0.02 to 0.04 kW per kW against 0.20 to 0.30 kW for a chiller with a water-cooled condenser, an order of magnitude.

For an absorption chiller there is effectively no choice of tower type. Per 1 kW of cooling, a compressor chiller sends ×1.25 heat to the tower and an absorption chiller ×2.4, and the machine needs 30 to 32 °C water. A dry cooler gives 33 to 38 °C and does not fit; a closed-circuit tower on double heat becomes huge. An open wet tower at a 19 to 21 °C wet bulb gives 23 to 26 °C with margin. The absorption chiller itself draws 5–10 kW of electricity per MW of cooling.

The water balance takes two minutes on a notepad. For 1,000 m³/h and a 10 K range: evaporation 17 m³/h, blowdown 8.5 m³/h at 3 cycles of concentration, make-up 25.5 m³/h. At 7 cycles make-up is 19.8 m³/h, 22 % less. The client should see this figure before the contract.

Without water treatment the tower loses capacity by year two. Orders of magnitude from industry sources (CTI, ASHRAE): 0.5 mm of scale costs 10 %, 1 mm costs 20 %, 0.5 mm of biofilm 30 %, clogged fill 25 %, plume recirculation 15 %, winter icing 35 %. At equal thickness, biofilm is worse than scale.

In an open circuit the pump dies of cavitation. Suction head comes only from the basin water level: NPSHa = Hatm (about 10.3 m of water) + Hstatic − Hlosses − Hvapour, and it must exceed NPSHr by 0.5 to 1 m. Hot return water at 35 to 40 °C, a pump above the basin and a clogged strainer eat the margin; the impeller is destroyed within weeks.

Who is it for?

• Design engineers and lead engineers issuing a cooling tower section.

• Owner's engineers comparing offers and accepting the design.

• Anyone choosing a heat rejection scheme for a chiller or an absorption chiller.

FAQ

What should go to the manufacturer with the brief?

Water flow and the heat load it comes from; entering and leaving water temperatures; the design wet bulb and its exceedance level; redundancy requirements; the site location with height and layout limits; the operating mode, year-round or seasonal.

How is pump head different in an open circuit?

In a closed loop the rise is offset by the fall. In an open circuit the water exits to atmosphere at the top of the tower, so the static lift from basin to water distribution, the pressure ahead of the nozzles and a fouling allowance are added to pipe losses.

Related articles

• 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

• What is an absorption chiller? How it works, when it beats a chiller

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