Heat Rejection Calculator — Dry Coolers & Cooling Towers

Heat rejection equipment is sized on approach temperature, not on the load alone. A small change in approach can double or halve the surface you need.

An estimate for comparing options. Final selection depends on the manufacturer’s coil data at your exact design condition, fluid and glycol concentration.

Approach is the whole design

Rejection equipment does not care much about the absolute load — it cares about how close you are asking the fluid to get to ambient. Going from a 10 K approach to a 5 K approach roughly doubles the coil surface for the same duty. Where the process tolerates it, raising the fluid temperature is by far the cheapest way to shrink the equipment.

Dry or evaporative

Dry coolers reject to dry bulb; cooling towers reject towards wet bulb, which in most climates is several degrees lower. That headroom is what makes evaporative rejection viable where dry cooling is not — at the cost of water consumption, treatment and a maintenance regime that dry equipment does not need.

Glycol has a price beyond the litres

Adding glycol for freeze protection lowers specific heat and raises viscosity, so the same duty needs more flow and more pumping power, and the coil performs slightly worse. Use the lowest concentration that actually protects your minimum ambient rather than a round number.

Link to or cite this tool

If this calculator is useful in a specification, tender document or forum answer, please link to it rather than copying the figures — the assumptions behind it are stated above and may be updated.

https://www.hvacrnexus.com/tools/heat-rejection-calculator/

Working out the fan side

Once the refrigeration duty is fixed, the evaporator fans and any condenser fans are a separate calculation. Longwell publishes a set of fan-side tools — airflow, static pressure, fan laws and motor sizing — at longwellfans.com.

Equipment this calculation points to

Questions about this calculation

What approach temperature should I design for?
It depends on what the process can accept. Anything below about 5 K becomes expensive in surface area, and below 3 K dry cooling generally stops being practical at summer ambients. Widening the approach by raising fluid temperature is usually the cheapest lever available.
When should I choose a cooling tower over a dry cooler?
When the approach to dry bulb is too tight at your design ambient, or when footprint is constrained — evaporative equipment rejects far more per unit area. Against that, weigh water cost, treatment, legionella management and winter operation.
Does glycol change the sizing?
Yes. It lowers specific heat, so the same duty needs a higher flow rate, and it raises viscosity, which increases pumping power and slightly degrades heat transfer. Specify the lowest concentration that covers your actual minimum ambient.

Send us the duty and we will quote matched equipment

Give us the figure this calculator produced, your room temperature and your location — we will come back with condensing unit and evaporator options, price and lead time, usually within one working day.