Choosing a Plant Cooling System: 9 Questions, 6 Schemes, 1 Algorithm

This is a 20-page guide for chief engineers, technical directors, designers and procurement teams who are choosing a cooling scheme for a plant and do not want to overpay over a 10 to 15 year horizon. In nine out of ten projects that reach us at the quotation stage, the scheme has already been chosen, often badly, and the customer pays for it for decades. The guide sets out my approach: 9 questions to answer before you open supplier quotes, 6 working schemes, and 1 algorithm that narrows the choice down to one or two suitable options. The PDF is in Russian; the key points are below in English.

What is inside the guide?

• Goal and risks: what you are optimising and which risk worries you most.

• Nine questions before the quotes: from payback and automation to the cost of downtime and energy use.

• Natural or mechanical cooling: why the wet-bulb temperature decides everything.

• Six schemes: open cooling tower, closed cooling tower, dry cooler with adiabatic pre-cooling, packaged air-cooled chiller, chiller with free cooling or with a water-cooled condenser and tower, and a lithium bromide absorption chiller.

• A comparison table of the schemes to use as a checklist in supplier meetings.

• A one-page algorithm: goal, site resources, load type, total cost of ownership.

• Five typical mistakes that cost millions.

• Three practical steps after reading.

Key takeaways

The line is drawn at wet-bulb plus 3 °C. If the coolant setpoint is below the wet-bulb temperature plus 3 °C, you need mechanical cooling: a vapour-compression or absorption chiller. If the setpoint is at or above that line, a cooling tower will do the job without a chiller. Design wet-bulb temperatures under SP 131.13330.2020 (Russian building climatology code) are 20 °C for Minsk, 21 °C for St. Petersburg and 22 °C for Moscow. A cooling tower's approach to wet-bulb is usually 3 to 5 °C, so in summer a wet tower delivers coolant at roughly 23 to 26 °C. For processes with a setpoint of 25 °C and above, a chiller is not needed at all.

Natural versus mechanical cooling changes costs by an order of magnitude. A cooling tower spends electricity only on fans and pumps: an open tower without a chiller uses about 5% of the electricity of a chiller scheme. If a chiller cannot be avoided, at loads from 1 MW a chiller with a water-cooled condenser and cooling tower runs at 20 to 40% lower operating cost than a packaged air-cooled unit.

A packaged chiller suits small and medium loads. An air-cooled chiller fits loads up to 1 to 2 MW of cooling when the site has no water or no room for a tower and the facility has to start quickly. The price of convenience is a low coefficient of performance (COP) and a high electricity bill in hot weather. Free cooling adds 15 to 30% to the capital cost compared with a packaged unit, but in a region with a long cold season the compressor stays off for most of the year.

An absorption chiller makes cooling from heat. A Thermax machine draws about 3 kW of electricity per 1 MW of cooling, tens of times less than a vapour-compression chiller of the same capacity. It makes sense when steam from a combined heat and power plant, flue gas or other waste heat is currently vented to the atmosphere, when cheap gas is available, or when the site has hit its grid connection limit. The limits: thermal COP of 0.7 to 0.8 for single-effect and 1.3 to 1.4 for double-effect machines, and about 30 minutes to reach rated output, so critical processes get a backup chiller sized at 5 to 20%.

Compare TCO, not the price in the quote. Total cost of ownership in the guide is CAPEX + 10 × annual OPEX + service, spare parts, water treatment and downtime. The option that wins on capital cost often loses on TCO by a factor of 1.5 to 2. If a supplier will not give you a 10-year calculation, that is a bad sign.

Water treatment is a separate line in the budget. Estimates for a wet tower often include only the tower itself. A year later come fouling, a 20% loss of capacity, emergency cleaning and a wastewater fine. As a rough estimate, a water treatment station costs RUB 300,000 to 400,000, plus about RUB 100,000 per year for chemicals.

Use hourly climate data. A design based on standard SNiP (legacy Russian building code) reference points, without the actual site, loses 20 to 30% of capacity in summer. You need multi-year hourly data for the specific location (bin data).

Size redundancy from the cost of downtime. For continuous production, plan 5 to 20% spare capacity or N+1 (duty units plus one standby) on key sections. Work out the cost of an hour of downtime before the project starts.

The narrower question, which heat rejection unit to use for a given fluid temperature, I cover separately: Dry cooler, cooling tower or chiller: choosing by dry-bulb and wet-bulb temperature.

Who is it for?

• Chief engineers and technical directors choosing a cooling scheme for the first time or after a long break.

• Designers who need to justify a scheme to the customer with numbers.

• Procurement teams: the comparison table works as a checklist in a supplier meeting.

FAQ

When can a cooling tower work without a chiller?

When the coolant setpoint is not below the wet-bulb temperature plus 3 °C. For most regions of Russia that means a setpoint of 25 °C and above. Below that line the evaporative limit cannot be reached, and you need mechanical cooling: a vapour-compression or absorption chiller.

Where do I start if supplier quotes have already arrived?

Do not choose the scheme from the quotes. First write down your answers to the 9 questions; one A4 sheet is enough. Then prepare a technical specification with setpoints, operating modes, available resources and site constraints, and send it to suppliers. That way you compare like with like instead of unrelated offers. Ask every supplier for a 10-year TCO.

Related articles

• How to cut industrial cooling costs: 8 measures with the numbers

• Chiller total cost of ownership: purchase vs 15 years of running

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

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